C-glycosylanthocyanidins synthesized from C-glycosylflavones

Abstract Nine C -glycosyldeoxyanthocyanidins, 6- C -β-glucopyranosyl-7- O -methylapigeninidin, 6- C -β-glucopyranosyl-7- O -methylluteolinidin, 6- C -β-(2″- O -β-glucopyranosylglucopyranosyl)-7- O -methylapigeninidin, 6- C -β-(2″- O -β-glucopyranosylglucopyranosyl)-7,4′-di- O -methylapigeninidin, 8- C -β-glucopyranosylapigeninidin, 8- C -β-(2″- O -α-rhamnopyranosylglucopyranosyl)apigeninidin, 8- C -β-(2″- O -α-(4″′- O -acetylrhamnopyranosyl)glucopyranosyl)apigeninidin, 6,8-di- C -β-glucopyranosylapigeninidin ( 8 ), 6,8-di- C -β-glucopyranosyl-4′- 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.