Azido-sialic acid

Figure 11.22 Azido-sialic acid-containing glycans can be labeled in vivo with biotin-PEG-phosphine using the Staudinger ligation reaction, which forms an amide bond.

The biotinylated glycans on the cell surfaces subsequently may be probed with (strept)avidin reagents to detect the azido-sialic acid modifications. Alternatively, the cells may be lysed and the glycoproteins isolated using an immobilized (strept)avidin or monomeric avidin affinity resin. 

Figure 17.19 An azido-sialic acid derivative that gets incorporated into glycans in cells can be labeled specifically with a biotin-phosphine tag using the Staudinger ligation process. The result is an amide bond linkage with the glycan.

Luchansky SJ, Bertozzi CR. Azido sialic acids can modulate cell-surface interactions. ChemBioChem. 2004 5 1706-1709. Hansen HC, Magnusson G. Synthesis of selected aminodeoxy analogues of galabiose and globotriose. Carb. Res. 1999 322 166-180. 

The synthesis of unsaturated 4-azido-sialic acid derivatives is covered in Chapter 10, and the synthesis of 2-acetamido-l,2-dideoxynojirimycin in Chapter 18. 

Azido-N-acetyl sialic acid derivative (SiaNAz)... 

The methods used for in vivo incorporation of azido-monomers and performing a labeling reaction with live cells are relatively simple. The following protocol is based on the methods of Saxon and Bertozzi (2000), which uses acetylated azidoacetylmannosamine as the azido-monomer source and a biotin-PEG-phosphine compound to biotinylate cell surface glycoproteins at the specific azide-sialic acid incorporation sites (Figure 17.19). 

The sialic acid aldolase-catalyzed condensation of D-mannose 8 and pyruvate led, in an excellent yield, to the synthesis of KDN 9 [33], a natural deaminated neuraminic acid first isolated from rainbow trout eggs [34] and then discovered in other species. The discovery that sialic acid aldolase accepts as substrates D-mannose substituted on the 2-position, even by bulky substituents such as phenyl, azido, or bromine, opened the route to novel unnatural sialic acid derivatives [35-39]. Pentoses also are substrates. N-Substituted neuraminic acids could be prepared either directly from the corresponding Af-substituted mannosamine, such as N-thioacyl derivatives [40], or after reduction and acylation of 5-azido-KDN [41]. Recently, AT-carbobenzyloxy-D-mannosamine was converted, in a good yield, into the N-carbobenzyloxy-neurarninic acid, further used as a precursor of a derivative of castanospermine [42]. 

D. C. M. Kong and M. von Itzstein, The first synthesis of a C-7 nitrogen-containing sialic acid analogue, 5-acetamido-7-azido-3,5,7-trideoxy-D-giycero-D-ga/acro-2-nonulopyranosonic acid (7-azido-7-deoxy-Neu5Ac), Tetrahedron Lett. 36 957 (1995). 

---