Synthesis of (−)-Cytisine Using a 6-endo aza-Michael Addition

An asymmetric synthesis of (−)-cytisine has been achieved. The piperidine C-ring was formed using a stereodivergent intramolecular 6-endo aza-Michael addition. The B-ring was established by intramolecular pyridine N-alkylation. The absolute stereochemistry was established by an Evans acyl oxazolidinone enolate alkylation reaction that proceeded with an unexpected stereochemical outcome due to participation of the pyridine nitrogen lone pair.