The different modes of interaction between achiral ketenes and homochiral aldimines derived from chiral aldehydes to give the two possible diastereoisomers of cis-beta-lactams have been examined using the AM1 method. It has been found that the step which determines the final stereochemistry is the conrotatory ring closure between C-3 and C-4 atoms. The high diastereoselectivity predicted by our calculations is in excellent agreement with available experimental data. The origin of this high stereocontrol is rationalized on the basis of the stabilizing interaction between the C-X sigma(*) orbital (X being an electronegative atom) and the p atomic orbital of the C-3 atom. A similar treatment has been applied to the interaction between achiral imines and homochiral ketenes. Our calculations predict high diastereomeric excesses which again are in good agreement with the experimentally found diastereoselection.