On the mechanism of conversion of N-acyl-4-acyloxy-beta-lactams into 2-substituted 1,3-oxazin-6-ones. Can a low-barrier transition state be antiaromatic?

The mechanism of the conversion of N-acyl-4-acyloxy-beta-lactams into 1,3-oxazin-6-ones has been investigated using ab initio and density functional theories. It has been found that two pseudopericyclic reactions are involved in the whole process. The first key reaction is a retro-[4-exo-dig] cyclization instead of a thermal conrotatory electrocyclic ring opening. Magnetic characterization of the corresponding transition structure shows antiaromatic character, despite the low activation energy associated with this process. The second step is very exothermic and has no activation barrier. It corresponds to another pseudopericyclic reaction instead of a six-electron disrotatory electrocyclization. These results confirm that there is no correlation between aromaticity and pseudopericyclic reactions. In contrast, thermal-symmetry-allowed pericyclic reactions are always aromatic. Therefore, magnetic analysis of the corresponding transition structures constitutes a useful tool to distinguish between both kinds of processes.

Conversion of N-acyl-4-acyloxy-beta-lactams into 1,3-oxazin-6-ones: two consecutive pseudopericyclic processes

N-Acyl-4-acyloxy-beta-lactams are converted into 1,3-oxazin-6-ones under basic conditions. This transformation is believed to proceed via N-acylazetones, which rearrange to the final products by a sequence of two electrocyclic processes. The calculated (RHF and B3LYP) transition structures of both concerted reactions are shown to present characteristic pseudopericyclic orbital topologies.