Glycosyl imidates, 69. Synthesis of flavone C ‐glycosides vitexin, isovitexin, and isoembigenin

2‐Hydroxy‐4,6‐dimethoxyacetophenone ( 4 ) was glycosylated with O ‐(2,3,4,6‐tetra‐ O ‐benzyl‐α‐ D ‐glucopyranosyl) trichloroacetimidate ( 5 ) and trimethylsilyl triflate as promoter to yield directly the C ‐glycoside 6 . Construction of the flavone system by application of a Baker‐Venkataraman‐type rearrangement followed by deprotection yielded isoembigenin ( 2 ). Glycosylation of 4,6‐bis( tert ‐butyldimethylsilyloxy)‐2‐hydroxyacetophenone ( 17 ) with the trichloroacetimidate 5 afforded the O ‐glycoside intermediate 18 which was converted via Fries rearrangement into the C ‐glycoside 21 . Applying again the Baker‐Venkataraman rearrangement and cyclization gave isovitexin and vitexin derivatives 25 and 26 , which were completely deprotected to yield isovitexin ( 1b ) and vitexin ( 1a ), respectively.