A complete study on the acidic hydrolysis of the azetidin-2-one ring,
an essential component of beta-lactam compounds, has been carried out
by means of ab initio calculations by using a 6-31G* type basis set.
The former reaction has been studied by means of an A-1 type unimolecu
lar mechanism, characterized by an N-protonation followed by an openin
g of the ring and further addition of water to the carbonyl group. The
system involving the azetidin-2-one ring, the H3O+ ion and a water mo
lecule has been considered, where three transition states have been id
entified, being the barriers corresponding to the addition of H+ to th
e nitrogen and the addition of water to the carbonyl group being pract
ically negligible (first and third reaction steps, respectively). The
energy barrier for the opening of the ring (second reaction step) was
14.23 kcal mol(-1). (C) 1998 Elsevier Science B.V.