A Chiral Pentenolide-Based Unified Strategy toward Dihydrocorynantheal, Dihydrocorynantheol, Protoemetine, Protoemetinol, and Yohimbane

An organocatalytic cross-aldol reaction of formaldehyde (formalin) with alkyl aldehydes, followed by the Z-selective Horner-Wadsworth-Emmons (HWE) reaction and immediate lactonization, afforded γ-alkylated pentenolides in good overall yields and excellent enantioselectivities. Based on this scalable sequence, five quinolizidine alkaloids were synthesized in a unified and concise manner. The development of an in situ activation of a tertiary amide to improve the efficiency of the Bischler-Napieraiski (B-N) reaction was also noteworthy due to the generality to sensitive substrates for a variety of target molecules.