Synthesis of isoflavanoid oligomers using a pterocarpan as inceptive electrophile

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Abstract
(3S)-2′,7-Dihydroxy-4′-methoxyisoflavan serves as bifunctional nucleophile at C-5′ and C-6, when condensed with the carbocation generated at C-11a of its (6aS, 11aS)-3-hydroxy-9-methoxypterocarpan analogue under mild acid conditions or by photolysis, to form isomeric (3S,4S)-[4,3′]- and -[4,6]-3,4-trans-bi-isoflavanoids, and hence the [4,3′ : 4,6]-tri-isoflavanoid homologue. Structural assignments are supported by an analogous series of condensations. Self-condensation of the pterocarpan generates both isoflavan-pterocarpan and ispflavan-isoflavene analogues in low yields. The [4,3′]-bi-isoflavanoid is identical with the first natural oligomer from Dalbergia nitidula.