A lactonization method to assign the anomeric configuration of the 3,4-unsaturated congeners of N-acetylneuraminic acid

Assigning the correct configuration at C2 in sialosides is a standing problem due to the absence of an anomeric hydrogen. All different empirical rules that have been proposed over the years lack general applicability. In particular, the correct configuration of several 3,4-unsaturated derivatives of N-acetylneuraminic acid, which have been recently shown to be novel sialidase/neuraminidase inhibitors, could only be tentatively assigned by similarity with the reported 3,4-unsaturated 2O-methyl sialosides. In this work, we overcome this hassle as we devised a rapid synthetic method to unequivocally resolve the anomeric configuration of the 3,4-unsaturated Neu5Ac derivatives through the synthesis of the corresponding inedited unsaturated 1,7-lactones. Moreover, we discovered a diagnostic 13C NMR signal that allows the formulation of a new empirical rule for the direct assignment of the C2 stereochemistry of these molecules, even when only one of the two C2 epimers is available.