SYNTHESIS OF NOVEL C-METHYLFLAVONES

If the Baker-Venkataraman rearrangement of C-methylphloracetophenone t riaroyl esters is carried out in DMSO with powdered NaOH, the result g reatly depends on the substitution pattern both of the phloracetopheno ne moiety possessing either one or two C-methyl groups and of the aroy l parts bearing a conjugated methoxy group (or not such a group) with respect to the carbonyl group. With benzoyl, 4-methoxy-benzoyl or 3,4- dimethoxybenzoyl as aroyl group the 3,5-di-methylphloracetophenone tri aroyl esters 10a-c directly yield the corresponding unsubstituted ring B or 4'-methoxy- and 3',4'-dimethoxy-substituted 5,7-dihydroxy-6,8-di methylflavones 11a-c. In contrast, the 3-methylphloracetophenone triar oyl esters 3a-c react quite differently, depending on the aroyl substi tution pattern. Thus, the triester 3a containing benzoyl groups gives the hemiketal Wessely-Moser isomers 5a, 5a' whereas the triesters 3b a nd 3c containing 4-methoxyoxy- or 3,4-dimethoxybenzoyl groups are conv erted into compounds existing as two equilibria of two beta-diketo and two beta-keto enol tautomers. Finally, dehydration of each mixture fu rnishes solely the corresponding unsubstituted ring B or 4'-methoxy- a nd 3',4'-dimethoxy-substituted 5,7-dihydroxy-6-methylflavones 8a-c. Me thylation of 11c affords 7-O-methyl and 5,7-di-O-methyl derivatives 12 a and b.