Flexible approach to stemona alkaloids: total syntheses of (-)-stemospironine and three new diastereoisomeric analogs.

Total syntheses of (-)-stemospironine and three new diastereoisomeric analogs have been completed through a flexible strategy devised for Stemona alkaloids. The azabicycle 7 is the pivotal intermediate, from which the sequence splits according to each particular target. The most remarkable differential feature for stemospironine is the installation of the spiranic γ-lactone through an intramolecular Horner-Wadsworth-Emmons olefination. The configuration of the stereogenic center at C-11 was controlled by fine-tuning of the synthetic sequence.