An NMR study of the lactonization of alpha-N-acetylneuraminyl-(2 --> 3)-lactose.

The composition of the products formed by treatment of commercial alpha-Neu5Ac-(2 --> 3)-beta-D-Galp-(1 --> 4)-D-Glc (3'-sialyllactose) with glacial acetic acid was investigated by 1H-13C one- and two-dimensional NMR spectroscopy and fast atom bombardment-mass spectrometry. The data confirmed that the major product of the reaction was alpha-Neu5Ac-(2 --> 3)-beta-D-Galp-(1 --> 4)-D-Glc-(1c --> 2b)-lactone, which reverted to the starting material on standing in aqueous solution at ambient temperature, but for which complete NMR assignments are reported. The NMR data led to the tentative conclusion that the reaction also yielded small amounts of lactose, and alpha-Neu5Ac-(2 --> 3)-beta-D-Galp-(1 --> 4)-D-Glc-(1c --> 4b)-lactone which was stable in aqueous solution.