The formation of beta-lactone through Lewis acid-promoted [2 + 2] cycloaddition is studied using semiempirical (AM1/RHF and AM1/CI) and nb initio (HF/6-31G* and MP2/6-31G*) calculations. After a preliminary semiempirical study of the BF3-catalyzed parent reaction through two distinct reaction paths, ab initio and/or semiempirical studies on solvent and Lewis acid (BH3 and BF3) effects concentrate on the mechanism involving the prior formation of the C-C bond. At the HF/6-31G* level of theory the introduction of BS induces a reduction of the activation energy from 40.8 to 11.9 kcal/mol, and calculations performed with AM1/COSMO showed that the introduction of a solvent results in the formation of an earlier transition state. The case of BH3 is somehow more complicated since the studied system induces, both at the semiempirical and nb initio levels, a hydride transfer leading to a very stable product.