Hydroboration of chromenes, flav-2-enes, flavylium salts, and flavones

Abstract

By hydroboration and oxidation chrom-2-ene gave chroman-3-ol, and chrom-3-ene gave chroman-4-ol with only a minor quantity of chroman-3- ol. Hydroboration of coumarin to chroman-3-ol therefore proceeds through chrom-2-ene instead of through chrom-3-ene as previously proposed. Successive hydroboration and oxidation of 4?-methoxyflav-2- ene and of 4?-methoxyflavylium chloride each gave a mixture of 3-(2- hydroxyphenyl)-1-(4-methoxyphenyl)propan-2-ol, 3-(2-hydroxyphenyl)-1- (4- methoxyphenyl)propan-1-ol, and 4?-methoxyflavan (formed by cyclization of the diarylpropan-1-ol), The same three products were obtained by hydroboration and oxidation of 4?-methoxyflavone, in this case accompanied by 2,3-trans-4?-methoxy-flavan-3-ol, 2,4-trans-4?- methoxyflavan-4-ol, and 2,3-trans-4?-methoxyflavan-3,4-trans-diol. N.m.r. data are presented on the compounds obtained.