C-glycosyl linkage

The naturally occurring C-nucleosides containing C-glycosyl linkages are shown in Table 1. 

The aim of this chapter is to describe the methods currently available for the stereoselective synthesis of compounds having 1,2-cz x-O-glycosylic or 1,2-czj-C-glycosylic linkages.11... 

Dehydration of pentitols, hexitols, 1-amino-1-deoxypentitols and 1-amino-1-deoxyhexitols by heating in strong acid media, affords anhydropentitols presenting in some cases 1,2-m-C-glycosylic linkages.82... 

Invariably, the predominant (or exclusive) product has a-stereochemistry at the newly formed C-glycosyl linkage (see Scheme 77), and disubstitution at C(l) has also been achieved (Scheme 25, Sect. 2.2.1). Allylstannanes serve as radical acceptors in intermolecular processes (Scheme 78) [122, 134, 135] and, under appropriate circumstances, dimerization of the anomeric radicals is observed, as in C-disaccharide 291 (Scheme 79) [136]. 

C-Glycosidation. Reaction of phenyl thioglycosides with dimethyl diazomalonate in the presence of rhodium(II) acetate allows for the construction of C-glycosyl linkages in moderate yields under neutral conditions (eq 14). The methodology is applicable to both thiofuranosides and thiopyranosides. A related insertion process allows for a mild carbon extension of 4-phenylthioazetidinones (eq 15). ... 

These observations have several implications for studies of glycoproteins (i) the resonances of anomeric carbon atoms that are involved in N-glycosylic linkages will be rather difficult to identify, because of their proximity to nonanomeric-carbon resonances and (ii) due to the proximity of the chemical shifts of C-l and C-5 (they are only 0.6 p.p.m. apart for / -d-G1cNAc — Asn), it may be difficult, but not impossible, to ascertain whether N-glycosylic linkages exist in the glycoprotein (see later). 

The anomeric linkages can be classified according to the relative and absolute configuration at C-l and C-2 (Fig. 1). 1,2-cis-Glycosylic linkages... 

C-Glycosidic linkages are present in various natural products and may inhibit glycosidases due to the structural analogy to their O-counterparts.42 Glycosyl phosphates 63 and 48 served successfully in the a-selective synthesis of aryl... 

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