The chemistry and bioactivity of Southern African flora II: flavonoids, quinones and minor compound classes

Abstract

This review is intended to highlight the relevance of natural products in drug discovery paying particular attention to those derived from Southern African medicinal plants with diverse biological activities. In this review series, a literature survey led to the collection of 864 secondary metabolites from 101 plant species from 57 plant families. A correlation between the known biological activities of isolated compounds and the ethnobotanical uses of the plants has been attempted. Part I focused on alkaloids and terpenoids, while this part is focused on the bioactivities of flavonoids, quinines and other minor, unique compound classes which correlate with their ethnobotanical uses in African traditional medicine (ATM).

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Smith B. Babiaka

Smith Babiaka studied Chemistry at the University of Buea, Cameroon, where he subsequently obtained his Bachelor of Science degree in 2005 and the Master of Science degree in 2011. Since 2012 he has been enrolled for a PhD in Chemistry at the same university working on natural products/organic synthesis. His previous work has been focused on the extraction, purification and characterisation of natural products from African medicinal plants. His current assignment at the Chemical and Bioactivity Information Centre consists in developing natural product databases for African medicinal plants and knowledge bases for environmentally harmful chemicals.

Fidele Ntie-Kang

Fidele Ntie Kang studied Chemistry at the University of Douala in Cameroon between 1999 and 2004, leading to BSc and MSc degrees. His PhD work at the Centre for Atomic Molecular Physics and Quantum Optics (CEPAMOQ) was based on computer-aided design of anti-tubercular agents. He has an experience in molecular modeling and has been involved in the design and management of 3D structural databases of natural products from African flora for virtual screening. He has previously worked as Scientific Manager at the Chemical and Bioactivity Information Centre (CBIC), hosted at the Chemistry Department of the University of Buea, Cameroon. He is currently a Georg Forster postdoctoral fellow, funded by the Alexander von Humboldt Foundation, Germany, hosted by Prof. Wolfgang Sippl.

Bakoh Ndingkokhar

Bakoh Ndingkokhar is currently a trainee in cheminformatics at the Chemical and Bioactivity Information Centre (CBIC), hosted at the Chemistry Department of the University of Buea, Cameroon. He was born in Cameroon on 1987 and obtained his undergraduate degree in Chemistry after successful studies from 2009 to 2014 at the University of Buea, Cameroon. His current assignments include outsourcing data from the chemical cosmetics database and building the natural products database for Southern African medicinal plants for virtual screening purposes.

James A. Mbah

James Mbah studied chemistry at the University of Buea in Cameroon where he obtained his BSc in Chemistry and later obtained an MSc in Organic Chemistry from the University of Dschang in Cameroon. He subsequently obtained a PhD in Organic Chemistry from the University of Yaoundé I, Cameroon in 2003, under the joint supervision of Professors Pierre Tane and Bonaventure Ngadjui. Both his doctoral and postdoctoral research projects have been focused on the search for drug leads from Cameroonian medicinal plants for the treatment of tropical diseases. He is currently a lecturer/research group leader in Phytochemistry and Head of Laboratories at the Department of Chemistry, University of Buea, Cameroon.

Wolfgang Sippl

Wolfgang Sippl studied Pharmacy at the University in Berlin. He later obtained a PhD in Pharmaceutical Chemistry at the University of Düsseldorf in the group of Hans-Deiter Höltje and was a post-doctoral fellow at the Université Louis-Pasteur in Strasbourg (France) where he worked with Camille G. Wermuth. He then took a senior researcher position in Düsseldorf before moving to the University of Halle-Wittenberg as a full professor for Medicinal Chemistry in 2003. Since 2010, he has been Director of the Institute of Pharmacy in Halle. His main interests are focused on computational chemistry and structure-based drug design.

Joseph N. Yong

Joseph Yong is currently a lecturer/Head of Department in the Chemistry Department of the University of Buea. He obtained a “Licence en Chimie” from the University of Yaoundé in 1979 and pursued further studies at the University of New Orleans and the University of Rhode Island, USA where he obtained MS and PhD degrees in 1986 and 1991, respectively. He started teaching at the University of Yaoundé in 1992 and moved to the University of Buea in 1998. His present area of research focuses on organic synthesis and the use of ethnomedical knowledge to search for drug leads from plants and mushrooms in Cameroon for the remedy of neglected tropical diseases particularly, malaria, tuberculosis and onchocerciasis.

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1 Introduction

Natural products isolated from African medicinal plants have provided a significant contribution to the discovery and development of new drugs used in the treatment of various ailments; the country of South Africa alone having about 30 000 plant species, about 10% of the world's higher plants.^1–3 Moreover, the majority of the population living in the less developed world totally rely on natural products for their health-care needs.^4,5 Thus there is a need to revisit natural products as a starting point for drug discovery. Recently, natural products have provided new leads for compounds that have good pharmacokinetic profiles.⁶ Several thousands of pure secondary metabolites have been isolated from African medicinal plants but the local populations solely depend on herbs as no approved drug has been derived from this flora.^7,8 These plant isolates have been employed in African traditional medicine (ATM), thus establishing the role of natural products in drug discovery programs. Southern Africa has a rich flora and fauna which have been recognized and exploited in the past decades given that 50% out of the 25 000 plant species on the earth surface are endemic to the region.^8–12 Thus there is a growing need to establish a report focused on biologically active secondary metabolites isolated from Southern Africa which correlates with their ethnobotanical uses in ATM from this region.

Recently, our research group has been involved in the documentation of knowledge from African flora, relevant for drug discovery programs in the continent. A number of reviews have been published in internationally recognized peer-reviewed journals on bioactive natural products and the pharmacokinetic profiles of the compounds from African medicinal plants focusing on the different countries/regions.^13–21 These have covered medicinal plants from central Africa,^13,14 West Africa,¹⁵ and Northern Africa.¹⁶ Other studies have been focused on developing three dimensional (3D) databases of bioactive natural product from the entire continent for virtual screening purposes¹⁷ and assessing their drug metabolism and pharmacokinetics profiles using in silico model.^17b,18 Other review articles have been focused on molecules with the potential to be developed into drugs for particular diseases like malaria,¹⁹ antimycobacterial infections like tuberculosis²⁰ and cancer.²¹ This has received significant attention from different collaborators involved in drug discovery from medicinal plants and thus has prompted the need for the series of reviews on Southern Africa. In the first part of this review series,²² emphasis was laid on unique compound classes from Southern African flora having remarkable biological activities, stressing on establishing a correlation between biological activities of the derived compounds (alkaloids and terpenoids) and the uses of the plants in African traditional medicine and their chemotaxonomic classifications. These are secondary metabolites isolated from plants growing in the Southern Africa region, which includes the following countries; Angola, Botswana, Madagascar, Malawi, Mozambique, Namibia, South Africa, Swaziland and Zimbabwe. In the present paper, our main focus would be on flavonoids, quinones and other minor compound classes to highlight the medicinal value and potentials of the isolated phytochemicals by discussing the bioactivity of the isolated principles versus ethnobotanical uses of the plant species.

2 Flavonoids from Southern African flora

Flavonoids continue to attract attention as potentially useful agents because they exhibit a broad spectrum of biological activities, including anti-inflammatory, anti-cancercinogenic, antiviral, anti-oxidant, anti-thrombogenic and anti-atherogenic properties.^23–34 In this report, summaries of the most interesting results for flavonoids which exhibit biological activities correlating with the ethnobotanical uses of the plant species of origin have been shown in Table 1, while the chemical structures of the isolated compounds are shown in Fig. 1–4. In Table 2, the biological activities which correlate with the ethnobotanical uses of the plants have been highlighted in bold.

Table 1 Bioactivity of derived flavonoids versus ethnobotanical uses of plant species

Compounds	Plant species (Country)	Family	Ethnobotanical use	Measured Activity	References
Piliostigmol (1), 6,8-di-C-methylquercetin-3,3′,7-trimethyl ether (2), 6,8-di-C-methylquercetin-3,3′-dimethyl ether (3), 3′,6,8,-tri-C-methylquercetin-3,7-dimethyl ether (4), 6-C-methylquercetin-3-methyl ether (5), 6,8-di-C-methylkaempferol-3-methyl ether (6) and 6-C-methylquercetin-3,3′,7-trimethyl ether (7)	Piliostigma reticulatum (South Africa)	Caesalpiniaceae	Leprosy, smallpox, coughs, ulcer, heart pain, gingivitis, snake bite, dysentery, fever, wounds	Antimicrobial activity	Babajide et al.,^41
Flavokawain (8), cardamomin (9), alpinetin (10) and pinocembrin (11)	Combretum apiculatum (South Africa)	Combretaceae	Abdominal disorders, backache, bacterial infections	Antimicrobial activity	Eloff et al.,^48
5-Hydroxy-7,4′-dimethoxyflavone (12), quercetin 5,3′-dimethylether (13), rhamnazin (14), rhamnocitrin (15), genkwanin (16), apigenin (17) and kaempferol (18)	Combretum erythrophyllum (South Africa)
Catechin (19)	Euclea divinorum (Zimbabwe)	Ebenaceae	Diarrhoea, convulsions, cancer, skin diseases and gonorrhoea	Cytotoxicity activity	Mebe et al.,^52
Epicatechin (20)	Euclea undulata (South Africa)	Ebenaceae	Diabetes	Hypoglycemic activity	Deutschländer et al.,^54
4,7,2′-Trihydroxy-4′-methoxyisoflavanol (21), 5,7,3′,4′-tetrahydroxy-5′-(2-epoxy-3-methylbutyl)isoflavanone (22), 5,7,2′,4′-tetrahydroxy-8,5′-di(γ,γ-dimethylallyl)-flavanone (23), 5,7,3′-trihydroxy-4′-methoxy-5′γ,γ-dimethylallylisoflavanone (24), 5,7,2′-trihydroxy-4′-methoxy-6,5′-di(γ,γ-dimethylallyl)-isoflavanone (25), 5,7,2′,4'-tetrahydroxy-8,3′-di(γ,γ-dimethylallyl)-isoflavanone (26), derrone (27) and Bolusanthols A to C (28 to 30)	Bolusanthus speciosus (Botswana)	Fabaceae	Emetic, abdominal pains, ornamental tree	Antibacterial activity	Bojase et al.,^58,59
5,7,3′,5′′,7′′,4′′′-Hexahydroxy (4′-O-3′′′)-biflavone (31), (−)-epicatechin (20), epiafzelechin (32), dihydrokaempferol (33), quercetin (34), luteolin (35), dihydroquercetin-3′-O-glucoside (36), daidzein (37) and genistein (38)	Vangueria infausta (Botswana)	Rubiaceae	Malaria, wounds, menstrual and uterine problems, and genital swelling among others	Antiplasmodial and antimicrobial activity	Mbukwa et al.,^62
Epiafzelechin (33)	Ficus lutea (South Africa)		Treatment of diabetes	Hypoglycemic activity	Olaokun et al.^63
3,5,7-Trihydroxy-4′-methoxyflavone (39), 5,7,4′-trihydroxy-3, 6-dimethoxyflavone (40), 5,7-dihydroxy-3,6,4′-trimethoxyflavone (41), 5-hydroxy −3,7,4′-trimethoxyflavone (42) and 3,4′,5,7-tetrahydroxy flavone (kaempferol) (18)	Dodonaea viscosa (South Africa)	Sapindaceae	Sore throat, wounds, fever, piles, fever, malaria, angina, cold, arthritis, sinusitis flu, and boils, skin diseases of the head and face	Antibacterial and antioxidant activity	Teffo et al.,^66
7,3′-Dihydroxy-4′-methoxy-5′-γ,γ-dimethylallylisoflavone (erylatissin A) (43), 7,5′-dihydroxy-6′′,6′′-dimethyl-4′′,5′′-dehydropyrano[2′′,3′′:4′,5′]isoflavone (erylatissin B) (44), (−)-7, 3′-dihydroxy-4′-methoxy-5′-γ,γ-dimethylallylflavanone (erylatissin C) (45), 7,4′-dihydroxyisoflavone (daidzein) (37), 7,4′-dihydroxy-3′-γ,γ-dimethylallylflavanone (abyssinone II) (46), 7,3′-dihydroxy-4′-methoxyisoflavone (calycosin) (47), 7, 4′-dihydroxy-3′-γ,γ-dimethylallyl isoflavone (neobavaisoflavone) (48), 3,9-dihydroxy-10-γ,γ-dimethylallylpterocarpan (phaseollidin) (49), 3,6a-dihydroxy-9-methoxy-10-γ,γ-dimethylallylpterocarpan (cristacarpin) (50), 4,2′,4′-trihydroxy-3′-γ,γ-dimethylallylchalcone (51), 5,7,4′-trihydroxyisoflavone (genistein) (38), 4,2′,4′-trihydroxychalcone (52), 3,9-dihydroxypterocarpan (demethylmedicarpin) (53)	Erythrina latissima (Botswana)	Fabaceae-Papilionoideae	Dressing open wounds	Antibacterial, antifungal and radical scavenging activity	Chacha et al.,^68
Davidigenin (54)	Mascarenhasia arborescens (Madagascar)	Apocynaceae	Intestinal disorders, intestinal spasms and diarrhoea	Antispasmodic and antioxidant activities	Désiré et al.,^85

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Fig. 1 Chemical structures 1 to 20.

Fig. 2 Chemical structures 21 to 33.

Fig. 3 Chemical structures 34 to 43.

Fig. 4 Chemical structures 44 to 54.

Table 2 Bioactivity of derived quinones versus ethnobotanical uses of plant species

Isolated metabolites	Plant species (Country)	Family	Ethnobotanical use	Measured activity	References
Isopinnatal (55)	Kigelia pinnata (Zimbabwe)	Bignoniaceae	Skin cancer	Cytotoxic activity	Jackson et al.,^86
Shinanolone (56), 7-methyljuglone (57) and diospyrin (58)	Euclea natalensis	Ebenaceae	Toothache, chest complaints and headache	Antimycobacterial activity	Bapela et al.,^88
Aloeresin A (59), aloesin (60), aloin A (61) and aloin B (62)	Aloe ferox (South Africa)	Xanthorrhoeaceae	The plant possesses laxative and cathartic effects	Not tested	Kanama et al.,^89

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Piliostigma reticulatum (Caesalpiniaceae) is a leguminous plant that is used in ethnomedicines for the treatment of leprosy, smallpox, coughs, dysentery, fever, wounds and a variety of closely related disease conditions.^35–40 Babajide et al. isolated the four novel compounds; piliostigmol (1), 6,8-di-C-methylquercetin-3,3′,7-trimethyl ether (2), 6,8-di-C-methylquercetin-3,3′-dimethyl ether (3), 3′,6,8-tri-C-methylquercetin-3,7-dimethyl ether (4) together with the known compounds; 6-C-methylquercetin-3-methyl ether (5), 6,8-di-C-methylkaempferol-3-methyl ether (6) and 6-C-methylquercetin-3,3′,7-trimethyl ether (7) from the leaves of this plant.⁴¹ Compound 1 demonstrated the highest antibacterial activity against E. coli (MIC = 2.57 μg mL⁻¹, 0.006 μmol), which is three times more active than amoxicillin and therefore, validates the traditional use of the plant.

Traditional healers in Eastern and Southern Africa have used Combretum species, for many applications including treating abdominal disorders, backache and several other diseases.^42–46 Antioxidant-directed fractionation led to the isolation of four antioxidant compounds; flavokawain (8), cardamomin (9) alpinetin (10) and pinocembrin (11) from ethyl acetate and butanol soluble fractions of the leaf extracts of Combretum apiculatum (Combretaceae).^47,48 Compounds 8, 10 and 11 demonstrated antibacterial activities on Staphylococcus aureus; Enterococcus aureus; Pseudomonas aeruginosa and Escherichia coli as described.^47,49

Martini et al. isolated 5-hydroxy-7,4′-dimethoxyflavone (12), quercetin-5,3′-dimethylether (13), rhamnazin (14), rhamnocitrin (15), genkwanin (16), apigenin (17) and kaempferol (18) from Combretum erythrophyllum using bioassay-guided fractionation.⁵⁰ The compounds demonstrated antibacterial activities on Staphylococcus aureus; Enterococcus aureus; Pseudomonas aeruginosa and Escherichia coli as described.^47,49 The activities of the isolated compounds correlate with the ethnomedicinal use of the plant in traditional medicines.

Euclea divinorum (Ebenaceae) root bark is used in traditional medicine for the treatment of diarrhoea, convulsions, cancer, skin diseases and gonorrhoea.⁵¹ Mebe et al. isolated the known flavonoids catechin (19) together with some known compounds from the chloroform extract of this plant.⁵² The isolated compounds were tested for their cytotoxic activity (ED₅₀ < 20 μg mL⁻¹) against a panel of cell lines using cell culture systems as described.⁵³ The cytotoxic activity displayed by some of the compounds confirms the ethnomedicinal use of the plant in the treatment of cancer.

Euclea undulata (Ebenaceae) is used by traditional healers in the Venda area, Limpopo Province in the treatment of diabetes. Deutschländer et al. isolated epicatechin (20) in addition to some other compounds from the crude acetone extract of the root bark of this plant.⁵⁴ The isolated compounds were evaluated for their hypoglycemic activities by executing in vitro assays on myocytes, as well as their ability to inhibit the carbohydrate hydrolysing enzyme α-glucosidase.⁵⁵ Compound 20 may have some ability to lower blood glucose levels. The hypoglycemic activity exhibited by the compound confirms the use of the plant in traditional medicines.

Bolusanthus speciosus (Fabaceae), also known as tree wisteria, is used as an ornamental tree in gardens and parks, because of its beauty.⁵⁶ The root infusion of this plant is also used by some communities as an emetic while the dried inner bark is used to relieve abdominal pains.⁵⁷ The dried inner bark of this plant has been used to relieve abdominal pains, emitism and tuberculosis.⁵⁸ Bojase et al. isolated the two new isoflavonoids: 4,7,2′-trihydroxy-4′-methoxyisoflavanol (21), 5,7,3′,4′-tetrahydroxy-5′-(2-epoxy-3-methylbutyl)isoflavanone (22) together with the known compounds 5,7,2′,4′-tetrahydroxy-8,5′-di(γ,γ-dimethylallyl)-flavanone (23), 5,7,3′-trihydroxy-4′-methoxy-5′-γ,γ-dimethylallylisoflavanone (24), 5,7,2′-trihydroxy-4′-methoxy-6,5′-di(γ,γ-dimethylallyl)isoflavanone (25), 5,7,2′,4′-tetrahydroxy-8,3′-di(γ,γ-dimethylallyl)isoflavanone (26) and derrone (27) from the combined ethyl acetate/methanolic extracts of the stem bark of this plant.⁵⁸ The authors further isolated the flavonoids bolusanthols A to C (28 to 30), along with 4 known flavonoids from the stem bark of the same plant species.⁵⁹ Compound 22 showed moderate activity against gram positive bacteria and weak activity against gram negative bacteria, while compound 21 was weakly active against both organisms in a TLC bioautography assay. The results are consistent with the traditional use of the plant in treatment of abdominal pains, associated mostly with bacterial infections.

Vangueria infausta (Rubiaceae) fruits are eaten by humans and wild animals and also used traditionally for the treatment of malaria, wounds among others.^60,61 Mbukwa et al. isolated the new biflavonoid 5,7,3′,5′′,7′′,4′′′-hexahydroxy(4′-O-3′′′)-biflavone (31) together with the known compounds (−)-epicatechin (20), epiafzelechin (32), dihydrokaempferol (33), quercetin (34), luteolin (35), dihydroquercetin-3′-O-glucoside (36), daidzein (37) and genistein (38) from aerial parts of Vangueria infausta (Rubiaceae).⁶² Compound 31 showed higher radical scavenging activity against 2,2-diphenyl-1-picrylhydrazyl (DPPH) reagent compared to ascorbic acid (standard) using a spectrophotometric method. But compound 31 was less sensitive to Gram-positive and Gram-negative bacterial strains and yeast (Candida mycoderma) compared to 34 and 35 on the Bioautographic Agar Overlay Assay. Compounds 20 and 32 were found to be active against E. coli at minimum loading of 50.0 and 100.0 μg, respectively. Moreover, the hypoglycemic activity of acetone leaf extract of Ficus lutea (Moraceae) could be partly explained by the presence of compound 32.⁶³

Dodonaea viscosa (Sapindaceae) leaves have traditionally been administered to treat sore throat, wounds, fever, piles, boils and other diseases.^64,65 Teffo et al. isolated the five known flavonoids; 3,5,7-trihydroxy-4′-methoxyflavone (39), 5,7,4′-trihydroxy-3,6-dimethoxyflavone (40), 5,7-dihydroxy-3,6,4′-trimethoxyflavone (41), 5-hydroxy-3,7,4′-trimethoxyflavone (42) and 3,4′,5,7-tetrahydroxy flavone (kaempferol) (18) from the leaves of this plant using bioassay guided fractionation.⁶⁶ Compounds 39 and 18 demonstrated antioxidant activity (EC₅₀ = 75.49 ± 1.76 μM and 35.06 ± 0.85 respectively) but lower than L-ascorbic acid (EC₅₀ = 13.55 ± 0.28 μM) used as a standard antioxidant agent. Compound 18 was in general the most active against all the test organisms with MIC values between 16 and 63 μg mL⁻¹ compared to the others.⁶⁶

Erythrina latissima (Fabaceae-Papilionoideae) stem and roots are burnt and used for dressing open wounds.⁶⁷ Chacha et al. isolated the three new flavonoids 7,3′-dihydroxy-4′-methoxy-5′-γ,γ-dimethylallylisoflavone (erylatissin A) (43), 7,5′-dihydroxy-6′′,6′′-dimethyl-4′′,5′′-dehydropyrano[2′′,3′′:4′,5′]isoflavone (erylatissin B) (44), (−)-7,3′-dihydroxy-4′-methoxy-5′-γ,γ-dimethylallylflavanone (erylatissin C) (45),⁶⁸ together with the ten known compounds; 7,4′-dihydroxyisoflavone (daidzein) (37), 7,4′-dihydroxy-3′-γ,γ-dimethylallylflavanone (abyssinone II) (46), 7,3′-dihydroxy-4′-methoxyisoflavone (calycosin) (47), 7,4′-dihydroxy-3′-γ,γ-dimethylallylisoflavone (neobavaisoflavone) (48), 3,9-dihydroxy-10-γ,γ-dimethylallylpterocarpan (phaseollidin) (49), 3,6a-dihydroxy-9-methoxy-10-γ,γ-dimethylallylpterocarpan (cristacarpin) (50), 4,2′,4′-trihydroxy-3′-γ,γ-dimethylallylchalcone (51), 5,7,4′-trihydroxyisoflavone (genistein) (38), 4,2′,4′-trihydroxychalcone (52), 3,9-dihydroxypterocarpan (demethylmedicarpin) (53) from the stem wood of this plant.^69–75 Compounds 45, 50, 51 demonstrated the highest antimicrobial activity in vitro against Escherichia coli, Staphylococcus aureus, Bacillus subtilis and Candida mycoderma using the procedures described.^76–84 Compounds 37, 49, 51 were the most active towards radical scavenging properties using DPPH test.^91–93

Désiré et al. isolated the dihydrochalcone, davidigenin (54) as the main active constituent using bioassay-guided fractionation from Mascarenhasia arborescens (Apocynaceae).⁸⁵ The antispasmodic activity demonstrated by compound 54 supports the use of the plant in traditional medicine in the treatment of intestinal spasms.

3 Quinones from Southern African flora

The summary of the most important findings on the bioactive quinones from Southern Africa flora have been given in Table 2, while the chemical structures are shown in Fig. 5. In Table 2, the biological activities which correlate with the ethnobotanical uses of the plants have been highlighted in bold.

Fig. 5 Chemical structures 55 to 62.

Kigelia pinnata (Bignoniaceae) juice or water extracts of the fruits or stem bark has been used in the treatment of skin cancer. Jackson et al. isolated the known compounds norviburtinal and isopinnatal (55)⁸⁶ from the crude dichloromethane extracts using bioassay-guided fractionation.⁸⁷ The results revealed that norviburtinal showed a much greater cytotoxic effect (IC₅₀ = 3.25 μg mL⁻¹), but little selectivity towards melanoma cell lines. Compound 55 (IC₅₀ = 11.13 μg mL⁻¹) demonstrated slightly greater cytotoxic activity against the melanoma cell lines but its high cytotoxicity against the non-cancer fibroblasts warrants further investigation as a novel lead anticancer agent. The cytotoxic activity of the crude extracts and pure compounds of the plant corroborates its medicinal use in the treatment of skin cancer.

Bapela et al. isolated shinanolone (56), 7-methyljuglone (57) and diospyrin (58) from the shoots, roots and seeds of Euclea natalensis (Ebenaceae).⁸⁸ The roots of this plant are used to relief toothache, headache and chest complaints amongst other uses.⁸⁹ The three naphthoquinones; shinanolone (56), 7-methyljuglone (57) and diospyrin (58) demonstrated significant activity against drug-sensitive and drug-resistant strains of Mycobacterium tuberculosis and lends credence to the ethnomedicinal use of the plant.⁹⁰

Ultra-high performance liquid chromatography coupled to mass spectrometry (UHPLC-MS) was investigated as an ultrafast, accurate, and sensitive method in the quantification of the main compounds in 101 Aloe ferox exudates, harvested in South Africa.⁹¹ The main compounds were shown to be the chromones; aloeresin A (59), aloesin (60) and the anthrones aloin A (61) and aloin B (62). Even though the identified compounds have not been tested, this plant is known to possess laxative and cathartic effects.

4 Other compound classes from Southern African flora

Table 3, is a summary of other bioactive compounds isolated from Southern African flora. The biological activities which correlate with the ethnobotanical uses of the plants have been highlighted in bold.

Table 3 Bioactivity of other derived compounds versus ethnobotanical uses of plant species

Isolated metabolites	Plant species (Country)	Family	Ethnobotanical use	Measured activity	References
Scopoletin (63)	Artemisia afra (South Africa)	Asteraceae	In southern Africa it is used to treat coughs, colds, diabetes, malaria, sore throat, asthma, headache, dental care, gout and intestinal worms	Antimicrobial properties	More et al.,^92 van Vyk and Gerick^93
2-Butanoyl-4-prenyl-1-methoxy phloroglucinol (64), 2-(2-methylpropanoyl)-4-prenylphloroglucinol (65), 2-(2-methyl-butanoyl)-4-prenylphloroglucinol (66)	Helichrysum paronychioides (Botswana)	Asteraceae	Constipation, coughs and analgesic	Antibacterial, antifungal, antiviral and antioxidant activities	Mutanyatta-Comar et al.,^94
Helihumulone (67)	Helichrysum cymosum (South Africa)	Asteraceae	Respiratory ailments and wound infections, malaria	Antimicrobial and antimalarial activity	van Vuuren et al.,^101
Drimenin or 5,10-dihydro-6,7-dimethyl-4H-benzo[5,6]cyclohepta[1,2-b]-furan (68)	Warburgia salutaris (South Africa)	Canellaceae	Yeast, fungal, bacterial and protozoal infections, expectorant and smoked for coughs and colds	Antimicrobial activity	Mohanlall et al.,^103
5-(Hydroxy methyl)furan-2(5H)-one (69) and 5-(hydroxymethyl)dihydrofuran-2(3H)-one (70)	Knowltonia vesicatoria (South Africa)	Ranunculaceae	Tuberculosis	Antimycobacterial activity	Labuschagné et al.,^104
Combretastatin B5 (71)	Combretum woodii (South Africa)	Combretaceae	Treatment of syphilis, abdominal pains, conjunctivitis, diarrhoea and toothache, among other ailments	Antimicrobial activities	Eloff et al.,^108
Goudotianone 1 (72), goudotianone 2 (73), 1,3,7-trihydroxy-2-isoprenylxanthone (74)	Garcinia goudotiana (Madagascar)	Clusiaceae	Antiparasitic, antitussive and antimicrobial properties	Antimicrobial and cytotoxic activity	Mahamodo et al.,^109
Uguenenazole (75) and uguenenonamide (76)	Vepris uguenensis	Rutaceae	Malaria	Antimalarial activity	Cheplogoi et al.,^110
Lippialactone (77)	Lippia javanica (South Africa)	Verbenaceae	Influenza, measles, rashes, stomach problems, headaches	Antiplasmodial activity	Ludere et al.,^114
Cassinidin A (80) and cassinidin B (81)	Cassia abbreviate (Botswana)	Caesalpinioideae	Bilharzias, skin diseases, cough, pneumonia, fever, gonorrhea, abdominal pains, headaches and snakebites, water fever and heart diseases, dysentery, diarrhoea, severe abdominal pain and toothache, oral and vaginal candidiasis particularly in HIV/AIDS patients	Antibacterial and antifungal activity	Erasto et al.,^123,125
Combretastatin B5 (71)	Combretum woodii (South Africa)	Combretaceae	Syphilis, abdominal pains, conjunctivitis, diarrhoea and toothache	Antibacterial activity	Eloff et al.,^128
(7E)(8,2′)-3,7,9,5′,9′-Pentahydroxy-4,4′-dimethoxyneolign-7-ene (76) and (9E,11Z)14-hydroxyoctadecan-9,11-dienoic acid (82)	Erythrina lysistemon (Botswana)	Leguminosae	Antiviral, anticancer and cytotoxic activities	Anti-microbial and antifungal activities	Juma et al.,^129
4-Ethyl-nonacosane (83)	Lippia javanica (Mozambique)	Verbenaceae	Influenza, measles, rashes, malaria, stomach problems, fever, colds, cough, headaches	Antitubercular and anti-HIV activity	Mujovo et al.,^132

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The coumarin scopoletin (63), along with betulinic acid and acacetin have been reported to three most active components of the ethanol extract of Artemisia afra (Asteraceae), attributable to the antimicrobial activity of the aforementioned plant extract.⁹² This could partly justify the uses of this plant in traditional medicine in Southern Africa, like in the treatment of coughs, colds, diabetes, malaria, sore throat, asthma, headache, dental care, gout and intestinal worms.⁹³

Mutanyatta-Comar et al. isolated the phloroglucinol derivatives; 2-butanoyl-4-prenyl-1-methoxy phloroglucinol (64), 2-(2-methylpropanoyl)-4-prenylphloroglucinol (65), 2-(2-methyl-butanoyl)-4-prenylphloroglucinol (66),⁹⁴ which are known to exhibit antibacterial, antifungal, antiviral and antioxidant activities.^95–100 The compounds were screened for antioxidant activity against Cu-induced low density lipoprotein (LDP) oxidation. The results showed that compound 67 was found to be the most active inhibiting LDL oxidation at all concentrations (0.5–10 μM) while compounds 64 and 65 showed moderate activities. The activities of the isolated compounds validate the ethnomedicinal uses of Helichrysum paronychioides (Asteraceae).

van Vuuren et al. isolated the known phloroglucinol derivative, helihumulone (67) from the aromatic plant Helichrysum cymosum (Asteraceae) used in traditional medicine to treat respiratory ailments, malaria, wound infections and other tropical diseases.^101,102 Compound 67 was active in inhibiting the growth of the malaria parasite (IC₅₀ = 14.89 μg mL⁻¹) even though it was toxic. The antimicrobial and antimalarial activities demonstrated by helihumulone (67) confirm that of the crude extracts of the plant.

Warburgia salutaris (Canellaceae) stem and leaves have been used to treat bacterial infections and the bark of this plant is smoked for coughs and colds amongst other uses. Previous screening of this plant demonstrated promising antibacterial activity which supports its use in traditional medicine. Mohanlall et al., using bioassay-guided fractionation, isolated the active antimicrobial agents drimenin, 5,10-dihydro-6,7-dimethyl-4H-benzo[5,6]cyclohepta[1,2-b]-furan (68) from the stem bark of this plant.¹⁰³ Labuschagné et al. isolated 5-(hydroxy methyl) furan-2(5H)-one (69) and 5-(hydroxymethyl)dihydrofuran-2(3H)-one (70) from Knowltonia vesicatoria (Ranunculaceae) which demonstrated antimycobacterial activity.¹⁰⁴ The activity validates the traditional use of the plant in the treatment of tuberculosis. Compound 70 was active against drug-sensitive M. tuberculosis with an MIC of 50.0 μg mL⁻¹.

Species of the two main genera, Combretum and Terminalia from Southern Africa have been used in the treatment of syphilis, abdominal pains, conjunctivitis, diarrhoea and toothache, among other ailments.^105–107 The stilbene 2′,3′,4-trihydroxyl-3,5,4′-trimethoxybibenzyl (combretastatin B5, 71) has been isolated from Combretum woodii (Combretaceae) leaves.¹⁰⁸ Compound 66 showed significant activity against S. aureus with an MIC of 16 μg mL⁻¹ but with lower activity towards P. aeruginosa (125 μg mL⁻¹), E. faecalis (125 μg mL⁻¹) and slight activity against E. coli (Fig. 6).¹⁰⁸

Fig. 6 Chemical structures 63 to 71.

The prenylated benzoylphloroglucinol derivatives, goudotianone 1 (72) and goudotianone 2 (73), the xanthone, 1,3,7-trihydroxy-2-isoprenylxanthone (74), along with other compounds have been isolated from the leaves of Garcinia goudotiana from Madagascar.¹⁰⁹ This plant is used traditionally for its antiparasitic, antitussive and antimicrobial properties. The compounds displayed a high antimicrobial activity against some Gram-positive bacteria with a moderate cytotoxicity.^110,111 The anti-microbial properties of the isolated compounds were assessed against Gram-positive bacteria, in particular Staphylococcus lugdunensis, Enterococcus faecalis and Mycobacterium smegmatis. The three compounds all demonstrated moderate or high selective significant antimicrobial activities against the tested species. In particular, compound 72 was very active against E. faecalis C159-6 (MIC = 39 μg mL⁻¹), whereas compound 68 showed a high activity against S. lugdunensis T29A3 (MIC = 39 μg mL⁻¹). The compound 1,3,7-trihydroxy-2-isoprenylxanthone (74) was very active against the three strains with an MIC inferior to 100 μg mL⁻¹. By contrast, the three compounds showed cytotoxicity more moderate than the extracts. The prenylated xanthone (74) was the least cytotoxic compound.¹⁰⁹

Vepris uguenensis (Rutaceae) is used traditionally in the treatment of malaria in Kenya.¹¹⁰ Species of the same genus have found applications in ethnomedicines.^111–113 Cheplogoi et al. isolated the novel compounds, methyl uguenesonate, uguenenazole (75) and uguenenonamide (76) from the root of this plant. Methyl uguenesonate showed mild antimalarial activity against 3D7 (chloroquine susceptible, CQS) and FCM29 (chloroquine resistant, CQR) strains of Plasmodium falciparum. It was found that while compounds 75, 76 was completely inactive against both strains of the parasite, methyl uguenesonate displayed mild activity, with IC₅₀ values of 10.4 ± 4.4, 29.2 ± 3.2 against the CQS and CQR strains, respectively.¹¹⁰

Ludere et al. isolated the new antimalarial α-pyrone, lippialactone (77) from the aerial parts of Lippia javanica (Verbenaceae). This plant is used in South Africa against various chest ailments, influenza, headaches.¹¹⁴ Compound 77 was active against the chloroquine sensitive D10 strain of Plasmodium falciparum with an IC₅₀ value of 9.1 μg mL⁻¹, and is also mildly cytotoxic.

Several Senna species (Leguminosae-Caesalpinioideae) are used as purgatives or laxatives depending on the dose.¹¹⁵ The acetone fraction of Senna singueana stem bark from South Africa have demonstrated anti-diabetic effects in a rat model against type 2 diabetes.¹¹⁶ This plant species is also used traditionally in some parts of Ethiopia, for the treatment of a form of skin cancer.¹¹⁷ Moreover, the inner bark of the plant is chewed fresh to soothe stomach spasm and smoke from the wood and bark is used as smoke baths.¹¹⁷ A recent study also showed that ethanol extracts of S. singueana from Ethiopia have both in vitro and in vivo antimalarial properties,¹¹⁸ in addition to the previously known in vitro radical scavenging activity.¹¹⁹ Studies carried out on sister species have led to the isolation of the stilbene 3,4′,5-trihydroxystilbene (resveratrol, 78) from S. italica, a plant used traditionally in the northern parts of the Limpopo province of South Africa for the treatment of STIs,^120,121 as well as D-pinitol (79) from Senna versicolor¹²² and other compounds like alkaloids, quinines and anthraquinones.¹²⁰ A recent study by Scherzberg et al. has shown that structural modification of resveratrol (78) leads to increased anti-tumor activity, but causes profound changes in the mode of action.^120b This is particularly the case with the analog (Z)-3,5,4′-trimethoxystilbene (Z-TMS). This analog has shown increased antiproliferative activity towards a number of cancer cell lines compared to resveratrol, which has been shown to inhibit tubulin polymerization in vitro. Cell growth inhibition was determined with IC₅₀ values for Z-TMS between 0.115 μM and 0.473 μM (resveratrol: 110.7 μM to 190.2 μM). Moreover, resveratrol derivatives have demonstrated potent inhibitory properties against influenza H1N1 neuraminidase.^120c

Erasto et al. isolated the two novel proanthocyanidins; cassinidin A (80) and cassinidin B (81) from the root bark of Cassia abbreviata (Caesalpinioideae) used in the treatment of various ailments.¹²³ Compounds 80 and 81 demonstrated moderate to high antimicrobial activity against both Gram-positive and Gram negative bacteria which validates the in vitro activity of the crude extract and the ethnomedicinal use of the plant.^124–127

Crude extracts from Combretum woodii (Combretaceae) have demonstrated antibacterial activity with MIC values in the order of 0.04 mg mL⁻¹.¹²⁸ Eloff et al. isolated the stilbene; 2′,3′,4-trihydroxyl-3,5,4′-trimethoxybibenzyl (combretastatin B5) (71) from the leaves of this plant. Compound 81 displayed a significant activity against S. aureus with an MIC of 16 mg mL⁻¹ which confirms the use of the species in the treatment of abdominal pains, toothache, and syphilis among other ailments (Fig. 7).⁴³

Fig. 7 Chemical structures 72 to 81.

Juma et al. isolated the new compounds; (7E) (8,2′)-3,7,9,5′,9′-pentahydroxy-4,4′-dimethoxyneolign-7-ene (82) and (9E,11Z) 14-hydroxyoctadecan-9,11-dienoic acid (84) from Erythrina lysistemon (Leguminosae).¹²⁹ Compound 84 showed quite appreciable activity against the Gram-positive bacteria Bacillus subtilis which correlates with the use of the extracts from this plant in traditional medicine and the antimicrobial activities of the isolated compounds.^130,131 Mujovo et al. isolated the long chain alkane; 4-ethyl-nonacosane (83) from Lippia javanica (Verbenaceae).¹³² Infusions of the leaves of this plant are commonly used in Africa as a tea against various ailments like malaria, cough, and headaches among other diseases.^133,134 The known triterpenoid euscaphic acid, isolated from this plant, displayed a minimum inhibitory concentration of 50 mg mL⁻¹ against a drug-sensitive strain of Mycobacterium tuberculosis which validates the use of the plant in ethnomedicine (Fig. 8).

Fig. 8 Chemical structures 82 to 84.

5 Conclusions

In this review, we have presented an overview of the results of biological activities of selected NPs (flavonoids, quinones and minor compound classes) isolated from plants used in traditional medicine in Southern Africa (covering 10 countries). The plant sources, geographical collection sites and chemical structures of pure compounds were retrieved from literature sources comprising data collected from articles from major peer-reviewed journals, MSc and PhD theses from university libraries within the region spanning the period 1971 to 2015. We also used the author queries in major natural product and medicinal chemistry journals. The report does not claim to be exhaustive. The goal has been to document the baseline knowledge and lay the foundation for subsequent investigations.

The collected data includes plant sources, uses of plant material in traditional medicine, plant families, region of collection of plant material, isolated metabolites and type (e.g. flavonoid, terpenoid, etc.), and measured biological activities of isolated compounds (as commented in the literature). The study has provided a survey of the biological activities of compounds derived from Southern African flora versus the ethnobotanical uses of the plant species from which the compounds have been isolated. This series of reviews dedicated to Southern African flora is also intended to give an in depth coverage of the chemotaxonomy of the flora of Southern African and a cheminformatics analysis of the derived natural products. In this study, 864 secondary metabolites have been identified from 101 plant species from 57 plant families. Only the most interesting compounds have been discussed in this review series. The rest of the compounds have been included in the database of NPs from Southern Africa, which is under development.

From the data presented in Tables 1–3, the biological activities of 62 out of the 117 plant metabolites indicated in the text could be used to validate the ethnobotanical uses of the plant species. The chemical structures of the secondary metabolites could be further modified to ameliorate biological activity and virtual screening methods could be used to enhance drug discovery by docking some of the compounds towards specific drug target sites. This first part focuses on alkaloids and terpenoids²² and the second part on flavonoids, quinines and other minor compound classes. The part III of this series of review would be focus on the cheminformatics analysis of the derived natural products.

Acknowledgements

Financial support is acknowledged from Lhasa Ltd, Leeds, UK through the Chemical and Bioactivity Information Centre (CBIC), University of Buea, Cameroon. Ms Irene N. Mukoko (Department of Chemistry, University of Buea) assisted in the data analysis. FNK acknowledges a Georg Forster fellowship for postdoctoral researchers from the Alexander von Humboldt Foundation.

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Footnote

† These authors contributed equally.

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