Synthesis of 4-substituted flavans from 4α-halogenoflavans

Abstract

The reactions of flavan-4α- or -4β-ols unsubstituted in ring A with phosphorus or similar halides give 4α-halogenoflavans which react with a variety of nucleophiles (phenolate ions, thiophenolate ions, amines, and alcohols) yielding the corresponding 4-substituted flavans whose stereochemistries depend upon the ratio of S_N1 to S_N2 reactions occurring. Biflavonoids have been synthesised by the reaction of 4α-halogenoflavans with flavonols under conditions of phase transfer catalysis.