A Kinetic Trapping Approach for Facile Access to 3F_axNeu5Ac and a Photo-Cross-Linkable Sialyltransferase Probe

Sialic acid (Neu5Ac) is installed onto glycoconjugates by sialyltransferases (STs) using cytidine monophosphate-Neu5Ac (CMP-β-d-Neu5Ac) as their donor. The only class of cell-active ST inhibitors are those based on a 3F_axNeu5Ac scaffold, which is metabolically converted into CMP-3F_axNeu5Ac within cells. It is essential for the fluorine to be axial, yet stereoselective installation of fluorine in this specific orientation is challenging. Sialic acid aldolase can convert 3-fluoropyruvate and 2-acetamido-2-deoxy-d-mannopyranose (ManNAc) to 3FNeu5Ac, but stereocontrol of the fluorine in the product has not been possible. We hypothesized that the 3F_ax kinetic product of a sialic acid aldolase reaction could be trapped by coupling with CMP-sialic acid synthetase to yield CMP-3F_axNeu5Ac. Here, we report that highly active CMP-sialic acid synthetase and short reaction times produce exclusively CMP-3F_axNeu5Ac. Removal of CMP from CMP-3F_axNeu5Ac under acidic conditions unexpectedly led to 3-fluoro-β-d-Neu5Ac 2-phosphate (3F_axNeu5Ac-2P). Alkaline phosphatase successfully converted 3F_axNeu5Ac-2P to 3F_axNeu5Ac, enabling stereochemically controlled access to 3F_axNeu5Ac, which is effective in lowering the sialoglycan ligands for Siglecs on cells. Moreover, our kinetic trapping approach could be used to access CMP-3F_axNeu5Ac with modifications at the C5, C9, or both positions, which enabled the chemoenzymatic synthesis of a photo-cross-linkable version of CMP-3F_axNeu5Ac that selectively photo-cross-linked to ST6GAL1 over two other STs.