Chemical shift non-equivalence of ring protons in N-substituted β-lactams

A study is reported on the chemical shift non-equivalence of the diastereotopic protons at C-4 in identically 3,3-disubstituted β-lactams having an asymmetric carbon atom in positions ranging from β to ε of the chain attached to the nitrogen atom. The non-equivalence has been correlated with the nature of the substituents bonded to the chiral centre and to its distance from the C-4 protons.