Access to Enantio-enriched Substituted α-Trifluoromethyl Azepanes from l-Proline.

4-Substituted α-trifluoromethyl azepanes C were synthesized via the ring expansion of trifluoromethyl pyrrolidines A, which were synthesized from l-proline via a regioselective ring-opening of a bicyclic azetidinium intermediate B by various nucleophiles. The regioselectivity of the ring expansion is induced by the presence of a trifluoromethyl group. The chirality of the starting material was transferred to the azepanes with high enantiomeric excess.