Reduction of an enaminone: Synthesis of the diamino alcohol core of ritonavir

The reduction of (SS)-2-amino-5-dibenzylamino-4-oxo-1,6-diphenylhex-2-ene was optimized for diastereoselectivity and overall conversion to (2S,3S,5S)-5-amino-2-dibenzylamino-3-hydroxy-1,6-diphenylhexane (2a), A two-step reduction sequence is described wherein the enamine is reduced with a borane-sulfonate derivative followed by reduction of the resulting ketone with sodium borohydride. The desired 2a was obtained with 84% diastereoselectivity and an acyclic 1,4 stereoinduction ratio of 14:1, This methodology has been used to produce multikilogram quantities of the diamino alcohol core of Ritonavir and should be general to the synthesis of related diamino hydroxyethylene isosteres.