Hydroxyoxiranone: an ab initio MO investigation of the structure and stability of a model for a possible α-lactone intermediate in hydrolysis of sialyl glycosides

The standard enthalpy of formation ΔH°f,298 for hydroxyoxiranone is estimated as –377 ± 10 kJ mol–1 by means of ab initio molecular orbital calculations at the QCISD(T)(full)/6-311G(2df,p)//MP2(full)/6-311G(d,p) level of theory, corresponding to a conventional ring strain energy of 104 kJ mol–1. The hydroxy substituent on Cα stabilizes the α-lactone by 65 kJ mol–1 with concomitant elongation of the bond from Cα to the endocyclic oxygen On. There is a much larger stabilization (205 kJ mol–1) by the hydroxy substituent upon the zwitterion obtained by heterolysis of the Cα–On bond, and upon the carbene obtained by decarboxylation of the zwitterion. The relative energies of the α-lactone, zwitterion and carbene have been determined by MP2(fc)/6-31+G(d) calculations in vacuo and with the IPCM method for aqueous solvation. Solvation by this continuum method preferentially stabilizes the zwitterion, although at the IPCM-MP2(fc)/6-31+G(d) level the α-lactone is still 21 kJ mol–1 lower in energy than the zwitterion.