A study of nuclear prenylation and allylation of isoflavones and synthesis of 4'-methyl ethers of osajin and warangalone

Abstract

Nuclear prenylation of 5,7-dihydroxyisoflavones with prenyl bromide in the presence of methanolic methoxide is found to give a mixture of three compounds viz., 6,8-di-C-prenyl (25%), 6-C-prenyl (l5%) and 7-O-prenyl (2%) derivatives. 6,8-Di-C-prenyl-4'-methoxy-5,7-dihydroxyisoflavone (IIa) thus obtained gives on oxidative cyclization with DDQ a mixture of two products; the major component agreed with 4'-O-methylosajin and the minor component should therefore be 4'-O-methylwarangalone. Nuclear allylation with allyl bromide in the presence of methanolic potash yields a mixture of six products viz, 6-C-allyl (17%), 6-C-allyl-7-O-allyl (5%), 8-C-allyl (12%), 8-C-allyl-7-O-allyl- (3%), 6,8-di-C-allyl (2%) and 7-O-allyl (3%) derivatives. The difference in the behaviour of prenyl and allyl bromides seems to be due to the greater reactivity of the former.