A Concise Total Synthesis of (±)-Trigonoliimine B

Trigonoliimine B, a hexacyclic alkaloid, is synthesized in seven steps from simple starting materials. The synthesis features the use of an alpha-isocyanoacetate as a glycine template for the preparation of an alpha,alpha-disubstituted alpha-amino ester that is appropriately functionalized for the construction of C, D, and E rings. Sulfolane was found to be the solvent of choice for the unprecedented Bischler-Napieralski reaction implemented for the construction of a seven-membered ring with concurrent formation of an exo-imine function.