Synthesis of naturally occurring o-heterocyclic compounds of biological activity

Abstract

The total synthesis of (-)-cabenegrin A-I was achived via (-)-6aR,11ar maackiain, which was obtained by optical resolution of racemic maackiain using S-(-)-α-methylbenzyl isocyanate. The synthesis of rac-maackiain was performed both with the Heck oxyarylation of 7-benzyloxy-2H-chromene and the BF3·OEt2 mediated ring closure of isoflavan-4-ol derivatives, the latter of which provided much higher yields. The first enantioselective synthesis of trans-6aS,11aR-pterocarpan and its conversion to cis-6aS,11aS-ptarocarpan was also presented starting from racemic 2′-benzyloxyflavanone. Their stereochemistry was deduced by circular dichroism (CD) as well as by X-ray analysis of the ketal intermediate.