A new entry to α-alkylidene-β-lactams by 4-exo-dig cyclization of carbamoyl radicals

A new synthetic method of α-alkylidene-β-lactam frameworks has been developed. Irradiation of carbamotelluroates 5 having a propargylic group at the nitrogen atom with visible light afforded α-telluromethylene-β-lactams 6 by 4-exo-dig cyclization of carbamoyl radicals, generated by homolytic cleavage of carbamoyl carbon–tellurium bond of 5, with internal C–C triple bond. Density functional theory (DFT) calculations on cyclization step indicate that 4-exo-dig cyclization of carbamoyl radicals is kinetically favored. The existence of a rapid equilibrium between carbamoyl and four-membered vinyl radicals was suggested by control experiments using the corresponding carbamoselenoate.