Sialic acid, common derivatives

TV-acetylneuraminic acid (the most common type of sialic acid = Sia), Sialic acid is a more general term including N- and O-substituted derivatives of neuraminic acid. Compounds in capital boldface type are the unmodified sugars or their amine derivatives. 

Ikeda et al. reported that a combined system of bismuth triflate [Bi(OTf)3] and BF3-Et20 in dichloromethane is an efficient promoter for the glycosylation of sialyl acetates among various Lewis acids, for example, BiCl3, Sc(OTf)2, Yb(OTf)3, Zn(OTf)2, and TMSCl, and their combinations, which showed the first example of utilization of Bi(OTf)3 in the glycosylation of common sialic acid derivatives (O Scheme 17) [22]. 

Natural sialic acids (Schauer 1982 1991) are derivatives of 5-amino-3,5-dideoxy- D-glycero-D-galacto-nonu osonic acid 12.1. This awkward name has been replaced by neuraminic acid . The most common derivative is N-acetyl-neuraminic acid 12.2 whose configuration is easy to memorize because, in the Fischer representation, 12.3, it is presented as an aldolic condensation product of N-acetylmannosamine (2-acetamido-2-deoxy-D-mannose) and pyruvic acid. When the expression sialic acid is used without any other precision, it is in reference to derivative 12.2. It exists in the free state or glycosidated in the d-conformation, which allows an equatorial disposition of the three-carbon side chain. Structure 12.2 represents the stable /3-anomer of the free sugar with an axial anomeric hydroxyl group and all-equatorial non-anomeric substituents. An X-ray spectrum of this crystallized /3- anomer confirms this conformation and reveals, moreover, that the side chain has the zig-zag conformation with two... 

The glycosphingolipids of plants tend to resemble those of animals in being largely derived from glucosylceramide, and their assembly does not add anything to the principles illustrated by their animal counterparts. Sialic acid again appears to be absent. Glycoglycerolipids are common. 

As with the uronic acids the symbols for these are derived from those of the corresponding simple sugars. All common amino- and acetamido-sugars found in polymers are substituted at C-2, with exception of sialic acids (see below), and so no symbol is used to denote the site of substitution. 

The final step in the activation of sialic acid to a nucleotide sugar occurs by the reaction shown above. It is clear that this reaction is unique in the sense that (1) the monosaccharide itself is the substrate rather than the more common 1-phosphate, and (2) the nucleotide sugar contains only one phosphate group rather than two. Like several of the other enzymes participating in sialic acid biosynthesis, CMP-sialic acid synthetase can utilize the N-glycolyl as well as the N-acetyl derivative. 

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