Treatment of protected N-acetylneuraminic acid 2-formyl-4-nitrophenyl glycoside with (diethyl-amino)sulfur trifluoride (DAST) and methanol resulted, apart from the known transformation of the formyl group into a difluoromethyl one, in an undocumented formation of 5-(O-methyl acetimidate) of Neu5Ac. This imidate was converted into the target 5-acetamido derivative under the action of aqueous TFA. The yield of the target product (94%) was twice as high as that reported earlier.