Synthesis and Application of 3,3-Difluoro Sialic Acid as a Sialyltransferase and Neuraminidase Inhibitor

Abstract

Abstract Sialic acid (Neu5Ac) is a monosaccharide terminating glycoconjugates, serving as ligands for lectins in health and disease. Tools to target sialyltransferases (STs) and neuraminidases (NEUs) are important in the study of sialoglycans. 3FaxNeu5Ac is a common ST inhibitor, while 2,3diFNeu5Ac is a covalent mechanism-based NEU inactivator. Here, we demonstrate the advantages of having two fluorines at C3 for both classes of inhibitors. Motivated by the finding that 3FaxNeu5Ac is transferred by STs slowly, we aimed to synthesize 3,3-difluoro-Neu5Ac (3,3diFNeu5Ac). Two fluorines at C3 successfully prevented ST-mediated transfer. Unexpectedly, CMP-3,3diFNeu5Ac is 5–40-fold more potent than CMP-3FaxNeu5Ac against five human STs. Moreover, CMP-3,3diFNeu5Ac was 6–11-fold more potent toward ST3GAL1 than the four STs tested. While protected 3,3diFNeu5Ac was not very active in cells, CMP-3,3diFNeu5Ac was surprisingly active, showing enhanced inhibition toward ST3GAL1 over ST6GAL1, translating to selective inhibition of Siglec-7 ligands over Siglec-2 ligands. To target NEUs, 2,3,3-trifluoro-Neu5Ac (2,3,3triFNeu5Ac) was synthesized as a mechanism-based covalent NEU inhibitor. For 2,3diFNeu5Ac, a covalent glycosyl-enzyme intermediate was formed that broke down, but covalent inhibition of bacterial and viral NEUs by 2,3,3triFNeu5Ac was more stable. Therefore, through synthetic access to 3,3diFNeu5Ac, more potent and stable inhibitors of STs and NEUs were developed, which can serve as better tools to probe the functions of sialoglycans.