Enantioselective preparation of 3-substituted-4-pentenoic acids via the claisen rearrangement

Asymmetric C-C bond formation via the diastereoselective aza-Claisen rearrangement of N-allylketene N,O-acetal 4 is described. The starting materials, allylic alkylating agent 1 and optically pure oxazoline 2, are easily prepared and, in a one-pot procedure, generate rearranged oxazolines 5 in 52-94% diastereomeric excess. The overall chemical yields for 2 → 5 range from 51 to 78%. The aza-Claisen rearrangement (4 → 5) proceeds with excellent N,O-acetal face selectivity and with good to excellent chair selectivity. Hydrolysis of rearranged oxazoline 5 completes an enantioselective synthesis of 3-substituted pent-4-enoic acids.