Some photochemical and oxidative conversions of pterocarpans and isoflavans: functional requirements for cyclization of isoflavans to pterocarpans

Abstract

Photolysis of pterocarpans in methanol or acetic acid provides the first general and also direct method of access to 4-functionalized 2′-hydroxy-3,4-trans-isoflavans by C-ring fission and solvolysis. Spontaneous recyclization to the pterocarpan is governed by functional-dependent factors such as the effective delocalization of the transient 4-carbocation and formation of quinone-methide intermediates. In isolated instances there is also photoreduction to isoflavans, or formation of an 8-hydroxy-9-methoxymethoxypterocarpan by solvolysis following fission of the 8,9-methylenedioxy-ring system. Oxidative conversions of 2′,7-dihydroxyisoflavans to pterocarpans with 2,3-dichloro-5,6-dicyanobenzoquinone in methanol may proceed via unstable quinone methides, or via 4-carbocation intermediates after hydride abstraction. However, appropriate 2′-hydroxy-4′-methoxy-disubstitution provides the first example of selective aromatic methoxylation of the B-ring under oxidative conditions.