Access to 3‑<i>O</i>‑Functionalized <i>N</i>‑Acetylneuraminic Acid Scaffolds

Direct access to 3-<i>O</i>-functionalized 2-α-<i>N</i>-acetylneuraminides and their corresponding 2,3-dehydro-2-deoxy-<i>N</i>-acetyl­neuraminic acid derivatives is described. Initially, a stereoselective ring-opening of the key intermediate <i>N</i>-acetylneuraminic acid (Neu5Ac) 2,3-β-epoxide with an alcohol provided the 3-hydroxy α-glycoside. <i>O</i>-Alkylation of the C3 hydroxyl group generated novel 3-<i>O</i>-functionalized Neu5Ac derivatives that provided the corresponding unsaturated derivatives upon elimination

Exploring the Interactions of Unsaturated Glucuronides with Influenza Virus Sialidase

A series of C3 <i>O</i>-functionalized 2-acetamido-2-deoxy-Δ⁴-β-d-glucuronides were synthesized to explore noncharge interactions in subsite 2 of the influenza virus sialidase active site. In complex with A/N8 sialidase, the parent compound (C3 OH) inverts its solution conformation to bind with all substituents well positioned in the active site. The parent compound inhibits influenza virus sialidase at a sub-μM level; the introduction of small alkyl substituents or an acetyl group at C3 is also tolerated

Structural Insights into Human Parainfluenza Virus 3 Hemagglutinin–Neuraminidase Using Unsaturated 3‑<i>N</i>‑Substituted Sialic Acids as Probes

A novel approach to human parainfluenza virus 3 (hPIV-3) inhibitor design has been evaluated by targeting an unexplored pocket within the active site region of the hemagglutinin–neuraminidase (HN) of the virus that is normally occluded upon ligand engagement. To explore this opportunity, we developed a highly efficient route to introduce nitrogen-based functionalities at the naturally unsubstituted C-3 position on the neuraminidase inhibitor template <i>N</i>-acyl-2,3-dehydro-2-deoxy-neuraminic acid (<i>N</i>-acyl-Neu2en), via a regioselective 2,3-bromoazidation. Introduction of triazole substituents at C-3 on this template provided compounds with low micromolar inhibition of hPIV-3 HN neuraminidase activity, with the most potent having 48-fold improved potency over the corresponding C-3 unsubstituted analogue. However, the C-3-triazole <i>N</i>-acyl-Neu2en derivatives were significantly less active against the hemagglutinin function of the virus, with high micromolar IC₅₀ values determined, and showed insignificant <i>in vitro</i> antiviral activity. Given the different pH optima of the HN protein’s neuraminidase (acidic pH) and hemagglutinin (neutral pH) functions, the influence of pH on inhibitor binding was examined using X-ray crystallography and STD NMR spectroscopy, providing novel insights into the multifunctionality of hPIV-3 HN. While the 3-phenyltriazole-<i>N</i>-isobutyryl-Neu2en derivative could bind HN at pH 4.6, suitable for neuraminidase inhibition, at neutral pH binding of the inhibitor was substantially reduced. Importantly, this study clearly demonstrates for the first time that potent inhibition of HN neuraminidase activity is not necessarily directly correlated with a strong antiviral activity, and suggests that strong inhibition of the hemagglutinin function of hPIV HN is crucial for potent antiviral activity. This highlights the importance of designing hPIV inhibitors that primarily target the receptor-binding function of hPIV HN