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
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 exo-imine. 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.