Novel isoflavan–pterocarpan interconversions: some structural requirements for cyclization

Abstract

Photolyses of pterocarpans in methanol and in acetic acid solutions provide novel synthetic access to 4-methoxy- and 4-acetoxy-2′-hydroxy-3,4-trans-isoflavans, respectively, and hence to 2′,4-dihydroxy-3,4-trans- analogues; easy, if not spontaneous, reversion of both 4-methoxy- and 4-hydroxyisoflavans to pterocarpans, compared with recalcitrant 2′-hydroxyisoflav-3-enes, indicates the significance of a suitable leaving group in the benzylic 4-position combined with a reduced heterocyclic ring at C-3 in promoting cyclization.