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Recent advances in the flavonoid chemistry of african plants
Pergamon
0031-9422.(94)EO19&W
Phytcxhemwry.
Vol. M. No. 5. pp. 1109-1115. 1994 Ellcncr Saena Lid Printed in Gilt Lkitaia 0031.9422/9437.00+0.al
REVIEW ARTICLE NUMBER 90 RECENT
ADVANCES IN THE FLAVONOID PLANTS NABIEL A. M.
CHEMISTRY
OF AFRICAN
SALEH
National Research Centre, El-Dokki, Cairo, Egypt
(Received 10 January 1994) IN HONOUR
OF PROFESSOR
RAGAI K. IBRAHIM’S SIXTY-FIFTH
BIRTHDAY
Key Word Index-African
proanthocyanidins;
plants; flavonoids; flavones; flavonols; chalcones; flavanones; flavan and biflavonoids; isoflavonoids; review.
Abstract-Recent reports of flavonoids from African plant species are reviewed with emphasis on newly described structures and including all the major flavonoid classes.
In a review of the flavonoids occurring in plants of African origin covering the period of 19651990, all common and new structures were recorded and tabulated [ 11. In the present article, an updating of recent information of flavonoids reported in plants of African origin is outlined, particularly newly reported structures. FLAVONES African plants are the main source of the so-called llavonoids’, which are 7- or 5,7-oxygenated compounds with unsubstituted B rings and a prenyl unit at C-8. Thirteen flavonoids belong to this class [l-8]. In Tephrosia praecans (Leguminosae) a new angular isomer of isopongaflavone has been characterized [9]. Recently, a compound with the rare 2’,4’,5’-B-ring oxygenation pattern 5,5’-dihydroxy-3,7,2’,4’-tetramethoxyflavone was isolated from the heartwood of Chlorophora excelsa (Moraceae) [lo]. This compound is isomerit to 5,2’-dihydroxy-3,7,4’,5’-tetramethoxytlavone previously isolated from Distemonanthus benthamianus (Leguminosae) [l 11, while Hoslundia opposita (Lamia ceae) was found to be the source of a series of rate 6-Cpyrone-substituted 5,7-dimethoxyflavones [12, 133, in which the 6-C-pyrone substituent is attached via a 6.5” or 6,6”-link (see oppositin, 1). Novel flavone glycosides reported in African plants include the 2”-glucosides and 2”-glucuronides of apigenin and luteolin 7-lactate from Marrubium oulgare (Labiatae) [14]. Two polyglycosides encountered in nature for the first time are hypolaetin 7-[6”‘-acetylallosyl( I -+2)-3”acetylglucoside] from Stachys aegyptiaca (Labiatae) [IS] and acacetin 7-0-[rhamnosyh 1+4)glucosyl( 1+6”)-6”‘0-acetylsophoroside] from Peganum harmala (Zygophyl‘Tephrosia
laceae) [16]. A new acetylated triglycoside was recently isolated from P. harmala and identified as acacetin 7-0[glucosyl(l -&“)-3”‘-0-acetylneohesperidoside] together with the novel acacetin 7-rhamnoside [17]. Among the C-glycosylflavonoids only common monoand di-C-glycosylflavones have been reported from African plants [l]. and only one new compound has been identified: luteolin 7,4’-dimethyl ether 6-C-glucoside from Parkinsonia acufeata (Leguminosae) [ 183. Similarly, there are only four new reports of flavone C-glycoside-Oglycosides. These include the 2”-0-xyloside and 2”-Oglucoside of isoswertisin from Gnetum species (Gnetaceae) [19] and the 3’-0-glucoside and 3’-0-gentiobioside of swertiajaponin from Phragmites australis (Gramineae)
lm. FLAVONOLS
Flavonols are the largest class of flavonoids, with 98 structures reported in African plants, 22 of which were reported for the first time in nature Cl]. Although most of the common flavonol aglycones have been detected in African plants, no flavonols containing isoprenoid, furano and pyrano groups have been reported so far. However, Alluaudia species (Didiereaceae) are the source of a series of unusual C-methylflavonols, with a total of 11 novel compounds [21-261. For example, in A. dumosa two novel pairs of enantiomers based on 3-methoxyflavones bearing a dihydroxystilbene in ring B have been characterized (see 2 and 3) [25], while another new flavonol, 3-hydroxy-4’-methoxyflavone has been reported from the flowers of Millettia zechiana (Leguminosae) and its structure proved by synthesis [273.
1109
N.
1110
A. M.
SALEH
OH
OH 1 Oppositin
2
(-)-Androyol
3
(-)-6-C-Methylandroyol
4 Schdflain
Of the 129 flavonol glycosides isolated from African plants, 34 are novel [I]. Several sulphated flavonols have been reported in African genera e.g. lphiona (Compositae) [28,29], Malca (Malvaceae) [30,31] and Tumarix (Tamaricaceae) [32-341. Novel structures include the 4”-pcoumaroyl derivatives (both cis and truns) of kaempferol and quercetin 3-robinobiosides and 3-robinobioside-7glucoside isolated from Strychnos oariobilis (Loganiaceae) [35,36]. Similarly, isorhamnetin 3-0-CC’-p-coumaroylrhamnosyl( 1-+6)galactoside] was isolated for the first time from Aerua jaounicu (Amaranthaceae) [37], and recently, 6”-[2-E-butenoyl]isorhamnetin 3-0-glucoside was isolated from Zygophyllum simplex (Zygophyllaceae)
C381. CHALCONES
0
AND DIHYDROCHALCONES
African plants contain a variety of chalcone and dihydrochalcone structures. Thus, besides the commonly encountered structures [I], a number of novel chalcones have been reported: 4,2’,6’-trihydroxy-4’-prenyloxychal-
5
and enantiomer (+)-androyol. and (+)-ensntiomer,
R=H R = Me
Isoschefflerin
cones from Helichrysum athrixifolium (Compositae) [39], 4,2’,4’-trihydroxy-3’-C-methylchalcone from Crinum augusturn (Liliaceae) [40], 4’-0-geranylisoliquiritigenin from Milletia ferruginea (Leguminosae) [41] and praecanosone-A and praecanosone-B from Tephrosia praecans (Leguminosae) [9, 421. Two further unique chalcones, schefilerin (4) and isoschefflerin (5) have been isolated from Uuariu schefleri (Annonaceae) [43], which are believed to be biogenetically derived from a Diels-Alder-like cyclization between a chalcone and the monoterpene ,!I-ocimene. Indeed, b’vuriu species have been the source of several new compounds. For example, a series of mono-C-substituted dihydrochalcones bearing a 2-hydroxybenzyl function at position 3’ or 5’ have been reported from U. angolensis C44-463, together with the 3’,5’-di-C-substituted derivative diuvarctin and the cyclized compound chamuvaritin [45, 461. Also, two novel dihydrochalcones, triuvaretin (6) and isotriuvaretin (7) were recently identified from the roots of U. leptocludon [47]. These were found to be dihydrochalcones with three 0-hydroxybenzyl moieties in each molecule. A novel
1111
Flavonoid chemistry of African plants
HO
HO
6 l’nuvaretin
7
Isotriuvareun
OH
HO
8
Dichamanetin
flavene was isolated from U. dependens accompanied by small amounts of a corresponding chalcone which appears to be an artifact formed by oxidation of the flavene [48]. Recently, the first member of the flavonoid-related but-Zenolides was isolated from the heartwood of Pericopsis elata (= Afiormosia elata) (Leguminosae) [49].
9
Sigmoidin G
been reported from Uuariu chatnae (e.g. see dichamanetin, 8) [56,57]. A new dihydroflavonol, eriotrinol, and a new prenylated flavanone, sigmoidin-G (9). were recently isolated from the stem bark of Eryrhrinu eriotricha and E. siymoidea, respectively [SS].
FI.AVANS AND PROAYTHOCYANIDINS FLAVANONES
The flavanones reported from African plants include all the classes so far encountered in nature. 0-Prenylation and C-prenylation are common features of flavanones. The first occurrence of a 5,7-dihydroxyflavanone with 70-prenyl, 6,8-di-C-prenyl, and 8-C-prenyl-7-0-prenyl substitution has been described in Helichrysum species of African origin [39, SO]. Similarly, Erythrina siymoidea (Leguminosae) is the source of a series of flavanones (sigmoidin-A-F) with C-prenylation or additional pyrano groups in ring B [Sl -551. As in the case of the dihydrochalcones, a series of flavanones C-substituted at position 6 and/or 8 with a 2-hydroxybenzyl group have
According to Porter [59] flavans include Bavans, flavan-3-ols, flavan-Cols, Ravan-3,4-dials, flav-3-ene and peltogynoid flavans. Flavans reported for the first time from African plants are: 4’-hydroxy-5,7-dimethoxy-%Cmethylflavan (tlavan) from Puncratium maritimum (Liliaceae) [6O]; robinetinidol-3-0-gallate (flavan-3-01) from Burkea ufricuna (Leguminosae) [61]; hildgardtol-A, -B and their 4-methoxyderivatives (flavan4ols) from Tephrosia hildebrandtii [62]; hildgardtene (flav-3-ene) from T. hildebrandtii [62] and 5,7,8-trimethoxyflav-3-e.ne from Uwrio dependens [48]. A peltogynan previously isolated from Cassine transraelensis (Celastraceae) [63] and C. habe [64], was isolated also from C. papillosa [65]
N. A. M. SALEH
OH OH
0
OR
10
11 Calodenone.R = Me
Bongosin
12 IDphironcA.
13
R=H
Enotriwhin
and its absolute contiguration established as 11.1 ldimethyl-1,3.8,10-tetrahydroxy-9-methoxypeltogyn~~n. African plants are the source of several proanthocyanidins. Of interest are a series of guibourtinidol+atechin or epicatechin dimers in which catcchin or epicatechin are carboxylated at C-6 or C-8 from Acacia luederiti-ii (Lcguminosae) [66]. Also a series of flavan-3-01s condensed to stilbenes has been reported from the heartwood of Guihourfio wleospertna (Leguminosae) [67], while a proanthocyanidin based on an isoflavan was recorded in Dalhrryitr nitiduln (Leguminosae) [68]. The heartwood of C’olophospermum mopanr (Leguminosae) proved to be the source of the first profisetinidins and proguibourtinidins based on C-8 [69] or C-6 [7O] substituted (-)-fisetinidol. Steynberg et (I/. [71] isolated (42,6:41.8)-bis-( -)fisetinidol-( + )-catechin triflavonoid from the same plant. The heartwood of c‘. mopune afforded a series of dimers based on mopanol and peltogynol [72]. These are the promopanidins: (+)-mopanane-(4P,6)-fisetinidols, the epimopanone-(4.6 and 4.8)-fisctinidols and the unique oxidatively coupled epimopanone-guibourtinidol. They are accompanied by the related propeltogynidins: (+ )pcltogynane-(4/$6)-fisetinidols, epipcltogynanc-(4r,6)( - )-fisetinidol and ( - )-lisetinidol-(4x,6,)-( + )-peltogynol. Finally. a new 4-arylflavan-R-01 based on tisetinidol substitutcd at position 4 with a 2.4.dihydroxy-3-mcth-
oxyphenyl afiicanutn
group has been reported (Leguminosae) [61].
from Peltophorum
BIFLAVONOIIX
Biflavonoids based on bichalcones and chalconeflavanone dimers reported in African plants all appear to be novel compounds [ 11. Brackenridyea zanquebarica (Ochnaceae) [73 753 and Lophira lanceolala (Ochnaceae) [76-783 are the main sources of this new class of biflavonoids. The novel bitlavanone isochamaejasmin was also reported from B. zungueharicu [74], while its 7,7”-diglucoside was described from Ormocarputn kirkii (Leguminosae [79]). Recently, I.. alata proved to be the source of the dimers, lophirone-F [78, 803 and the new dimer, azobechalcone A [80]. and a tetramer isolated from the bark [80, 813. The same plant was also the source of a novel chalcone-flavanone dimer, bongosin (lo), in which the carbonyl group on the flavanone C ring was reduced [82]. It is suggested to be biosynthetically derived from lophirone B reported from the same plant, and first isolated from 15. lanceolara [76]. Finally, a new isobiflavonoid, calodenone (I 1). has been reported from the stem bark of Ochna ca~odendrott (Ochnaccae) [83]. Also isolated from the same plant was lophirone A (12) lirst reported from L. lonwol~ra (Ochnaccae) [84].
Flavonoid chemistry of African plants lSOFLAVONOlDS
Of the 138 isoflavonoids reported from African plants, 59 are novel structures [l]. Among them, several structures with an uncommon B ring hydroxylation pattern (position 3’ or 2’,3’) have been characterized from Salsola somalensis (Chenopodiaceae) [SS, 863. Novel C-substituted isoflavones of African origin include barbigerone from Tephrosia barbigera (Leguminosae) [87], senegalensin from Erythrina senegalensis (Leguminosae) [883, eriotiochin (13) and 8-prenylluteone from E. eriotriocha [89, 901, the pumilaisoflavone series from Tephrosia pumila (Leguminosae) 191, 921, and S-methoxydurmillone from Milletia ferruginea (Leguminosae) [933. Three further new isoflavones have been isolated from the roots of Polygala viragata (Polygalaceae) and identified as 5,7dihydroxy-8,4’-dimethoxy-, 5,7,4’-trihydroxy-6,8-dimethoxy- and 5,7-dihydroxy-6,8,4’-trimethoxy-isoflavone [94]. The heartwood of Pycnanthus anyolensis (Myristicaceae) was found to contain the new 2’-hydroxy-7,4’dimethoxyisoflavone [SS], while a novel isoflavanone, 2,3-dihydroauriculatin, has been reported from Erythrina senegalensis [96]. New isoflavone glycosides include the 7-0-glucoside of 5,7-dihydroxy-6,4’-dimethoxyand 5,7,3’-trihydroxy4’,5’-dimethoxyisoflavone from Iris germanica (Iridaceae) [97] and the 7-apiosyl( 1+6)glucoside and 7-glucoside-4’apioside of both daidzein and gentistein from Neorautanenia amboensis (Leguminosae) [98]. Auriculatin 4’glucoside was recently isolated as a new glycoside from Erythrina eriotricha [99]. Rotenoids and pterocarpans reported from African plant are restricted to the Leguminosae. Thus, two new rotenoids, 9-demethyl-dihydrostemonal and dihydrostemonal have been reported from Tephrosia pentaphyllo [lOO]. Neorautanenia amboensis was found to synthesize the new 12a-hydroxyrotenoids, neobanone, 12ahydroxyisomillettone [lOl], 12a-hydroxyrotenone, 12a0-methylrotenone Cl023 and the dehydrorotenone, rotenonone [loll. Several novel pterocarpans were also isolated: 3,8-dihydroxy-9-methoxypterocarpan and 8hydroxy-3,9-dimethoxypterocarpan from Pterocarpus soyauxii [103], ficifolinol, folitenol and folinin from N. ficifolia [ 1041, homoedudiol, neodunol and neorautenol from N. edulis [lOS] and 3,4-dihydroxy-9methoxypterocarpan from Taoeriniera obyssinica [ 1061. The presence of the new pterocarpenes neoduleen and neorauteen in N. edulis [105] and the novel 6a-hydroxypterocarpans, hildecarpidin and hildecarpin in Tephrosia hildebrundtii [107-1093 have also been reported.
Acknowledgements The author thanks Prof Dr D. Ferreira, Department of Chemistry, The University of the Orange Free State, South Africa and Prof. Dr Z. T. Fomum, Dr A. Nkengfack and Dr S. F. Kimbu, Department of Chemistry, University of Yaounde, Cameroon, for supplying recent publications. PHY 36:5-C
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N. A., Bates. R. B. and Linz, G. S. (1984) Tetrahedron Letters 25, 105. Drewes, S. E.. Hudson, N. A.. Bates, R. B. and Linz, G. S. (1987) J. Chem. SW. Pzrkin Trans I 2809. Tih, R. G., Sondcngam, B. L.. Martin. M. T. and Bodo, B. (1989) Phytochemistry 28. 1557. Tih, R. G., Sondengam, B. L., Martin, M. T. and Bodo, B. (1989) J. XUI. Prods 52, 284. Tih, R. G., Sondengam, B. I.., Martin M. T. and Bodo, B. (1990) Phyrochcmistry 29, 2289. Nyandat, E., Hassanali. A., de Vicentc, Y., Multari, G. and Galeffi, C. ( 1990) Phytorhemistry 29, 2361.
Flavonoid chemistry of African plants
80. Murakami, A., Tanaka, S., Ohigashi, H., Hirota, M., Irie, R., Takeda, N., Tatematsu, A. and Koshimizu, K. (1992) ~~yfoc~e~~~f~y 31, 2689. 81. Tih, A., Tih, R. G., Sondengam, B. L., Martin, M. T. and Bodo, B. (1992) ~~yfoc~e~isrry 31,981. 82. Tih, A. E., Tih, R. G., Sondengam, B. L., Martin, M. T. and Bodo, B. (1990) J. Nnt. Prods 53,964. 83. Messanga, B. B., Tih, R. G., Kimbu, S. F., Sondengam, B. L., Martin, M. T. and Bodo, E. (1392) J. Nat. Prods 55, 245. 84. Tih, R. G., Sondengam, B. L., Martin, M. T. and Bodo, B. (1987) Tetrahedron Letters 23, 2967. 85. Abegaz, B. M. and Woldu, Y. (1991) Phylochemisrry 30, 1281. 86. Woidu, Y. and Abegaz, 8. M. (1990) Phyz~chemisrry 29, 2013. 87. Vilain, C. (1980) Phytochem~srry 19, 988. 88. Wandji, J., Nkengfack, A. E., Fomum, Z. T., Ubillas, R., Killday, K. 13.and Tempesta, M. S. (1990) J. Nat. Prods 53, 1425. 89. Nkengfack, A. E., Sanson, D. R., Fomum, Z. T. and Tempesta, M. S. (1989) Phytochemistry 28, 2522. 90. Nkengfack,A. E., Fomum,Z.T., Ubillas, R.,Sanson, D. R. and Tempesta, M. S. (1990) J. Nat. Prods 53, 509. 91. Dagne, E., Dinku, B., Gray, A. I. and Waterman, P. G. (1988) Phytochemistry 27, 1503. 92. .‘enesew, A., Dagne, E. and Waterman, P. G. (1989) Phytochemistry 28, 129 1. 93. Dagne, E., Bekele, A. and Waterman, P. G. (1989) Phytochemistry 28, 1897. 94. Bashir, A., Hamburger, M., Msonthi, J. D. and Hostettmann, K. (1992) Phyro~hemis~ry 31, 309.
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95. Omobuwajo,
0. R., Adesanya, S. A. and Babalola, G. 0. (1992) Phytochemistry 31, 1013. 96. Taylor, R. B., Corley, D. G., Tempesta, M. S., Fomum, Z. T., Ayafor, J. F. and Wandji, J. (1986) .I. Nat. Prods 49, 670. 97. Ali, A. A., El-Emary, N. A., El-Moghazi, M. A., Darwish, F. M. and Frahm, A. W. (1983) Phytochemistry 22, 2061. 98. Breytenbach, J. C. (1986) .I. Nat. Prods 49, 1003. 99. Nkengfack, A. E., Meyer, M., Tempesta, M. S. and F mum, Z. T, (1991) Ptanta Med. 57,488.
100. Dagne, E., Yenesew, A. and Waterman, P. 6. (1989) Phytochemistry 28, 320.
101. Oberholzer, M. E., Rail, G. J. H. and Roux, D. G. (1976) ~hy~oc~ernjsrry 15, 1283. 102. Oberholzer, M. E., Rail, G. J. H. and Roux, D. G. (1974) Tetrahedron Letters 25, 2211. 103. Bezuidenhoudt, B. C. B., Brandt, E. V. and Ferreira, D. (1987) Phytochemistry 26, 531. 104. Brink, C. M., Engelbrecht, J. P, and Graham, D. E. (1970) J. South African Chem. Institute 23, 24. 105. Brink, A. J., Rail, G. J. H. and Engelbrecht, J. P. ( 1974) P~y~ochem~s~ry13, 158 1. 106. Duddeck, H.,Yenesew, A. and Dagne, E. (1987) Bull. Chem. Sac. Ethiopia 1, 36. 107. Lwande, W., Bentley, M. D. and Hassanali, A. (1986) insect Sci. Applic. 7, 501. 108. Lwande, W., Bentley, M. D., Macfoy, C., Lugemwa, F. N., Hassanali, A. and Nyandat, E. (1987) Phytochem~sfry 26, 2425. 109. Lwande, W., Hassanali, A., Njoroge, P. U., Bentley,
M. D., Delle Monache, F. and Jondiko, J. I. (1985) Inseci Sci. Appk. 6, 537.
0031-9422.(94)EO19&W
Phytcxhemwry.
Vol. M. No. 5. pp. 1109-1115. 1994 Ellcncr Saena Lid Printed in Gilt Lkitaia 0031.9422/9437.00+0.al
REVIEW ARTICLE NUMBER 90 RECENT
ADVANCES IN THE FLAVONOID PLANTS NABIEL A. M.
CHEMISTRY
OF AFRICAN
SALEH
National Research Centre, El-Dokki, Cairo, Egypt
(Received 10 January 1994) IN HONOUR
OF PROFESSOR
RAGAI K. IBRAHIM’S SIXTY-FIFTH
BIRTHDAY
Key Word Index-African
proanthocyanidins;
plants; flavonoids; flavones; flavonols; chalcones; flavanones; flavan and biflavonoids; isoflavonoids; review.
Abstract-Recent reports of flavonoids from African plant species are reviewed with emphasis on newly described structures and including all the major flavonoid classes.
In a review of the flavonoids occurring in plants of African origin covering the period of 19651990, all common and new structures were recorded and tabulated [ 11. In the present article, an updating of recent information of flavonoids reported in plants of African origin is outlined, particularly newly reported structures. FLAVONES African plants are the main source of the so-called llavonoids’, which are 7- or 5,7-oxygenated compounds with unsubstituted B rings and a prenyl unit at C-8. Thirteen flavonoids belong to this class [l-8]. In Tephrosia praecans (Leguminosae) a new angular isomer of isopongaflavone has been characterized [9]. Recently, a compound with the rare 2’,4’,5’-B-ring oxygenation pattern 5,5’-dihydroxy-3,7,2’,4’-tetramethoxyflavone was isolated from the heartwood of Chlorophora excelsa (Moraceae) [lo]. This compound is isomerit to 5,2’-dihydroxy-3,7,4’,5’-tetramethoxytlavone previously isolated from Distemonanthus benthamianus (Leguminosae) [l 11, while Hoslundia opposita (Lamia ceae) was found to be the source of a series of rate 6-Cpyrone-substituted 5,7-dimethoxyflavones [12, 133, in which the 6-C-pyrone substituent is attached via a 6.5” or 6,6”-link (see oppositin, 1). Novel flavone glycosides reported in African plants include the 2”-glucosides and 2”-glucuronides of apigenin and luteolin 7-lactate from Marrubium oulgare (Labiatae) [14]. Two polyglycosides encountered in nature for the first time are hypolaetin 7-[6”‘-acetylallosyl( I -+2)-3”acetylglucoside] from Stachys aegyptiaca (Labiatae) [IS] and acacetin 7-0-[rhamnosyh 1+4)glucosyl( 1+6”)-6”‘0-acetylsophoroside] from Peganum harmala (Zygophyl‘Tephrosia
laceae) [16]. A new acetylated triglycoside was recently isolated from P. harmala and identified as acacetin 7-0[glucosyl(l -&“)-3”‘-0-acetylneohesperidoside] together with the novel acacetin 7-rhamnoside [17]. Among the C-glycosylflavonoids only common monoand di-C-glycosylflavones have been reported from African plants [l]. and only one new compound has been identified: luteolin 7,4’-dimethyl ether 6-C-glucoside from Parkinsonia acufeata (Leguminosae) [ 183. Similarly, there are only four new reports of flavone C-glycoside-Oglycosides. These include the 2”-0-xyloside and 2”-Oglucoside of isoswertisin from Gnetum species (Gnetaceae) [19] and the 3’-0-glucoside and 3’-0-gentiobioside of swertiajaponin from Phragmites australis (Gramineae)
lm. FLAVONOLS
Flavonols are the largest class of flavonoids, with 98 structures reported in African plants, 22 of which were reported for the first time in nature Cl]. Although most of the common flavonol aglycones have been detected in African plants, no flavonols containing isoprenoid, furano and pyrano groups have been reported so far. However, Alluaudia species (Didiereaceae) are the source of a series of unusual C-methylflavonols, with a total of 11 novel compounds [21-261. For example, in A. dumosa two novel pairs of enantiomers based on 3-methoxyflavones bearing a dihydroxystilbene in ring B have been characterized (see 2 and 3) [25], while another new flavonol, 3-hydroxy-4’-methoxyflavone has been reported from the flowers of Millettia zechiana (Leguminosae) and its structure proved by synthesis [273.
1109
N.
1110
A. M.
SALEH
OH
OH 1 Oppositin
2
(-)-Androyol
3
(-)-6-C-Methylandroyol
4 Schdflain
Of the 129 flavonol glycosides isolated from African plants, 34 are novel [I]. Several sulphated flavonols have been reported in African genera e.g. lphiona (Compositae) [28,29], Malca (Malvaceae) [30,31] and Tumarix (Tamaricaceae) [32-341. Novel structures include the 4”-pcoumaroyl derivatives (both cis and truns) of kaempferol and quercetin 3-robinobiosides and 3-robinobioside-7glucoside isolated from Strychnos oariobilis (Loganiaceae) [35,36]. Similarly, isorhamnetin 3-0-CC’-p-coumaroylrhamnosyl( 1-+6)galactoside] was isolated for the first time from Aerua jaounicu (Amaranthaceae) [37], and recently, 6”-[2-E-butenoyl]isorhamnetin 3-0-glucoside was isolated from Zygophyllum simplex (Zygophyllaceae)
C381. CHALCONES
0
AND DIHYDROCHALCONES
African plants contain a variety of chalcone and dihydrochalcone structures. Thus, besides the commonly encountered structures [I], a number of novel chalcones have been reported: 4,2’,6’-trihydroxy-4’-prenyloxychal-
5
and enantiomer (+)-androyol. and (+)-ensntiomer,
R=H R = Me
Isoschefflerin
cones from Helichrysum athrixifolium (Compositae) [39], 4,2’,4’-trihydroxy-3’-C-methylchalcone from Crinum augusturn (Liliaceae) [40], 4’-0-geranylisoliquiritigenin from Milletia ferruginea (Leguminosae) [41] and praecanosone-A and praecanosone-B from Tephrosia praecans (Leguminosae) [9, 421. Two further unique chalcones, schefilerin (4) and isoschefflerin (5) have been isolated from Uuariu schefleri (Annonaceae) [43], which are believed to be biogenetically derived from a Diels-Alder-like cyclization between a chalcone and the monoterpene ,!I-ocimene. Indeed, b’vuriu species have been the source of several new compounds. For example, a series of mono-C-substituted dihydrochalcones bearing a 2-hydroxybenzyl function at position 3’ or 5’ have been reported from U. angolensis C44-463, together with the 3’,5’-di-C-substituted derivative diuvarctin and the cyclized compound chamuvaritin [45, 461. Also, two novel dihydrochalcones, triuvaretin (6) and isotriuvaretin (7) were recently identified from the roots of U. leptocludon [47]. These were found to be dihydrochalcones with three 0-hydroxybenzyl moieties in each molecule. A novel
1111
Flavonoid chemistry of African plants
HO
HO
6 l’nuvaretin
7
Isotriuvareun
OH
HO
8
Dichamanetin
flavene was isolated from U. dependens accompanied by small amounts of a corresponding chalcone which appears to be an artifact formed by oxidation of the flavene [48]. Recently, the first member of the flavonoid-related but-Zenolides was isolated from the heartwood of Pericopsis elata (= Afiormosia elata) (Leguminosae) [49].
9
Sigmoidin G
been reported from Uuariu chatnae (e.g. see dichamanetin, 8) [56,57]. A new dihydroflavonol, eriotrinol, and a new prenylated flavanone, sigmoidin-G (9). were recently isolated from the stem bark of Eryrhrinu eriotricha and E. siymoidea, respectively [SS].
FI.AVANS AND PROAYTHOCYANIDINS FLAVANONES
The flavanones reported from African plants include all the classes so far encountered in nature. 0-Prenylation and C-prenylation are common features of flavanones. The first occurrence of a 5,7-dihydroxyflavanone with 70-prenyl, 6,8-di-C-prenyl, and 8-C-prenyl-7-0-prenyl substitution has been described in Helichrysum species of African origin [39, SO]. Similarly, Erythrina siymoidea (Leguminosae) is the source of a series of flavanones (sigmoidin-A-F) with C-prenylation or additional pyrano groups in ring B [Sl -551. As in the case of the dihydrochalcones, a series of flavanones C-substituted at position 6 and/or 8 with a 2-hydroxybenzyl group have
According to Porter [59] flavans include Bavans, flavan-3-ols, flavan-Cols, Ravan-3,4-dials, flav-3-ene and peltogynoid flavans. Flavans reported for the first time from African plants are: 4’-hydroxy-5,7-dimethoxy-%Cmethylflavan (tlavan) from Puncratium maritimum (Liliaceae) [6O]; robinetinidol-3-0-gallate (flavan-3-01) from Burkea ufricuna (Leguminosae) [61]; hildgardtol-A, -B and their 4-methoxyderivatives (flavan4ols) from Tephrosia hildebrandtii [62]; hildgardtene (flav-3-ene) from T. hildebrandtii [62] and 5,7,8-trimethoxyflav-3-e.ne from Uwrio dependens [48]. A peltogynan previously isolated from Cassine transraelensis (Celastraceae) [63] and C. habe [64], was isolated also from C. papillosa [65]
N. A. M. SALEH
OH OH
0
OR
10
11 Calodenone.R = Me
Bongosin
12 IDphironcA.
13
R=H
Enotriwhin
and its absolute contiguration established as 11.1 ldimethyl-1,3.8,10-tetrahydroxy-9-methoxypeltogyn~~n. African plants are the source of several proanthocyanidins. Of interest are a series of guibourtinidol+atechin or epicatechin dimers in which catcchin or epicatechin are carboxylated at C-6 or C-8 from Acacia luederiti-ii (Lcguminosae) [66]. Also a series of flavan-3-01s condensed to stilbenes has been reported from the heartwood of Guihourfio wleospertna (Leguminosae) [67], while a proanthocyanidin based on an isoflavan was recorded in Dalhrryitr nitiduln (Leguminosae) [68]. The heartwood of C’olophospermum mopanr (Leguminosae) proved to be the source of the first profisetinidins and proguibourtinidins based on C-8 [69] or C-6 [7O] substituted (-)-fisetinidol. Steynberg et (I/. [71] isolated (42,6:41.8)-bis-( -)fisetinidol-( + )-catechin triflavonoid from the same plant. The heartwood of c‘. mopune afforded a series of dimers based on mopanol and peltogynol [72]. These are the promopanidins: (+)-mopanane-(4P,6)-fisetinidols, the epimopanone-(4.6 and 4.8)-fisctinidols and the unique oxidatively coupled epimopanone-guibourtinidol. They are accompanied by the related propeltogynidins: (+ )pcltogynane-(4/$6)-fisetinidols, epipcltogynanc-(4r,6)( - )-fisetinidol and ( - )-lisetinidol-(4x,6,)-( + )-peltogynol. Finally. a new 4-arylflavan-R-01 based on tisetinidol substitutcd at position 4 with a 2.4.dihydroxy-3-mcth-
oxyphenyl afiicanutn
group has been reported (Leguminosae) [61].
from Peltophorum
BIFLAVONOIIX
Biflavonoids based on bichalcones and chalconeflavanone dimers reported in African plants all appear to be novel compounds [ 11. Brackenridyea zanquebarica (Ochnaceae) [73 753 and Lophira lanceolala (Ochnaceae) [76-783 are the main sources of this new class of biflavonoids. The novel bitlavanone isochamaejasmin was also reported from B. zungueharicu [74], while its 7,7”-diglucoside was described from Ormocarputn kirkii (Leguminosae [79]). Recently, I.. alata proved to be the source of the dimers, lophirone-F [78, 803 and the new dimer, azobechalcone A [80]. and a tetramer isolated from the bark [80, 813. The same plant was also the source of a novel chalcone-flavanone dimer, bongosin (lo), in which the carbonyl group on the flavanone C ring was reduced [82]. It is suggested to be biosynthetically derived from lophirone B reported from the same plant, and first isolated from 15. lanceolara [76]. Finally, a new isobiflavonoid, calodenone (I 1). has been reported from the stem bark of Ochna ca~odendrott (Ochnaccae) [83]. Also isolated from the same plant was lophirone A (12) lirst reported from L. lonwol~ra (Ochnaccae) [84].
Flavonoid chemistry of African plants lSOFLAVONOlDS
Of the 138 isoflavonoids reported from African plants, 59 are novel structures [l]. Among them, several structures with an uncommon B ring hydroxylation pattern (position 3’ or 2’,3’) have been characterized from Salsola somalensis (Chenopodiaceae) [SS, 863. Novel C-substituted isoflavones of African origin include barbigerone from Tephrosia barbigera (Leguminosae) [87], senegalensin from Erythrina senegalensis (Leguminosae) [883, eriotiochin (13) and 8-prenylluteone from E. eriotriocha [89, 901, the pumilaisoflavone series from Tephrosia pumila (Leguminosae) 191, 921, and S-methoxydurmillone from Milletia ferruginea (Leguminosae) [933. Three further new isoflavones have been isolated from the roots of Polygala viragata (Polygalaceae) and identified as 5,7dihydroxy-8,4’-dimethoxy-, 5,7,4’-trihydroxy-6,8-dimethoxy- and 5,7-dihydroxy-6,8,4’-trimethoxy-isoflavone [94]. The heartwood of Pycnanthus anyolensis (Myristicaceae) was found to contain the new 2’-hydroxy-7,4’dimethoxyisoflavone [SS], while a novel isoflavanone, 2,3-dihydroauriculatin, has been reported from Erythrina senegalensis [96]. New isoflavone glycosides include the 7-0-glucoside of 5,7-dihydroxy-6,4’-dimethoxyand 5,7,3’-trihydroxy4’,5’-dimethoxyisoflavone from Iris germanica (Iridaceae) [97] and the 7-apiosyl( 1+6)glucoside and 7-glucoside-4’apioside of both daidzein and gentistein from Neorautanenia amboensis (Leguminosae) [98]. Auriculatin 4’glucoside was recently isolated as a new glycoside from Erythrina eriotricha [99]. Rotenoids and pterocarpans reported from African plant are restricted to the Leguminosae. Thus, two new rotenoids, 9-demethyl-dihydrostemonal and dihydrostemonal have been reported from Tephrosia pentaphyllo [lOO]. Neorautanenia amboensis was found to synthesize the new 12a-hydroxyrotenoids, neobanone, 12ahydroxyisomillettone [lOl], 12a-hydroxyrotenone, 12a0-methylrotenone Cl023 and the dehydrorotenone, rotenonone [loll. Several novel pterocarpans were also isolated: 3,8-dihydroxy-9-methoxypterocarpan and 8hydroxy-3,9-dimethoxypterocarpan from Pterocarpus soyauxii [103], ficifolinol, folitenol and folinin from N. ficifolia [ 1041, homoedudiol, neodunol and neorautenol from N. edulis [lOS] and 3,4-dihydroxy-9methoxypterocarpan from Taoeriniera obyssinica [ 1061. The presence of the new pterocarpenes neoduleen and neorauteen in N. edulis [105] and the novel 6a-hydroxypterocarpans, hildecarpidin and hildecarpin in Tephrosia hildebrundtii [107-1093 have also been reported.
Acknowledgements The author thanks Prof Dr D. Ferreira, Department of Chemistry, The University of the Orange Free State, South Africa and Prof. Dr Z. T. Fomum, Dr A. Nkengfack and Dr S. F. Kimbu, Department of Chemistry, University of Yaounde, Cameroon, for supplying recent publications. PHY 36:5-C
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