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.

Total synthesis of the putative structure of stemonidine: the definitive proof of misassignment.

[structure: see text] The total synthesis of the putative structure of the Stemona alkaloid stemonidine has been completed. The key transformations include a 1,3-dipolar cycloaddition of a chiral nitrone derived from (S)-prolinol and a spirolactonization process involving the generation of the critical stereocenter. The NMR data of the synthetic material do not match those reported for the natural product.