Development of a Divergent Synthetic Route to the Erythrina Alkaloids: Asymmetric Syntheses of 8-Oxo-erythrinine, Crystamidine, 8-Oxo-erythraline, and Erythraline.

A general synthetic methodology toward the erythrina alkaloids has been developed. Inspired by a proposed biosynthetic mechanism, the medium-sized chiral biaryl lactam was asymmetrically transformed into the common core A-D rings by a stereospecific singlet oxygen oxidation of the phenol moiety, followed by a transannular aza-Michael reaction to the dienone functionality. The late-stage manipulation of the oxidation and oxygenation states of the functional groups on the peripheral moieties enabled the flexible syntheses of the erythrina alkaloids.