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 C_α stabilizes the α-lactone by 65 kJ mol^-1 with concomitant elongation of the bond from C_α to the endocyclic oxygen O_n. There is a much larger stabilization (205 kJ mol^-1) by the hydroxy substituent upon the zwitterion obtained by heterolysis of the C_α-O_n 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 a-lactone is still 21 kJ mol^-1 lower in energy than the zwitterion.