A convenient and versatile synthesis of 6,5- and 7,5-fused bicyclic lactams as peptidomimetics

Chemo-selective reduction of the pyroglutamate ring carbonyl in a serine-pyroglutamate or homoserine to pyroglutamate dipeptide gave a hemiaminal. Treatment of the hemiaminal with a catalytic amount of TFA in CH2Cl2 generated an N-acyliminium ion that was intramolecularly trapped by the side-chain hydroxyl to give a 6,5 or 7,5-bicyclic lactam.