Hydroboration of Flav-3-enes, and oxidation to 2,4-trans-Flavan-4-ols, 2,3-trans-Flavan-3-ols, and 1,3-Diarylpropane derivatives

Abstract

Hydroboration of flav-3-enes has been found to be sensitive to electron release from substituents in the A-ring. Flav-3-enes which lack electron-releasing substituents in the A-ring give 2,4-trans-flavan-4- ols as the principal non-phenolic products. Electron-releasing substituents in the A-ring favour boration at the 3-position, and the boranes so formed suffer ring fission, which leads to the isolation of phenolic 1,3-diarylpropane derivatives as well as 2,3-trans-flavan-3- ols. Ring fission of the intermediate borane is minimized by reducing the temperature of hydroboration. N.m.r. data are presented on the compounds obtained.