Stereoselectivity in N-Iminium Ion Cyclization: Development of an Efficient Synthesis of (±)-Cephalotaxine
1CAS Key Laboratory of Synthetic Chemistry of Natural Substances and ‡CAS Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences , 345 Lingling Road, Shanghai 200032, China.
Abstract:
A stereoselective N-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 N-acyliminium ion cyclization was rationalized by the stereoelectronic effect of the Z- or E-allylsilane. Two diastereomers arising from the cyclization were merged into the formal synthesis of (±)-cephalotaxine.
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