Neuraminic acid anion

Vasella has applied the concept of anomeric anion stabilization by a nitro group to the /3-D-JV-acetyl-D-glucosamine derivative 177, available in four steps from N-acetyl-n-glucosamine [52] (Scheme 39). Reaction of the tetraethylammonium nitronate derived from 177 with aldehyde 178 provides anti-179 which then undergoes stereoselectively reduction (see Sect. 2.2.1) to provide -C-glycoside 180, intermediate in a synthesis of N-acetyl-neuraminic acid. 

On the other hand, the sugar moiety of the glycosphingolipids represents the hydrophilic part of the molecule. The solubility in water is also dependent on the number of monosaccharide residues bound in the molecule. Furthermore, such strongly anionic components as neuraminic acids, and sulfate or phosphate groups, influence the ratio of the solubility in water and fat. These two kinds of solubility characteristics are probably essential in the biological roles of the glycosphingolipids when they function as biomembrane components. 

Holmquist, L., and Brossmer, R., 912a. On the specificity of neuraminidase. The carboxymethyl a-ketoside of N-acetyl-D-neuraminic acid, a Vibrio cholerae neuraminidase substrate having two anionic sites, FEBS Lett. 22 46. 

Sialosides have a distinct mechanism of hydrolysis for its unusual sugar structure of sialic acid. For example, the large 8-dideuterium and small primary kinetic isotope effects observed at the anomeric carhon and the large secondary kinetic isotope effect observed at the carboxylate carbon in the acid-catalyzed solvolysis of CMP-Af-acetyl neuraminate 24 support an oxocarbenium ion-like transition state 25 having the 5S conformation without nucleophilic participation of carboxylate and with the carboxylate anion in a looser environment than in the ground state [15] (O Fig. 3). Such a zwitterion structure is consistent with the results from calculations using the COSMO-AMI method for aqueous solutions [16]. 

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