Identification of the Erythrina phytoalexin cristacarpin and a note on the chirality of other 6a-hydroxypterocarpans

Cristacarpin, a new phytoalexin from Erythrina crista-galli is assigned the structure (?)-3,6a-dihydroxy-9-methoxy-10-γ,γ-dimethylallyl-cis-pterocarpan. It is accompanied by the known phytoalexins phaseollidin and demethylmedicarpin in this plant. Cristacarpin, phaseollidin and demethylmedicarpin were also obtained from E. sandwicensis and (together with isomedicarpin) from the related legume, Psophocarpus tetragonolobus. A compilation of selected optical rotation, NMR and conformational data for all known 6a-hydroxypterocarpans is presented and it is concluded that the previously assigned chiralities of neobanol, glyceollins I–IV and 3,6a,9-trihydroxypterocarpan should be reversed. Chirality assignments are made for a number of previously unassigned compounds.