underwent non-concerted stereo specific [2 + 2] cycloadditions with the (Z)-imine moiety of diazepines 2, leading thereby with good diastereoselection to the trans-β-lactam adducts 3 (major) and 4 (minor). The absolute configuration of the major cycloadduct 3a was determined by an X-ray analysis. Its formation is discussed in terms of minimisation of steric interaction in the two transition states which