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 α-isocyanoacetate as a glycine template for the preparation of an α,α-disubstituted α-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 <i>exo</i>-imine function