HYDROXYOXIRANONE - AN AB-INITIO MO INVESTIGATION OF THE STRUCTURE ANDSTABILITY OF A MODEL FOR A POSSIBLE ALPHA-LACTONE INTERMEDIATE IN HYDROLYSIS OF SIALYL GLYCOSIDES

The standard enthalpy of formation Delta H(f,298)degrees for hydroxyox iranone is estimated as -377 +/- 10 KJ mol(-1) by means of ab initio m olecular orbital calculations at the CISD(T)(full)/6-311G(2df,p)//MP2( full)/6-311G(d,p) level of theory, corresponding to a conventional rin g strain energy of 104 kJ mol(-1), The hydroxy substituent on C-alpha stabilizes the alpha-lactone by 65 kJ mol(-1) with concomitant elongat ion of the bond from C-alpha 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-alpha-O-n bond, a nd upon the carbene obtained by decarboxylation of the zwitterion, The relative energies of the a-lactone, zwitterion and carbene have been determined by MP2(fc)/6-31 + G(d) calculations in vacuo and with the I PCM method for aqueous solvation. Solvation by this continuum method p referentially stabilizes the zwitterion, although at the IPCM-MP2(fc)/ 6-31 + G(d) level the alpha-lactone is still 21 kJ mol(-1) lower in en ergy than the zwitterion.