Clerodane diterpenes: sources, structures, and biological activities

Abstract

The clerodane diterpenoids are a widespread class of secondary metabolites and have been found in several hundreds of plant species from various families and in organisms from other taxonomic groups. These substances have attracted interest in recent years due to their notable biological activities, particularly insect antifeedant properties. In addition, the major active clerodanes of Salvia divinorum can be used as novel opioid receptor probes, allowing greater insight into opioid receptor-mediated phenomena, as well as opening additional areas for chemical investigation. This article provides extensive coverage of naturally occurring clerodane diterpenes discovered from 1990 until 2015, and follows up on the 1992 review by Merritt and Ley in this same journal. The distribution, chemotaxonomic significance, chemical structures, and biological activities of clerodane diterpenes are summarized. In the cases where sufficient information is available, structure activity relationship (SAR) correlations and mode of action of active clerodanes have been presented.

1. Background and introduction

1.1. The sources of clerodane diterpenes

Clerodane diterpenes are a large group of naturally occurring secondary metabolites found in several hundreds of plant species from various families and in organisms from other taxonomic groups, such as fungi, bacteria, and marine sponges. Table 1 illustrates the occurrence of clerodane diterpenes in the plant kingdom and marine animals.

Table 1.

The occurrence of clerodane diterpenes in the plant kingdom and marine animals

Division	Class	Family	Genus	Number of species
Magnoliophyta (flowering plants)	Dicotyledon	Lamiaceae	Ajuga, Ballota, Elsholtzia, Glossocarya, Gomphostemma, Kinostemon, Nepeta, Otostegia, Plectranthus, Salvia, Scutellaria, Teucrium	81
		Verbenaceae→Lamiaceae	Callicarpa, Clerodendrum, Cornutia, Premna, Vitex	7
		Euphorbiaceae	Aparisthmium, Cleidion, Croton, Macaranga	27
		Compositae or Asteraceae	Aster, Baccharis, Conyza, Haplopappus, Microglossa, Nannoglottis, Pulicaria	18
		Flacourtiaceae→Salicaceae	Casearia, Laetia, Zuelania	16
		Menispermaceae	Burasaia, Fibraurea, Tinospora	10
		Annonaceae	Polyalthia	7
		Portulacaceae	Portulaca	3
		Caesalpiniaceae	Detarium, Hymenaea	2
		Meliaceae	Amoora	2
		Araliaceae	Cussonia	1
		Chrysobalanaceae	Licania	1
		Combretaceae	Bucida	1
		Loganiaceae→Stilbaceae	Nuxia	1
		Mimosaceae	Entada	1
		Rutaceae	Clausena	1
	Monocotyledon	Dioscoreaceae	Dioscorea	2
		Alismataceae	Echinodorus	1
		Hydrocharitaceae	Halophila	1
Pteridophyta (ferns)		Gleicheniaceae	Dicranopteris	5
Marchantiophyta (liverworts)		Geocalycaceae	Heteroscyphus	2
		Jungermanniaceae	Jamesoniella	2
		Scapaniaceae	Scapania	2
		Adelanthaceae	Adelanthus	1
		Lejeuneaceae	Thysananthus	1
Marine animals		Marine sponge	Agelas	3
		Marine mollusk	Syphonota	1

Clerodane diterpenes have attracted interest in recent years as a result of their noteworthy biological activities, particularly as agents modifying the feeding behavior of many economically important insect phytophagous pests. Various genera from the plant family Lamiaceae have been identified as rich sources of antifeedant clerodanes, with species of the genus Scutellaria producing some of the most potent clerodane antifeedants known so far. In addition, the major active clerodanes of Salvia divinorum can serve as opioid receptor probes, enabling better understanding of opioid receptor-mediated phenomena, as well as providing additional areas for chemical investigation.

1.2. The basic structures of clerodane diterpenes

Clerodane diterpenes are bicyclic diterpenoids. The basic skeleton is divided into two fragments: a fused ring decalin moiety (C-1–C-10) and a six-carbon side chain at C-9 (C-11–C-16, with C-16 attached at C-13, i.e., 3-methylpentyl). The remaining four carbons (C-17–C-20) are attached at C-8, C-4, C-5, and C-9, respectively, on the decalin system as illustrated below.

Approximately 25% of clerodanes have a 5 : 10 cis ring fusion as represented by columbin (1). This diterpenoid furanolactone has been isolated from several plants, including Sphenocentrum jollyanum Pierre (Menispermaceae) and Jateorhiza calumba Miers (Menispermaceae). It is sold in a crude drug preparation called calumbae radix or tinosporae radix. Columbin exhibited dose dependent anti-inflammatory activity as well as chemopreventative activity against colorectal cancer.^1–3 The remaining 75% of clerodanes have a 5 : 10 trans ring fusion as exemplified by clerodin (2). This compound was originally isolated from Clerodendrum infortunatum L. (Lamiaceae) and has potential as a natural pesticide due to its insect antifeedant activity.⁴ Clerodanes with a 5 : 10 trans ring junction are characteristic of the Lamiaceae family and to a lesser extent the Compositae (Asteraceae) family, while clerodanes with a 5 : 10 cis ring junction are more commonly found in the Euphorbiaceae, Flacourtiaceae (Salicaceae), and Menispermaceae families.

In addition to the relative configuration of the trans or cis junction of the fused rings, clerodanes are further classified by their relative configurations at C-8 and C-9. Consequently, as shown in Fig. 1, four types of clerodane skeletons are defined with respect to the configuration at the ring fusion and of the substituents at C-8 and C-9: trans–cis (TC), trans–trans (TT), cis–cis (CC), and cis–trans (CT).⁵ In the majority of clerodanes, the C-17 and C-20 substituents on C-8 and C-9 are cis.⁵

Fig. 1.

Stereochemical variety in clerodane diterpenoids.

In their 1992 review, Merritt and Ley noted that confusion exists in the literature over the absolute stereochemistry of the clerodanes.⁶ The absolute stereochemistry of clerodin (2), the first member of the clerodane series, was revised leading to the following terminology. neo-Clerodanes (formerly ent-clerodanes) have the same absolute stereochemistry as clerodin, while entneo-clerodanes are enantiomeric to clerodin.^6,7 The former compounds predominate in number over the latter compounds.⁵ We have used neo-clerodane in this paper, except for compound names already given with the ent-clerodane terminology.

Biosynthetically, the clerodanes likely arise from geranylger-anylpyrophosphate (3) as shown in Scheme 1. However, this overall biogenetic route is simplified as many parallel pathways are needed to yield the multiplicity of clerodane natural products.⁸ Initially, plant cyclases catalyze a proton-initiated cationic cycloisomerization of 3 to generate a labdane-type precursor skeleton, such as 4 [one of four possible structures (‘normal’, ‘ent’, ‘syn-normal’, ‘syn–ent’), depending on the conformation of 3].^8b Subsequently, this intermediate can undergo either a concerted or stepwise migration process of methyl and hydride shifts. A concerted process, with a C-4α to C-5α methyl group migration,⁹ gives clerodane-type intermediate 5 and results in only trans clerodanes. A stepwise process ‘pauses’ at a halimane-type intermediate (6) that retains both gem-dimethyl groups on C-4. Intermediate 6 can then progress to either cis or trans clerodanes. The general scheme without specific stereochemistry is also shown. Examples to support this proposed biosynthetic pathway include the isolation of the partially rearranged labdane compounds chettaphanin (7) from Adenochlaena siamensis (Compositae)¹⁰ and salmantic acid (8) from Cistus laurifolius (Cistaceae).¹¹

Scheme 1.

1.3. The biological activities of clerodane diterpenes

The most important biological activities of clerodanes are insect antifeedant effects¹² and action as novel opioid receptor probes.¹³

1.3.1. Insect antifeedant activity

Clerodane diterpenes are best-known and most extensively studied for their insect anti-feeding and related insecticidal properties, with an emphasis placed on the safety aspects of such natural insect antifeedants in relation to the lives of mammals and fish. To date, over 400 natural and semi-synthetic clerodanes have been examined in laboratory assays, yielding several compounds with potent antifeedant activity against various insect species.¹²

1.3.2. Probes in opioid pharmacology

In 2002, opioid receptors were implicated in the actions of the psychoactive mint Salvia divinorum.¹³ The main active constituent isolated from the leaves of S. divinorum is the neoclerodane diterpene salvinorin A (9). This molecule is interesting to pharmacologists, because it is a non-serotonergic hallucinogen that lacks a basic nitrogen and is a potent and selective agonist for κ opioid receptors. Synthetic organic chemists also find it an attractive target because of its unique structure containing seven chiral centers and a diterpene scaffold.

Opioid agonists based on 9 have the potential to treat pain, cough, diarrhea, stimulant dependence, and mood disorders. Antagonists derived from 9 have potential use in treating several medical conditions, including drug dependence, depression, opioid-induced constipation, and obesity. Thus, analogues of 9 may prove to be excellent research tools and provide greater insight into opioid receptor-mediated phenomena.

1.3.3. Other bioactivities

Besides insect antifeedant activity and opioid receptor agonist effects, clerodane diterpenes can exhibit other pharmacological activities, including antitumor,¹⁴ antifungal, NGF-potentiating, antibiotic, anti-peptic ulcer, antiplasmodial, as well as hypoglycemic, hypolipidemic, and anti-thrombin inhibitory activity.

This review provides extensive coverage of naturally occurring clerodane diterpenes discovered in the last 25 years (1990–2015) along with their various bioactivities. The distribution, chemotaxonomic significance, chemical structures, and biological activities of clerodane diterpenes are summarized. In the cases where sufficient information is available, the structure activity relationship (SAR) correlations and mode of action of active clerodanes have been presented.

2. Structure classifications and sources of clerodane diterpenes

During the last 25 years, over 1300 diterpenoids and nor-diterpenoids with the clerodane carbon skeleton have been isolated. For clarity and the purposes of this review, they have been grouped together by particular structural features as described below.

Firstly, the C-11–C-16 fragment can be acyclic or occur as several different bi- and mono-cyclic substructures (Fig. 2). Substructures a–c contain a bicyclic furofuran system, either tetrahydro (a) or hexahydro (b and c). Moreover, the two latter systems can have oxygen moieties present on C-13 and C-14 (b) or C-15 (c), forming a hemiacetal or acetal. Furthermore, when OR is methoxy or ethoxy in substructure c, the compound could be an artifact resulting from the use of methanol or ethanol using the isolation procedure. Substructure d possesses one furan ring (C-11–C-13, C-16) and a two-carbon open chain system (C-14–C-15), formally arising from the opening of the acetal moiety and subsequent reduction of the C-15 aldehyde to a primary alcohol. Alternatively, carbons C-11 and C-12 are present as a two-carbon ethyl chain, while carbons C-13–C-16 form an attached single ring, either an α,β-unsaturated-γ-lactone (e) or lactol (f). Sometimes, Δ^11,12 unsaturation is present or carbons C-11 and C-12 can bear oxygenated groups. Finally, bicyclic spiro substructures can be found. The tetrahydropyran incorporates C-8 and C-9, as well as C-11–C-13, and the γ-lactone (C-13–C-16) can assume both possible configurations (g, h) at C-13.

Fig. 2.

C₁₁–C₁₆ moiety of clerodane diterpenoids.

Secondly, the decalin moiety contains some consistent functional features. The decalin junction is mostly trans, and the two groups (C-17 and C-20) on positions C-8 and C-9, respectively, are primarily cis, as well as α-oriented (type TC neo-clerodanes in Section 1.2). Six classifications (A–F) have been made based on the formal oxidation number of carbon C-18 in the decalin moiety (Fig. 3). Groups A and B contain a C-4α/C-18 epoxide, while C-18 is present as a hydroxymethyl in group C. Furthermore, in groups A and B, carbon C-19 is often hydroxylated, either esterified (A) or forming a hemiacetal or acetal bridge with the α-hydroxy group on carbon C-2 (B). In a few cases, carbon C-19 is a methyl or a carboxylic group. In substructure D, C-4 and C-18 form an exocyclic double bond. In this case, carbon C-19 is always a methyl. When C-18 is a methyl, C-19 is also always a methyl. Finally, carbon C-18 can have a higher oxidation number as in an aldehyde (E) or acetal (F). Again, in this case, carbon C-19 is always a methyl.

Fig. 3.

The decalin moiety of clerodane diterpenoids.

Finally, all of the natural neo-clerodanes have been classified into seven different types (I–VII, Fig. 4) on the basis of their two fragments, the C-11–C-16 moiety (a–h) and the decalin moiety (A–F). Unless otherwise indicated, the diterpenes possess the neo-clerodane absolute stereochemistry.

Fig. 4.

Basic skeletal classifications of clerodane diterpenoids.

2.1. Type I with an acyclic side chain at C-9

Type I clerodane diterpenoids are characterized firstly by having an acyclic side chain at C-9, and then are further divided into three subtypes related to the decalin system. The first subtype has an O-containing five-membered cyclic ring attached to the decalin ring A (18,19-oxide), the second subtype has a double bond between C-3 and C-4 or at another (or no) position of either decalin ring (without an 18,19-oxide), and the third subtype has an epoxy ring in the decalin system.

2.1.1. Type I subtype I with an O-containing five-membered ring at C-18 and C-19

In this subtype, most of the representative compounds are based on two derivations of the 18,19-oxide clerodane nucleus – zuelanin (double bonds at C-12/C-13 and C-14/C-15) and isozuelanin (double bonds at C-13/C-16 and C-14/C-15) (Fig. 5). Various substituents are found at C-2, C-6, C-7, C-18 and C-19, as well as sometimes at C-12, in the isozuelanin subtype, and the decalin system can be saturated. Compounds with other skeletons also exist.

Fig. 5.

Zuelanin and isozuelanin skeletons.

2.1.1.1. Type I subtype Ia with the isozuelanin skeleton^15–33 (Table 2 – compounds 10–86 found in ESI^†)

The new clerodanes from type I subtype Ia are all 5 : 10 cis and 17 : 20 trans, mostly isolated from genuses Casearia and Zuelania in the family Salicaceae. The structures of zuelaguidins A–D (17–20) from Z. guidonia are typical of this subtype.¹⁶ Corymbulosins A–C (25–27) from Laetia corymbulosa were elucidated as clerodane diterpenes unsaturated at C-3, C-13(16), and C-14.¹⁸ Corymbulosin A (25) has a decadienoate ester at C-2, while corymbulosins B and C (26–27) have a saturated decanoate ester at C-6. The latter two compounds are identical except for the configuration at C-2. As based on coupling constant and NOE data, H-2 is equatorial in the former and axial in the latter. However, the study was unable to assign the absolute or relative stereochemistry of the three compounds. Corymbulosin A was the most cytotoxic with IC₅₀ values of 0.6 μM against SF539 human CNS tumor cells and 8 μM against the LOX melanoma cell line in two-day cytotoxicity tests.¹⁸

Two other compounds in this structural subtype, intrapetacins A (35) and B (36) from Licania intrapetiolaris, with a p-hydroxybenzoate group at C-2, displayed moderate cytotoxicity against KB cells, with IC₅₀ values of 2.0 and 0.8 μg mL⁻¹.²⁰ Caseanigrescens A–D (37–40) from C. nigrescens showed moderate cytotoxicity against the A2780 human ovarian cancer cell line, with an IC₅₀ range of 0.83–1.4 μM.²¹ Unlike most compounds in this subtype, compounds 37–39 are substituted at C-7 (37, acetoxy; 38–39, hydroxy). When 37–40 were stored in CDCl₃ for varying times during NMR analysis, their hemiacetal resonances disappeared and aldehyde resonances appeared. This result indicated that all four compounds slowly hydrolyzed to corresponding unstable dialdehydes. The hydrolysis was likely caused by traces of acid in the specific CDCl₃ used, and did not occur when the compounds were allowed to stand in a fresh sample of CDCl₃.²¹ Argutins F–H (41–43) with a unique hydroperoxide moiety at C-12 were isolated from C. arguta.²²

It should be noted that the clerodane absolute stereochemistry was not determined in every structural characterization study. For example, esculentin A (76) has been reported in both ent–neo¹⁶ and neo³² configurations. In addition, while caseargrewiin A (74) was shown as a neo-clerodane, caseargrewiins B–D (1256–1258, structures in Section 3.5) co-isolated from C. grewiifolia were shown as ent-neo-clerodanes. The absolute configuration of C-2 in 1258 established as R by a modified Mosher's method, and the absolute stereochemistry in the rest of the molecule based on NMR coupling constants and NOESY correlations.³¹ Generally, this review has focused on relative configurations only.

Caseabalansin A and 18-epicaseabalansin A (85–86) are the first examples of clerodane diterpenoids with an oxygen bridge between C-2 and C-19.³³ They were initially isolated as an inseparable 1.3 : 1 isomeric mixture from C. balansae and identified based on NMR spectroscopy. Conversion to the 18-acetates allowed separation of the two compounds by HPLC.

2.1.1.2. Type I subtype Ib with the zuelanin skeleton^{14,22,23,25,28,34–40} (Table 3 – compounds 87–135 found in ESI^†)

The newly reported type I subtype Ib compounds are also 5 : 10 cis and 17 : 20 trans, mostly isolated from the family Salicaceae. They are structurally similar to type I subtype Ia compounds, but with a different double bond pattern in the C-11–C-16 acyclic side chain. Casearvestrins A–C (87–89) from Casearia sylvestris show the typical zuelanin skeleton, with acetoxy groups at both C-18 and C-19, in addition to a 18,19-oxide. In these three compounds, C-2 and C-6 are also substituted with various four to six carbon esters and a hydroxy group, respectively. Compounds 87–89 displayed promising bioactivity in cytotoxicity assays against a panel of tumor cell lines and antifungal assays against Aspergillus niger in a disk diffusion assay.³⁴ Argutins A–E (90–94) from C. arguta contain the same cyclic ether with varying combinations of decadienoyloxy, hydroxy, and hydrogen at C-2, C-6, and C-7. Among them, argutin B (91) showed the highest degree of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) sensitization. Furthermore, the synergistic effect of 91 and TRAIL together was three-fold greater than that of 91 alone.²² Like the type I subtype Ia compounds 35–36 mentioned above, type I subtype Ib case-arborins A–E (96–100) from C. arborea contain structurally novel aromatic esters, either at C-2 or C-6. When evaluated against LOX and SF539 cell lines using a two-day cytotoxicity assay, compounds 96–100 exhibited IC₅₀ values ranging from 0.29 to 9.7 μM.³⁵

Caseobliquins A (127) and B (128) from C. obliqua have different substituents at C-6, a p-hydroxybenzoate moiety in 127 and a cinnamoate moiety in 128.³⁹ Meanwhile, the acetoxy groups on C-18 and C-19 are trans, rather than cis as in the above-mentioned compounds.

2.1.1.3. Type I subtype Ic with other skeletons^16,41,42 (Table 4 – compounds 136–155 found in ESI^†)

Type I subtype Ic includes compounds with acyclic side chains different from 3-methylenepent-4-ene or (E)-3-methylpenta-2,4-diene, which are found in subtypes Ia and Ib, respectively. Most of the type I subtype Ic compounds contain a 18,19-lactone ring. Compounds 136–138 from the aerial parts of Olearia teretijdia contain a Δ^13E double bond, while this position is saturated in 139–149. The terminal carbon in the side chain is generally hydroxymethyl, malonyloxymethyl, or carboxy rather than methyl. Compound 149 also has a Δ² rather than Δ³ double bond.⁴¹

Baccharis linearis was the source of three new clerodanes: baclinal (150) with a 3-formyl-3E-pentenyl side chain and epimeric baclinepoxides (151–152) with an interesting 13,16-spiro-oxirane in the C-9 side chain.⁴² Zuelaguidins E, G, and H (153–155), isolated from Zuelania guidonia (family Salicaceae), were the first reported diterpenoids containing a 3,6-dihydro-1,2-dioxin moiety. This endoperoxide may result from a Diels–Alder reaction between zuelaguidin A (17, see Section 2.1.1.1), also found in Z. guidonia, and molecular oxygen.¹⁶ Compounds 153–155 are 5 : 10 cis and 17 : 20 trans. The remaining compounds in type I subtype Ic are 5 : 10 trans and 17 : 20 cis, as typical of clerodanes isolated from the family Asteraceae.

2.1.2. Type I subtype II with a double bond between C-3 and C-4 or another position

2.1.2.1. Type I subtype IIa with a double bond between C-3 and C-4 (ref. 14, 15, 18, 26, 41 and 43–88) (Table 5 – compounds 156–253 found in ESI^†)

Type I subtype IIa compounds contain many of the same acyclic side chains as type I subtype Ia–Ic compounds. For instance, both 3-methylenepent-4-ene (156–158) and 3-methylpenta-2,4-diene (164–167) side chains are found in type I subtype IIa. Interestingly, compounds 164–166 with a Δ^12Z double bond were isolated from both Schistochila acuminate and Heteroscyphus planus,^45–47 while heteroscyphol (167) with an assigned Δ^12E double bond was found only in Heteroscyphus planus.⁴⁷

Terpenetetriene (159) was produced from a transformant of Streptomyces lividans and postulated to be a probable biosynthetic intermediate of terpentecin (160), a diterpenoid antibiotic previously isolated from the bacterium Streptomyces griseosporus.⁴³ This study was the first to report a bacterial diterpene cyclase. Compound 159 was also isolated together with 161 from Jungermannia infusca. These two compounds have the same planar structure but different stereochemical structures. Both are 5 : 10 trans, but the former compound is 17 : 20 trans,⁴³ while the latter is 17 : 20 cis.⁴⁴

Compounds 193 and 194 have a neo-clerodane skeleton with a Δ^3,4 C=C bond and a C=O group at C-2.⁶³ Compounds 197 (2-oxokolavenic acid) and 198, isolated from different plant species, are identical, except for the orientation of the C-20 methyl group, β in the former compound and α in the latter compound.^65,66 The cis-decalin (5α Me, 10α H) and trans orientation of C-8 and C-9 (17α Me, 20β Me) in 197 were confirmed by X-ray crystallographic analysis. Furthermore, 2-oxokolavenic acid with a trans-decalin (5β Me, 10α H) and cis orientation of C-8 and C-9 (17β Me, 20β Me) was co-isolated at the same time from the fruits of Detarium microcarpum, as well as previously from the bark and leaves of the same plant.⁶⁵

Portulene acetal (199) with a caged hemiacetal [6.6]-ring was isolated as a minor constituent from Portulaca grandiflora.⁶⁷ The structures of 200 and 201 were quite similar, except for a tigloyl group in 200 and an angeloyl group in 201.⁶⁸ Seven new clerodanes, exemplified by 207, were obtained from Baccharis trinervis.⁷² Although no double bond is present between C-13/C-14, the stereochemistry of C-13 in these compounds and their analogue 214 from B. gaudichaudiana⁷³ could not be deduced spectroscopically.

The four new clerodanes (227–230) isolated from Nuxia sphaerocephala were assigned to the ent-series (now neo-series) based on optical rotation values, plus the absolute configuration of C-13 in 227 was established as S based on its phenylglycine methyl ester (PGME) amide derivatives.⁷⁸ In addition to having a free carboxylic acid rather than ethyl ester at the side chain terminus, compounds 231 and 232 have a formyloxy group and an acetyloxy group, respectively, on C-2 compared with the hydrogen in 233.⁷⁹ All three compounds were isolated from Clausena dunniana.

Compounds 240–242 were established as hydroxy and peroxy derivatives of the 2-oxo group 239, based on X-ray and CD analysis.⁸¹ Compounds 249–252, isolated as 3 : 1 or 4 : 1 mixtures from Linaria saxatilis, are the (12E)- and (12Z)-stereo-isomers of the Δ³-endocyclic analogues.⁸⁷

2.1.2.2. Type I subtype IIb with a double bond at another (or no) position^{36,55,63,65,74,75,79,82,83,88–102} (Table 6 – compounds 254–289 found in ESI^†)

The decalin double bonds in type I subtype IIb compounds can be either endocyclic at C-1/C-2 (e.g., 277), C-2/C-3 (276), or C-7/C-8 (272) or exocyclic at C-4/C-18 (281) or C-8/C-17 (258). Alternatively, some compounds in this subtype do not have a decalin double bond, but instead often have dihydroxy substitution (e.g., 266–267). Like in type I subtype IIa, various acyclic side chains are present.

Leojaponin A (276), characterized by a C4-C7 oxa-bridge and a double bond between C-2 and C-3, is the first clerodane diterpenoid obtained from Leonurus japonicus.⁹⁶ Palmadorins A and B (288–289), from the Antarctic nudibranch Austrodoris kerguelenensis, were the first two of a new series of clerodane diterpene glycerides.⁸⁸

2.1.3. Type I subtype III with an epoxy ring^{46,65,90,99,103–105} (Table 7 – compounds 290–296 found in ESI^†)

Among all compounds with an acyclic side chain, seven compounds are classified as type I subtype III with a epoxy ring either at C-3/C-4 (290–294)^{46,65,90,103,104} or C-4/C-18 (295–296).^99,105 The β-orientations of the four methyl groups (C17–C20), as well as H-10, in 291 from Detarium microcarpum are shown as reported.⁶⁵ The assignments were based on NOE correlations, but seem uncommon from a biogenetic viewpoint. Compound 292 from Heteroscyphus planus is a possible intermediate in the biosynthesis of diterpenes that have a spiro-γ-lactone group at the C-9 position.⁴⁶ Compounds 293 from Jungermannia paroica and 294 from Stachys glutinosa have almost identical structures with a hydroxy group at C-13, but a hydrogen and α-hydroxy group, respectively, at C-2.^103,104 The orientation of the epoxy methylene H₂-18 in 295 from Polyalthia longifolia was deduced to be β, based on comparison of the chemical shifts and coupling constants with those of similar structures in the literature.⁹⁹ Highly oxygenated compound 296 from Ajuga decumbens inhibited lipopolysaccharide (LPS)-induced nitric oxide production in RAW 264.7 macrophages.¹⁰⁵

2.2. Type II with a 2-ethylfuran-based side chain at C-9

Type II clerodane diterpenoids are characterized initially by a 2-ethylfuran-based, rather than acyclic, side chain at C-9. When viewed simplistically, an oxygen atom has been inserted between C-15 and C-16 of a 3-methylenepent-4-ene side chain.

Type II compounds then are further split into various subtypes. Subtypes Ia and Ib generally have more complex or multiple O-containing rings at various positions of the decalin moiety, as compared with subtype Ic with only one simple epoxy ring on the decalin moiety. Subtypes IIa and IIb do not have an O-containing ring, but instead have one or more double bonds in the decalin moiety (IIa) or saturated decalin or oxodecalin moiety (IIb). Finally, subtype III compounds have a distinctive tetrahydrofuran rather than furan in the C-9 side chain, along with various decalin moieties.

2.2.1. Type II with various O-containing rings

2.2.1.1. Type II subtype Ia with various O-containing rings^75,106–127 (Table 8 – compounds 297–323 found in ESI^†)

Nasimalun A (297) from Barringtonia racemosa illustrates a type II subtype Ia clerodane with a C-18/C-19 γ-lactone ring,¹⁰⁶ while teumassilenin B (302) from Teucrium massiliense is a type II subtype Ia with a similar γ-lactol ring.¹¹¹ T. massiliense also yielded new clerodane diterpenes from two additional type II subtypes: teumassilenin C (336) (type II subtype Ib with an oxetane ring, see Section 2.2.1.2) and teumassilenin A (423) (type II subtype IIb with an oxodecalin, see Section 2.2.2.2; the first example of an 18β-aldehyde from Teucrium species).¹¹¹

A furan ring is present between C-18 and C-6 in clerodanes 314–316,^122,123 while plaunol E (306) has a γ-lactone ring at this same position, as well as a δ-lactol ring between C-20 and C-19.¹¹⁵ Compound 306 significantly inhibited LPS-induced NO production with an IC₅₀ value of 2.79 μM.¹¹⁵

Other new clerodanes in this subtype also had oxygenated rings incorporating C-20. Sacacarin (318) from Croton cajucara has a C-19/C-20 δ-lactone ring. Compound 319 from Salvia miniata has an oxygenated structure containing C-19/C-20 and C-7/C-20 acetalic bridges.¹⁰⁸ The C-20/C-19 δ-lactone in 321 (ref. 116) from Teucrium oxylepis and the C-20/C-6 δ-lactol ring in 323 (ref. 75) from Pteronia eenii are also accompanied by C-4/C-18 and C-3/C-4 epoxide rings, respectively.

2.2.1.2. Type II subtype Ib with other O-containing rings^{84,111,128–140} (Table 9 – compounds 324–355 found in ESI^†)

Clerodane diterpenoids with an axial oxyfunction at the C-7 position are rare, but a few examples, such as 326–329, have been reported. The structure of 326 from Tecrium cossonii distinctly contained a 4,18-spiro-oxirane compared with those of 327–329 from Ptychopetalum olacoides.^129,130 Methyl dodonates A–C (330–332), three new modified clerodanes containing a tricyclo [5.4.0.0^1.3]undecane ring system, were isolated from Dodonaea viscosa.¹³¹ They have been proposed as putative intermediates in the biogenetic pathway to diterpenes possessing a bicyclo[5.4.0]undecane or bicyclo[5.3.0]decane ring system.

Teucrolin E (335) from Teucrium oliverianum contains an oxo group (C=O) at C-7.^133,134 Its originally proposed structure also contained an oxetane ring with the oxygen connecting C-4 to C-10, leaving C-18 as a hydroxymethyl group.¹³³ However, additional NMR analysis of the diacetate, including NOE studies, indicated that C-18 is instead part of a tetrahydrofuran ring that includes C-10 and a tertiary OH group is present at C-4.¹³⁴ As mentioned previously, teumassilenin C (336) from Teucrium massiliense does possess an oxetane ring; however, the oxygen connects C-4α and C-19.¹¹¹ Anastreptin (337) from Adelanthus lindenbergianus contains a cyclic ketal moiety with oxygen bridges from C-12 to C-7 and C-12 to C-8 of the decalin moiety.⁸⁴ Three type II subtype Ib clerodanes (bafoudiosbulbins A, D, and E; 339–341) were isolated from Dioscorea bulbifera.^135,136 Compound 339 is a stereoisomer of 340; both compounds contain a lactone bridge (OC=O) between C-2 and C-5, as well as between C-6 and C-8.^135,136 Compound 341 is identical to 340, but also contains a 3α,4α-epoxide.¹³⁶

Scaparvin A (343), a novel caged cis-clerodane diterpenoid with an unprecedented C-6/C-11 bond, was isolated together with scaparvins B–E (344–346, 350), without the C-6/C-11 bond, from the Chinese liverwort Scapania parva.¹³⁸ Their absolute structures were elucidated by analysis of NMR and CD data coupled with electronic circular dichroism (ECD) calculations. The authors proposed an enzymatic intramolecular aldol reaction as the key step in the biogenetic pathway of 343.¹³⁸ Parvitexins A–C (347–349) from S. parvitexta were the first natural products identified with an unusual 2,8-dioxobicyclo[3.2.1] octane moiety.¹³⁹

2.2.1.3. Type II subtype Ic with a simple epoxy ring^{123,129,139,141–148} (Table 10 – compounds 356–376 found in ESI^†)

Clerodane diterpenes in this subtype generally have either a 3,4 epoxide (e.g., 357 from Scapania parvitexta,¹³⁹ 359 from Croton eluteria,¹⁴² 364 from Thysanathus spathulistipus¹⁴³) or a 4,18-spiro-oxirane (e.g., 369 from Teucrium oliverianum,¹⁴⁶ 375 from T. fruticans¹²³). Their structural differences are mainly in the linkages between the carbocyclic and heterocyclic moieties and the functionalization of the decalin core. However, the β-oriented epoxide of phlomeoic acid (376) from Phlomis bracteosa is uniquely at C-1/C-10.¹⁴⁸

2.2.2. Type II with or without a C=C double bond in the decalin moiety

2.2.2.1. Type II subtype IIa with one or more decalin C=C double bonds^{74,106,110,130,149–168} (Table 11 – compounds 377–419 found in ESI^†)

Vishautriwatic acid (377) from Dodonaea viscosa was identified with different stereochemistry at C-5 and C-9 from the known neo-clerodane hautriwatic acid (378) found in Eremocarpus setigerus.^149,150 The cis-relationship of H-10 and Me-20 in 377 is uncommon compared with the trans-relationship found in 378. Crotonolide G (404, Croton laui), with a unique 2,5-dihydrofuran rather than furan in the C-9 side chain, displayed significant antibacterial activity with an MIC value of 43.4 μM against four strains of Gram-positive bacteria, including Staphylococcus aureus ATCC 25923, Staphylococcus epidermidis ATCC 12228, Micrococcus luteus ATCC 9341, and Bacillus subtilis CMCC 63501.¹⁶⁵ Crotomembranafuran (414, Croton membranaceus), which has a less common ethanone rather than ethyl or ethylene linkage between the decalin and furan ring systems, had an IC₅₀ value of 10.6 μM against PC-3 cells.¹⁶⁸

In addition to the C-3/C-4 double bond, compounds 415 and 418 found in Conyza hypoleuca also contain a second double bond (C-1/C-2 in 415;¹⁵⁵ C-1/C-10 plus C-2 oxo in 418 (ref. 169)). Finally, eluterin B (419) from Croton eluteria contains an exocyclic C-4/C-18 double bond.¹⁴²

2.2.2.2. Type II subtype IIb without a decalin C=C double bond^{89,111,142,151,165,170–172} (Table 12 – compounds 420–428 found in ESI^†)

Crolechinic acid (422) is representative of type II subtype IIb compounds and was found as a minor constituent in Croton lechleri based on TLC profiles and NMR spectra.⁸⁹ Eluterin A (425) differs structurally from eluterin B (419) only in the functionalities at C-3, C-4, and C-18: oxo and β-methyl in the former, but α-hydroxy and exocyclic double bond in the latter.¹⁴² Four new tricyclic clerodane type diterpene aldehydes (423 and 426–428) were characterized through modern spectroscopic techniques and comparison with literature data.^{111,170–172} 20α-Aldehydes are present in 426 and 427 from C. hovarum.^170,171 Compounds 423 and 428 both contain a 18-aldehyde and the same relative stereochemistry, but the former was reported as a neo-clerodane from Teucrium massiliense and the latter as an ent-neo-clerodane from Nepeta juncea.^111,172

2.2.3. Type II subtype III with a tetrahydrofuran ring^{72,89,149,173–179} (Table 13 – compounds 429–445 found in ESI^†)

Compounds 429 and 430 from Baccharis trinervis were determined as trinerolide and 15-epi-trinerolide, respectively, based on the number of split peaks and coupling constant of the acetal H at C-15.⁷² Compound 434 was a possible artifact from B. articulata, with the true natural product being its hemiacetal analogue.¹⁷³ Two new neo-clerodane diterpenoids with multiple O-containing rings, compounds 439 and 440, were isolated from an acetone extract of the aerial parts of Scutellaria galericulata.^176,177

2.3. Type III with a 3-ethyl-2-butenolide-based side chain at C-9

Type III clerodane diterpenoids bear a 3-ethyl-2-butenolide-based side chain at C-9, with various 6O-containing rings or double bonds at different positions. Comparison of the type II and type III general structures shows that the furan ring in the former has been replaced by a furan-2(5H)-one (or 2-butenolide) in the latter.

2.3.1. Type III subtype I with O-containing rings

2.3.1.1. Type III subtype Ia with five-membered cyclic O-containing rings^{84,173,180–191} (Table 14 – compounds 446–480 found in ESI^†)

Amphiacrolides A–E, I, J, L, and M (446–454), isolated from Amphiachyris dracunculoides, have an ethyl butenolide side chain attached at C-9, as supported by characteristic MS fragments (m/z 111, 98, and 97) and ¹H and ¹³C-NMR peaks.^180–182 The stereochemistries of the amphiacrolides were established from the chemical correlation of these compounds to gutierolide, a compound with absolute stereochemistry determined by X-ray analysis.^180–182

The planar structures of 460 and 461, isolated from different Baccharis species are identical, but the compounds are epimeric at C-8.^173,186 The absolute stereochemistry (neo-series, 5 : 10 trans, 17 : 20 trans) of gaudichanolide A (461) from B. gaudichaudiana was established by X-ray crystallographic analysis.¹⁸⁶

Several new clerodanes, exemplified by 463 from Cephaloziella kiaeri, have a unique unsaturated γ-lactone moiety incorporating C-18 and C-6.^{84,182,187–191} Three 1 : 1 mixtures (470–475) of epimeric clerodane diterpenes with a C-8/C-12 ether bridge were isolated from Adelanthus lindenbergianus.⁸⁴ Structures 474–476 contain a second ether bridge between C-12 and C-7 forming a cyclic ketal at C-12.⁸⁴ A C-1/C-12 ether bridge is present in 477 also from A. lindenbergianus,⁸⁴ while a C-10/C-12 ether bridge is found in 478–480 from Scapania ciliata.¹⁹¹

2.3.1.2. Type III subtype Ib with other O-containing rings^{52,181,192–214} (Table 15 – compounds 481–527 found in ESI^†)

Many compounds in this subtype contain the typical functional groups of neo-clerodane diterpenoids, including a C-4/C-18 (e.g., 505) or C-3/C-4 epoxide (e.g., 515, 516) and cis C-17α and C-20α methyl groups.^192–209 Among them, hastifolin A (505) from Scutellaria hastifolia showed significant antifeedant activity against larvae of Spodoptera littoralis at a concentration of 100 ppm; its feeding index was 60 ± 15.2 and FI₅₀ concentration was 45 ppm.²⁰⁴ Seguiniilactones A and B (515–516) from Colquhounia seguinii differ structurally only in where the butenolide ring is connected to C-12. This connection is at the β position of the butenolide ring in the former compound and at the α position in the latter compound.²⁰⁹ Thus, the carbonyl moiety of the lactone ring is at C-15 (a 15,16-olide) in 515 and at C-16 (a 16,15-olide) in 516. A β-substituted α,β-unsaturated γ-lactone functionality was found to be crucial for the strong antifeedant activity of this compound class, and 515 was approximately 17-fold more potent than commercial neem oil insecticide against the generalist plant-feeding insect Spodoptera exigua.^208,209

The C-2/C-19 ether bridges of amphiacrolide K (518; Amphiachyris dracunculoides) and conyzalactone (519; Conyza blinii) have opposite relative configurations, and these two compounds are also the two ent-neo-clerodane exceptions in this subtype.^181,210 neo-Clerodane-type diterpenoid 520 from Ajuga decumbens has a C-1/C-19 ether bridge.²¹¹ Compounds 521–525 contain a C-20/C-7 γ-lactone/lactol bridge,^52,212 compounds 526 has a C-18/C-7 δ-lactone bridge,²¹³ and finally, microdon B (527) possesses a C-17/C-12 δ-lactone bridge.²¹⁴

2.3.1.3. Type III subtype IIa with C3/C4 double bond^{58,70,72,155,163,164,175,183,189,213,215–228} (Table 16 – compounds 530–575 found in ESI^†)

A wide range of substituents are found on the decalin and butenolide moieties in type III subtype IIa compounds. With CO₂H at the decalin C-4 position and OMe or H, respectively, at the butenolide C-4 position, limbatolides B (533) and C (534) from Otostegia limbata inhibited acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) enzymes in a concentration-dependent manner with IC₅₀ values of 47.2, 103.7 and 17.5, 14.2 μM, respectively.¹⁸⁹ Polylongifoliaon B (543) from Polyalthia longifolia is one of a few type III subtype IIa compounds with an α,β-unsaturated ketone in the decalin ring A.²²⁰ This compound improved the viability of human neuroblastoma cells (SK-N-MC cells) under Aβ-induced neurotoxicity with an IC₅₀ value of 3.75 μM.²²⁰

The H-10 of compounds 558–571 has the more unusual α-orientation (ent-neo-clerodane).^{163,164,183,213,226} Both cis–trans (e.g., 558–564), trans–cis (e.g., 566), and cis–cis (e.g., 571) configurations of the decalin ring junction and C-17/C-20 orientation, respectively, are found. Solidagoic acids C–I (558–564) from Solidago virgaurea contain a carboxylic acid at C-19, a motif that is relatively uncommon among the clerodanes.¹⁸³ Compared with the standard drug deoxynojirimycin (425.6 ± 8.1 μM), compound 566 from Duranta repens showed significant α-glucosidase inhibitory activity (IC₅₀ 577.7 ± 19.0 μM).²²⁶ Structurally, it has a 6β-OH and opposite stereochemistry at C-8 to C-10 compared with 534 mentioned above.

Scutebata A (574), as well as scutebatas B and C (994 and 995; see Section 2.9.1), from Scutellaria barbata possess a rare hydroxy group at the α-position of the α,β-unsaturated lactone ring. Scutebata A showed weak cytotoxic activity against SK-BR-3 cells with an IC₅₀ value of 15.2 μM.²²⁸ Compound 575 from Baccharis trinervis has a saturated, rather than unsaturated, γ-lactone in its side chain.⁷²

2.3.1.4. Type III subtype IIb with double bonds in other positions^{117,194,205,206,208,217,229–248} (Tables 17 and 18 – compounds 576–614 found in ESI^†)

Compounds 577–579, 584, and 888 (see Section 2.7) from Scutellaria barbata showed significant cytotoxic activities against three human cancer lines, HONE-1 nasopharyngeal, KB oral epidermoid carcinoma, and HT29 colorectal carcinoma, with IC₅₀ values in the range of 2.0–8.1 μM.^229,230,232 Compound 584 has a 2,3-epoxy-2-isopropyl-n-propoxy moiety attached at C-6, and its possible biosynthesis was proposed.²³⁰ Salvidivins C (587) and D (588) from Salvia divinorum are unique neo-clerodane diterpenes that possess a γ-hydroxy-α,β-unsaturated γ-lactone moiety, and are geometrical isomers at the γ-lactone moiety.²³⁵

Several compounds in this subtype have a 4,18-exo-methylene group (e.g., 589, 597, 598–600),^{217,231,236–242} while compounds 612–614 have a unique α-oriented cyclopropane ring formed from C-4, C-5, and C-19.^247,248 The latter three compounds were isolated from two different marine organisms, an Okinawa tunicate Cystodytes sp and a Formosan gorgonian coral Echinomuricea sp. Echinoclerodane A (614) was found to be the C-8 epimer of dytesinin A (612), and the chiral carbons in 614 were assigned as 4S*, 5S*, 8S*, 9S* and 10R*.²⁴⁸

2.4. Type IV with a 5-(3-furyl)-δ-valerolactone-based side chain at C-9

Compounds in this group are characterized by a 5-(3-furyl)-δ-valerolactone-based side chain at C-9, together with lactone and epoxy rings, hydroxy and acetoxy groups, as well as double bonds. Both type II and type IV compounds contain a furan ring in the C-9 side chain but differ by the presence of a 17,12-δ-lactone ring in the latter class. Thus, as seen below, position 17 is generally a free methyl group in type II compounds, while it is incorporated into the δ-lactone ring as the carbonyl group in type IV compounds.

2.4.1. Type IV subtype I with O-containing rings^{108,109,113,120,249–262} (Table 19 – compounds 615–655 found in ESI^†)

In addition to the 17,12-δ-lactone ring characteristic of type IV clerodanes, many new subtype I compounds from Salvia species have a 18,19-γ-lactone ring. Their decalin moieties also contain various numbers and locations of double bonds (see 615, 617, 622–624) and oxygenated groups. The 1β,10β-epoxy group of 627–630 from Salvia herbacea was deduced by analysis of spectroscopic data.²⁴⁹ Tehuanin G (630) exhibited anti-inflammatory activity (IC₅₀ 0.24 μM/ear) comparable to that of indomethacin, the reference compound.²⁴⁹ In contrast, the C-1(2)-epoxy group of 632–634 from S. reptans has an α-orientation.^252,253 Except for the stereochemistry of the C-18–C-19 lactone ring fusion (trans in 632, cis in 633), compounds 632 and 633 have identical structures with both A/B and B/C cis ring fusions, as established by X-ray analysis.^252,253 Furthermore, tehuanins A–C (635–637) from S. herbacea contain a 1,8-oxygen bridge; this unusual structural feature was confirmed by X-ray diffraction of 635.²⁴⁹

In addition to the C-12–C-17 δ-lactone ring found in the type IV class, compounds 638–640 from Jamesoniella autumnalis have a second lactone ring at C-18/C-6,¹²⁰ while fibrauretin A (641) from Fibraurea tinctoria has a second lactone ring at C-1/C-18 as well as an epoxide ring at C-2/C-3.²⁵⁴ Compound 647, a stereoisomer of 8-hydroxycolumbin at the C-12 position, contains the same C-1(18)-lactone ring as 641 rather than the C-2(18)-lactone ring of compounds 648 and 649.²⁵⁸ Compounds 650–653 are novel furano-clerodanes from Dioscorea antaly and D. bulbifera with a second δ-lactone ring bridging carbonyl C-19 to C-2.^259,260

2.4.2. Type IV subtype II other compounds^{84,108,132,157,165,214,235,257,263–274} (Table 20 – compounds 656–691 found in ESI^†)

Compound 665 from Salvia divinorum is a C-17 epimeric mixture of the hemiacetal salvinorin J, and is an example of a neo-clerodane hemiacetal (lactol) susceptible to mutarotation with the formation of an equilibrium mixture of C-17 epimers.²⁶⁶ Salvinicins A (684) and B (685) from the same plant are unique neo-clerodanes with a 3,4-dihydroxy-2,5-dimethoxytetrahydrofuran ring.²⁷³ Their absolute stereochemistry (neo-series, A/B ring trans, B/C ring trans) was based on X-ray crystallographic analysis. Interestingly, salvinicin A (684) exhibited partial κ agonist activity with an EC₅₀ value of 4.1 ± 0.6 μM (E_max = 80% relative to (-)-U50, 488H), while salvinicin B (685) exhibited antagonist activity at μ receptors with a K_i of >1.9 μM. This report provided a new lead in the development of opioid receptor antagonists.²⁷³ Salvidivins A (686) and B (687) are a pair of geometrical isomers of the γ-hydroxy-α,β-unsaturated γ-lactone, differing from each other in the linkage position to C-12. It appears that 686 and 687 are important precursors of 684 (a partial agonist of the κ-opioid receptor) and 685 (the first μ-opioid antagonist with a neo-clerodane skeleton).²³⁵

Compounds 688–691 are type IV subtype II compounds with variations on the δ-lactone structure.^84,108,274 Compound 688 from Cornutia grandifolia and 689 from Adelanthus lindenbergianus are distinguished by a unique ether linkage between C-1 and C-12.^84,274 Remarkably, the structures of orcadensin (690) also from A. lindenbergianus, as well as salvianduline D (691) from Salvia miniata, contain cyclic ketal functions with two oxygen bridges from C-12 to different positions of the decalin moiety.^84,108

2.5. Type V with an α-spiro-attached 4-(3-furyl)-γ-butyrolactone-based side chain at C-9

Clerodane diterpenoids of this group contain an unusual C-9-spiro-γ-lactone substituted at C-12 with a furan ring or are compounds arising from rearrangements of this structure. As contrasted in the below figure, the γ-lactone ring of type V compounds includes C-20 (C=O), C-9 (as the one carbon link to the decalin system), C-11, and C-12, while the δ-lactone ring of type IV compounds incorporates C-17 (C=O), C-8, C-9, C-11, and C-12. Type V compounds have a free Me-17, while type IV compounds have a free Me-20.

2.5.1. Type V subtype I with various O-containing rings^{75,108,113,165,187,275–281} (Table 21 – compounds 692–719 found in ESI^†)

Compound 692 from Salvia fulgens exemplifies a variation on the basic type V structure. In it, C-20 is connected to C-7 as well as C-12 by oxygen atoms, forming two tetrahydrofuran rings joined at the C-20 acetal. This interesting structure was confirmed by X-ray crystallographic analysis as a dihydro derivative of the known salvifaricin, which has a double bond at C-1/C-2, as well as C-3/C-4.²⁷⁵ Like salvifaricin, salvifolin (695; S. tiliaefolia)²⁷⁶ and dugesin F (696; S. dugesii),¹¹³ also have two double bonds in the decalin A-ring. However, unlike salvifaricin and 692, the second ether bridge from C-20 to C-7 is missing in 695 and 696, and instead a cyclic hemiacetal (spiro-γ-lactol) and C-7 oxo groups are present. An A/B cis ring fusion was elucidated in 695, in comparison with linearolactone whose structure was established by X-ray diffraction analysis, while the structurally similar 696 has a trans decalin ring fusion. Dugesin F (696) exhibited an inhibitory effect on influenza virus FM1, a strain that causes a cytopathic effect (CPE) in MDCK cells.¹¹³ The results [TC₅₀ 45.67 μg mL⁻¹, IC₅₀ 9.43 μg mL⁻¹, therapeutic index (TI) 4.84] implied that 696 is a non-toxic antiviral compound against influenza virus FM1.

Teusandrins A–D (708–711) isolated from Teucrium sandrasicum contain a non-rearranged C-9-spiro-γ-lactone. Notably, such diterpenoids containing an oxetane ring involving positions 4α,19 (e.g., 708 and 709) and 4β,10β (e.g., 710 and 711), as well as 4β,6β (not illustrated) of the neo-clerodane skeleton are relatively frequent among the constituents of Teucrium plants.²⁸¹

Several new clerodane diterpenoids, crotonolides A–D (713–714, 716, 718) and isocrotonolides B–D (715, 717, 719), were isolated from the aerial parts of Croton laui.¹⁶⁵ They contain a variation on the C-9-spiro-γ-lactone with the two oxygen atoms on C-20 incorporated into both a tetrahydrofuran ring through C-12 and a six-membered lactone/lactol ring between C-19 and C-20. Crotonolide A (713) also contains a Δ^3,4 double bond, a Δ^8,17 exocyclic double bond, and a 18,6-γ-lactone. In 714–719, the latter lactone ring is absent, and also C-19 is hydroxylated rather than present as an oxo group. Compounds 714/715, 716/717, and 718/719 are epimeric pairs at C-19 and were obtained as 3 : 1 interconverting mixtures.

2.5.2. Type V subtype II with 4,18-; 3,4-; or 8,17-oxirane moieties^{75,123,126,128,142,144,145,262,280–294} (Table 22 – compounds 720–767 found in ESI^†)

Most of the new compounds in this subtype have a C-12 furan, a spiro-20,12-hemiacetal function involving the C-9, C-11, C-12, and C-20 carbons, a 4α,18-oxirane, and a trans decalin ring junction. Teumassin (720) from Teucrium massiliense contains the rare feature of a C-2 hydroxy group.²⁸² The diterpenes 729–732 isolated from T. polium possess the same absolute configuration, and belong to the neo-clerodane series.²⁸⁵ In this clerodane subtype, the C-12 stereocenter can have an R-configuration (e.g., 743 from T. maghrebinum), as well as an S-configuration (e.g., 729).^126,144,280 New C-10 oxygenated type V subtype III neo-clerodane derivatives, sandrasin A (744) and 6-deacetylsandrasin A (745) were isolated from the aerial parts of T. sandrasicum.²⁸⁹ Analysis of spectroscopic data revealed ether linkages between both C4α,C18 and C19α,C20α in 750 obtained from T. abutiloides.²⁸⁷

The investigation of different Pteronia species afforded 28 new diterpenes, including five cis-clerodanes in this subtype (751–753, 758–759).⁷⁵ Compounds 751–753 have a 8,17-oxirane and C-10 is hydroxylated. Compounds 758–759 have a 3,4- oxirane and C-10 bears a hydrogen. An extract of the aerial parts of Microglossa pyrrhopappa afforded cis-clerodanes 760–763 as well as 767 with a Δ^1,10 double bond.¹²⁸

Cascarilla, the bitter bark of the South American tree Croton eluteria, is a commercially available and inexpensive source of polyfunctionalized clerodane diterpenoids. In addition to the bitter triol cascarillin, ten additional new diterpenoids, including eluterins J and I (756–757) in this subtype, were isolated and characterized.¹⁴² The structural differences among cascarilla clerodanes mainly involve the linkage between the carbocyclic and the heterocyclic moieties and the functional groups on C-3, C-4, and C-6. Although cascarillin was previously reported to be a γ-hydroxyaldehyde, this study showed that it is actually a mixture of interconverting γ-lactols.¹⁴² Compound 756 is set apart by the oxygenation of C-11, a very unusual feature in furoclerodanes.¹⁴²

Crotonpene A (764), which has a rare 2,3-dihydrofuran ring with a spiro-carbon at C-9 and an oxygen connecting C-12 and C-20, may be formed by oxidation or enzyme catalysis of crotonpene B (765). Both compounds are found in Croton yanhuii.²⁹⁴

2.5.3. Type V subtype III with a C-9-spiro-γ-lactone/lactol moiety and opened furan ring^47,61 (Table 23 – compounds 768–773 found in ESI^†)

Clerodane-type diterpenoids (768–773) with a C-9-spiro-γ-lactone/lactol moiety bearing an opened furan ring (2-hydroxy-3-buten-2-yl) at C-12 are rare and found only in Heteroscyphus plants. In the decalin portion, compounds 768 and 769 possess a 3,4-epoxide, 770 contains a 3,4-epoxy and 2-oxo groups, and compounds 771–773 have a 3,4-double bond and 2-oxo moiety.^47,61

2.5.4. Type V subtype IV other compounds^{75,142,277,289,292,295–299} (Table 24 – compounds 774–793 found in ESI^†)

Compounds in this subtype possess the furanyl substituted C-9-spiro-γ-lactone/lactol together with various substituents and unsaturation (Δ^3,4 777;²⁹⁷ Δ^4,18 778;²⁹⁷ Δ^1,10 779;⁷⁵ Δ^1,2 780;²⁷⁷ saturated 781 (ref. 297)) in the decalin system. A small coupling constant between H-6 and H-7 proved that these protons were in α,α-equatorial positions in 777, which consequently contains a cis-6β,7β-diol. Teulolin B (778) is the first neo-clerodane diterpene with an exocyclic double bond at C-4/C-18 isolated from Teucrium species.²⁹⁷ In sandrasin B (781), C-4 bears α-OH and β-CH₂OH groups, rather than being involved in a spiro-oxirane with C-18 as found in 744 and 745, which are type V subtype II compounds (see Section 2.5.2.) co-isolated from T. sandrasicum.²⁸⁹

2.6. Type VI with a furofuran-based side chain at C-9

Type VI compounds contain a bicyclic furofuran system, either hexahydro or tetrahydro, attached at C-9. As contrasted below, an oxygen bridge between C-11 and C-16 differentiates the bicyclic type IV from the monocyclic type II compounds.

2.6.1. Type VI subtype I with a hexahydrofurofuran-based side chain at C-9 (ref. 176, 193, 195, 207 and 300–319) (Table 25 – compounds 794–845 found in ESI^†)

Most neo-clerodanes in this group possess a hexahydrofurofuran side chain at C-9 and a 4α,18-spiro-oxirane group, while some compounds also contain an additional C-19,2α-hemiacetal function (compare the structures of 815 and 834 obtained from Scutellaria discolor).³¹¹ Interestingly, compound 819, isolated from S. columnae, was the first neo-clerodane diterpene reported to have a hexahydrofurofuran moiety with an 11R-configuration.³¹³

Generally, clerodin hemiacetal derivatives are found as C-15 epimeric mixtures. Scupolin K (811) from S. polyodon was found as a mixture of the C-15 epimers of the 14,15-dihydro- 15-hydroxy derivative of scupolin J,³¹⁰ and scutalsin (818) from S. altissima was also a 1 : 1 epimeric mixture of the C-15 hemiacetal function.³¹² Compounds 812–814 and 832–833, which have ethoxy acetal groups, are considered to be artifacts from Scutellaria discolor formed in the course of extraction or separation using ethanol.³¹¹ Compounds 824 and 825 from Ajuga salicifolia are the C-15 epimers of the 14,15-dihydro-15-hydroxy derivative of 826.³¹⁶ Compound 831 from Clerodendrum inerme was assigned as a mixture of C-15 epimers of 14,15-dihydro-15-hydroxy-3-epicaryoptin.³¹⁹ Two pairs of diastereomeric hemiacetals, scutecyprols A (838) and B (810), were detected in the aerial parts of S. cypria. After oxidation, they were isolated as their γ-lactone derivatives.³⁰⁹

Compound 837 from S. alpine contains an isobutyroyloxy group at C-19, while a rarer propanoyloxy substituent is present in 836 from S. barbata; however, their absolute configurations were not ascertained.^195,321 Scupontins C, D, and F (839–841) from S. pontica possess unusual [(3′S,3″S)-3′-[(3″-acetoxybutyryl) oxy]butyryloxy and [(3′S,3″S,3‴S)-3′-[[3‴-[(3‴-hydroxybutyryl) oxy]butyryl]oxy]butyryl]oxy substituents, respectively, attached to the C-19 position of the neo-clerodane nucleus.³²² Scutalpin M (842) also from S. alpine is the first 14-oxidized hexahydrofuro-furan-neo-clerodane derivative isolated from natural sources.¹⁹³ Compound 843 from A. lupulina has a C-4/18 exocyclic double bond, which is unusual in this type of clerodane diterpenes.³¹⁵ Inermes A (844) and B (845) from C. inerme are dimeric neo-clerodanes with the two hexahydrofurofuran rings joined through an ether linkage at C-15, the latter compound contains a C-1 methoxy group not found in the former compound.³¹⁹

2.6.2. Type VI subtype II with a tetrahydrofurofuran-based side chain at C-9 (ref. 207, 300–302, 306, 310, 311, 320 and 322–328) (Table 26 – compounds 846–874 found in ESI^†)

The tetrahydrofurofuran system with a 14,15 double bond is the same in compounds of this subtype, and their clerodendrin skeletons also contain a 4α,18-spiro-oxirane. They differ in other substitutions on the decalin system. Certain compounds [e.g., jodrellins A and B (860–861) from Scutellaria species] also contain an additional C-19,2α-hemiacetal function,^{306,310–312,326–328} as found in the type VI subtype I compounds mentioned in the prior section. The spectroscopic differences observed between 853 and 854 found in S. laterifora suggested the presence of an acetoxy substituent in the former and a 2-methylbutanoyloxy group in the latter.³²⁰ Clerodendrum trichotomum yielded clerodendrins I, E, F, G (868–871) all having a double bond at C-7/C-8 in their decalin skeleton are substituted with 2α-hydroxy, 4α,18-epoxy, 6α,19-diacetoxy, 7,8-ene and 11,12,13,16-tetrahydrofurofuran functions, but with different 3β-acyloxy groups.^325,328 Like 839–841, scupontins A, B, and E (872–874) from S. pontica are esterified at C-19 with diand tri-esters of 3-hydroxybutanoic acid.³²²

2.7. Type VII with a 13-spiro-15,16-γ-lactone moiety^{192,193,201,204,208,329–338} (Table 27 – compounds 875–902 found in ESI^†)

The defining structural characteristics of type VII neo-clerodane structures are a 8,13-ether bridge creating a tetrahydropyran that incorporates C-8 and C-9, as well as C-11–C-13, and a 13-spiro-15,16-γ-lactone moiety. Both possible configurations are found at the spiro C-13. A comparison with type III compounds is shown below.

Scutorientalin C (876) is the first neo-clerodane with a free C-11 axial hydroxy group in ring C (tetrahydropyran) to be isolated from a Scutellaria species.¹⁹² The observed spectroscopic differences between 877 and 878 were consistent with the presence of a C-6α isobutyric ester in the former compound rather than the tigloyloxy group found in the latter.^201,329 From a chemotaxonomic point of view, compound 879 is the first 8β,13S-epoxy-neo-clerodan-15,16-olide derivative found in European Scutellaria species, although these structural features are shown by several neo-clerodanes isolated from Asian Scutellaria species.³³⁰

Opposite absolute configurations have been found at C-13 (e.g., 13S and 13R in 884 and 898, respectively).^208,337 The aerial parts of Scutellaria hastifolia yielded several clerodanes similar structurally to the known scuteparvin, but distinguished by being trans-cinnamoyl derivatives. Some of these compounds are epimeric at C-13, and it was not possible to separate the 4 : 1 mixture of hastifolin G (886) and hastifolin F (894).²⁰⁴ Likewise, barbatellarine E (888) is a C-13 epimer of barbatellarine F (892), as confirmed by NOESY and optical rotation data.^333,334 Comparison of spectroscopic data for 896 and 897 indicated the presence of C-6α and C-7β equatorial isobutyryloxy groups and a free C-19 hydroxy group in the former compound instead of C-6α tiglate and C-7β and C-19 acetates in the latter compound.^332,336

2.8. Clerodane diterpene glycosides^{62,102,254,255,257,268,339–361} (Table 28 – compounds 903–981 found in ESI^†)

The clerodane diterpene glycosides come from many of the above types but have been placed into a separate category based on the presence of one or more sugar groups at various positions on both the decalin and C-9 side chain. Gleichenia japonica and Dicranopteris pedata yielded new glycosylated type I clerodane diterpenes with an acyclic C-9 side chain (903–904 and 905–906, 910–913, respectively).^339,340 The only structural difference between 903 and 904 is the presence of only glucopyranosyl at C-6 in the former, but glucopyranosyl linked to rhamnopyranosyl in the latter. However, compound 903 inhibited the growth of lettuce, whereas 904 accelerated growth.³³⁹ The related glycoside 905 with sugars on both C-6 and C-13 also accelerated lettuce stem growth, but inhibited root growth.³⁴⁰ Compounds 907–909 are the first clerodane diterpenes with l-arabinoside at C-13 isolated from the family Compositae (species Nannoglottis carpesioides).³⁴¹ Compounds 910–913 are monodesmosidic clerodane diterpene glycosides containing two monosaccharides, glucopyranosyl and rhamnopyranosyl.³⁴² Compounds 918–922 possess a 1,4-dihydroxy-2-buten-2-yl-ethyl group at C-9, which is characteristic of the diterpenoids found in Portulaca and Salvia plants.^62,102,344

Examples of glycosylated clerodanes with type II structures are 924–925 from Elsholtzia bodinieri, 926 from Salvia amarissima, and 927–928 from Tinospora tuberculata.^345–347 Amarisolide (926) was the first reported diterpene glucoside found in Salvia species.³⁴⁶

Compounds 930 and 931, both found in Baccharis sagittalis, were separated and characterized as C-18 β-d-glucopyranosyl peracetylated derivatives.³⁴⁹ The former compound can be classed as a type I clerodane glycoside, while the latter compound is a type III clerodane glycoside with a 3-ethyl-2-butenolide side chain.

Clerodane diterpene glycosides with various five-membered O-containing rings attached at C-12 were isolated from three Tinospora species.^350–352 Compounds 934 and 935 with an unsubstituted butenolide ring exhibited moderate anti-settling activity against the sea barnacle Balanus amphitrite, and are the first clerodane diterpenes to be reported with antifouling activity.³⁵⁰ Rumphiosides A and B (936 and 937) contain hydroxy- and methoxy-butenolide rings, while the tetrahydrofuran rings attached to C-12 in the epimeric 939 and 940 were possibly artifacts formed from a dialdehyde during the extraction of the plant material with methanol.³⁵¹ Compounds 941 and 942 have a 17,6-γ-lactone,³⁵¹ while compounds 936–937, 939–940, and 943–944 contain a 17,12-δ-lactone as well as an 18β,1β-δ-lactone. In cordifolides B and C (943 and 944) the butenolide ring located on C-12 is rotated nearly 180° from the C-12/C-13 bond, resulting in different orientations.³⁵² Their structures were determined on the basis of spectroscopic data interpretation.³⁵²

In addition to cordifolides B and C, Tinospora cordifolia also yielded a novel unique sulfur-containing type IV clerodane furanoditerpene glycoside, cordifolide A (948).³⁵² Its structure and configurations at chiral centers were confirmed by single-crystal X-ray crystallographic analysis. Cordioside (949) is a 19-nor-clerodane furanoditerpene glucoside with a C-3/C-4 double bond, and hence, no C-19 carbon.³⁵⁴ Phytochemical investigations on the aerial parts of Tinospora crispa led to the isolation of several new cis-clerodane type IV furanoditerpene glycosides (e.g., 950, 966, 967). In addition, spectroscopic assignments of a previously reported compound, borapetoside A (947), were revised on the basis of HMQC and HMBC correlations.³⁵³ Type IV glucoside 968 adopted a unique all boat conformation of its tricyclic ring system, as also indicated by energy calculations.²⁵⁵

Compounds 970 and 973 are type IV 18-nor-clerodane glucosides, whereas 971 and 972 are 18,19-dinor-clerodane-type diterpene glucosides.^254,357 Compounds 970–972, isolated from Tinospora sinensis, were subjected to an α-glucosidase inhibition assay, and exhibited IC₅₀ values of 2.9, 3.8, 3.3, and 1.9 mM, respectively. Meanwhile, the positive control, acarbose, demonstrated an IC₅₀ value of 0.84 μM.³⁵⁷ Compounds 974–976 are three new type V 19-nor-neo-clerodane diterpene glucosides.³⁵⁸

Compounds 978–981 are described by a novel macrocyclic skeleton containing an neo-clerodane diterpenoid moiety, one or two d-glucose units, and a 3-hydroxy-3-methylglutaric residue.^360,361 Two long nine-atom extended strands are con nected by two “cyclohexane-chairlike” two atom junctions to create a unique three-dimensional construction. The structure and the absolute stereochemistry of 978 were elucidated through a combination of spectroscopic techniques, degradation reactions, and conformational analysis methods. Compound 978 inhibited high density induced apoptosis in several human and murine carcinoma cell lines.

2.9. Clerodane derivatives

2.9.1. N (or S or Cl)-containing derivatives^{218,228,230,232–234,333,338,362–379} (Table 29 – compounds 982–1054 found in ESI^†)

Compounds 982 and 983 from Polyalthia longifolia contain a succinic diamide moiety attached at C-12; the former compound also has a double bond between C-13 and C- 14.³⁶² Other N-containing clerodane diterpenes have a heterocylic group attached at the terminal carbon (C-15) of a 3-methyl-3-pentenyl side chain. For example, when the marine sponge Agelas axifera was investigated for cancer cell growth inhibitory constituents, pyrimidine diterpenes (e.g., 986) were isolated.³⁶⁴ Other compounds from Agelas species possess both a 9 N-methyladenine moiety at C-15 as well as a 2-carboxy-4-bromopyrrole linked through an ester at C-18 (e.g., 990), whereas compound 991 with a C4/C18 exocyclic methylene has only the former moiety.³⁶⁵

Scutellaria barbata is a major source of neo-clerodane diterpenoid alkaloids.^{228,232,233,338,366–372} Nicotinic acid (also known as niacin or vitamin B3) is a frequent N-containing component of ester groups found at various positions on the decalin, as shown in the following examples: 994–995 (mentioned in Section 2.3.1.3.), 998, 1008, 1012.

Two similar N-containing clerodanes (1013 and 1014) were isolated from P. longifolia.³⁶² The former compound has a molecular weight two units greater than the latter, consistent with a pyrrolidine-15,16-dione in 1013 and a 1H-pyrrole-15,16-dione attached at C-12 in 1014. New clerodanes 1015–1021 with either a dihydro-2H-pyrrol-15-one, dihydro-2H-pyrrol-16-one, or 1H-pyrrole-15,16-dione at C-12 were isolated from Echinodorus macrophyllus and Casearia sylvestris.^218,373,374

Compounds 1022–1024 isolated from the twigs and leaves of Cleidion brevipetiolatum have a type IV clerodane skeleton with an infrequent methylsulfinyl group present at C-3.³⁷⁵ Rare Cl-containing clerodanes 1025 and 1026 were isolated from Teucrium pernyi and T. racemosum, respectively.^376,377 The Cl is part of a chlorohydrin in both compounds, with the CH₂Cl at C-17 in 1025 and at C-18 in 1026. Because they were present in acetone extracts of the plant material, these two compounds were not regarded as artifacts of the isolation procedure.

Other compounds with nicotinoyl esters were isolated from S. barbara as described below. Compound 1031, with an α-configuration of the ethoxy group, is the epimer of 1030.³⁷⁰ Compared with 1033, compound 1032 lacks a 13-spiro-15,16-γ-lactone moiety, as the result of oxidative cleavage between C-13 and C-14.³⁶⁸ NMR spectroscopy confirmed the presence of hydroxy and hydroxymethyl groups at C-13 in 1032, as well as the absence of carbon signals for C-14 and C-15. The originally reported structures of 1036–1043 were revised.³⁶⁶ The absolute stereochemistry 1R,5R,6R,7S,8R,9R,10R,13S was assigned to compounds 1045 (1048) and 1049, whereas compound 1050 has the same 5R,6R,7S,8R,9R,10R,11S,13R absolute configuration as 1051.³⁷⁹ Barbatine A (1050) also showed significant capability to protect cells against H₂O₂ with an ED₅₀ value of 16.8 μM.³⁷⁹ Barbatellarine C (1053) is a C-13 epimer of barbatellarine D (1054), as confirmed by a NOE difference experiment and the respective NOESY spectra.³³³

2.9.2. Degraded derivatives44,79,92,98,132,133,146,158,163,170,191,219,227,258,277,278,280,293,342,380–392 (Table 30 – compounds 1055–1097 found in ESI^†)

Compounds in this subtype have fewer than the normal 20 carbons of the basic clerodane skeleton. Firstly, in pentandranoic acid B (1055) from Callicarpa pentandra, a new contracted ring-A (cyclopentanone rather than cyclohexanone) is present.⁹⁸ Secondly, various one-carbon substituents can be absent, primarily, but not exclusively, C-18 or C-19. Compounds 1056–1061 are rare 18-nor-clerodane diterpenoids with a C-4 oxo or hydroxy group.^{258,293,380,381}

19-nor-Clerodanes constitute the majority of the degraded clerodanes, and the following examples come primarily from Croton and Teucrium species. One of the simplest 19-nor-clerodanes is cajucarin B (1062) isolated from Croton cajucara.¹⁵⁸ The 19-nor-clerodane 1063 with a C-5/C-10 double bond could be formed by a retro Diels–Alder reaction.¹⁷⁰ Except for an opened 17,12-γ-lactone ring and C-12 oxidation, compound 1064 from C. euryphyllus is quite structurally similar to 1070 and 1071, which have a butenolide moiety spanning C-19 to C-6, from Teucrium viscidum.^278,382 Crassifolin H (1073) from C. crassifolius has a similar structure except for the presence of a C-5/C-10 rather than C-4/C-5 double bond.³⁸⁷ It demonstrated anti-angiogenic activity by reducing vessel formation to 59.3% of the control value at a concentration of 15 μg mL⁻¹. Notably, the bioactive type V 19-nor-clerodane-type diterpenoid trans-dehydrocrotonin (1076), with a cyclohexenone decalin ring A, is one of the most investigated clerodanes in the current literature.³⁸⁹ Two of the three hydroxy groups in syspiresin A (1078) from T. chamaedrys are replaced by hydrogen (C-2) and a methoxy group (C-6) in teupolin IX (1079) from T. polium.^92,277 Crotoeurin A (1084) from C. euryphyllus was the first nor-clerodane diterpenoid dimer connected through a unique cyclobutane ring via a [2 + 2] cycloaddition; its structure was confirmed by single-crystal X-ray diffraction analysis.³⁸²

Compound 1085 is a 14,15,16-trinor-clerodane isolated from Sindora sumatrana.¹⁶³ Isolated from several different plant species, compounds 1086–1089 are unusual 14,15-bisnor clerodanes with a C-4/C-5 double bond,^{44,79,219,391} and compounds 1090–1096 are 13,14,15,16-tetranor-clerodanes. Among the latter tetranor-clerodanes, C-12 is present as a free carboxylic acid (e.g., 1090–1092)^227,342 or as part of a lactone ring. Croinsulactone (1093) from C. insularis contains a 12,13-γ-lactone with a spiro-carbon at C-9,¹³² while ciliatolide A (1094) from Scapania ciliata contains a 12,10-γ-lactone as well as a 14,6-γ-lactone (the latter ring is comparable with a 18,6-γ-lactone in a non-degraded clerodane).¹⁹¹ Teucrolin D (1095) and teucrolivin F (1096) from T. oliverianum also contain a 12,10-γ-lactone as well as a 4,14-spiro-oxirane (identical to a 4,18-spiro-oxirane in a non-degraded clerodane).^133,146 Compound 1097 from Jamesoniella colorata is a degraded clerodane (rearranged drimane)-type sesquiterpenoid.³⁹²

2.9.3. Ring-seco derivatives75,108,119,121,128,131,144,155,187,250,252,263,270,393–398 (Table 31 – compounds 1098–1145 found in ESI^†)

seco-Derivatives have an opened ring at some point in the structure, creating multiple interesting compounds, depending on which bond is broken and what additional rearrangements are present. The first examples are 9,10-seco-clerodane diterpenoids.^{119,128,393–395} Although the configuration of the C-8/C-9 epoxide in jamesoniellide F (1098) from the liverwort Jamiesoniella autumnalis could not be determined absolutely by NMR, this 9,10-seco-clerodane likely is a biogenetic product of cis-clerodane 1110 found in the same plant.¹¹⁹ Salvianduline A (1102) and pyrrhopappolide (1104) from Salvia lavanduloides and Microglossa pyrrhopappa, respectively, are both 9,10-seco-clerodanes, but contain a 17,12-δ-lactone and a 20,12-γ-lactone, respectively. Compound 1111 is a 9,10-seco-clerodane with a fully saturated benzofuran rather than decalin bicyclic skeleton.¹⁰⁸ A C-7/C-10 ether bridge plus a hydroxyl on C-10 forms a hemiketal. Other 9,10-seco-clerodanes contain a a C-8/C-10 ether bridge.^119,187,393 Among several compounds of this type from Cephaloziella kiaeri, cephaloziellins H and I (1115–1116) are C-8/C-10 hemiketals, while one additional degree of unsaturation in cephaloziellin J (1117) was indicative of a second ether bridge between the C-10 ketal carbon and the C-11 oxygenated methine.¹⁸⁷ Furthermore, in cephaloziellin K (1118), the hemiketal bridge found in 1115 and 1116 is absent.¹⁸⁷ Both jamesoniellides B (1121) and A (1120), from J. autumnalis, have a C-8/C-10 ether bridge; however, the B-ring in the former compound is opened between C-9 and C-10, while that in the latter compound is fragmented between C-8 and C-9.¹²¹

In various 5,10-seco-neo-clerodanes,^{131,155,250,263,270} the unsaturated patterns of the “opened” decalins include 2,4,10(1)-triene (cyclodeca-1,3,5-triene) (e.g. 1122),²⁵⁰ 10-oxo-2,4-diene (cyclodeca-3,5-dien-1-one) (e.g., 1123),²⁵⁰ and 1,3,5(19)-triene (5-methylenecyclodeca-1,3-diene) (e.g., 1124)²⁶³ conjugated systems. Salvimicrophyllin A (1122) from S. microphylla was unstable in solution and decomposed on exposure to light and heat.²⁵⁰ The relative configuration of salvimicrophyllin B (1123) was confirmed by single-crystal X-ray diffraction crystallography.²⁵⁰ The epimeric hemiacetals 1125/1126 and acetals 1127/1128 could not be separated when isolated from Conyza welwitschii.²⁶³ Compounds 1132 and 1135 from Conyza and Dodonaea species, respectively, have the same ethylfuran side chain at C-9 but different double bond patterns in the cyclodecane ring, 1,3,5(19) and 2,4,10(1), respectively.^131,155 Tonalensin (1136) from S. tonalensis is an interesting 5,10-seco-neo-clerodane containing an acetal at C-20 with oxygens from C-7 and C-11 forming two fused tetrahydrofuran rings. Its trans,cis,cis-cyclodecatriene ring adopts a boat-chair conformation in which the Δ^1,3,5-triene system is no longer coplanar.³⁹⁶

Tinosporafuranol (1137) and tinosporafuranadiol (1138), obtained from the stem bark of Tinospora cordifolia, are 4,5-seco-clerodane-type diterpenoids.³⁹⁷ The single-crystal X-ray diffraction analysis of 5,6-seco-compound 1139 established a 5α-configuration for the C-19 acetoxymethylene group, as well as the absolute stereochemistry. In the crystalline state, the substituted cyclohexane ring of 1139 is in a chair conformation, with a mean torsion angle of 57°.¹⁴⁴ Both rhyacophiline (1140) and salvireptanolide (1141) from S. rhyacophila and S. reptans, respectively, have novel skeletons characterized by cleavage of the C-5/C-6 bond and aromatization of the A-ring.^252,398 Compound 1140's structure was fully established by spectroscopic and X-ray diffraction analyses.³⁹⁸ Jamesoniellide J (1142) from J. autumnalis is a seco-clerodane diterpenoid cleaved between C-8 and C-9.^187,393

2.9.4. Rearranged derivatives55,75,81,98,113,128,164,187,219,220,240,271,276,290,392,399–415 (Table 32 – compounds 1146–1224 found in ESI^†)

The clerodanes in this group have a vast scope of ring numbers, sizes, and fusions. The “usual” decalin systems are especially affected, but rearranged C-9 side chains also occur. In various abeoclerodane diterpenes with a (4→2) rearranged ring A moiety, C-3 is generally an aldehyde (e.g., 1149) or carboxylic acid (e.g., 1152) attached to the five-membered ring A.^{55,81,98,219,220,240,399} However, compound 1146 from Solidago altissima is also a homoditerpene with 21 carbons [a methyl (C-21) is attached to a C-3 oxo group] and could have resulted from reaction of a C-3 aldehyde with diazomethane used to esterify the extract.⁵⁵ In addition to the rare contracted ring A, pentandranoic acid A (1153) from Callicarpa pentandra also contains an 12-oxo-13(16)-methylene in a C-9 pentanoic acid side chain rather than the C-9 ethyl-α,β-unsaturated-γ-lactone of pentandralactone (1154), also found in the same plant.⁹⁸ Compound 1155 was isolated from Polyalthia viridis as a 1 : 1 mixture of C-16 epimers.²¹⁹

Compounds 1157–1160 contain an expanded seven-membered A-ring where C-19 is inserted between C-4 and C-5.^164,400,401 In 1157 from Portulaca pilosa, this novel bicyclo[5.4.0] undecane trans-clerodane skeleton also has an ether bridge at C-3→C-5 in the A-ring.⁴⁰⁰ Scapanialide B (1160) from Scapania parva is the first cis-clerodane diterpenoid with a bicyclo[5.4.0] undecane skeleton isolated from liverworts.¹⁶⁴ Compounds 1161–1163 from two Salvia species^402–404 also contain a seven-membered A-ring; however, this ring includes C-6, rather than C-19. In addition, C-11 has been inserted between C-8 and C-9, creating a phenyl B-ring fused to a γ-lactone, in a tricyclic skeleton.

Notably, compounds 1164–1166 from three different Salvia species have a novel rearranged A/B ring spiro-fused neo-clerodane skeleton with the spirocyclic junction at C-10.^113,402,405 The structure of spiroleucantholide (1164) might be derived biogenetically by a ring contraction of the seven-membered ring in a precursor benzocycloheptatriene skeleton to form a spiro-fused junction. This report was the first to identify a spiro-6/6 A/B ring diterpenoid derived from a neoclerodane skeleton.⁴⁰²

Compounds 1167–1171, also from various Saliva species, contain a salvigenane skeleton with a seven-membered B-ring, which contains C-11 inserted between C-8 and C-9.^{113,403,404,406,407} Like in 1161–1163, a fused γ-lactone is also present. However, the A/B ring sizes are 7/6 in 1161–1163, but 6/7 in 1167–1171.

X-ray crystal structure determination of blepharolide A (1172) showed a 5,6-unsaturated octahydro-1H-cyclopropa[α] naphthalene derivative, which presented a new rearranged clerodane structure with a C6–C6–C3 ring system. This new skeleton was named isosalvigenane. The isolation of salvigenane (blepharolide B, 1167) and isosalvigenane (blepharolide A, 1172) diterpenoids from Salvia blepharophylla suggested a botanical chemotaxonomic relation between sections Fulgentes, Albolanatae and Brandegeia of subgenus Calosphace.⁴⁰⁶ Tilifodiolide (1173) from Salvia dugesii was the first natural product with a substituted tetralin skeleton; its unusual structure was confirmed by X-ray diffraction analysis.²⁷⁶ A biogenetic derivation of both 1173 and the co-isolated 1161 from a clerodane precursor was postulated.

The rearranged clerodane diterpenes baenzigerides A (1174) and B (1176) and the related open-ring glucosides, baenzigerosides A (1175) and B (1177), were isolated from the stems of Tinospora baenzigeri.^408,409 The novel skeleton of baenzigeride A (1174) could be produced from the cis-ent-neoclerodane epoxide (A) (Scheme 2). Rearrangement with ring-A contraction through migration of either the 1–2 or 3–4 bond would give the aldehyde (B). Reduction of B should give the primary alcohol (C), which could lactonize to 1174. Although epoxides such as (A) have not been found as natural products, the known clerodane 2,3-epoxides, e.g., jateorin, do have a 1,18-lactone group.⁴⁰⁸

Scheme 2.

Possible biogenesis of baenzigeride A (1174).

The vapor and condensed solid resulting from heating salvinorin A (9) to 245 °C for 10 min were combined as a chloroform extract, from which two new rearranged clerodanes (1178–1179), were isolated. The major structural changes to 9 were epimerizations at C-2 and C-8, eliminations of acetoxy and methyl ester groups, and carbon–carbon rearrangements at C-1, C-2, and C-10. Compounds 1178 and 1179 are unique salvinorin derivatives with interlocking five-membered cyclopentane and six-membered anhydride rings; the 3-oxabicyclo[3.2.1]octane-2,4-dione systeme was confirmed by X-ray analysis.²⁷¹

Microphyllandiolide (1180) from Salvia microphylla represents the first example of a new framework with a 9/3 bicyclic ring system (microphyllane skeleton). A plausible biogenetic pathway to this new skeleton arises from proposed transformations of other neo-clerodane diterpenes isolated from Salvia species, involving pericyclic reactions. As shown in Scheme 3, an electrocyclic opening of the decalin ring of the diene (a) would give the triene (b). Cyclopropanation of (b) would give (c), and finally, allylic oxidation of (c) would provide the C-3 hydroxy group of 1180.⁴¹⁰

Scheme 3.

A plausible biogenetic pathway to 1180.

Compound 1181 from Teucrium betonicum is a biogenetically unexpected diterpenoid with a new 7β-homo-19(5→18)abeo-neo-clerodane skeleton.⁴¹¹ Its structure was established by spectroscopic means, including an X-ray diffraction analysis.

Both Microglossa pyrrhopappa (e.g., 1182, 1186) and Pteronia divaricata (e.g., 1190) provided rearranged clerodanes with a 2-oxabicyclo[2.2.1]heptan-3-one system most likely formed by Wagner–Meerwein rearrangement; compound 1182 also has a 7,8-epoxide.^75,128 Ring-A of 1194 (M. pyrrhopappa) and 1195 (P. incana) is a cyclopentene ring substituted with hydroxymethyl (C-3), and the former compound has an additional hydroxy group at C-6.^75,128 Pteroniatrilactone (1198) from P. eeni has the same rearranged carbon skeleton as 1186 with the addition of a third lactone between C-2 and C-19 (OC=O). An inseparable mixture of related 3 : 1 epimeric hemiacetals 19-α- and 19-β-hydroxypteronia dilactone (1196, 1197) was also isolated.⁷⁵ Eeniolide (1199), also from P. eeni, was probably formed biogenetically from an 8,10-dihydroxyclerodane diterpenoid, with cleavage of the C-9/C-10 bond and formation of a C-9/C-8 double bond, followed by ring-A/B reclosure at C-17/C-10.⁷⁵

Jamesoniellide C (1200) with the same side chain (furanylmethylene-dihydrofuranone) as 1199 but a 5/6 rather than 6/6 skeleton was isolated from the liverwort Jamesoniella autumnale along with the interesting 8,9 and 9,10-seco-clerodanes 1120 and 1121 (see Section 2.9.3).⁴¹² Compounds 1201–1206 with a novel rearranged tetrahydrobenzofuran fused to the decalin through a C₈–C₁₆ bond were isolated from Teucrim alyssifolium.^413,414 Although 1205 and 1206 were structurally quite similar to 1201–1204, the presence of a C-7/C-19 hemiketal bridge in the former compounds was the main difference between these two related groups.^413,414 Salvilanguiduline A (1207) from Salvia languidula illustrates a rearranged clerodane skeleton containing an epoxy spiro γ-lactone function and a C₁–C₁₃ bond.⁴¹⁵ The structure of the rearranged clerodane cephaloziellin O (1215) isolated from the Chinese liverwort Cephaloziella kiaeri was confirmed by single-crystal X-ray diffraction analyses, and the absolute configurations of 16 new clerodane diterpenoids, cephaloziellins A–P, were established by comparing experimental and calculated electronic circular dichroism spectra.¹⁸⁷

Teucrium brevifolium yielded several clerodanes containing an unusual rearranged skeleton with an eight-membered ring carbocycle (e.g., 1217, 1219, 1221).²⁹⁰ The ring conformation in each compound was established by exhaustive NMR spectroscopic studies as well as X-ray data on 1221. A biogenetic pathway from teubrevin D (747) was postulated to explain the formation of the 5,10-seco-9(8→19)abeo-neo-clerodane skeleton (1219, 1221) and the 7,8,17-trinor derivatives (1217).²⁹⁰

The leaves of Salvia xalapensis yielded two new clerodane-type diterpenoids with an opened (C₅–C₆) unsaturated ring-B skeleton, salvixalapadiene (1223) and isosalvixalapadiene (1224).⁴⁰⁷ Both compounds have a phenyl A-ring and two double bonds in the opened B-ring at C6-7/C8-11 and C7-8/C9-11, respectively. Their unprecedented rearranged skeleton may be derived biogenetically from a salvigenane precursor. Salvixalapoxide (1222) with a languidulane skeleton was also isolated at the same time.⁴⁰⁷

2.10. NMR features of clerodane diterpenes

NMR spectroscopy, especially ¹³C NMR analysis, has been widely employed for the characterization of the different skeletons of clerodane diterpenes. The assignments of carbon signals of a new clerodane diterpene by comparison with the data of known compounds require the ¹³C data of appropriate model compounds. It would appear to be of value to provide an easy access to an extensive list of ¹³C data of these diterpenes. Inspection of the chemical shifts of different types of clerodanes revealed that the main differences among the seven structural types (I–VII) are related to the number of methyl groups (C-17 or C-20 are absent, in the IV and V structural types, respectively), and chemical shifts of C-8, C-12, C-17, and C-20. The characteristic spectroscopic features associated with these seven different structural types I–VII are summarized in Table 2.

Table 2.

Characteristic ¹³C NMR chemical shifts of types I–VII (δ_C, ppm)

	I	II	III	IV	V	VI	VII
C-5	37→40	35→55
C-8	31→38	31→45		46→52	35→47		79
C-9			38→40		51→54		37
C-10	10β-H: 46	10β-H: 42→50	48→52
	10α-H: 50
C-11						84→87
C-12	32→38	17→27		70→73	68→75
C-13		123→129	168→176	120→125	125→130	40→42	75→77
C-14		108→111	112→117	108→110	108→117
C-15		136→146	172→175	144→146	138→145	68	173→176
C-16		137→148	70→74	138→140	138→145	102→108	76→80
C-17	16→18	13→18		169→177	14→20
C-20	18	16→18		14→26	172→178
		Δ5,10: 140		Δ7,8: 170, 100

Clerodanes that possess a furan ring at C-12 show carbon resonances between δ_C 123→132 (C-13), δ_C 107→114 (C-14), δ_C 138→146 (C-15), and δ_C 137→148 (C-16). In clerodane diterpenes containing a Δ³ double bond (C-3: δ_C 120→130; C-4: δ_C 130→145), the chemical shifts of these carbons are affected by the presence of the substituents at C-18. When this carbon contains a carboxyl or carboxymethyl group, C-3 (ca. δ_C 133) and C-4 (ca. δ_C 149), are deshielded. However, when C-18 is an aldehyde group, C-3 (δ_C 152.4) and C-4 (δ_C 151.7) are more deshielded. On the other hand, in clerodane diterpenes containing a Δ^1,3 double bond and a carboxymethyl group at C-18, resonances occur between δ_C 133→134 (C-1), δ_C 123→125 (C-2), δ_C 132→136 (C-3), and δ_C 134→137 (C-4). In the structural type IV containing a Δ⁷ double bond, carbon resonances appear at δ_C 172 (C-7) and δ_C 100 (C-8). For compounds having a Δ⁴ double bond and C-18 as a methyl group, C-4 and C-5 resonate at δ_C 125.7 and 134, respectively. If C-18 is part of a lactone ring, these carbons are deshielded [δ_C 127.6 (C-4) and 162 (C-5)]. An epoxy ring at C-4 and C-18 leads to carbon signals at δ_C 63→67 (C-4) and δ_C 43→51 (C-18).

2.11. Abbreviation of functional groups

3. Biological activities

Clerodane secondary metabolites may benefit a plant species by acting as a chemical defense mechanism against phytophagous animals or diseases. Apart from insect antifeedant properties, several clerodane diterpenes display other effects against insects. Insecticidal activity has been reported for ajugarins I and IV.^416,417 Ajugarin IV also displays insect growth regulating activity, as do 3-epi-caryoptin⁴¹⁸ and the 19-nor-clerodanes cis-and trans-dehydrocrotonin.⁴¹⁹ Fungicidal activity against plant pathogenic fungi has been reported for clerodin and the related jodrellins A and B.³²⁶

Besides insect antifeedant and antifungal activities, many clerodane diterpenes possess various pharmacological activities beneficial to humans, including action as opioid receptor probes, as well as NGF-potentiating, anti-ulcer, cytotoxic, anti-inflammatory, antiparasitic, and antibacterial activities.

3.1. Insect antifeedant activities

3.1.1. Insect antifeedant and related plant protective activities of clerodane diterpenes

Clerodane-type secondary metabolites have attracted considerable attention as insect antifeedants, which is by far the most extensively studied bioactivity of these diterpenes. Clerodane diterpenoids have been found in several hundreds of plant species from various families and in organisms from other taxonomic groups, such as fungi, bacteria, and marine sponges.^420–422 Especially, various genera from the plant families Lamiaceae have been identified as rich sources of antifeedant clerodanes, with species of the genus Scutellaria producing some of the most potent clerodane antifeedants known so far.^{306,330,423–426} The most active compounds from these species against larvae of Spodoptera littoralis (cotton leafworm) were scutalpin C (497), scutecyprol B (810), jodrellin A (860), jodrellin B (861), and dihydroclerodin (1225), with corresponding feeding index (FI) values of 92 ± 8, 100 ± 0, 95 ± 16, 100 ± 0, and 97 1, respectively. In addition, 810 showed potent activity against ± several other species of Lepidoptera.⁴²⁶

Ajubractins A–E (855–856, 795–797) and 15-epi-lupulin B (821) were isolated from a dichloromethane extract of Ajuga bracteosa.³⁰¹ Among them, 795–797 and 821 showed moderately high antifeedant activities (FR = 0.14–0.15). Data analysis from the behavioral responses of S. littoralis exposed to the clerodane diterpenoids hativenes A–C (827–829), from Ajuga pseudoiva, showed that the all of the compounds tested had strong antifeedant activity at 100, 10, and 1 mg⁻¹, which began to dissipate at 0.1 mg⁻¹.³¹⁷ Comparison of the antifeedant index at the latter two concentrations also indicated that a change in the relative configuration of carbons C-12 and C-15 did not modify the activity considerably. 14,15-Dehydroajugareptansin (859), from Ajuga reptans, had significant activity against sixth stadium larvae of Spodoptera littoralis.²⁰⁷ Overall, these data, compared to those of other clerodanes isolated from Ajuga and Salvia species, helped to clarify different authors' suggestions and conclusions, which related the antifeedant activity of clerodanes to the presence of a perhydrofuranofuran moiety, a trans decalin ring system bearing an epoxide, and acetate groups.

Clerodin (2), 15-methoxy-14,15-dihydroclerodin (1226), and 15-hydroxy-14,15-dihydroclerodin (1227) exhibited growth inhibitory activity against Helicoverpa armigera (cotton boll-worm) with growth inhibition index (GI₅₀) values of 13, 21, and 11 ppm, respectively, compared to azadirachtin, the active ingredient of many pesticides, with a GI₅₀ value of 15 ppm.⁴²⁷ Clerodin (2), clerodendrin B (849), 3-epicaryoptin (1228), and 15-hydroxyepicaryoptin (1229) were effective antifeedants at 10 μg cm⁻³ of diet against Earias vitella and at 10 μg cm⁻² of leaf against Spodoptera litura.⁴²⁸ In addition, 2, 122, 849–850, 1229, and 2-acetoxyclerodendrin B (1230) showed good insect growth inhibitory activity, even at lower concentrations. In contrast, clerodendrin H (851), from Clerondendron trichotomum, distinctly stimulated feeding activity in adult turnip sawflies.³²⁵

Jodrellins A and B (860–861) and the reference compound clerodin (2) also reduced the growth of the plant pathogenic fungus Fusarium oxysporum f. sp. Lycopersici after 18 h, and 861 and 2 maintained growth inhibition at 50 and 100 ppm after 66 h. Compound 2 delayed germination of Verticillium tricorpus spores at 25, 50 and 100 ppm after 42 h, and 861 had similar effects even at 66 h. This study suggests that certain neo-clerodane diterpenoids may contribute to antifungal, as well as anti-insect, protection in plants.³²⁶ Clerod-14-ene-3α,4β,13ζ-triol (1231) from Viguiera tucumanensis inhibited both germination and root growth of Sorghum halepense and Chenopodium album and also slightly inhibited Ipomoea purpurea.⁴²⁹ Allelopathic agents with phytogrowth inhibitory effects are attractive new leads for development of agrochemicals.

3.1.2. Compilation of structure–activity relationships of clerodane insect antifeedants

To date, over 300 natural and semi-synthetic clerodanes have been examined in laboratory assays, yielding several compounds with potent antifeedant activity against various insect species, but it is not known whether this activity will be retained under field conditions. A comprehensive compilation of all test results on the insect antifeedant activity of clerodane diterpenes resulted in some interesting trends based on only the 10–20% of the most active clerodanes per insect species.^12,430

(i) Active clerodanes generally have a trans-decalin neo-clerodane skeleton.

(ii) In the most potent clerodanes, the C-9 side chain fragment contains an oxygenated ring system. A furofuran-based structure appears to be most favorable for strong activity against many Lepidopteran insect species. Furan and butenolide side chains are also frequently found in the most potent clerodanes, and hydrogenation of these unsaturated moieties usually results in lower potency.

(iii) Structural elements from (i) and (ii) must be present simultaneously to produce high activity. However, a number of exceptions to this general observation are known. For example, while decalin 1233, which has the more common 4α-O-epoxide, but no C-9 side chain, was inactive against L. migratoria, the decalin 1232 with a 4β-O-epoxide showed equal antifeedant activity under no-choice conditions as the clerodin derivative 1234.¹²

(iv) In addition to (iii), the key structural elements from (i) and (ii) must be able to adopt a certain spatial orientation for the compound to exert high activity. Indeed, the stereoelectronic factors are more important than the hydrophobic aspects as determinants of antifeedant activity. In addition, a furan ring in the side chain and a carbonyl α,β-unsaturated (or spiro-epoxide) group appear to be crucial. A conformational study indicated that the optimum interatomic distance between these two moieties ranged from 9.5 to 10.5 Å.^431,432

For example, bacchotricuneatin A (1235), 7α-hydroxybacchotricuneatin A (1236), 1237, azadirachtin (1238), and the D-ring aromatic withanolide 1239 exhibited remarkable feedant-deterrent activity against T. molitor with PFI values of 22.62, 26.60, 25.03, 29.57, and 29.89, respectively.⁴³¹

(v) Both rings in the decalin fragment are often substituted with hydroxy, epoxy, or ester groups, but none of these moieties can be concluded to be essential for antifeedant activity. However, slight changes in the exact identities and orientations of such groups can affect the potency of the antifeedance effect.

The general observations in (i)–(v) represent a reasonable summary of the structural features involved in the overall antifeedant activity of clerodane diterpenes. However, the underlying mechanisms of insect antifeedant activity are certainly more complex than implied by this abridged picture.

3.2. Opioid receptor agonist

3.2.1. Salvinorin A as a probe in opioid pharmacology and other salvinorin analogues

The discovery of salvinorin A (9) as the first non-nitrogenous natural product with high affinity and efficacy at the κ-opioid receptors (KORs or KOP receptors) led to a reevaluation of whether a basic nitrogen is necessary for opioid receptor affinity and efficacy. With similar potency to LSD, compound 9 is one of the most potent naturally occurring hallucinogens, and appears to have distinctive properties at KOR, including ultra-high efficacy in particular transduction systems and a lower tendency to cause receptor desensitization.^433–442 In mice, it produces antinociception that can be blocked by KOR antagonists.^443,444 It also produced an aversive response in the conditioned place preference assay,⁴⁴⁵ blocked the locomotor-stimulant effects of cocaine,⁴⁴⁶ and did not exert 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM)-like effects in nonhuman primates.⁴⁴⁷ Interestingly, compound 9 has low affinity for the μ-opioid receptor (MOR), although it does show allosteric MOR modulation.⁴⁴⁸ It exhibited deleterious effects on learning and memory, acting via a KOR mechanism.^448–450

Phytochemical studies on S. divinorum led to the isolation of many clerodanes, including salvinorins C–J (659–662, 1240, 663–665), divinatorins A–F (405–407, 409–410, 1241), salvinicins A (684) and B (685), and salvidivins A–D (686–687, 587–588).^{166,167,235,263–266,273,451} (The structures of 409–410, 659, 686, 1240, and 1241 are specifically shown, the remaining compounds can be found in the Structure Tables in ESI^†.) In a modification study of 9, salvinorin C (659) had 250-fold lower KOR affinity compared with 9 (K_i = 1022 nM vs. K_i = 4 nM).⁴⁵² Divinatorins D (409) and E (410) also had reduced KOR affinity compared with 9 (K_i = 230 nM and K_i = 418 nM, respectively, vs. K_i = 1.0 nM).¹⁶⁷ Uniquely, salvidivin A (686) was identified as the first naturally occurring neoclerodane with KOR antagonist activity (K_e = 440 nM).⁴⁵³ Among dicarboxylic acid esters of 9, the methyl malonyl derivative (1242) showed the highest binding affinity (K_i = 2 nM),although analogues 1243–1245 still exhibited significant KOR affinity (K_i = 21, 36, and 39 nM).⁴⁵⁴ 12-epi-Salvinorin A (1246), synthesized in four steps from 9, was a selective partial KOR agonist. It partially activated signaling through G proteins, yet acted as a full agonist in the β-arrestin 2 DiscoveRx assay.^455,456

Of the derivatives of 9 reported to date, salvinorin B ethoxymethyl ether (EOM–SB) is the most potent. It exhibited ten-fold greater potency than 9 in vitro and in rodents, and had a longer duration of action in rodents.^457–460 In addition, 22-thiocyanato-(RB–64) and 22-chloro-salvinorin A (RB–48) were both extremely potent and selective KOR agonists in vitro and in vivo.⁴⁶¹

3.2.2. Structure–activity relationships of salvinorin A analogues

The general SAR studies of 9 have been performed by semi-synthetic structure modifications, and have mainly focused on 9's high affinity and selectivity for the KOP receptor. Some of these analogues have interesting pharmacological profiles, from full KOR agonist to partial δ-opioid receptor (DOR) or μ-opioid receptor (MOR) agonists and antagonists.^{13,448,453,462–464} The SAR is summarized in Fig. 5.^{13,457,465–471}

Fig. 5.

General SAR for salvinorin A activity at KOP receptors.

[Adapted from Prisinzano TE and Rothman RB, Chem Rev., 2008, 108, 1732-1743.]

At the C-1 position, the reduction or removal of the carbonyl is tolerated, and introduction of a 1,10-alkene increases the possibility of antagonist activity.

At the C-2 position, (1) the size and electronegativity of the substituent at the C-2 position is critical for activity at opioid receptors. Bioisosteric replacements of the ester moiety are tolerated. (2) α-Substituents are preferred over the corresponding β-substituents. (3) In general, small alkyl esters favor binding to KORs. The compounds with methoxymethyl (1247) and ethoxymethyl (1248) ethers at this position are among the most potent 9-derived KOR agonists reported to date, while aromatic esters favor MOR binding.^13,472 Different aromatic groups attached directly to the decalin core can be allowed by KOR.⁴⁷³

At the C-4 position, (1) small alkyl chains are preferential for KOR binding, while (2) hydrolysis or reduction of the carbomethoxy group leads to reduced KOR affinity, and (3) conversion to an amide is generally not tolerated.^13,472

Furthermore, the furan ring at C-12 may be reduced or replaced, but KOR affinity is reduced. The reduction or removal of the carbonyl at C-17 and the introduction of an 8,17-alkene are tolerated.^13,472

Finally, degradation of the furan ring and/or replacement with other heterocycles is tolerated. Moreover, such a modification produced the first DOR selective ligand (1249) with the scaffold of 9.^13,472

3.3. NGF-potentiating activities

With EC₅₀ values of 20.2, 12.7, 4.0 and 2.4 μg mL⁻¹, balanspenes D–G (13–16), which have a 18,19-expoxy clerodane-type diterpene skeleton with an additional double bond at C-3 and C-4, markedly increased the nerve growth factor NGF (20 ng mL⁻¹)-induced quantity of neurite-bearing cells.¹⁵ An equilibrium mixture of ptychonal (380) and ptychonal hemiacetal (329), 6α,7α-dihydroxyannonene (382), and 7α,20-dihydroxyannonene (383), significantly enhanced NGF-mediated neurite outgrowth in PC12 cells at concentrations ranging from 0.1 to 50.0 μM, 0.1 to 30.0 μM, and 0.1 to 10.0 μM, respectively.^130,474 At 10, 30, and 100 μmol, compounds 435 and 436 had no effect on neurite outgrowth from PC12D cells in the absence of NGF, but at 100 μmol, decidedly increased the NGF (2 ng mL⁻¹)-induced proportion of neurite-bearing PC 12D cells by 49% and 53%, respectively.¹⁷⁴

Both crotopenes A and B (764 and 765) exhibited potentiating activities on NGF-mediated neurite outgrowth from PC12 cells.²⁹⁴ At a concentration of 10 μM, degraded clerodane compounds 1064, 1083, and 1084 exhibited neurite outgrowth-promoting activity in NGF mediated PC12 cells.³⁸²

13-epi-15,16-Epoxy-15α-methoxy-ent-clerod-3-en-18-oic acid (1250) and 15,16-epoxy-7α,18-dihydroxy-15-methoxy-ent-clerod-3-ene (1251) also markedly increased the NGF (2 ng mL⁻¹)-induced proportion of neurite-bearing cells by 49%, 53%, and 68%, respectively⁴⁷⁵

3.4. Antiulcer activities^476–482

trans-Dehydrocrotonin (1076), trans-crotonin (1252), cordatin (1253), and aparisthman (1254)⁴⁸¹ exhibited significant anti-ulcer activities, comparable with those of cimetidine. At a dose of 100 mg kg⁻¹ (p.o.), the four compounds significantly reduced gastric injury induced by stress (67% or 72%), indomethacin/bethanechol (29%, 78%, 71%, 78%, respectively), ethanol (71% or 76%), pylorus ligature (30%, 35%, 59%, 50%, respectively), and hypothermic restraint (50% or 66%) in mice and rats. In the HCl/ethanol-induced gastric ulcer model, at 100 and 250 mg kg⁻¹ (p.o.), gastric lesion formation was decreased by 48.1%, 52.3%; 50.8%, 55.8%; 70%, 59% and 77%, 66%, respectively, when compared to the control group. In the pylorus-ligature model, 1076 and 1252 (p.o.), like cimetidine, increased the volume of gastric fluid when compared to the control group, while 1253 and 1254 (p.o.) decreased the volume of gastric fluid. Compound 1076 exhibited low acute toxicity in mice (LD₅₀ 876 mg kg⁻¹) when administered orally, however, hepatotoxicity resulted from its long-term use.

3.5. Cytotoxic activities

Casearupestrins A (21), B (22) and D (24) showed significant cytotoxicity against four cell lines (HL-60, HCT-8, MDA/MB-435, and SF-295), with IC₅₀ values ranging from 0.10 to 1.3 μM.¹⁷ Intrapetacins A (38) and B (39) displayed moderate cytotoxicity against KB cells, with IC₅₀ values of 2.0 and 0.8 μg mL^±1.²¹ In addition, compound 39 caused a significant (21 mm) zone of inhibition of fungal growth. Bioassay-guided fractionation of the EtOAc extracts of Casearia membranacea afforded case-amembrins A–E (60–64), M–O (65–67), and caseamembrols A (119) and B (68) as active principles.^26–28 Compounds 60 and 62–64 exhibited cytotoxic activity against PC-3 and Hep 3B, with IC₅₀ values below 3 μM. Compounds 66 and 67 showed significant activity against KB, DLD-1, and Med tumor cell lines (1.94–8.94 μg mL⁻¹). Compounds 119 and 68 were cytotoxic against PC-3 human prostate cancer cells with IC₅₀ values of 2.45 and 5.66 μM, respectively.²⁸

Laetiaprocerines A–D (101–103, 274) were cytotoxic toward the MCF7 human tumor cell line.³⁶ The structurally similar zuleanin-type compound caseamembrin G (1255) was cytotoxic against KB, HeLa, and Hep59T/VGH carcinoma cell lines.⁴⁸³

Esculentin B (95) from Casearia esculenta and caseargrewiins A–H (74, 1256–1258, 104–107) from C. grewiifolia showed significant cytotoxicity against three cancer cell lines (KB, BC1, and NCI-H187) with IC₅₀ values ranging from 0.1 to 8.7 μg mL⁻¹.^31,37 The most potent compounds were 104 and 106 against KB (IC₅₀ 0.66, 0.67 μg mL⁻¹), 1258, 95, 105, and 107 against BC1 (IC₅₀ 0.1, 0.17, 0.20, 0.21 μg mL⁻¹), and 104 and 1257 against NCI-H187 (IC₅₀ 0.15, 0.3 μg mL⁻¹), respectively, similar values to those for the control drug ellipticine. Bucidarasins A–C (133–135) showed potent cytotoxicity against nine human tumor cell lines with IC₅₀ values ranging from 0.5 to 1.9 μM.¹⁴

Compound 203 from Brazilian propolis reduced the incidence of skin tumors by inhibiting DNA synthesis via a de novo pathway, and suppressed the tumor growth by decreasing DNA synthesis via a salvage pathway.⁶⁹ Premnones A–C (234–236) exhibited cytotoxic activity when evaluated against three human cancer cell lines (Lu1, LNCaP, and MCF-7), and one normal cell line (HUVEC) in the range of ED₅₀ of 0.7–7.0 μg mL⁻¹.⁸⁰ However, 234 was not active when evaluated in a follow-up in vivo hollow fiber assay at the highest dose tested (50 mg kg⁻¹), using LNCaP, Lu1, and MCF-7 cells.

Crispene E (468) inhibited STAT3 dimerization in a cell-free fluorescent polarization assay and displayed significant toxicity against the STAT3-dependent MDA-MB 231 breast cancer cell line with selective inhibition of the expression of STAT3 and STAT3 target genes cyclin D1, fascin and bcl-2.¹⁹⁰ Ajugalide B (507) exhibited broad spectrum antiproliferative activity against A549, AGS, HepG2, and HT29 human cancer cell lines, with GI₅₀ values ranging from 3.18 to 5.94 μM.⁴⁸⁴ Below its cytotoxic concentration, 507 also reduced the tumorigenic and metastatic ability of A549 cancer cells by inhibiting anchorage-independent growth and cell migration; thus, 507 could be a lead for development of potential cancer chemotherapy.

Clerodermic acid (548) induced potent apoptosis against human leukemia HL60 cells.⁴⁸⁵ Compounds 590, 609, and 1260 from Polyalthia barnesii exhibited their highest potency against LNCaP and U373 cell lines, but generally showed broad spectrum cytotoxicity, with ED₅₀ values of less than 4 μg mL⁻¹ toward several cell lines.²³⁷ Scutebata L (597) exhibited moderate activity against several human cancer cell lines with IC₅₀ values ranging from 12.6 to 26.1 μM.²³¹ Calcicolins A (598) and C (600) showed significant cytotoxic effects against the D.mel-II and HepG2 cell lines with IC₅₀ values of 2.06, 2.10, and 9.04, 8.30 μg mL⁻¹, respectively.²⁴² Calcicolin B (599) also showed good toxicity against D.mel-II cells (IC₅₀ = 3.09 μg mL⁻¹) but was not as potent against HepG2 cells (IC₅₀ = 16.16 μg mL⁻¹).²⁴² Echinoclerodane A (614) exhibited moderate cytotoxicity against MOLT-4, HL-60, DLD-1 and LoVo tumor cells and inhibited superoxide anion generation and elastase release by human neutrophils.²⁴⁸ Tinosporin A (643) showed low cytotoxicity against HL-60 and MCF-7 cells, with IC₅₀ values of 18.63 and 23.58 μM, respectively.²⁵⁶ Crotonlide A (713) exhibited moderate cytotoxicity against HL-60 (IC₅₀ 9.42 μM) and P-388 (IC₅₀ 7.45 μM) tumor cell lines.¹⁶⁵

Many neo-clerodane diterpene alkaloids from Scutellaria barbata showed significant cytotoxic activity against three human cancer lines (HONE-1 nasopharyngeal, KB oral epidermoid carcinoma, and HT29 colorectal carcinoma cells) (IC₅₀ 2.0–8.1 μM) in various studies.^{230,232,233,367,370,371,378} Examples are shown below (1001, 1004, 1027, 1034, 1052) as well as in prior Section 2.9.1 (998, 1008, 1009, 1012, 1030, 1031, 1036–1042).

The rearranged clerodane polylongifoliaic A (1156) exhibited potent activity against SK-N-MC human neuroblastoma cells with an IC₅₀ value of 1.64 μM.²²⁰ 16-Oxo-cleroda-3,13(14)E-dienl5-oic acid (1259)⁴⁸⁶ and polyalthialdoic acid (189) showed antiproliferative activity against human leukemia HL-60 cells, with IC₅₀ values of 13.7 and 21.8 μM, respectively, compared with 5-fluorouracil's IC₅₀ of 9.5 μM.^60,487

Table 3 lists cytotoxicity results found for casearins A–R (1261–1278) against V-79 cells.^488,489 Three compounds without an oxygenated substituent at C-6, casearins G (1267), H (1268) and I (1269) showed the highest potency (IC₅₀ 0.17–0.51 μM). Casearins J (1270) and K (1271) with a hydroxy moiety at C-6 exhibited similar or slightly weaker activity (IC₅₀ 0.52 and 1.1 μM, respectively) than 1267–1269. Casearins L (1272) and M (1273) have a hydroxy moiety at C-7 rather than C-6, and showed slightly reduced activity (IC₅₀ 1.6 and 1.8 μM, respectively). Casearin C (1263), with an interesting decanoate at C-7 exhibited significant potency (IC₅₀ 0.77 μM), while a related compound, casearin E (1265) with the same decanoate group but slightly different oxygenated substituents at C-6 and C-18 was much less potent (IC₅₀ 4.7 μM). The authors postulated that the greater hydrophobicity of 1263 might increase its affinity for the V-79 cell membrane leading to significant activity. Furthermore, casearins A (1261) and D (1264) were converted to derivatives A_a (1279), A_b (1280), A_c (1281) with oxo, propionate, and butanoate groups, respectively, at C-6 and D_a (1282) with oxo groups at both C-2 and C-6. Only compound 1279 retained cytotoxic potency (IC₅₀ 0.55 μM); compounds 1280–1282 had greatly reduced activity (IC₅₀ 17, 38, 19 μM, respectively). Thus, the bulkiness of the C-6 group could greatly influence the activity.^489,490 In other studies, the structurally similar casearin X (1283) and caseargrewiin F (105), which do not have an oxygenated group at C-7, showed cytotoxic activity against MOLT-4, MDA-MB-435, HCT-8, and SF-295 human cell lines, with IC₅₀ values from 0.22 to 0.97 and 0.09 to 0.17 μM, respectively. Their lower cytotoxicity against L-929 cells (IC₅₀ 1.52 and 1.06 μM) perhaps indicated a more selective cytotoxic response to tumor cell lines.⁴⁹⁰ Other zuleanin-type clerodanes, case-arvestrins A–C (87–89), had comparable IC₅₀ values between 0.2 and 0.8 μM against a panel of tumor cell lines, including LX-1, HCT116, and A2780.³⁴

Table 3.

Structures and cytotoxic data (V-79 cells) of casearins and their derivatives^a

Casearin	R1	R2	R3	R4	R5	IC50 (μmol L−1)
A (1261)	OCH3	Ac	Ac	OH	OBu	1.0
B (1262)	OCH3	Ac	Ac	OAc	OBu	8.5
C (1263)	OH	Ac	Ac	OAc	ODc	0.77
D (1264)	OH	Bu	Ac	OH	OBu	1.8
E (1265)	OH	Et	Ac	OH	ODc	4.7
F (1266)	OH	Et	Ac	OH	OBu	29
G (1267)	OCH3	Ac	Ac	H	OBu	0.17
H (1268)	OH	Ac	Ac	H	OBu	0.37
I (1269)	OH	Ac	Bu	H	OBu	0.51
J (1270)	OCH3	Bu	Ac	OH	OBu	1.1
K (1271)	OAc	Ac	Ac	OH	OBu	0.52
L (1272)	OCH3	Bu	Ac	OAc	OH	1.6
M (1273)	OH	Bu	Bu	OAc	OH	1.8
N (1274)	OCH3	Ac	Bu	OAc	OBu	5.9
O (1275)	OCH3	Bu	Ac	OAc	OBu	6.0
P (1276)	OCH3	Ac	Ac	OAc	OAc	7.8
Q (1277)	OH	Ac	Ac	OAc	OBu	4.3
R (1278)	=O	Ac	Ac	OH	OBu	5.4
Aa (1279)	OCH3	Ac	Ac	=O	OBu	0.55
Ab (1280)	OCH3	Ac	Ac	OPr	OBu	17
Ac (1281)	OCH3	Ac	Ac	OBu	OBu	38
Da (1282)	=O	Bu	Ac	=O	OBu	19
X (1283)	Bu	Bu	Ac	OH	H

^aAc = acetate, Bu = butanoate, Dc = decanoate.

Mechanistic investigations showed that 1264 can protect DNA against different types of damage and act as an antioxidant by inducing detoxificant enzymes in HepG2 cells; these actions give rise to interesting chemopreventive characteristics in both HepG2 cells and the Salmonella typhimurium bacterial strain.⁴⁹¹ Finally, casearin X (1283) showed potent cytotoxic effects against CEM and HL-60 lines (IC₅₀ 0.4 μM) and PBMC cells (IC₅₀ 1.2 μM) and caused cell death via apoptotic pathways. These data further substantiated the promising antitumor-related properties of casearins.⁴⁹² In addition, 1283 exhibited chemopreventive activity against DNA damage induced by the particulates formed from burning sugarcane, which implied that 1283 can act by different mechanisms to protect DNA against damage, including repairable and non-repairable damages.⁴⁹³

Compounds 1284–1286 showed potent and selective cytotoxicity against P-388, A-549, HT-29, Mel-28 cell lines, with IC₅₀ values of 0.2–2.4 μM.⁴⁹⁴ (−)-Kolavenol (1287) increased lifespan (I.L.S.) in mice with IMC carcinoma, and was twice as effective (I.L.S. 98%, 41 mg per kg per day, 4 days) as 5-FU (46%, 30 mg per kg per day, 4 days).⁴⁹⁵ (+)-7β-Acetoxy-15,16-epoxycleroda-3,13(16),14-trien-18-oic acid (1288) strongly inhibited P-glyco-protein and likely has promise for development as an MDR-reversing agent.⁴⁹⁶ (5R,10R)-4R,8R-Dihydroxy-2S,3R:15,16-diepoxycleroda-13(16),17,12S:18,1S-dilactone (1289) exhibited a preventive effect against chemically-induced hepatocellular carcinoma (HCC) in rats, and could be a potent chemopreventive drug for HCC.⁴⁹⁷

trans-Dehydrocrotonin (1076) and trans-crotonin (1252) (see structures in Section 3.4) were evaluated for their effects on the survival of mice bearing Sarcoma 180 and Ehrlich carcinoma ascitic tumors, as well as the proliferation of cultured Ehrlich cells and TNFα activity. When the mice were treated with 80 and 120 mg kg⁻¹ of 1076 or 38 mg kg⁻¹ of 5-FU, substantial antitumor activity was observed (%T/C 128–140). Both compounds showed a cytotoxic value of 16 μM against Ehrlich carcinoma in 48 h cell culture. In vitro electrophoresis of DNA extracted from the treated tumor cells showed no apoptosis. But significant TNFα activity was detected in Ehrlich tumor-bearing mice treated with 1076, likely due to enhanced immune function.⁴⁹⁸

3.6. Anti-inflammatory activities

The hallucinogenic compound salvinorin A (9) (see structure in Section 3.2) is a potent KOR agonist. However, its multiple pharmacological effects are likely due to action on other targets, such as the cannabinoid CB1 receptor (CB1R). For instance, it exerted potent anti-inflammatory and antinociceptive effects, as mediated by KOR and CB1R,⁴⁹⁹ and thus, is an interesting new lead compound for the development of novel anti-inflammatory agents targeting KOR and CB1R. This observation also encourages further studies on 9 leading to the development of novel peripherally restricted derivatives with similar potency and selectivity. Thus, 9-related compounds may be useful future drugs, especially in patients with inflammatory bowel disease, in which pain is the most pronounced symptom during maintenance of remissions.⁵⁰⁰

Compound 485 inhibited LPS-induced NO production in BV-2 cells dose-dependently with an IC₅₀ value of 28.6 ± 2.6 μM.¹⁹⁴ Tinospinosides B (956) and C (957) also showed inhibitory effects against NO production.³⁵⁵

E-Isolinaridial (1290) and E-isolinaridial methylketone (1291) inhibited human synovial sPLA₂ in a concentration-dependent manner with IC₅₀ values of 0.20 and 0.49 μM, respectively, similar to that of scalaradial, an anti-inflammatory marine natural product that selectively inhibits 14 kDa type II phospholipase A2(PLA2).⁵⁰¹ In addition, these compounds reduced cell-free 5-lipoxygenase activity and A23187-induced neutrophil LTB4 biosynthesis, and significantly decreased receptor-mediated degranulation. 6-Oxocleroda-3,13(14)E-dien-15-oic acid methyl ester (1292) and 16-hydroxycleroda-3,13(14) E-dien-15-oic acid (1293), displayed significant activity against fMLP/CB induced superoxide generation by neutrophils with IC₅₀ values of 0.6 ± 0.09 and 1.49 ± 0.28 μg mL⁻¹, respectively.^246,436 The suppressive effects of 1293 on human neutrophil respiratory burst and degranulation were due, at least partially, to inhibition of calcium, AKT, and p38 signaling pathways.^457,502 3β,4β:15,16-Diepoxy-13(16),14-clerodadiene (1294) and thysaspathone (365) inhibited NO production in LPS-stimulated RAW 264.7 cells with IC₅₀ values of 20.1 and 11.6 μM, respectively.^143,438

3.7. Antiparasitic/antiprotozoal activities

Caseargrewiins A–D (74, 1256–1258, see structures in Section 3.5) were active in vitro against Plasmodium falciparum in vitro, with respective IC₅₀ values of 2.9, 2.4, 3.0, and 3.3 μg mL⁻¹.³¹ Clerodane diterpenoids 227–230 also inhibited the growth of the chloroquine-resistant strain FeB1, with IC₅₀ values between 4.3 and 14.6 μg mL⁻¹.⁷⁸ Penianthic acid (1295) and epicordatine (1296) exhibited weak activity against chloroquine-resistant strain K1.⁵⁰³ Casearlucine A (1297), caseamembrol A (119, see structure in Section 3.5), and laetiaprocerines A–D (101–103, 274, see structures in Section 3.5) displayed activity against P. falciparum with IC₅₀ values as low as 0.5 μM against FeB1 and F-32 strains.³⁶ In addition, compounds 119 and 1297 also showed activity against Leishmania amazonensis amastigote axenic stages and promastigote.³⁶ Ajugarin-1 (1298) showed moderate in vivo antiplasmodial activity, with an IC₅₀ of 23.0 ± 3.0 μM, against FCA 20/GHA P. falciparum.^504,505 At a dose of 200 mg kg⁻¹ in mice, the clerodane diterpenoid gomphostenin-A (1299) exhibited an impressive 93% chemosuppression against P. berghei.⁵⁰⁶

Clerodane 182, a diastereoisomer of kolavenol, exhibited trypanocidal activity (IC₅₀ 2.5 μg mL⁻¹) against Trypanosoma brucei rhodesiense, the parasitic cause of acute human African trypanosomiasis (sleeping sickness).⁵⁶ This compound was isolated from the root bark of Entada abyssinica, which is used by traditional healers in Uganda to treat sleeping sickness.

Infuscatin (626) and sepulturins A, C, D and E (1300, 1302–1304) showed antiprotozoal activity against clinically isolated strains of Entamoeba histolytica and Giardia lamblia with similar potency to (+)-catechin and tyramine, but much less than metronidazole. Sepulturins A–F (1300–1305) were isolated from Saliva shannoni J.D. Smith, which is used as a traditional medicine in El Salvador against malaria.⁵⁰⁷ The structure of 626 (ref. 251) was also revised based on NOESY data and structural similarity to 1302. These new clerodane diterpenes contain a tertiary hydroxy group at C-8 or C-10 or both positions.

3.8. Antifungal and antibacterial activities

A fruit pulp extract of Detarium microcarpum inhibited the growth of the plant pathogenic fungus Cladosporium cucumerinum and of the enzyme acetylcholinesterase, which has been implicated in Alzheimer's disease. Fractionation of this extract led to the isolation of four new clerodane diterpenes, 197 (see Section 2.1.2.1), 266–267 (see Section 2.1.2.2), and 291 (see Section 2.1.3). Their structures were elucidated from spectroscopic data and X-ray crystallography of 197 and 266. Compounds 197, 267, and 291 showed both antifungal activity and inhibition of acetylcholinesterase.⁶⁵

Caseanigrescen C (39) caused a significant (21 mm) zone of inhibition of fungal growth,²¹ and 16-oxo-cleroda-3,13(E)-diene-15-oic acid (204) also exhibited high antifungal activities, with MIC values of 25 to 50 μg, as compared with fungicide Dithane M-45.⁷⁰ Caseargrewiins A–D (74, 1256–1258, see Section 3.5) exhibited moderate activity against Mycobacterium tuberculosis, with MIC values of 12.5, 12.5, 25.0, and 12.5 μg mL⁻¹, respectively.³¹

Compounds 205 and 206 were isolated as active antibacterial principles from Haplopappus foliosus.⁷¹ They were active against six Gram-positive bacteria, but inactive against five Gram-negative bacteria. 18-Acetoxy-cis-cleroda-3-en-15-oic acid (226) also showed antibacterial activity against five Gram-positive bacteria.⁷⁷ 16α-Hydroxy-cleroda-3,13(Z)-diene-15,16-olide (551) was highly active against Gram-negative bacteria, including Escherichia coli, Klebsiella aerogenes, and Pseudomonas species, with MICs in the range 0.78–1.5 mg.⁷⁰ Among Gram-positive bacteria, compound 551 was also highly active against Bacillus species with MIC of less than 2 μg. Interestingly, against Gram-positive methicillin-resistant Staphylococcus aureus, compound 551 showed both in vivo efficacy in infected mice due to bacterial membrane disruption as well as synthergistic activity with clinically used antibiotics, such as tetracycline, linezolid, and daptomycin.⁵⁰⁸

Furthermore, neo-clerodanes-(5R,8R,9S,10R)-15,16-diol-15,16-dihydrohardwickiic acid (1306) and (2S,5R,8R,9S,10R)-2β-hydroxy-16-oxo-15,16-dihydrohardwickiic acid (1307) from Salvia adenophora showed strain-dependent activity against Gram-positive Staph. epidermidis.⁵⁰⁹ In contrast, lupulin A (822) showed clear activity against Gram-negative Ps. aeruginosa and E. coli (inhibitory zone 3–5 mm),³¹⁵ and hativenes A–C (827–829) showed potent activity against three Gram-negative rods (E. coli, Ps. aeruginosa and Salmonella typhimurium).⁵¹⁰ Lupulin F (794) also showed antibacterial activity against Ps. aeruginosa and E. coli.³⁰⁰

3.9. Other bioactivities

trans-Dehydrocrotonin (t-DCTN, 1076, see structure in Section 3.4) exhibits multiple biological effects, including antitumor, antiulcerogenic, hypolipidaemic, antiatherogenic, anti-oestrogen, antigenotoxicity, anti-inflammatory, and insect growth inhibitory property activities.^511–518 The anti-hyperglycemic potential of this compound was supported by its reported hypoglycemic effect, which was almost comparable to that produced by glibenclamide (2 mg kg⁻¹), a clinically useful drug.⁵¹⁹ Compound 1076 can also be used as a potent analgesic agent in case of peripheral algesia, without CNS effects.⁵²⁰ The hypotensive and bradycardia effects of 1076 are possibly related to some extent to the release of nitric oxide as well as direct effects on vascular smooth muscle, and cardiac pacemaker activity.⁵²¹

Casearinols A (1308) and B (1309) and casearinones A (1310) and B (1311), inhibited the binding of T-cell leukocyte function antigen 1 to intercellular adhesion molecule 1, with an IC₅₀ of 50 μM.⁵²² This report is the first to link this diterpene class with immunomodulatory activity. Diterpene 1312, an active principle of Baccharis trimeta, blocked vascular smooth muscle contractions induced by extracellular Ca²⁺ in KCl-depolarized preparations.⁵²³

Interestingly, 16α-hydroxycleroda-3,13Z-dien-15,16-olide (551, see structure in Section 3.8) is the first clerodane diterpene reported to have potential as a lipid lowering agent. It represents a new structural class of HMG-CoA reductase inhibitor.⁵²⁴

3.10. Toxicity

Some neo-clerodane diterpenoids, especially furan-containing diterpenoids, are highly hepatotoxic in mice, causing mid zonal hepatic necrosis. They have also been alleged to cause fulminant hepatitis, chronic hepatitis, or cirrhosis in humans.

Diosbulbin D (1313, DBD) was the first hepatotoxic furano norclerodane diterpenoid isolated from Dioscorea bulbifera.^525,526 The effects of DBD on the growth of normal human liver L-02 cells may be due to its induction of cell apoptosis, which may also explain the toxicity observed with plants containing furano clerodane diterpenoids.⁵²⁷ Diosbulbin B (1314, DB) is another hepatotoxic compound found in high quantity in D. bulbifera.^528,529 The hepatotoxic effects of the ornamental and medicinal germander plant (Teucrium chamaedrys) were attributed to the furano neo-clerodanes teucrin A (1315) and teuchamaedryn A (1316).⁵³⁰ Plants in the genus Teucrium have been used as erroneously “safe” herbal hypoglycemic and slimming aids.^530–532 Other furanoclerodane diterpenoids have not been assayed for hepatotoxicity, with more attention put on other pharmacological properties. Determination of toxicity is an important criterium for both consumer safety and clinical candidacy. For instance, the possible development of 8-epidiosbulbin E acetate (1317) (with a δ-lactone cis-fused to the decalin system rather than trans-fused as in the hepatotoxic 1313, DBD)⁵²⁶ as a potential plasmid-curing agent against multidrug-resistant bacteria, which pose a tremendous current health challenge.⁵³³

4. Conclusion

In this review, discoveries of clerodane diterpenes from 1990–2015 were categorized by their chemical structures. During the last 25 years, over 1300 diterpenoids and nor-diterpenoids with the clerodane carbon skeleton have been isolated. These natural neo-clerodanes have been classified into seven different groups on the basis two fragments, the C-11–C-16 moiety and the decalin moiety. In addition, clerodane-type diterpene glycosides and clerodane derivatives, including N-containing derivatives, degraded derivatives, ring-seco derivatives, and rearranged derivatives reported in the literature were considered in this review. Although their insect antifeedant activity and opioid receptor agonist effects are generally considered most important, clerodane diterpenes exhibit many other pharmacological activities. The distribution, chemotaxonomic significance, biological activity, structure activity relationship correlations, and modes of action of active clerodanes have also been summarized.

The body of knowledge on the chemistry, biological activity, and pharmacology of clerodane diterpenes continues to grow rapidly. Despite such advances, continued investigations into the biological mechanisms involved in insect antifeedant activity, as well as extended research into the chemistry and pharmacology of opioid receptor ligands, such as salvinorin A (9) or other natural products, may provide the means to differentiate among different types of antifeedants and give better defined sets of clerodanes with a distinct mode of action from which more detailed structure–activity relationships can be deduced, and may yet yield the holy grail of opioids.

Biography

Rong-Tao Li received her Ph.D at Kunming Institute of Botany, Chinese Academy of Science (CAS) in 2004, under the guidance of Prof. Han-Dong Sun. She worked with Prof. Kuo-Hsiung Lee from March 2013 to June 2014, as a Visiting Professor at the UNC Eshelman School of Pharmacy, University of North Carolina at Chapel Hill. She has been awarded several prizes conferred by national, CAS, provincial, or ministerial authorities, including the “National Excellent Doctoral Dissertation of P.R. China” in 2006, the “New Century Excellent Talents on University” in 2006, the “Young Academic and Technical Leader of Yunnan Province” in 2009, and the “Excellent Doctoral Dissertation of the Chinese Academy of Science” and “CPA-Servier Yong Investigator Award in Medicinal Chemistry” 2005. She has performed phytochemical studies on about 50 plants, published 12 patents, and over 130 scientific papers. She is now Professor, Kunming University of Science and Technology (2005-present). Her current research interests involve the activity-directed isolation, structure elucidation and the structure–activity relationships of natural products from medically important plants.

Susan L. Morris-Natschke received her B.S. in chemistry from the University of Maryland-College Park in 1975 and her Ph.D. in organic chemistry from the University of North Carolina-Chapel Hill (UNC-CH) in 1982. She is currently Research Professor in the Division of Chemical Biology and Medicinal Chemistry, UNC Eshelman School of Pharmacy, UNC-CH, where she has been on the faculty since 1983. Her interests include scientific writing/editing, as well as the synthesis and structure-activity relationships of bioactive natural products.

Kuo-Hsiung Lee received his B.S. in pharmacy from Kaohsiung Medical University, Taiwan (1961), M.S. in pharmaceutical chemistry from Kyoto University, Japan (1965), and Ph.D. in medicinal chemistry from University of Minnesota, Minneapolis (1968). He joined the faculty of UNC Eshelman School of Pharmacy, University of North Carolina-Chapel Hill, in 1970 and is now Kenan Distinguished Professor of Medicinal Chemistry and Director of the Natural Products Research Laboratories. He has published over 848 research articles, been granted over 114 patents, and received numerous awards, including most recently, the Third Cheung On Tak International Award for Outstanding Achievement in Chinese Medicine from Hong Kong Baptist University, School of Chinese Medicine in 2016.