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.