Anomeric anions

The main problem of these protocols is (3-elimination which occurs if a leaving group such as an alkoxy is present in the adjacent position. The elimination is prevented if the anomeric anion is strongly stabilized, for example by a nitro group,31 or in presence of a bad leaving group, such as the anions -CC32 or -NAc (Fig. 13).33... 

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. 

Prandi, J, Audin, C, Beau, J M, Synthesis of C-glycosides boron trifluoride etherate-promoted opening of epoxides by anomeric anions. Tetrahedron Lett., 32, 769-772, 1991. 

C-Glycosylation Oxocarbenium cation Anomeric radical Anomeric anion Stereoelectronic effect... 

In contrast to C-glycosylation by nucleophilic adchtion to an electrophilic carbohydrate, the use of an anomeric anion has found limited use for such a reaction due to instability. However, C-glycosylation by means of anomeric anion species is of signihcance in some special cases [74], The anomeric anion intermediate can be stereoselectively prepared by (i) reduc-hve metalation of anomeric halides or sulfones, (ii) transmetalation of glycosyl stannanes, and (iii) direct deprotonation of the anomeric proton. This section deals with three types of the reaction using different anomeric anions. 

As already alluded to in Schemes 3.2.5 and 3.3.2, the carboxyl group is an excellent stabilizing group for anomeric anions. Ci carboxyl glycosides are easily prepared from a number of methods discussed in Chapter 2. Additionally, many naturally occurring Ci carboxyl glycosides are known. 

The complexation in A increases the leaving group potential of the carbohydrate by a pre-arranged neutralization of the escaping anomeric anion. In addition, the elimination might also be favored by the higher local concentration of counteranions in the neighbourhood of the coordinatively fixed cation. 

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