Divergent Stereocontrolled Synthesis of the Enantiopure Tetracyclic Cores of Asparagamine A and Stemofoline via an Intramolecular 2‑Propylidine-1,3-(bis)silane Bicyclization

A concise and highly diastereoselective synthesis of the polyfused tetracyclic cores of the Stemona alkaloids asparagamine A and stemofoline that relies on a 2-propylidine-1,3-(bis)­silane bicyclization onto a enantiodefined pyrrolidine 2,5-di­(cation) equivalent derived from l-malic acid is reported. A crucial feature of this divergent synthetic approach involves the solvolysis of a transient and highly labile tertiary-propargylic hydroxylactam trifluoroacetate in the strongly ionizing medium 5 M LiClO4/Et2O. The acyliminium ion generated in this manner undergoes stereospecific interception by the aforementioned (bis)­silane nucleophile.