The hydroxyl stretching frequencies and conformations of Flavan-4-ols and Thiaflavan-4-ols

Abstract

The hydroxyl stretching frequencies of some 2,4-cis and 2,4-trans flavan-4-ols, thiaflavan-4-ols, and related compounds have been measured in dilute carbon tetrachloride solution. Characteristic frequencies are observed at 3626±2 cm^-1 (free hydroxyl), 3616±1, and 3597±2 cm^-1. The 3616 and 3597 cm^-1 bands are assigned to pseudo-axial and pseudo-equatorial hydroxyl groups respectively. Evidence is presented to show that the frequency shifts arise from differences in the interaction of the hydroxyl group with the π-electrons of the fused benzene ring, and that these flavans and thiaflavans exist in half- chair, rather than sofa conformations.