Stereoselectivity in <i>N</i>‑Iminium Ion Cyclization: Development of an Efficient Synthesis of (±)-Cephalotaxine

A stereoselective <i>N</i>-iminium ion cyclization with allylsilane to construct vicinal quaternary–tertiary carbon centers was developed for the concise synthesis of (±)-cephalotaxine. The current strategy features a TiCl₄-promoted cyclization and ring-closure metathesis to furnish the spiro-ring system. The stereochemical outcome in the <i>N</i>-acyliminium ion cyclization was rationalized by the stereoelectronic effect of the <i>Z</i>- or <i>E</i>-allylsilane. Two diastereomers arising from the cyclization were merged into the formal synthesis of (±)-cephalotaxine