From racemic to enantioselective total synthesis of trigonoliimines via development of an organocatalytic enantioselective Michael addition of α-aryl-α-isocyanoacetate to vinyl phenyl selenone.

Abstract

Trigonoliimines are hexacyclic bisindole alkaloids isolated recently by Hao and co-workers. A synthesis of (±)-trigonoliimine B was accomplished in seven steps from simple starting materials featuring the Bischler-Napieralski reaction for closing the seven-membered ring with concomitant formation of an exoimine. Sulfolane was found to be the solvent of choice for this unprecedented transformation. An organocatalytic enantioselective synthesis of α,α'-disubstituted α-amino acids was subsequently developed using methyl α-aryl-α-isocyanoacetates as glycine templates and vinyl phenyl selenone as a Michael acceptor. Using one of this Michael adducts as a starting material, total synthesis of both (+)- and (-)-trigonoliimine A was subsequently realized.