Acid-catalysed coupling reactions and conversions of isoflavone epoxides

Abstract

Whereas 4′,7-dimethoxy-(5) and 4′-methoxy-2′,7-ditosyloxy-isoflavone epoxides (7) are subject to regioselective acid-mediated methanolysis to yield 2-hydroxy-3-methoxy- and 3-hydroxy-2-methoxyisoflavanones, the 2′7-dibenzyloxy-4′-methoxy analogue (6) is transformed regiospecifically into the 2-hydroxy-3-methoxyisoflavanone (17). The course of these coupling reactions is dependent on the B-ring oxygenation pattern.

Epoxide (5) reacts with m-methoxyphenol at ambient temperature to give a 3-aryl-2-hydroxyisoflavanone (34). At 0 °C the latter compound is accompanied by two regioisomeric O–C-coupled analogues (38) and (40). With phloroglucinol both epoxides (5) and (7) afford 2,3-diarylbenzofurans [(27) and (29) respectively] which presumably originate via acid-catalysed conversion of intermediate 3-aryl-2-hydroxyisoflavanones (21) and (22). Differences regarding regioselectivity between the respective nucleophiles (methanol vs. phenolic units), and between the phenolic moieties mutually, are rationalised in terms of the effect of nucleophilicity and of steric constraints imposed on the transition states leading to the respective regioisomers.