Structure and synthesis of polyphenols from honeybush tea (cyclopia intermedia) and the potential of flavonoids as active oxygen scavengers
Kamara, Bukirwa Irene
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Cyclopia intermedia E. Mey (Fabaceae), from which Honeybush tea is brewed, is one of approximately 24 Cyclopia species of woody legumes endemic to the Cape fynbos (Cape macchia) region of South Africa. Supported by results from our initial investigations on the plant, demonstrating the presence of phenolic compounds including coumestans, isoflavones, flavanones, xanthones, a flavone, pinnitol and p-coumaric acid, the tea is gaining popularity as a health beverage. Presence of these compounds that are claimed to have interesting pharmacological properties, supported by belief that the tea contains very little, if any, caffeine and a low tannin content, as well as its usage as a medicinal concoction by the people of the Western 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 of polymerisation was accomplished by cleavage of the crude tannin fraction in dilute acid with benzyl mercaptan and phloroglucinol as capture nucleophiles. Stuctures of the compounds were elucidated and characterised as their full acetate derivatives by high resolution (300 MHz) IH NMR speetometry (including COSY and NOESY experiments), Circular Dichroism and Electron Impact Mass spectrometry. Along with tyro sol and its 3-methyl ether analogue, two new C6.C2 and C6.CI monoaryls with the same , β-apiofuranosyl-4-0-,β-D-glucopyranosyl unit were isolated. These were accompanied by the known flavonoids, 3',4',7-trihydroxy- and 3',5,7-trihydroxy-4'-methoxy flavones, 6-C-,β-D- and 5-0-a-D-glucopyranosylkaempferol, the flavanones 7-0-,β-Dglucopyranosylnaringenin and eriodictyol, 5-0-,β-D-glucopyranosyleriodictyol and the isoflavone, 7-0-,β-D-glucopyranosyl-4' ,6-di-0-methylafrormosin. The new compounds isolated 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 -Drutinosylflavan were the new 4-thiobenzylflavans isolated the benzyl mercaptan reaction. Absence of the heterocyclic protons of the C-ring in the coumestan structure precluded structural elucidation by NMR alone and necessitated confirmation by synthesis. The three coumestans were, therefore, synthesized via a novel route by directly transforming the appropriate 2,2' -dihydroxychalcones into coumestans. Since the flavonoids are presumed to contribute significantly towards the scavenging effects of the active oxygen species, flavonols (quercetin, myricetin, fisetin, and robinetin) and flavan-3- ols (catechin and epicatechin), with similar hydroxylation pattern were selected and reacted with super oxide. Besides expected products resulting from cleavage of the molecule, a unique deoxygenation of the 3' -OH on B-ring occurred with some of the compounds. These results clearly indicate that the tentative claimed health promoting properties of Honeybush tea may at least, in part, be attributed to the presence of these and other phenolics in C.intermedia.