Alkaline and acidic hydrolysis of the b-lactam ring

• A complete study of the alkaline and acidic hydrolysis of the ß-lactam ring of azetidin-2-one was carried out using ab initio molecular orbital calculations at the RHF/6-3l+G* and RHF/6-31G** levels, respectively. Alkaline hydrolysis has been studied through a BAC2 mechanism characterized by a nucleophilic attack on the ß-lactam carbonyl group, formation of the tetrahedral intermediate and cleavage of the C-N bond until the formation of the final product of the reaction, this being the limiting step of the reaction. On the other hand, the acidic hydrolysis has been studied by means of a A- 1 type unimolecular mechanism, characterized by a nitrogen-protonation followed by an opening of the ring and further addition of water to the carbonyl group. The system involving the azetidin-2-one ring, the H30+ ion and a water molecule has been considered. Three transition states have been identified; the barriers corresponding to the addition of H+ to the nitrogen and the addition of water to the carbonyl group are practically negligible (first and third reaction steps, respectively).