Chemo- and diastereoselective control for a flexible approach to (5S,6S)-6-alkyl-5-benzyloxy-2-piperidinones

Chemo- and diastereoselective transformation of the N,O-acetals and their chain tautomers (4/5), readily derived from protected 3-hydroxyglutarimide 1a, was studied. It was uncovered that while the reaction with a combination of boron trifluoride etherate/zinc borohydride led to cyclic products (5S,6S/R)-6-alkyl-5-benzyloxy-2-piperidinones 3/2, and 6 in modest chemo- and diastereoselectivities, the reaction of 4/5 with zinc borohydride led exclusively to the formation of the ring-opening products 6 in excellent anti-diastereoselectivities. On the basis of the latter reaction, a flexible approach to (5S,6S)-6-alkyl-5-benzyloxy-2-piperidinones 3 was disclosed.