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. 2022 Dec 13;27(24):8868. doi: 10.3390/molecules27248868

Comment on Pyrrolizidine Alkaloids and Terpenes from Senecio (Asteraceae): Chemistry and Research Gaps in Africa

Nicholas John Sadgrove 1
Editor: Kemal Husnu Can Baser1
PMCID: PMC9781224  PMID: 36558004

Abstract

The genus Senecio is one of the largest in Asteraceae. There are thousands of species across the globe, either confirmed or awaiting taxonomic delimitation. While the species are best known for the toxic pyrrolizidine alkaloids that contaminate honeys (as bees select pollen from the species) and teas via lateral transfer and accumulation from adjacent roots of Senecio in the rhizosphere, they are also associated with more serious cases leading to fatality of grazing ruminants or people by contamination or accidental harvesting for medicine. Surprisingly, there are significantly more sesquiterpenoid than pyrrolizidine alkaloid-containing species. The main chemical classes, aside from alkaloids, are flavonoids, cacalols, eremophilanes, and bisabolols, often in the form of furan derivatives or free acids. The chemistry of the species across the globe generally overlaps with the 469 confirmed species of Africa. A small number of species express multiple classes of compounds, meaning the presence of sesquiterpenes does not exclude alkaloids. It is possible that there are many species that express the pyrrolizidine alkaloids, in addition to the cacalols, eremophilanes, and bisabolols. The aim of the current communication is, thus, to identify the research gaps related to the chemistry of African species of Senecio and reveal the possible chemical groups in unexplored taxa by way of example, thereby creating a summary of references that could be used to guide chemical assignment in future studies.

Keywords: pyrrolizidine, alkaloid, cacalol, eremophilane, contamination, adulteration, toxin, poison

1. Introduction

According to ‘Plants of the World Online’ (https://powo.science.kew.org/, accessed on 15 October 2022) (POWO), there are 1477 accepted species of Senecio in the world, with a further 3490 tentative species or synonyms, making it one of the largest genera in Asteraceae. According to POWO, 477 of the accepted species are native to Africa, but if the recognized synonyms are excluded [1,2,3], the total number of recognized African Senecio is reduced to 469, many of which are reported as a traditional medicine or a dangerous contaminant of foods and medicines [4,5,6].

It is the pyrrolizidine alkaloids in several species of Senecio that underly the poisonings occurring in ruminants [7] and people, leading to hepatomegaly (enlarged liver), ascites (abdominal fluid), and cirrhosis [4]. The toxicosis of people has been noted in cases of accidental contamination of a medicinal species [8], contamination of honey by bees [9], or by lateral transfer of toxic alkaloids in tea plantations [6], and water contamination [10,11,12].

In South Africa, Senecio angustifolius (Thunb.) Willd., contaminates Rooibos tea (Aspalathus linearis (Burm.f.) R.Dahlgren). Unfortunately, S. angustifolius has a similar growth habit and flower color as A. linearis, making it difficult to eliminate from Rooibos plantations. As the invading species grows among the Rooibos plants, it secretes pyrrolizidine alkaloids into the rhizosphere, where they are enter the root system of A. linearis and accumulate in the tea leaves [6].

Another example from South Africa is related to therapeutic use of Senecio coronatus (Thunb.) Harv. The roots of the species are used in traditional medicine. A suppository of the aqueous extract is given to infants as a means to confer strength to the child during weaning. Unfortunately, several infants have succumbed to hepatic sinusoidal obstruction syndrome, which is known to have been occurring since the 1980s [13], and possibly much longer. Due to superstition around the declaration of materials to forensics, it was not until 2017, following a new wave of deaths, that an examination of the biota could be made, revealing that the material associated with poisoning contained biota that was morphologically different, possibly representative of another species as a contaminant [8] or a toxic genotype currently incorrectly circumscribed as S. coronatus [14].

There are several species of Senecio that are associated with the toxication of honey. The presences of pyrrolizidine alkaloids in honeys has been reported in Europe [15], Brazil [9], China [16], North America [17], and via the South African species S. inaequidens DC [18], which is now naturalized in Italy (and Europe). The honey is toxified by the pollen from species of Senecio, harvested by bees, and carried back to the hive. The issue in Europe has prompted the European Food and Safety Authority to elaborate on the health risks associated with the consumption of honeys or plant products known to express or be contaminated by pyrrolizidine alkaloids [19]. The risks are also evident in Australia, since Echium plantagineum L., known by the vernacular ‘Patterson’s Curse’, expresses pyrrolizidine alkaloids that also find their way into honey. While the local honey, known as ‘Patterson’s Curse Honey’ is popular, it is contaminated with pyrrolizidine alkaloids [20].

Although the chemistry of Senecio is well-known to include pyrrolizidine alkaloids, and the genus is chemically varied. Surprisingly, there are many more terpenoid taxa than toxic species in Senecio. Furthermore, there are several chemical studies of species that were previously circumscribed as Senecio, but they are now revised to such genera as Caputia or Othonna [1] (among others). While the taxonomy has changed, the chemical similarities to species in Senecio are evident, which can be ascertained by reading the previous papers by Bohlmann [21,22]. In another example, the macrolide platyphylline and the pyrrolizidine alkaloid seneciphylline were first discovered after isolation from Senecio platyphyllus D.C. [1]. The etymology of the vernacular names given to these alkaloids is related to the botanical name of the species. However, S. platyphyllus was renamed to Caucasalia macrophylla (M.Bieb.) B.Nord. There are many examples of species revisions that complicate the interpretation of the etymology of the compound names.

The current communication is a summary of the chemistry and toxicity of African species of Senecio. This work is intended to serve as a reference in guiding the further chemical prospection of the species in southern Africa. The correct names of all taxa were determined using the POWO database to remain up to date with the taxonomic status of all species listed. A literature search was conducted on each species individually to ascertain if the chemical characterization was retrievable, to identify what is known, the chemotaxonomic implications thus far, and to reveal research gaps. The literature search was also extended to species synonyms whenever old or outdated names were realized in the course of compiling data.

2. Phytochemistry of African Senecio

A search of the 469 species of Senecio from Africa was conducted on Google Scholar [23] to ascertain if phytochemical studies exist, individually searching the genus and species against the words ‘chemistry’, ‘phytochemistry’, ‘sesquiterpene’, or ‘pyrrolizidine alkaloid’. The species (or subspecies) searched are listed in Table 1, and the tentative number of species associated with phytochemical studies amounted to 83 (Table 2).

Table 1.

A list of all of the currently accepted species of Senecio that are native to Africa, according to POWO (477 species, 469 after synonyms are subtracted), conveying those with phytochemical information and those with no records listed here, as they were not identified in the literature search. Y = phytochemical study found, N = no phytochemical study found.

Species and Author Y/N Species and Author Y/N Species and Author Y/N
S. abbreviatus S.Moore N S. glutinosus Thunb. Y S. paniculatus P.J. Bergius Y
S. abruptus Thunb. N S. gossweileri Torre N S. parascitus Hilliard N
S. acetosifolius Baker N S. gossypinus Baker Y S. parentalis Hilliard and B.L. Burtt N
S. achilleifolius DC. N S. gramineticola C. Jeffrey N S. parvifolius DC.
(Synonym of S. carroensis)		 N
S. actinoleucus F.Muell. N S. gramineus Harv. N S. paucicalyculatus Klatt Y
S. acutifolius DC. N S. grandiflorus P.J. Bergius Y S. pauciflorus Baker N
S. adenostylifolius Humbert N S. gregatus Hilliard N S. pauciflosculosus C. Jeffrey N
S. adnatus DC. Y S. hadiensis Forssk. Y S. pearsonii Hutch.
(Synonym of S. asperulus)		 N
S. adscendens
(syn. andinus) Bojer		 N S. halimifolius L. Y S. pellucidus DC. N
S. aegyptius L. Y S. harveyanus MacOwan N S. peltophorus Brenan N
S. aequinoctialis R.E.Fr. N S. hastatus L.
(Synonym of S. erosus and S. robertiifolius)		 N S. penninervius DC. N
S. aetfatensis B.Nord. N S. hastifolius (L.f.) Less. N S. pentactinus Klatt N
S. affinis DC. Y S. haygarthii
M.Taylor ex Hilliard		 N S. pentecostus Hiern N
S. agapetes C.Jeffrey N S. hebdingii (Rauh and Buchloh) G.D. Rowley N S. perralderianus Coss. N
S. albanensis DC. N S. hedbergii C. Jeffrey N S. perrieri Humbert N
S. albanopsis Hilliard N S. hederiformis Cron N S. perrottetii DC. N
S. albifolius DC. N S. heliopsis Hilliard and B.L. Burtt Y S. persicifolius L. N
S. albopunctatus Bolus N S. helminthioides (Sch.Bip.) Hilliard Y S. petiolaris DC. N
S. aloides DC. N S. hermannii B. Nord. N S. petraeus Boiss. and Reut. N
S. altissimus Mill. N S. hesperidum Jahand, Maire, and Weiller N S. phalacrolaenus DC. N
S. ambositrensis Humbert N S. hieracioides DC. Y S. pillansii Levyns N
S. amplificatus C.Jeffrey N S. hildebrandtii Baker N S. pinifolius (L.) Spreng. N
S. anapetes C.Jeffrey N S. hirsutilobus Hilliard N S. pinnatifidus Less. N
S. andapensis Humbert N S. hirtifolius DC N S. pinnatipartitus Sch.Bip. ex Oliv. N
S. andohahelensis Humbert N S. hirto-crassus Humbert N S. pinnulatus Thunb. N
S. angulatus L.f. Y S. hochstetteri
Sch.Bip. Ex A.Rich.		 N S. piptocoma O.Hoffm. N
S. angustifolius (Thunb.) Willd. Y S. hoggariensis Batt. and Trab. Y S. pirottae Chiov. N
S. anomalochrous Hilliard N S. hollandii Compton N S. plantagineoides C. Jeffrey N
S. antaisaka Humbert N S. holubii Hutch. and Burtt Davy N S. pleistophyllus C. Jeffrey N
S. antambolorum Humbert N S. humidanus C. Jeffrey N S. poggeanus Mattf. N
S. antandroi Scott Elliot N S. hypochoerideus DC. Y S. polelensis Hilliard N
S. anthemifolius Harv. N S. ilicifolius Thunb. Y S. polyadenus Hedberg N
S. antitensis Baker N S. ilsae A.Santos and ReY-Bet N S. polyanthemoides Sch.Bip. Y
S. aquifoliaceus DC. N S. immixtus C. Jeffrey N S. polyodon DC. N
S. arabidifolius O.Hoffm N S. inaequidens DC. Y S. poseideonis Hilliard and B.L. Burtt N
S. arenarius Thunb. N S. incomptus DC. N S. praeteritus Killick N
S. arniciflorus DC N S. incrassatus Lowe N S. propior S. Moore N
S. asperulus DC Y S. infirmus C. Jeffrey N S. prostratus Klatt N
S. auriculatissimus Britton N S. ingeliensis Hilliard N S. pseudolongifolius
Sch.Bip ex J. Calvo		 N
S. austromontanus Hilliard N S. inornatus DC. Y S. pseudosubsessilis C. Jeffrey N
S. balensis S.Oritz and Vivero N S. intricatus S.Moore N S. ptarmicifolius Bory N
S. bampsianus Lisowski N S. isatideus DC. Y S. pterophorus DC. Y
S. barbatus DC. N S. isatidoides E. Phillips and C.A.Sm. N S. puberulus DC. N
S. baronii Humbert N S. jacksonii S. Moore N S. pubigerus L. Y
S. barorum Humbert N S. junceus (Less.) Harv. N S. purpureus L. Y
S. basalticus Hilliard N S. juniperinus L.f. N S. purtschelleri Engl. N
S. baurii Oliv. N S. junodii Hutch. and Burtt Davy N S. qathlambanus Hilliard N
S. belbeysius Delile N S. kacondensis S. Moore N S. quartziticola Humbert N
S. bellis Harv. N S. kalambatitrensis Humbert N S. quinquelobus (Thunb.) DC. N
S. bipinnatus Less. N S. kalingenwae Hilliard and B.L. Burtt N S. quinquenervius Sond. N
S. Bollei Sunding and C.Kunkel Y S. karaguensis O. Hoffm. N S. ragazzii Chiov. N
S. boutonii Baker N S. katangensis O. Hoffm. N S. randii S.Moore N
S. brachyantherus (Hiern) S.Moore N S. kayomborum Beentje N S. rehmannii Bolus N
S. brachypodus DC Y S. keniophytum R.E.Fr. N S. repandus Thunb. N
S. brevidentatus M.D. Hend N S. kerdousianus Gomiz and Llamas N S. reptans Turcz. N
S. brevilorus Hilliard N S. kuluensis S.Moore N S. resectus Bojer ex DC. N
S. brittenianus Hiern N S. kundelungensis Lisowski N S. retortus Benth. N
S. bryoniifolius Harv. N S. kuntzeanus Dinter N S. retrorsus DC. Y
S. bulbinefolius DC. N S. laevigatus Thunb. N S. rhammatophyllus Matt.f. N
S. bupleuroides DC. Y S. laevis Humbert N S. rhomboideus Harv. Y
S. burchellii DC. Y S. lamarckianus Bullock N S. rhyncholaenus DC. N
S. burtonii Hook.f. Y S. lanceus Aiton N S. rigidus L. N
S. byrnensis Hilliard N S. latecorymbosus Gilli N S. robertiifolius DC. N
S. cakilefolius DC.
(Synonym of S. arenarius)		 N S. latibracteatus Humbert N S. roseiflorus R.E.Fr. Y
S. caloneotes Hilliard N S. laticipes Bruyns N S. rosmarinifolius L.f. Y
S. canabyi Humbert N S. latifolius DC. Y S. rugegensis Muschl. N
S. canaliculatus Bojer ex DC. N S. latissimifolius S. Moore N S. ruwenzoriensis S. Moore Y
S. canalipes DC. N S. lawalreeanus Lisowski N S. sabinjoensis Muschl. N
S. capuronii Humbert N S. laxus DC. N S. saboureaui Humbert N
S. cardaminifolius DC. N S. leandrii Humbert N S. sakalavorum Humbert N
S. carnosus Thunb. N S. lejolyanus Lisowski N S. sakamaliensis (Humbert) Humbert N
S. carroensis DC. N S. lelyi Hutch. N S. sandersonii Harv. Y
S. cathcartensis O. Hoffm. Y S. leptopterus Mesfin N S. saniensis Hilliard and B.L. Burtt N
S. caudatus DC. N S. lessingii Harv. N S. schimperi Sch.Bip. ex Hochst. N
S. cedrorum Raynal N S. letouzeyanus Lisowski N S. schultzii Hochst. ex A. Rich. N
S. chalureaui Humbert N S. leucadendron (G. Forst.) Hems.L. N S. schweinfurthii O.Hoffm. Y
S. chrysocoma Meerb. Y S. leucanthemifolius Poir. Y S. scitus Hutch. and Burtt Davy N
S. cinerascens Aiton N S. lewallei Lisowski N S. scoparius Harv. N
S. citriceps Hilliard and B.L.Burtt N S. lineatus DC. N S. semiamplexifolius De Wild. N
S. cochlearifolius Bojer ex DC. N S. linifolius L. Y S. seminiveus J.M. Wood and M.S. Evans N
S. coleophyllus Turcz. N S. Lisowskii Long Wang and Beentje N S. serratuloides DC. Y
S. comptonii J.C.Manning and Goldblatt N S. litorosus Fourc. N S. serrulatus DC. N
S. confertus Sch.Bip. Ex A.Rich. N S. littoreus Thunb. N S. serrurioides Turcz. N
S. conrathii N.E.Br. Y S. lividus L. Y S. shabensis Lisowski N
S. consanguineus DC. Y S. lobelioides DC.
(Synonym of S. flavus)		 N S. simplicissimus Bojer ex DC. N
S. cordifolius L.f. N S. longiscapus Bojer ex DC. Y S. sisymbriifolius DC. N
S. cornu-cervi MacOwan N S. luembensis De Wild. and Muschl. N S. skirrhodon DC. N
S. coronatus (Thunb.) Harv. Y S. lycopodioides Schltr. N S. snowdenii Hutch. N
S. cotyledonis DC. N S. lydenburgensis Hutch. and Burtt Davy Y S. sociorum Bolus N
S. crassissimus Humbert Y S. lygodes Hiern N S. sophioides DC. N
S. crassiusculus DC. N S. lyratus Forssk. Y S. sororius C. Jeffrey N
S. crassorhizus De N S. mabberleyi C.Jeffrey N S. sotikensis S. Moore N
S. crenatus Thunb. N S. MacOwanii Hilliard N S. spartareus S. Moore N
S. crenulatus DC. N S. macrocephalus DC. Y S. speciosissimus J.C.Manning and Goldblatt N
S. crispatipilosus C.Jeffrey N S. macroglossoides Hilliard N S. speciosus Willd. Y
S. crispus Thunb. Y S. macroglossus DC. N S. spiraeifolius Thunb. N
S. cristimontanus Hilliard N S. macrospermus DC. Y S. squalidus L. Y
S. cryphiactis O.Hoffm. N S. madagascariensis Poir. Y S. stella-purpurea V.R.Clark, J.D.Vidal, and N.P.Barker N
S. cryptolanatus Killick N S. malacitanus Huter Y S. steudelii Sch.Bip. ex A. Rich. N
S. cyaneus O.Hoffm. N S. malaissei Lisowski N S. striatifolius DC. N
S. cymbalariifolius (L.) Less. N S. mandrarensis Humbert N S. strictifolius Hiern N
S. decaryi Humbert N S. maranguensis O.Hoffm. N S. subcanescens (DC.) Compton N
S. decurrens DC. N S. margaritae C.Jeffrey N S. subcoriaceus Schltr. N
S. deltoideus Less. Y S. marginalis Hilliard N S. subfractiflexus C. Jeffrey N
S. denisii Humbert N S. mariettae Muschl. Y S. submontanus
Hilliard and B.L. Burtt		 N
S. dentatoalatus
Mildbr. Ex C.Jeffrey		 N S. maritimus L.f. Y S. subrubriflorus O.Hoffm. Y
S. depauperatus Mattf. N S. marnieri Humbert N S. subsessilis Oliv. and Hiern N
S. diffusus L.f. N S. marojejyensis Humbert N S. subsinuatus DC. N
S. digitalifolius DC. N S. massaicus (Maire) Maire Y S. sylvaticus L. Y
S. dilungensis Lisowski N S. matricariifolius DC. N S. syringifolius O.Hoffm. Y
S. diodon DC. N S. mattirolii Chiov. N S. tabulicola Baker N
S. diphyllus De Wild. and Muschl. N S. mauricei Hilliard and B.L. Burtt Y S. tamoides DC. Y
S. discodregeanus
Hilliard and B.L.Burtt		 N S. maydae Merxm.
(Synonym of S. albopunctatus)		 N S. tanacetopsis Hilliard N
S. discokaraguensis C.Jeffrey N S. mbuluzensis Compton N S. teixeirae Torre N
S. dissidens Fourc. N S. melastomifolius Baker N S. telekii O.Hoffm. N
S. dissimulans Hilliard and B.L.Burtt N S. mesembryanthemoides Bojer ex DC. N S. telmateius Hilliard N
S. doryphoroides C.Jeffrey N S. mesogrammoides O.Hoffm. N S. tenellus DC. N
S. doryphorus Mattf. N S. meuselii Rauh N S. teneriffae Sch.Bip. ex Bolle N
S. dracunculoides DC. N S. meyeri-johannis EngL. N S. thamathuensis Hilliard N
S. dregeanus DC. N S. microalatus C. Jeffrey N S. thunbergii Harv. N
S. dumeticola S.Moore N S. microglossus DC. Y S. torticaulis Merxm. N
S. dumosus Fourc. N S. microspermus DC. N S. tortuosus DC. N
S. eenii (S.Moore) Merxm. N S. mimetes Hutch. and R.A. Dyer N S. trachylaenus Harv. N
S. elegans L. N S. mitophyllus C. Jeffrey N S. trachyphyllus Schltr. N
S. ellenbeckii O.Hoffm. N S. mlilwanensis Compton N S. transmarinus S. Moore N
S. eminens Compton N S. monticola DC. N S. triactinus S. Moore N
S. emirnensis DC. N S. mooreanus Hutch. and Burtt Davy N S. trilobus L. N
S. englerianus O.Hoffm. N S. moorei R.E.Fr. N S. triodontiphyllus C. Jeffrey N
S. eriobasis DC.
(Synonym of S. erosus)		 N S. mooreioides C. Jeffrey N S. triplinervius DC. N
S. eriopus Willk. N S. morotonensis C. Jeffrey N S. triqueter Less. N
S. erlangeri O.Hoffm. N S. muCronatus Willd. N S. tsaratananensis Humbert N
S. erosus L.f. N S. multibracteatus Harv. N S. tugelensis J.M. Wood and M.S. Evans N
S. erubescens Aiton Y S. multicaulis DC. N S. tysonii MacOwan N
S. erysimoides DC. N S. multidenticulatus Humbert N S. ulopterus Thell. N
S. esterhuyseniae J.C.Manning and Goldblatt N S. muricatus Thunb. N S. umbellatus L. Y
S. euriopoides DC. N S. myriocephalus Sch.Bip. ex A.Rich. N S. umbricola Cron and B. Nord. N
S. evelynae Muschl. N S. nanus Sch.Bip. ex A. Rich. N S. umgeniensis Thell. N
S. exarachnoideus C.Jeffrey N S. napifolius MacOwan N S. unionis Sch.Bip. ex A. Rich. N
S. exuberans R.A.Dyer N S. natalicola Hilliard N S. urophyllus Conrath N
S. fanshawei Beentje N S. navicularis Humbert N S. urundensis S.Moore N
S. farinaceus
Sch.Bip. Ex A.Rich.		 N S. navugabensis C. Jeffrey N S. vaingaindrani Scott Elliot N
S. flavus (Decne.) Sch.Bip. Y S. neo Bakeri Humbert N S. variabilis Sch.Bip. N
S. foeniculoides Harv. N S. neoviscidulus Soldano N S. venosus Harv. N
S. forbesii Oliv. and Hiern N S. ngandae Beentje N S. verbascifolius Burm.f. N
S. francoisii Humbert N S. ngoyanus Hilliard N S. vernalis Walds. and Kit. Y
S. fresenii Sch.Bip. N S. nyangani Beentje N S. vestitus P.J.Bergius N
S. gallicus Vill. Ex Chaix Y S. nyungwensis P. Maquet N S. vicinus S.Moore N
S. gariepiensis Cron N S. ochrocarpus Oliv. and Hiern N S. villifructus Hilliard N
S. garnieri Klatt N S. odontopterus DC. N S. vimineus DC. N
S. gazensis S.Moore N S. oederifolius DC. N S. vira-vira Hieron. Y
S. geniorum Humbert N S. ornatus S.Moore N S. vitellinoides Merxm. N
S. gerrardii Harv. Y S. othonniflorus DC. N S. vittarifolius Bojer ex DC. N
S. giessii Merxm. N S. oxyodontus DC. Y S. voigtii van Jaarzv. N
S. glaberrimus DC. N S. oxyriifolius DC. Y S. volcanicola C. Jeffrey N
S. glanduloso-lanosus Thell. N S. paarlensis DC. N S. vulgaris L. Y
S. glandulo-pilosus
Volkens and Muschl.		 N S. pachyrhizus O. Hoffm. N S. waterbergensis S. Moore N
S. glastifolius L.f. N S. paludaffinis Hilliard Y S. windhoekensis Merxm. N
S. glaucus L. Y S. panduratus Less.
(Synonym of S. erosus)		 N S. wittebergensis Compton N
S. glutinarius DC. N S. panduriformis Hilliard N S. xenostylus O. Hoffm. N
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Table 2.

Details of phytochemical studies of the 83 species (or subspecies) of Senecio in Africa, for which records could be found.

Species Details
S. adnatus DC. Toxic alkaloid (macrolide): Platyphylline [24].
S. aegyptius L. Toxic alkaloids: Senecionine (pyrrolizidine) and otosenine (macrolide) [25].
S. aegyptius L.
var. discoideus Boiss.		 Sesquiterpenes: 1,10-epoxyfuranoeremophilane (in essential oil), with traces of monoterpenes [26]. Non-volatiles include 1-β-hydroxy-8-oxoeremophila-7,9-dien-12-oic acid, rutin, and quercetin-3-O-glucoside-7-O-rutinoside [27]. Novel eremophilane lactones also described [28].
S. affinis DC. Sesquiterpenes: Cacalols and bisabolols [29].
S. angulatus L.f. Toxic alkaloid: Angularine (pyrrolizidine) [30]. Phenols: Cynarin, chlorogenic acid and trans-ferulic acid [31]. Essential oils: α-Pinene, β-pinene, limonene, camphene, germacrene D, viridifloral, β-caryophyllene [32].
S. angustifolius
(Thunb.) Willd.		 Toxic alkaloids (pyrrolizidines): Senecionine N-oxide, retrorsine N-oxide, retrorsine, seneciphyline, senecionine, senkirkine [6].
S. asperulus DC. Possible chemotypes. Terpenes: furoeremophilanes, α -humulene, ent-kaurenic acid, ent-kaurenol [33]. Toxic alkaloids: Pyrrolizidine alkaloid N-oxides (exact identity not known) [34].
S. bollei
Sunding and C.Kunkel		 Toxic alkaloid (pyrrolizidine): Senecivernine [35].
S. brachypodus DC. Toxic alkaloid (pyrrolizidine): Rosmarinine [36].
S. bupleuroides DC. Toxic alkaloid (pyrrolizidine): Retrorsine [37].
S. burchellii DC. Toxic alkaloids (pyrrolizidine): Senecionine N-oxide and senkirkine [6].
S. burtonii Hook.F. Sesquiterpene: Cacalolide derivative, 4α-[2′-hydroxymethylacryloxy]-1β-hydroxy-14-(5 → 6) abe oeremophilan-12,8-olide. Shikimic acid derivative, (3′E)-(1α)-3-hydroxymethyl-4β,5α-dimethoxycyclohex-2-enyloctadec-3′-enoate. Fatty acid derivatives, octacosan-1-ol, 3β-hydroxyolean-12-en-28-oic acid, and 3β-acetoxyolean-12-en-28-oic acid [38].
S. cathcartensis
O.Hoffm.		 Sesquiterpenes: Eremophilene derivatives [21].
S. chrysocoma
Meerb.		 Toxic alkaloids: 7-angelylplatynecine, 9-angelylplatynecine, sarracine, neosarracine [39], and other 7-Angelyl-1-methylenepyrrolizidines (pyrrolizidines) [40].
S. conrathii N.E.Br. Sesquiterpenes: β-Farnesene, furoeremophilane derivatives, and germacrene D-4-ol [41]. Additionally, a nickel hyperaccumulator.
S. consanguineous DC. Toxic alkaloid: Retrorsine (pyrrolizidine), very low concentration [42].
S. coronatus
(Thunb.) Harv.		 Species did not contain toxic alkaloids [8], or mere traces, but further work necessary to know of the chemistry.
S. crassissimus
Humbert		 Sesquiterpenes: Germacrene D, bicyclogermacrene, Z-caryophyllene epoxide and other epoxides. Triterpenes: Lupeol, its acetate, lupeone, β-amyrin acetate, β-amyrenone, glutin-5(6)-en-3β-ol, 28-oxo-β-amyrenone, and the angelate [43].
S. crispus Thunb. Sesquiterpene dimers: Disesquiterpenoid derivative [44].
S. deltoideus Less. Unusual sesquiterpenes, linear diterpenes, and polyunsaturated alkenes and -kynes [45]. Used as a medicine to treat gynaecological and obstetric disorders [46].
S. erubescens Aiton Sesquiterpenes: Bisabolanes and eremophilanes [29].
S. flavus
(Decne.) Sch.Bip.		 Sesquiterpenes (Oxyeuryopsin derivatives): 3β-Methylbutyryloxyeuryopsin, 3β-angeloyloxyeuryopsin, 3β-senecioyloxyeuryopsin, 3β-hydroxyeuryopsin, euryopsin-3-one, furanoligularenone, and others [47].
S. gallicus
Vill. Ex Chaix		 Toxic alkaloids (pyrrolizidine): Ligularizine, senkirkine and senecionine N-oxide [48]. Essential oil: β-Phellandrene, apinene, germacrene-D, myrcene, α-copaene, sabinene, (Z)-β-ocimene, β-caryophyllene, p-cymene, β-pinene, α phellandrene, α-terpinolene, (E)-β-ocimene, α-humulene, azingiberene, and caryophyllene oxide [49].
S. gerrardii Harv. Sesquiterpenes: Eremophilene derivatives [21].
S. glaucus L. Essential oils: Isolongifolen-9-one, longiverbenone, 4-carene, p-cymene, thujone [50], m-mentha-1(7),8-diene, cis-m-mentha-2,8-diene, dehydrofukinone, α-terpinolene, 2,5-cyclohexadiene-1,4-dione,2-(1,1-dimethylethyl)-5-(2-methyl-2-propen-1-yl), sabinene, α-Fenchene and 1,3,8-p-menthatriene [51]. Phenols: Vanillic acid and gallic acid [52]. Flavonoids: Isorhamentin 3-O-β-D-glucoside, and isorhamentin 3-O-β-D-rutinoside. Benzofuran glucosides: 2,3-Dihydro-3β-hydroxyeuparin 3-O-glucopyranoside, isorhamentin 3-O-β-D-glucoside, and isorhamentin 3-O-β-D-rutinoside [53].
S. glaucus L. subsp. coronopifolius (syn. S. desfontainei). Toxic alkaloids (pyrrolyzidine): Seneciphylline [25].
S. glutinosus Thunb. Sesquiterpenes (seco-eremophilanes): Senglutinosin, 3α-hydroxy-10β-H-eremophil-11(13)-en-9-one, and nor-seco- glutinosone [33].
S. gossypinus Baker Flavonoid: Kaempferol-3-O-α-L-arabinopyranoside. Triterpenes: α-Amyrin and β-amyrin [54].
S. grandiflorus
P.J.Bergius		 Sesquiterpenes (furano): Cacalol derivatives [55].
S. hadiensis Forssk. Toxic alkaloids (macrolides): Rosmarinine, 12-O-acetylrosmarinine, neorosmarinine, hadiensine (1α-hydroxyplacyphylline), 12-O-acetylhadiewine, 12-O-acetylneohadiewine, and petitianine (2α-hydroxy-1,2-dihydroretronine) [56]. Sesquiterpenes (tricyclic): presilphiperfolan-2α,5α,8α-triol and presilphiperfolan-2α,5α,8α,10α-tetraol [57].
S. halimifolius L. Sesquiterpenes (furano): Furanoeremophilanes [58].
S. heliopsis
Hilliard and B.L.Burtt		 Sesquiterpenes: Furanoeremophilanes and cacalols [29].
S. helminthioides
(Sch.Bip.) Hilliard		 Phenylpropenes: Trimethoxy-phenylpropenes [22].
S. hieracioides DC. Sesquiterpenes (furo): 9,10-Dehydrofuranoeremophilane, ligularenolide, eremophil-7(11)-en-8,12-olide, 8,8′-epimeric dimers. Shikimic acid derivatives [59].
S. hoggariensis
Batt. and Trab.		 Phenols: 3,4-Dihydroxybenzoic acid, 4-hydroxybenzoic acid, 6,7-dihydroxycoumarin, vanillic acid, caffeic acid, p-coumaric acid, and ferulic acid [60].
S. hypochoerideus DC. Sesquiterpenes (furano): Furanoeremophilanes [61].
S. ilicifolius Thunb. Toxic alkaloid: Senecionine ‘responsible for bread-poisoning’ [62].
S. inaequidens DC Toxic alkaloids (pyrrolizidine): Retrorsine, senecionine [42], senecivernine, integerrimine, and an analogue of retrorsine [63]. Possibly another chemotype with furanosesquiterpenes [58].
S. inornatus DC. Toxic alkaloid (pyrrolizidine): O7-Angeloylretronecine [64]. Sesquiterpenes (furano): Furanoeremophilanes [58].
S. isatideus DC. Vinyl-olefins: Polyunsaturated alkenes [65].
S. latifolius DC. Toxic alkaloids: Retrorsine, isatidine, sceleratine, chlorodeoxysceleratine (merenskine), and the N-oxides of sceleratine and merenskine [66].
S. leucanthemifolius
Poir.		 Toxic alkaloids (pyrrolizidine): Integerrimine and senecionine [67]. Essential oils: α-hydroxy-p-cymene, carvacrol, nerol, carveol, and cis-α-bisabolene [68].
S. linifolius L. Sesquiterpenes: furanoeremophilanes, i.e., maturinone and seven cacalohastin derivatives [69].
S. lividus L. Sesquiterpenes: Eremophilanes [29]. Possibly essential oils: [70].
S. longiscapus
Bojer ex DC.		 Essential oils: Sabinene, elemicin, β-pinene, methyleugenol, α-pinene, and myrcene [71].
S. lydenburgensis
Hutch. and Burtt Davy		 Sesquiterpenes: Multiple cacalol derivatives [72].
S. lyratus Forssk. Sterols: Sitosterol and stigmasterol. Triterpene: β-Amyrin [73].
S. macrocephalus DC. Toxic alkaloids (pyrrolizidine): Traces of 7-senecioyl-9-sarracinylheliotridine and 7-angelyl-9-sarracinyl-heliotridine [74].
S. macrospermus DC. Sesquiterpenes: Cacalohastin derivatives [75].
S. madagascariensis
Poir.		 Toxic alkaloids (pyrrolizidine): Senecivernine, senecionine, integerrimine, senkirkine, mucronatinine, retrorsine, usaramine, otosenine, acetylsenkirkine, desacetyldoronine, florosenine, and doronine [76].
S. malacitanus Huter Toxic alkaloids (pyrrolizidine): Unnamed derivatives [77].
S. mariettae Muschl. Toxic alkaloid (pyrrolizidine): Retrorsine [78].
S. maritimus L.f. Alkanes: Polyunsaturated aklenes and -ynes. Furanosesquiterpene derivatives [79].
S. massaicus
(Maire) Maire		 Essential oil: p-Cymene, n-hexadecanoic acid, and docosane-11-decyl [80].
S. mauricei
Hilliard and B.L.Burtt		 Sesquiterpenes: Senmauricinol-(2-methyacrylat), hilliardinolisobutyrat, hilliardinol-(2-methylacrylat), 10β-hydroxy-1-oxo-6β-isobutyryloxy-2,3-dehydro-furanoeremophil-9-on, 2,3-desoxyhilliardinol-isobutyrat, 2,3-desoxohilliardinol-(2-methylacrylat), and 8,12-dioxo-7,11,9,10-tetradehydroeremophilan [81].
S. microglossus DC. Sterols: Stigmasterol, sitosterol, dammaradienol, its 3-epimer, and the angelate. Sesquiterpenes: Germacrene D,
y- and δ-cadinene, bisabolol, the angelate [41].
S. oxyodontus DC. Sesquiterpene: Pentaynene sesquiterpene, bisabolene derivatives [82], and triquinane sesquiterpenes [83] p-Hydroxyacetophenone [82].
S. oxyriifolius DC. Sesquiterpenes: Tricyclic bisabolol derivatives (epoxides) [84].
S. paludaffinis
Hilliard		 Sesquiterpenes: Cacalol and bisabolol derivatives [85].
S. paniculatus
P.J.Bergius		 Toxic alkaloids (pyrrolizidine): 7β-Angelyl-1-methylene-8α-pyrrolizidine, 7β-angelyl-1-methylene-8α-pyrrolizidine, 7β-angelyl-1-methylene8α-pyrrolizidine-4-oxide, 7-angelylhastanecine, 9-angelylhastanecine, 7-angelylplatynecine, 9-angelylplatynecine, 9-angelylplatynecine-4-oxide, sarracine, neosarracine, and retrorsine [86].
S. paucicalyculatus
Klatt		 Toxic alkaloids (pyrrolizidine): Retrorsine and isatidine [87].
S. polyanthemoides
Sch.Bip.		 Sesquiterpenes (furano): Cacalol derivatives [55]. Essential oils: Limonene, p-cymene, β-selinene, α-pinene, β-pinene, 1,8-cineole, caryophyllene oxide, and humulene epoxide II [88].
S. pterophorus DC. Toxic alkaloids (pyrrolizidine): Retronecine, otonecine, platynecine, and rosmarinecine derivatives [89].
S. pubigerus L. Sesquiterpenes: Germacrene D, bicyclogermacrene, beta-farnesene, and bisabolol derivatives. Toxic alkaloid (pyrrolizidine): acylpyrrole [90].
S. purpureus L. Sesquiterpenes (eremophilenes): Diesters of seneremophilondiol, senescaposone and isosenescaposone, esterified with 4-methyl-5-acetoxy-pent-2-enoic acid [21].
S. retrorsus DC. Toxic alkaloids (pyrrolizidine): Retrorsine, isatidine, and isatinecic acid [91].
S. rhomboideus Harv. Sesquiterpenes: Eremophilene derivatives [21].
S. roseiflorus R.E.Fr. Flavonoids: O-methylated, i.e., 5,4′-dihydroxy-7-dimethoxyflavanone [92].
S. rosmarinifolius L.f. Toxic alkaloid (macrolide): Rosmarinine [62].
S. ruwenzoriensis
S.Moore		 Toxic alkaloids (pyrrolizidine): Isoline and bisline [93].
S. sandersonii Harv. Sesquiterpenes: Diester and germacrene derivatives [94].
S. schweinfurthii
O.Hoffm.		 Toxic alkaloid (pyrrolizidine): 7β-Hydroxy-1-methylene-8α-pyrrolizidine N-oxide [95].
S. serratuloides DC. Sterols: Phytosteroids and estran-3-one, 17-(acetyloxy)-2-methyl-, (2à,5à,17á) [96]. Used as a medicine to treat gynaecological and obstetric disorders [46].
S. speciosus Willd. Toxic alkaloids (pyrrolizidine): 7-Senecioyl-9-sarracinylheliotridine and 7-angelyl-9-sarracinyl-heliotridine [74].
S. squalidus L. Essential oils: p-Cymene and α-phellandrene [97].
S. subrubriflorus
O.Hoffm.		 Diterpenes: Sandaracopimarene derivatives. Sesquiterpene: Bisabolene derivative (4,7-oxide) [98].
S. sylvaticus L. Sesquiterpenes: Alkenes, bicyclic derivatives, and furanosesquiterpenes [79].
S. syringifolius
O.Hoffm.		 Toxic alkaloids (pyrrolizidine): Angularine, rosmarinine, and 12-O-acetylrosmarine, together with their N-oxides [99].
S. tamoides DC. Flavonoids: di-C-rhamnosylapigenin, mangiferin, and isomangiferin [100].
S. umbellatus L. Sesquiterpenes: Furanoeremophilane derivatives [101].
S. vernalis
Walds. and Kit.		 Sesquiterpenes (furano): Cacalol derivatives [55].
S. vira-vira
Hieron.		 Toxic alkaloids (pyrrolizidine): Anacrotine, neoplatyphylline, uspallatine. Flavonoids: Quercitrin, rutin, isorhamnetin 3-O-β-robinobioside. Sterols: Sitosterol, campesterol, stimasterol, stimasta-3,5-dien-7-one, and stimasta-4,6-dien-3-one. Triterpenes: α-/β-amyrins [102].
S. vulgaris L. Toxic alkaloids (pyrrolizidine): Senecionine, senecionine N-oxide, integerrimine N-oxide, seneciphylline N-oxide, retrorsine N-oxide, and spartioidine N-oxide [103]. Essential oils: α-Humulene, (E)-β-caryophyllene, terpinolene, ar-curcumene, and geranyl linalool [104].
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The species with phytochemical records, listed in Table 1, have been elaborated upon in Table 2, with details related to the chemical classes identified.

The most common classes of compound identified and published from African Senecio include pyrrolizidine alkaloids, eremophilanes, biasabolols, cacalols, and flavonoids (Figure 1). Derivatives of these include furans, oxides, O-linked moieties, and mere stereoisomers. Other groups of compounds that are less frequently reported include phenylpropanes, essential oils, triterpenes, sterols, oxyeuryopsins, diterpenes, dimers of sesquiterpenes, fatty acid derivatives, and polyunsaturated alkynes and alkenes (Table 2).

Figure 1.

Figure 1

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Common classes of compound in African species of Senecio.

The vast majority of the chemical work performed on the world’s species of Senecio was conducted in Germany by Bohlmann and collaborators [21,22,33,41,43,45,55,58,59,61,72,75,79,81,82,84,85,90,94,98]. This group covered a number of the African species, sometimes focused exclusively on those from South Africa (südafrikanischen) [21,22,90]. The majority of the work by Bolmann and his group characterized a significant number of new and existing sesquiterpenes, particularly the cacalols and their derivatives. The cacalols were named from the species Cacalia delphiniifolia Siebold and Zucc. [105], and although the species was revised to Japonicalia delphiniifolia (Siebold and Zucc.) C. Ren and Q.E. Yang, the cacalols are now widely characterized in the genus Senecio (Table 2).

The contents of Table 2 are not representative of exhaustive chemical studies of the respective species. For example, where essential oils have been tentatively characterized, the fixed components of the material have not been studied. Furthermore, in a minority of cases, multiple types of chemical classes are identified, sometimes within a single study. For example, S. vira vira was exhaustively studied, and flavonoids, sterols, triterpenes, and pyrrolizidine alkaloids were identified in that single biota. This underscores the possibility of other classes of metabolite in the other species in Table 2. When chemists study pyrrolizidine alkaloids, they focus on an ‘alkaloids’ extract produced via acid-base partitioning [106]. By following such an approach, non-basic metabolites are excluded from the extracts. Thus, the species listed in Table 2 may be more complex than is realized.

A chemotaxonomic review of Senecio divides the genus into sections, corresponding to the different types of sesquiterpenes, i.e., section bisabolene, sect. cacalol, sect. eremophilane, sect. furanoeremophilane, sect. eremophilanolide, and sect. germacrane [29]. The approach to recognize taxa, according to a specific group of metabolites (the sesquiterpenes), to the exclusion of other metabolites that may be present in the biota, reduces the complexity of the analysis. This approach may have been informed by the impractical division of alkaloids verses terpenes, due to the technical difficulty mentioned above. However, to know if the clustering tree of sesquiterpenes in Senecio is of true value, it may be necessary to test for agreement of a molecular phylogeny [107].

3. Conclusions

The genus Senecio is one of the largest in the family Asteraceae. Species in the genus are chemically varied, and previous scholars have attempted to understand this from a chemotaxonomic perspective. Several of the species have been associated with poisonings of both ruminants and humans, ranging from mild toxicosis to fatality. The pyrrolizidine alkaloids are the toxic principle of the poisonous species, and while there are many species that are not reported to contain this class of compound, studies do not always focus on this class, nor do they confirm that they searched for them in extracts or qualify that they are not present. Thus, the chemical information that has been gathered in this communication is not a robust guide to the exhaustive chemistry of the respective species, but rather a preliminary finding that can be elaborated upon with further study. Thus, it is of essence to re-examine the species that reportedly contain sesquiterpenes to know if pyrrolizidine alkaloids are absent generally, or if the co-occurrence of the two classes of compound is common.

Acknowledgments

The author would like to acknowledge Ben-Erik van Wyk, who motivated this communication.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The author declares no conflict of interest.

Sample Availability

Samples of the compounds are not available from the author.

Funding Statement

This research received no external funding.

Footnotes

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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