Studies on lactams—VIII : The conformation of n-aryl lactams

Abstract

N-Aryl-β-lactams have been found to be characterized by strong UV maxima at about 250 mμ. Some N-aryl-γ-lactams also show similar absorption max but heavily substituted γ-lactams or N-phenyl δ-lactam display only low absorption in this region. A study of UV and NMR spectra indicates that in N-aryl β-lactams, the heterocyclic ring and the three valences of nitrogen are planar and the N-aryl ring lies in the plane of the β-lactam. This planarity as well as the strong UV absorption are the result of an extended conjugation between the aryl ring and the amide function. Ortho substitution on the aryl ring causes a slight departure from this planarity and reduces the intensity of the UV absorption. In case of N-aryl γ-lactams, ortho substitution on the aryl ring causes enough departure from planarity to eliminate altogether the absorption max near 250 mμ.