Structure and synthesis of polyphenols from honeybush tea (cyclopia intermedia) and the potential of flavonoids as active oxygen scavengers

[摘要] Cyclopia intermedia E. Mey (Fabaceae), from which Honeybush tea is brewed, is one ofapproximately 24 Cyclopia species of woody legumes endemic to the Cape fynbos (Capemacchia) region of South Africa. Supported by results from our initial investigations on theplant, demonstrating the presence of phenolic compounds including coumestans, isoflavones,flavanones, xanthones, a flavone, pinnitol and p-coumaric acid, the tea is gaining popularity asa health beverage. Presence of these compounds that are claimed to have interestingpharmacological properties, supported by belief that the tea contains very little, if any, caffeineand a low tannin content, as well as its usage as a medicinal concoction by the people of theWestern and Eastern Cape, prompted further investigations of the plant.A senes of enriched fractions of the methanol extracts of the fermented shoots of C.intermedia followed by chromatographic (column Sephadex LH-20 and preparative thin layer)separations afforded flavones and glycosylated flavonols, flavanones, isoflavones and C6.C2-and Cx.Cj-type compounds. Determination of the tannin content and the average degree ofpolymerisation was accomplished by cleavage of the crude tannin fraction in dilute acid withbenzyl mercaptan and phloroglucinol as capture nucleophiles. Stuctures of the compoundswere elucidated and characterised as their full acetate derivatives by high resolution (300 MHz)IH NMR speetometry (including COSY and NOESY experiments), Circular Dichroism andElectron Impact Mass spectrometry.Along with tyro sol and its 3-methyl ether analogue, two new C6.C2 and C6.CI monoaryls withthe same , β-apiofuranosyl-4-0-,β-D-glucopyranosyl unit were isolated. These wereaccompanied by the known flavonoids, 3',4',7-trihydroxy- and 3',5,7-trihydroxy-4'-methoxyflavones, 6-C-,β-D- and 5-0-a-D-glucopyranosylkaempferol, the flavanones 7-0-,β-Dglucopyranosylnaringeninand eriodictyol, 5-0-,β-D-glucopyranosyleriodictyol and theisoflavone, 7-0-,β-D-glucopyranosyl-4' ,6-di-0-methylafrormosin. The new compoundsisolated were 5-0-a-D-rutinosylnaringenin, 8-C-,β-D-glucopyranosyl- and 3-0-,6-C-di-,β-Dglucopyranosylkaempferol,6 -0-, β -apiofuranosyl-7 -0-,β-D-glucopyranosyl-4' -0-methylisoflavone and 6-O-β -apiofuranosyl-6-0- β -D-glucopyranosyl-3' -4'-methylenedioxyflavonol.While cleavage of the tannin fraction afforded three new 4-arylflavans, 4-(2,4,6-trihydroxyphenyl)-4' ,5,7-trihydroxyflavan, its 5-0- β -D-glucopyranosyl analogue and 4-(2,4,6-trihydroxyphenyl)-3',4' ,5,7-tetrahydroxyflavan from the phloroglucinol reaction, 4-thiobenzyl-4',7 -dihydroxy-5-0- β -D-glw:opyranosylflavan and 4-thiobenzyl-4', 7-dihydroxy-5-0- β -Drutinosylflavanwere the new 4-thiobenzylflavans isolated the benzyl mercaptan reaction.Absence of the heterocyclic protons of the C-ring in the coumestan structure precludedstructural elucidation by NMR alone and necessitated confirmation by synthesis. The threecoumestans were, therefore, synthesized via a novel route by directly transforming theappropriate 2,2' -dihydroxychalcones into coumestans.Since the flavonoids are presumed to contribute significantly towards the scavenging effects ofthe active oxygen species, flavonols (quercetin, myricetin, fisetin, and robinetin) and flavan-3-ols (catechin and epicatechin), with similar hydroxylation pattern were selected and reactedwith super oxide. Besides expected products resulting from cleavage of the molecule, a uniquedeoxygenation of the 3' -OH on B-ring occurred with some of the compounds.These results clearly indicate that the tentative claimed health promoting properties ofHoneybush tea may at least, in part, be attributed to the presence of these and other phenolicsin C.intermedia.

[发布机构] University of the Free State