Synthesis of Novel C-Methylflavones

If the Baker‐Venkataraman rearrangement of C‐methylphloracetophenone triaroyl esters is carried out in DMSO with powdered NaOH, the result greatly depends on the substitution pattern both of the phloracetophenone moiety possessing either one or two C‐methyl groups and of the aroyl parts bearing a conjugated methoxy group (or not such a group) with respect to the carbonyl group. With benzoyl, 4‐methoxybenzoyl or 3,4‐dimethoxybenzoyl as aroyl group the 3,5‐dimethylphloracetophenone triaroyl esters 10a–c directly yield the corresponding unsubstituted ring B or 4′‐methoxy‐ and 3′,4′‐dimethoxy‐substituted 5,7‐dihydroxy‐6,8‐dimethylflavones 11a–c. In contrast, the 3‐methylphloracetophenone triaroyl esters 3a–c react quite differently, depending on the aroyl substitution pattern. Thus, the triester 3a containing benzoyl groups gives the hemiketal Wessely‐Moser isomers 5a, 5a′ whereas the triesters 3b and 3c containing 4‐methoxy‐ or 3,4‐dimethoxybenzoyl groups are converted into compounds existing as two equilibria of two β‐diketo and two β‐keto enol tautomers. Finally, dehydration of each mixture furnishes solely the corresponding unsubstituted ring B or 4′‐methoxy‐ and 3′,4′‐dimethoxy‐substituted 5,7‐dihydroxy‐6‐methylflavones 8a–c. Methylation of 11c affords 7‐O‐methyl and 5,7‐di‐O‐methyl derivatives 12a and b.