Reductive Oxy-Nazarov Cyclization toward Expedient Construction of a Cyclopenta[1,2-b]pyrrolo[1,2-a]azepine Ring System: Formal Total Syntheses of Stemonamine and Cephalotaxine.

Formal total syntheses of stemonamine and cephalotaxine bearing the core cyclopenta[1,2-b]pyrrolo[1,2-a]azepine ring skeleton were achieved. The general synthetic strategy in the synthesis features the reductive oxy-Nazarov cyclization as key step, leading to the versatile construction of N-substituted spiro quaternary stereogenic centers from readily available starting materials.