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 pseudo-pericyclic reactions are involv ed in the whole process. The first key reaction is a retro-[4-exo-dig] cycl ization instead of a thermal conrotatory electrocyclic ring opening. Magnet ic characterization of the corresponding transition structure shows antiaro matic character, despite the low activation energy associated with this pro cess. 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 corr elation between aromaticity and pseudopericyclic reactions. In contrast, th ermal-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.