Asymmetric Synthesis of Diazepino-β-lactams by [2 + 2] Cycloaddition of an ‘Evans-Sjogven’ Ketene with 1H-1,2-Diazepines

The ketene derivative of the chiral oxazolidinone 1 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 give sequencially the zwitterionic intermediates and the final cycloadducts.