THE N-ACETYL NEURAMINYL OXOCARBENIUM ION IS AN INTERMEDIATE IN THE PRESENCE OF ANIONIC NUCLEOPHILES

Solvolysis of CMP N-acetyl neuraminate (CMP-NeuAc) in 1.8 M acetate bu ffer at pH 5 containing 0.9 M azide results in the formation of both a nomers of 2-deoxy-2-azido N-acetyl neuraminic acid in addition to N-ac etyl neuraminic acid as determined by H-1-NMR product analysis. A rate dependence on [azide] was observed with an apparent bimolecular rate constant of (2.1 +/- 0.3) x 10(-3) M-1 min(-1) which could only accoun t for half of the azido-NeuAc formed. Comparison of rate, product rati o, and stereochemical data indicate that concurrent pathways for forma tion of N-3-NeuAc are operative, with 17% of product forming from reac tion of azide and the tight ion pair, 12% via the solvent separated io n pair, and 6% from the free NeuAc oxocarbenium ion. From the correcte d product ratio data, the lifetime of the oxocarbenium ion was estimat ed to be greater than or equal to 3 x 10(-11) s. Solvolysis of CMP-Neu Ac at pL = 5.0 afforded an observed solvent deuterium isotope effect ( SDIE) k(H2O)/k(D2O) = 0.45, consistent with specific acid catalysis of glycosidic bond cleavage. A SDIE of 0.66 for the apparent bimolecular azide trapping pathway was also observed. An apparent isotope effect of similar to 1.1 for trapping of the N-acetyl neuraminyl oxocarbenium ion by water was determined by product analysis of azide trapping in H2O and D2O. An ab initio transition state for attack of water on an N -acetyl neuraminyl oxocarbenium ion model was located which featured a hydrogen bond between the oxocarbenium ion carboxylate and water; pro ton transfer was not part of the reaction coordinate. It is proposed t hat the N-acetyl neuraminate carboxylate group stabilizes an intermedi ate oxocarbenium ion, but the barrier for capture by water is lowered by a transition state hydrogen bond.