Flavan derivatives. XXIV. cis- and trans-3-Methoxyflavanones

Abstract

Oxidation of three 2,3-cia-3,4-cis-3-methoxyflavan-4-ols with active manganese dioxide gave the corresponding 2,3-cis-3-methoxyflavanones which were equilibrated in deuterochloroform containing trifluoroacetic acid to mixtures of cis- (c.33%) and trans-3-methoxyflavanones (c.67%). Tominaga's base-catalysed cyclization of 2'-hydroxy-α-methoxycha1cones to 3-methoxyflavanones has been found to yield the 2,3-trans isomers. The higher stability of the 2,3-trans-configuration of 3-hydroxy- flavanones (dihydroflavonols) compared with 3-methoxyflavanones is attributed to hydrogen bonding; no cis-isomer was detected when acid-catalysed epimerization of a trans-3-hydroxyflavanone was attempted. M.m.r. data are reported for the compounds described.