2β,3β-Difluorosialic acid derivatives structurally modified at the C-4 position: synthesis and biological evaluation as inhibitors of human parainfluenza virus type 1

A series of 4-O-substituted 2β,3β-difluorosialic acid derivatives (3a–d) has been synthesized. A key intermediate was synthesized efficiently by the electrophilic syn-addition of fluorine to the double bond of a glycal precursor using molecular fluorine or xenon difluoride in the presence of BF3·OEt2. Among compounds 3a–d, the 4-O-thiocarbamoylmethyl derivative 3c showed the most potent inhibitory activity against sialidase of human parainfluenza virus type 1.