Flavan derivatives. XXXII. Structures and N.M.R. spectra of Flav-2-enes and Flav-3-enes; Hydroboration and hydroxylation of Flav-3-enes

Reduction of 3-alkoxyflavylium salts with complex metal hydrides is shown to yield 3-alkoxyflav-3-enes directly without the intermediate formation of 3-alkoxyflav-2-enes which has been demonstrated in the similar reduction of 3-alkoxyflavones. Reduction with complex metal hydrides of simple flavylium salts lacking a 3-alkoxyl group gives flav-2-enes and is a convenient route to flav-2-enes. Flavylium salts lacking a 3-alkoxyl group undergo nucleophilic attack by hydride reagents at the 4-position (to yield flav-2-enes) but 3-alkoxyflavylium salts undergo nucleophilic attack at the 2-position to yield flav-3-enes (with hydrides) or 2-alkoxyflav-3-enes (with alcohols). Hydroboration of 4'-methoxyflav-3-ene gives no detectable flavan-3-01 (i.e. homobenzyl alcohol) and yields mainly 2,4-trans-4'-methoxyAavan-4-01 in a useful addition to synthetic routes to CL-flavan-4-01s (2,4-trans isomers). Hydroxylation of flav-3-enes with peracetic acid leads mainly to 2,3-trans- flavan-3,4-cis-diol derivatives accompanied by minor quantities of 2,3-cis-3,4-trans derivatives which are easily removed by recrystallization. N.m.r. data are presented on all the compounds examined.