Pyranosides anomerization

This ether, developed for the protection of a pyranoside anomeric hydroxyl, is prepared via a Konigs-Knorr reaction from the glycosyl bromide and 2-(ben-zylthio)ethanol in the presence of DIPEA. It is cleaved, after oxidation with dimethyldioxirane, by treatment with LDA or MeONa. ... 

In consonance with this heuristic, a phenolic TBS derivative has been shown to migrate to a primary alcohol. A pyranoside anomeric hydroxyl is more acidic than the 2-OH, and thus treatment of the disaccharide with NaH and BnBr results in migration of the silyl group and protection of the anomeric center with a benzyl group. ... 

The /7-nitrophenyl ether was used for the protection of the anomeric position of a pyranoside. It is installed using the Konigs-Knorr process and can be cleaved by hydrogenolysis (Pd/C, H2, AC2O), followed by oxidation with ceric ammonium nitrate (81-99% yield). ... 

Indeed, a variety of Lewis acids have been shown to effed glycosylation with hemiacetal donors. Ernst and coworkers have used 5mol% of [Rh(III)(MeCN)3 (triphos)] tris(triflate) with 4 A molecular sieves to prepare glycoconjugates 18 and 19 [26]. Mukaiyama s group has used trityl tetrakis(pentafluorophenyl)borate (3-5 mol%) with Drierite in the preparation of disaccharides 20 and 21 [27,28]. In the synthesis of 21, the a-seledivity was shown to arise from in situ anomerization of the (1-pyranoside over time. 

Consequences of the conformational anomeric effect are largely expressed in monosaccharides and their derivatives. One recognizes the conformational endo-anomeric effect for pyranosides with a polar X group at C(l) (contrasteric electronic stabilization effect Fig. 7A) and conformational exo-anomeric effect for glycosides (acetals) in which the alkyl group of the exocyclic moiety is synclinal (Fig. 7B, C). 

Fig. 7 (A) Conformational endo-anomeric effect (B) conformational exo-anomeric effect in axial O-pyranosides (X = OR) (C) conformational exo-anomeric effect in equatorial O-pyranosides (X = OR).

Pikho and co-workers50 found that nonanomeric [6,5]-spiroketals (having a pyranoside moiety with an equatorial CO acetal bond) can be formed under conditions of kinetic control. For instance, 62 undergoes acid catalyzed spiroacetalization giving the anomeric (most stable) acetal 63... 

The stereochemistry of the reaction of glycosyl radicals is strongly influenced by the anomeric effect. Glucopyranosides and manno-pyranosides afford stereo selectively the a-C-glycosyl compounds whereas in furanosidic structures the stereochemistry is not always predictable. 

A parameter that has been useful for defining the anomeric stereochemistry of glycopyranosides is the chemical shift of the anomeric carbon atom, C-l. Similarly, the 13C NMR shifts of C-l in methyl septanosides also diagnostic for their anomeric configuration. In general, the Sc.i values for a-septanosides are slightly upfield (99-104 ppm in completely deprotected septanosides) relative to (S-septanosides (104-111 ppm) (Table I). This distribution of chemical shifts mirrors the trends for pyranosides namely, the C-l chemical shifts for a-pyranosides lie upfield from those of p-pyranosides. This observation suggests that the anomeric effect is operative in... 

Anomeric triphenylphosphonium salts have been used as well as phenylsul-fides,but in the latter case extra stabilization is necessary (see below). Anomeric nitrosugars, which have been extensively studied in C-glycosylation reactions by Vasella, will be covered in Sect. 2.2.1 and ester enolates derived from 3-deoxy-2-ketoulosonic acids (sialic acid and KDO derivatives), which bear a structural similarity to 2-deoxy pyranosides, will be covered in Sect. 4.4. Deprotonation of anomeric phenylsulfones has been discussed in Sect. 2.1.1 and additional transformations on closely related compounds are presented in Scheme 14 [20]. Alkylation of phenylsulfone 54 with epoxide 55 provides adduct 56 which eliminates benzenesulfinic acid at room temperature to give the C(l)-alkylated glycal 57 a similar elimination is also observed with adducts derived from... 

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