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.