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.