Reversible Diastereoselective Photocyclization of Phenylglyoxylamides of α-Amino Acid Methyl Esters to 3-Hydroxy β-Lactams

Phenylglyoxylamides 1a-m were prepared from enantiomerically pure (1b-l) α-amino acid methyl esters in high yields and irradiated in acetonitrile solutions with λ = 300 nm to give the β-lactams 2a-m as photocyclization products in moderate (38 and 40% for 2j and 2k) to excellent yields (85-100%) as racemic mixtures. The photocyclization is completely reversible in the absence of trace amounts of acid and the diastereoselectivity of the lactam formation largely depends on the amount of acid in the reaction medium. The isoleucine and a dipeptide derivative 1f and 5, respectively, enabled NMR-spectral evidence for the zero degree of chirality memory in the photocyclization.