C-glycosylanthocyanidins synthesized from C-glycosylflavones

Abstract

Nine C-glycosyldeoxyanthocyanidins, 6-C-b-glucopyranosyl-7-O-methylapigeninidin, 6-C-b-glucopyranosyl- 7-O-methylluteolinidin, 6-C-b-(200-O-b-glucopyranosylglucopyranosyl)-7-O-methylapigeninidin, 6- C-b-(200-O-b-glucopyranosylglucopyranosyl)-7,40-di-O-methylapigeninidin, 8-C-b-glucopyranosylapigeninidin, 8-C-b-(200-O-a-rhamnopyranosylglucopyranosyl)apigeninidin, 8-C-b-(200-O-a-(400 0-O-acetylrhamnopyranosyl) glucopyranosyl)apigeninidin, 6,8-di-C-b-glucopyranosylapigeninidin (8), 6,8-di-C-bglucopyranosyl- 40-O-methylluteolinidin (9), have been synthesized from their respective C-glycosylflavones (yields between 14% and 32%) by the Clemmensen reduction reaction using zinc-amalgam. The various precursors (C-glycosylflavones) of the C-glycosylanthocyanidins were isolated from either flowers of Iris sibirica L., leaves of Hawthorn ‘Crataegi Folium Cum Flore’, or lemons and oranges. This is the first time C-glycosylanthocyanidins have been synthesized. The structures of all flavonoids including the flavone rotamers were elucidated by 2D NMR techniques and high-resolution electrospray MS. The distribution of the various structural forms of 8 and 9 are different at pH 1.1, 4.5, and 7.0, however, the two pigments undergoes similar structural transformations at the various pH values. Pigments 8 and 9 with C–C linkages between the sugar moieties and the aglycone, were found to be far more stable towards acid hydrolysis than pelargonidin 3-O-glucoside, which has the typical anthocyanidin C–O linkage between the sugar and aglycone. This stability may extend the present use of anthocyanins as nutraceuticals, pharmaceuticals or colorants.