Energetics of the metabolic production of (E,E)-muconaldehyde from benzene via the intermediates 2,3-epoxyoxepin and (Z,Z)- and (E,Z)-muconaldehyde: ab initio molecular orbital calculations.

To assess the feasibility of the pathway set out in the title, we have carried out ab initio molecular orbital calculations of the energies of cisoid- and transoid-2,3-epoxyoxepin and the eZzZz-, eZeZz-, eEeZz-, eEeEz-, and eEeEe-conformers of muconaldehyde at the MP2/6-31G* (frozen core, valence orbitals active) level with full geometry optimization using the split-valence RHF/6-31G* basis set. Including thermal energies, derived from vibrational frequencies obtained at the RHF/6-31G*//RHF/6-31G* level, reaction energies (that would correspond to gas phase data at 298 K) have been evaluated. The muconaldehyde conformers are lower in energy than the epoxyoxepin, i.e., are formed exothermically, and in accord with experiment, the Z,Z-conformers are less stable than the E,Z, and the E,Z less stable than the E,E. In addition, we have characterized the transition state for the fission of the C-O bond in the epoxy ring and the C-O bond in the oxepin ring of transoid-2,3-epoxyoxepin, thereby generating (eZzZz)-muconaldehyde. With an activation energy of 16.5 kcal mol-1 the half-life of the epoxyoxepin is very short at the temperatures employed in the experiments, less than 1 min, which explains why it has not been detected. To gain some insight into the isomerization about the carbon-carbon double bonds in the muconaldehyde conformers, we have examined the possible involvement of 2-formyl-2H-pyran and cyclobutene-3,4-dicarboxaldehyde as intermediates. In model reactions simulating the microsomal monooxygenase system, the formation of benzene oxide and of the epoxyoxepin are found to be very favorable exothermic processes.