α-Quaternary Proline Derivatives by Intramolecular Diastereoselective Arylation of N-Carboxamido Proline Ester Enolates

Pyrrolidine-2-carboxylate esters substituted in the 3-, 4- or 5-positions were converted to their N'-aryl urea derivatives. Deprotonation at the 2-position to form a potassium enolate led to migration of the N'-aryl substituent to the 2 position of the pyrrolidine ring, followed by cyclization of the resulting urea to give bicyclic alpha-aryl hydantoin derivatives of substituted prolines. Depending on the substitution pattern of the starting material, high diastereoselectivity was observed in the aryl migration, allowing formation of the products in enantiomerically enriched form, despite the intermediacy of a planar enolate. The hydrolysis of the bicyclic hydantoins under basic conditions gave a range of enantiopure and enantioenriched quaternary alpha-aryl proline derivatives.