Asymmetric Synthesis of Sappanin-Type Homoisoflavonoids

Homoisoflavanones, 3-benzylchroman-4-ones, which are naturally occurring sappanin-type homoisoflavonoids, possess a 16-carbon skeleton and a single chiral center. In this paper, we present various asymmetric synthesis of homoisoflavanones that have been developed. The first approach is a lipase-catalyzed desymmetrization of 2-benzylpropane-1,3-diol and its diacetate. Three approaches for the synthesis of an optically active 3-benzylchroman-4-one are asymmetric protonation of the silyl enol ether derived from a racemic homoisoflavanone, iridium-catalyzed asymmetric hydrogenation of alpha-phenoxymethylcinnamic acid, and Evans asymmetric aldol reaction. Recently, our group performed Noyori ruthenium-catalyzed asymmetric transfer hydrogenation with dynamic kinetic resolution, followed by alcohol oxidation of 3-benzylchroman-4-ol.