Chemistry of the Genus Plectranthus

Abstract

This review presents the phytochemical constituents of the genus Plectranthus reported up to 1999. Only a tetrameric derivative of caffeic acid was isolated from P. japonicus, but a group of long-chain alkylphenols, of possible taxonomic significance in the genus, was also isolated. As a genus of the subfamily Nepetoideae, Plectranthus is free from iridoid glycosides and rich in essential oil (i.e. > 0.5% volatile oil on a dry weight basis). Diterpenoids are the more common secondary metabolites in Plectranthus. The majority of them are highly modified abietanoids. This seems to be similar to the pattern of diterpenoids observed for Salvia, but no clerodane diterpenoids were found in Plectranthus.

Introduction

Labiatae is a large family that occurs worldwide and has species that are adapted to almost all habitats and altitudes. The genus Plectranthus L' He'r. belongs to subfamily Nepetoideae of tribe Ocimeae [1]. It comprises about eighty species worldwide, as indicated in this review. Taxonomically, Coleus Lour. is the closest to Plectranthus [2]. Coleus species are now generally accepted as belonging to either Plectranthus or to Solenostemon Thonn. (eds.) [3], and some confusion can arise distinguishing between Plectranthus and Coleus species [4,5]. In Plectranthus, the upper lip of the flower is unusually four-lobed and the large shoe-shaped lower lip is formed from a single lobe, while in Labiatae the upper lip often consisting of two lobes and the lower consisting of three [6].

Many Plectranthus species are plants of economic and medicinal interest. Several species may be grown as ornamentals, such as P. tenuiflorus in Saudi Arabia. The tubers of an unidentified Plectranthus species are eaten in Swaziland [7]. Livingstone potato tubers, P. esculentus is cultivated in tropical Africa for its edible tubers [8,9]. P. floribundus is cultivated in Nigeria for its edible tubers, also relished in Natal [10,11]. In Polynesia, the seed-oil of P. amboinicus is applied to the ear for treatment of acute edematous otitis acuta [12]. The leaf extract of P. tenuiflorus is also used in Saudi Arabia to treat ear infections [13]. The leaves of P. asirensis are used as an antiseptic dressing for wounds in Saudi Arabia [13]. The leaves of P. caninus are chewed in Africa to relieve toothache [14]. In East Africa the leaves of P. elegans are used as a vermicide [14]. P. vettiverioides is prescribed in Indian ayurvedic medicine as a remedy for vomiting and nausea [15]. The East African medicinal plant P. barbatus is used as a remedy for stomachache and as a purgative. It is also resistant to insect attack, and an aphid antifeedant diterpene has been isolated from it [16].

The chemistry of Plectranthus is still not well known. This is the first review of chemical constituents of Plectranthus species. The main phytochemical constituents of the genus Plectranthus are diterpenoids, essential oils and phenolics.

Diterpenoids

About 140 diterpenoids were identified from the colored leaf-glands of Plectranthus species. The majority of them are highly modified abietanoids, in addition to some phyllocladanes (structures D140-D146), ent-kaurenes (structures D147-D154) and a seco-kaurene (structure D155). The abietanoids, in turn, could be classified, according to structure variation, into royleanones (structures D1-D37), spirocoleons (structures D38-D66), vinylogous quinones (also named extended quinines, structures D67-D76), quinone methides (structures D77-D93), acylhydroquinones (structures D94-D117), (4→3) abeo-acylhydroquinones (structures D118, D119), phenolic abietanoids (structures D120-D122), 1,4-phenanthraquinones (structures D123-D127), dimeric abietanoids (structures D128-D136) and seco-abietanoids (structures D137-D139). Distribution of these diterpenoids and other constituents in species of Plectranthus are shown in Table 1. The names of these diterpenoids are listed in Table 2.

Table 1.

Alphabetical list of Plectranthus species and compounds isolated from them.

Plectranthus species	Isolated chemical constituents	References
Abyssinian P. sp.	D1, D5, D9, D10, D12-D14, D21, D30	37
P. albidus	L1-L8, L10-L12	28
P. aliciae	───	62
P. alloplectus	───	63
P. ambiguus	D141-D146, flavonoid M10	33
P. amboinicus	Essential oil	19
P. argentatus	D4, D5, D8, D21, D25, D101, D102, D112	38
P. asirensis	───	13
P. australis	───	64
P. barbatus	D29, D65, D75, D76, D115, D117	39
	D65	16
P. burorum	───	65
P. caninus	D108, D109	40
	D59-D64	86
P. ciliatus	───	61
P. coesta	D148	41
	D147	42
P. coetsa	───	66
P. coetsoides	D147, D149-D154	43
P. coleoides	Essential oil	25
P. cyrpiculoides	───	67
P. defoliatus	Essential oil	26
P. ecklonii	D86, M12-M15	68
P. edulis	D17, D18, D21, D22, D23, D24, D38-D48, D50, D55, D56, D66, D67, D69, D70, D94-D100, D106, D107, D118, D119, D137, D138	44
	D118	45
P. elegans	D93, D120	46
P. esculentus	───	69
P. fasciculatos	───	70
P. floribundus	───	11
P. fruticosus	Essential oil	20
P. gandicalyx	───	65
P. garckeanus	───	65
P. geradianus	───	71
P. glandulosus	Essential oil	23
P. glaucocalyx	An antimicrobial diterpenoid	47
P. grandidentatus	D68, D101, D102, D128-D134	48
	D5, D11	49
P. gratus	───	63
P. hadiensis	───	67
P. hereroensis	D9, D35, D36	50
	D37	51
	D9, D16	52
	Sesquiterpene M1	30
P. hilliardiae	───	61
P. incanus (= P. mollis)	Essential oil	27, 79
	Fatty acids	35
P. inflexus	───	72
P. japonicus	D155	53
	Caffeic acid derivative	36
P. japonicus var. glaucocalyx	───	77
P. kapatensis	───	65
P. lanuginosus	D17, D30-D34, D45, D47, D52-D54, D57, D58, D67, D71, D74, D88-D90, D100, D103, D110, D111	54
P. lucidus	───	62
P. madagascariensis	Essential oil	24
P. malvinus	───	62
P. marrubioides	Flavonoid M11	34
P. melissoides	───	79
P. mollis (= P. incanus)	───	79
P. mollis	───	79
P. myrianthus	D128	48
	D2, D4, D96, D98, D101	87
P. neochilus	───	73
P. nilgherricus	D82, D83, D139, D140	88
P. oribiensis	───	61
P. ornatus	───	74
P. parviflorus	D77, D82-D86	55
P. pentheri	───	62
P. porpeodon	───	65
P. pseudobarbatus	───	65
P. puberulentus	───	65
P. purpuratus	D72, D73, D77, D79, D91, D92, D121, D122, D140	56
P. purpuratus subsp. montanus	───	62
P. purpuratus subsp. tongaensis	───	62
P. reflexus	───	61
P. rugosus	Essential oil	18
	Triterpenoids M2-M6 & β-sitosterol	31, 32
	Triterpenoids M7-M9, β-sitosterol & hexacosanol	85
P. saccatus subsp. pondoensis	───	62
P. saccatus var. longitubus	───	61
P. sanguineus	D3, D4-D7, D9, D15, D21, D25, D26, D68, D99, D102, D128-D131, D139	57
P. schimperi	───	65
P. sp. from the borders of Lake Kiwu, Rwanda	D19-D21, D27-D29, D49, D51, D75, D76, D104, D105, D113-D116, D123-D127D123-D126	58
		59
P. spectabilis	───	63
P. stenophyllus	───	75
P. stocksii	───	79
P. strigosus	D77, D78, D82-D87	60
P. sylvestris	L9, L13-L18	29
P. tenuiflorus	Essential oil	21,22
P. vestitus	Essential oil	76
P. vettiveroides	───	78
P. zatarhendi	───	67
P. zatarhendi var. tomentosus	───	67
P. zuluensis	───	61

Table 2.

Names of diterpenoids encountered in Plectranthus species.

Diterp.	Name of diterpenoid	Diterp.	Name of diterpenoid
D1	Royleanone	D79	(11-Hydroxy-19-isovaleroyloxy-5,7,9(11),13-abietatetraen-12-one)
D2	6β, 7α-Dihydroxy-royleanone	D80	Fuerstione
D3	7-O-Formylhorminone	D81	3β-Acetoxyfuerstione
D4	6β-Hydroxy-7α-formyloxyroyleanone	D82	Parviflorone C
D5	6β-Hydroxy-7α-acetoxyroyleanone	D83	Parviflorone E
D6	6β-Hydroxyroyleanone	D84	Parviflorone B
D7	5,6-Dihydrocoleone U	D85	Parviflorone D
D8	6β-Formyloxy-7α-hydroxyroyleanone	D86	Parviflorone F
D9	Horminone	D87	Parviflorone G
D10	7α-Acetoxyroyleanone	D88	Lanugone M
D11	6β-Hydroxy-7α-acyloxyroyleanone	D89	Lanugone L
D12	Taxoquinone (= 7β-Hydroxyroyleanone)	D90	Lanugone N
D13	7-Oxoroyleanone	D91	6α,11-Dihydroxy-19-isovaleroyloxy-7, 9(11), 13-abietatrien-12-one
D14	8α,9α-Epoxy-7-oxoroyleanone	D92	6α,11-Dihydroxy-19-senecioyloxy-7,9(11), 13-abietatrien-12-one
D15	6β,7α-Dihydroxy(allyl)royleanone	D93	11-Hydroxy-12-oxo-7,9(11),13-abietatriene
D16	7α,12-Dihydroxy-17(15°16)-abeo-abieta-8,12,16-trien-11,14-dione	D94	(2'ξ,3aR,10bR)-8-(2'-Acetoxy-1'-methyl-ethyl)-3,3a-dihydro-7,9,10-trihydroxy-3a, 10b-dimethyl-1H-phenanthro[10,1bc]-furan-4(2H),6(10bH)-dione
D17	Lanugone A	D95	16-O-Acetylcoleon C
D18	(4bS,7R,8aR)-7-Formyloxy-4b,5,6,7,8,8a-hexahydro-3-hydroxy-4b,8,8-trimethyl-2-(2-propenyl)phenanthren-1,4-dione	D96	Coleon U
D19	Plectranthone F	D97	Coleon C
D20	Plectranthone G	D98
D21	6β,7α-Dihydroxyroyleanone	D99	16-O-Acetylcoleon D
D22	(4bS,7R,8aR,9S,10S)-7-Formyloxy-4b,5,6, 7,8,8a,9,10-octahydro-3,9,10-trihydroxy-4b, 8,8-trimethyl-2-(2-propenyl)-phenanthren-1,4-dione	D100	(15S)-Coleon D
D23	(4bS,7R,8aS,9S,10S)-4b,5,6,7,8,8a,9,10-Octahydro-3,9,10-trihydroxy-4b,7-dimethyl-8-methyliden-2-(2-propenyl)-phenanthren-1,4-dione	D101	Coleon V
D24	(2'ξ,4bS,7R,8aS,9S,10S)-4b,5,6,7,8,8a,9, 10-Octahydro-3,9,10-trihydroxy-2-(2'-hydroxypropyl)-4b,7-dimethyl-8-meth-ylidenphenanthren-1,4-dione	D102	Coleon U
D25	Coleon-U-quinone	D103	(15S)-Coleon C
D26	8α,9α-Epoxy-8,9-dihydrocoleon-U-quinone	D104	(15S)-2α-Acetoxycoleon C
D27	Plectranthone H	D105	(15S)-Coleon H
D28	Plectranthone I	D106	(2'ξ,4aS,10aS)-1,2,3,4,4a,10a-Hexahydro-5,6,8-trihydroxy-7-(2'-hydroxypropyl)-1,1,4a-trimethylphenanthren-9,10-dione
D29	Plectranthone J	D107	(2'ξ,4aR)-2,3,4,4a-Tetrahydro-5,6,8,10-tetrahydroxy-7-(2'-hydroxypropyl)-1,1,4a-trimethylphenanthren-9(1H)-one
D30	6,7-Didehydroroyleanone	D108	Coleon T
D31	Lanugone B	D109	Coleon S
D32	Lanugone C	D110	Lanugone R
D33	Lanugone D	D111	Lanugone S
D34	Lanugone E	D112	5,6-Dihydrocoleon U
D35	3β-Acetoxy-6β,7α,12-trihydroxy-17-(15°16);18(4°3)-bisabeo-abieta-4(19), 8.12,16-tetraen-11,14-dione	D113	(15S)-2α-Acetoxycoleon D
D36	16-Acetoxyhorminone	D114	(15S)-Coleon I
D37	16-Acetoxy-7α,12-dihydroxy-8,12-abieta-dien-11,14-dione	D115	Plectrinone B
D38	(2R,2'S,3'R,4'bS,7'R,8'aS,9'S,10'S)-3',10'-Diacetoxy-4'b,5',6',7',8',8'a,9',10'-octa-hydro-9'-hydroxy-2,4'b,7'-trimethyl-8'-methylidenspiro[cyclopropan-1,2'(1'H)-phenanthren]-1',4'(3'H)-dione	D116	(16S)-Plectrinone A
D39	(2R,2'S,3'R,4'bS,7'R,8'aS,9'S,10'S)-3'- Acetoxy,10'-formyloxy-4'b,5',6',7',8',8'a,9',10'-octahydro-9'-hydroxy-2,4'b,7'-trimethyl-8'-methylidenspiro[cyclopropan-1,2'(1'H)-phenanthren]-1',4'(3'H)-dione	D117	(16R)-Plectrinone A
D40	(2R,2'S,3'R,4'bS,7'R,8'aS,9'S,10'S)-10'-Acetoxy-4'b,5',6',7',8',8'a,9',10'-octahydro-3',9'-dihydroxy-2,4'b,7'-trimethyl-8'-methylidenspiro[cyclopropan-1,2'(1'H)-phenanthren]-1',4'(3'H)-dione	D118	Edulone A
D41	(2R,2'S,3'R,4'bS,7'R,8'aS,9'S,10'S)-3'- Acetoxy-4'b,5',6',7',8',8'a,9',10'-octahydro-9',10'-dihydroxy-2,4'b,7'-trimethyl-8'-methylidenspiro[cyclopropan-1,2'(1'H)-phenanthren]-1',4'(3'H)-dione	D119	(1'S,10bS)-7,9,10-Trihydroxy-8-(2'-hydroxy-1'-methylethyl)-3,10b-dimethyl-1H-benzo[g]cyclopenta[de][1]benzopyran-4(2H),6(10bH)-dione
D42	(2R,2'S,3'R,4'bS,7'R,8'aS,9'S,10'S)-10'-Formyloxy-4'b,5',6',7',8',8'a,9',10'-octa-hydro-3',9'-dihydroxy-2,4'b,7'-trimethyl-8'-methylidenspiro[cyclopropan-1,2'(1'H)-phenanthren]-1',4'(3'H)-dione	D120	7α,11-Dihydroxy-12-methoxy-8,11,13-abietatriene
D43	(2R,2'S,3'R,4'bS,7'R,8'aS,9'S,10'S)-4'b,5', 6',7',8',8'a,9',10'-Octahydro-3',9',10'-tri-hydroxy-2,4'b,7'-trimethyl-8'-methyliden-spiro[cyclopropan-1,2'(1'H)-phenanthren]-1',4'(3'H)-dione	D121	11,12-Dihydroxy-19-isovaleroyloxy-8,11,13-abietatrien-7-one
D44	(2R,2'S,3'R,4'bS,7'R,8'aR,9'S,10'S)-9-Acetoxy-7'-formyloxy-4'b,5',6',7',8',8'a, 9',10'-octahydro-3',10'-dihydroxy-2,4'b, 8',8'-tetramethylspiro[cyclopropan-1,2' (1'H)-phenanthren]-1',4'(3'H)-dione	D122	11,12-Dihydroxy-19-senecioyloxy-8,11,13-abietatrien-7-one
D45	Lanugon G	D123	Plectranthone B
D46	(2R,2'S,3'R,4'bS,7'R,8'aR,9'S)-7'-Formyl-oxy-4'b,5',6',7',8',8'a,9',10'-octahydro-3',9'-dihydroxy-2,4'b,8',8'-tetramethylspiro-[cyclopropan-1,2'(1'H)-phenanthren]-1',4' (3'H)-dione	D124	Plectranthone A
D47	Lanugone F	D125	Plectranthone C
D48	(2R,2'S,3'R,4'bS,7'R,8'aR,9'S,10'S)-7',10'-Bisformyloxy-4'b,5',6',7',8',8'a,9',10'-octa-hydro-3',9'-dihydroxy-2,4'b,8',8'-tetra-methylspiro[cyclopropan-1,2'(1'H)-phenanthren]-1',4'(3'H)-dione	D126	Plectranthone D
D49	Plectranthone K	D127	Plectranthone E
D50	(2R,2'S,3'R,4'bS,7'ξ,8'aR,9'S,10'S)-7',10'-Diacetoxy-4'b,5',6',7',8',8'a,9',10'-octa-hydro-3',9'-dihydroxy-2,4'b,7'-trimethyl-8'-methylidenspiro[cyclopropan-1,2'(1'H)-phenanthren]-1',4'(3'H)-dione	D128	Grandidone A
D51	Plectranthone L	D129	7-Epigrandidone A
D52	Lanugone H	D130	Grandidone B
D53	Lanugone I	D131	7-Epigrandidone B
D54	Lanugone J	D132	Grandidone D
D55	(2S,2'S,3'R,4'bS,7'R,8'aR,9'S,10'S)-7-Formyloxy-4'b,5',6',7',8',8'a,9',10'-octa-hydro-3',9'-diacetoxy-10'-hydroxy-2,4'b,8', 8'-tetramethylspiro[cyclopropan-1,2'(1'H)-phenanthren]-1',4'(3'H)-dione	D133	7-Epigrandidone D
D56	(2S,2'S,3'R,4'bS,7'R,8'aR,9'S,10'S)-7',10'-Bisformyloxy-4'b,5',6',7',8',8'a,9',10'-octa-hydro-3',9'-dihydroxy-2,4'b,8',8'-tetra-methylspiro[cyclopropan-1,2'(1'H)-phenanthren]-1',4'(3'H)-dione	D134	Grandidone C
D57	Lanugone K	D135	Nilgherron A
D58	Lanugone K'	D136	Nilgherron B
D59	Coleon R	D137	(3R)-6,9-Dihydroxy-3,4-dimethyl-7-(1-methylethyl)-3-(2-propenyl)naphtho[2,3-b]- furan-2-(3H),5,8-trione
D60	Coleon M	D138	(2'ξ,3R)-7-(2'-Acetoxy-1'-methylethyl)-6,9-dihydroxy-3,4-dimethyl-3-(2''-propenyl)-naphtho[2,3-b]furan-2-(3H),5,8-trione
D61	7,12-Diacetylcoleon J	D139	Sanguinon A
D62	Coleon N	D140	(16R)-17,19-Diacetoxy-16-hydroxy-13β-kauran-3-one
D63	Coleon Q	D141	(16R)-2α-Senecioyloxy-3α-acetoxyphyllocladan-16,17-diol
D64	Coleon P	D142	(16R)-2α-Senecioyloxy-3α,17-diacetoxy-16-hydroxyphyllocladane
D65	Plectrin	D143	(16R)-2α-Isovaleroyloxy-3α-acetoxyphyllocladan-16,17-diol
D66	Coleon Z	D144	(16R)-2α-Isovaleroyloxy-3α,17-diacetoxy-16-hydroxyphyllocladane
D67	(15S)-Lanugone O	D145	(16R)-3α-Acetoxyphyllocladan-16,17-diol
D68	14-Hydroxytaxodione	D146	(16R)-2α-Senecioyloxy-16,17-dihydroxyphyllocladan-3-one
D69	(4bS,8aS)-2-(2-Acetoxypropyl)-4b,5,6,7,8, 8a-hexahydro-1,4-dihydroxy-4b,8,8-tri-methylphenanthren-3,9-dione	D147	Plecostonol (= coetsidin A)
D70	(2'ξ,4bS,8aS)-4b,5,6,7,8,8a-Hexahydro-1,4-dihydroxy-2-(2'-hydroxypropyl)-4b,8,8-trimethylphenanthren-3,9-dione	D148	Coestinol
D71	Lanugone P	D149	Coetsidin B
D72	19-Isovaleroyloxytaxodione	D150	Coetsidin C
D73	19-Senecioyloxytaxodione	D151	Coetsidin D
D74	Lanugone Q	D152	Coetsidin E
D75	Coleon F	D153	Coetsidin F
D76	(16S)-Coleon E	D154	Coetsidin G
D77	Parviflorone A (= 11-hydroxy-19-senecioyl-oxy-5,7,9(11),13-abietatetraen-12-one)	D155	Rabdosin B
D78	Parviflorone H

Diterpenoids isolated from Plectranthus

Essential oils

Plectranthus is one of the oil-rich genera belonging to the subfamily Nepetoideae [17]. Table 1 lists Plectranthus species that have been investigated for essential oils. The main constituents of essential oils of Plectranthus are mono- and sesquiterpenes. For example, constituents of essential oil of P. rugosus [18], as eluted from fused silica capillary column, are α-pinene, camphene, β-pinene, sabinene, 3‑carene, myrcene, α-phellandrene, α-terpinene, limonene, β-phellandrene, cis-β-ocimene, γ-terpinene, trans-β-ocimene, p-cymene, terpinolene, thujone, 1-nonen-3-ol, α-copane, β-bourbonene, β-cubebene, linalool, caryophyllene, terpinen-4-ol, humulene, γ-muurolene, germacrene D, piperitone epoxide, α‑muurolene, bicyclogermacrene, δ-cadinene, γ-cadinene, α-curcumene, caryophyllene oxide, T‑cadinol, torreyol and α-cadinol. On the same GC column (fused silica capillary), essential oil of P. amboinicus [19] was separated into α-pinene, camphene, 1-octen-3-ol, β-pinene, myrcene, α‑phellandrene, Δ-3-carene, α-terpinene, p-cymene, limonene, (Z)-β-ocimene, (E)-β-ocimene, α‑phelandrène, γ-terpinene, α-terpinolene, linalool, camphor, 1-terpinen-4-ol, α-terpineol, thymol, carvacrol, α-cubebene, β-cubebene, β-elemene, β-caryophyllene, α-bergamotene, (Z)- β-farnesene, α‑humulene, β-guaiene, (-)-α-selinene, β-bisabolene, δ-cadinene, caryophyllene oxide, δ-cadinol, α‑cadinol, farnesol, calamenol and (-)-4β-7β-aromadendrandiol. Also on fused silica capillary column, essential oil of P. fruticosus [20] gave α-thuyene, sabinene, γ-terpinene, β-bourbonene, linalool, terpinen-4-ol, sabinyl acetate, α-humulene, aromadendrene, α-cubebene, β-bisabolene, γ-cadinene, α‑elemene, trans-farnesol and trans-copaene.

Long-chain alkylphenols

A group of long-chain alkylphenols, of possible taxonomic significance in the genus, has been isolated [28,29]. Long-chain alkylphenols L1-L8, L10-L12 were isolated from P. albidus and showed a significant in vitro antioxidant activity [28]. Antioxidant activity guided fractionation of extracts of P. sylvestris [29] and HPLC separation yielded the oxygenated long-chain alkylcatechols L9, L13-L18.

Long-chain alkylphenols

Miscellaneous constituents

Only one aristolane sesquiterpene, namely 1(10)-aristolen-13-al (M1), was isolated from P. hereroensis [30]. Five triterpenoids, named plectranthoic acid (M2), acetylplectranthoic acid (M3), plectranthadiol (M4), plectranthoic acid A (M5) and plectranthoic acid B (M6), in addition to β‑sitosterol were isolated from P. rugosus [31,32]. From the same species Misra et al. [85] isolated the triterpenoids oleanolic acid (M7), ursolic acid (M8) and betulin (M9), in addition to β-sitosterol and hexacosanol.

Flavonoids seem to be rare in Plectranthus. Only two flavonoids were identified, 4',7-dimethoxy-5,6-dihydroxyflavone (M10) from P. ambiguus [33] and chrysosplenetin (M11) from P. marruboides [34]. From P. mollis (= P. incanus), Mahmoud et al. reported the isolation of vernolic and cyclopropenoid fatty acids [35]. From P. japonicus (= Rabdosia japonica), a tetrameric derivative of caffeic acid was isolated [36].

Miscellaneous constituents

Conclusions

Although the genus Plectranthus comprises many plants of medicinal and economic interest [80], its chemistry remains poorly known. Caffeic acid and its derivatives are of widespread occurrence in the Labiatae family and of particular attention as chemotaxonomic markers. Chlorogenic acid appears to be of almost universal occurrence within this family, whereas rosmarinic acid is restricted to the subfamily Nepetoideae [81]. Only a tetrameric derivative of caffeic acid was isolated from P. japonicus [36]. But a group of long-chain alkylphenols, of possible taxonomic significance in the genus, was isolated [28,29]. Generally, the subfamily Lamioideae is rich in iridoid glycosides, whereas they are absent from the Nepetoideae [82]. No iridoid glycosides were isolated from Plectranthus.

Generally, Plectranthus species are essential-oil-rich (i.e. > 0.5% volatile oil on a dry weight basis), in agreement with the general situation that the Nepetoideae are oil-rich, whilst the Lamioideae are oil-poor [83].

Diterpenoids are the more common secondary metabolites in Plectranthus. The majority of them are highly modified abietanoids, in addition to some phyllocladanes and ent-kaurenes. It seems to be similar to the pattern of diterpenoids of Salvia [84], but no clerodane diterpenoids were found in Plectranthus.