Ab initio calculations provide with the models to explain the main features of the cycloaddition between olefins and isocyanates to yield 2-azetidinones (beta-lactams). It is found that the reaction takes place via concerted transition structures involving retention of configuration in the starting olefins. These transition structures have zwitterionic character. The presence of olefins having pi-donating groups and/or isocyanates with electron-withdrawing groups diminish the synchronicity as well. as the activation energy of the reaction, yielding exclusively the 4-substituted regioisomers. The solvent enhances the asynchronicity of the reaction and can modify its profile from a concerted to a two-step process, thus explaining the loss of stereospecificity observed in the reaction between sulfonyl isocyanates and vinyl ethers.