Pyranose Sugars

The incorporation of heteroatoms can result in stereoelectronic effects that have a pronounced effect on conformation and, ultimately, on reactivity. It is known from numerous examples in carbohydrate chemistry that pyranose sugars substituted with an electron-withdrawing group such as halogen or alkoxy at C-1 are often more stable when the substituent has an axial, rather than an equatorial, orientation. This tendency is not limited to carbohydrates but carries over to simpler ring systems such as 2-substituted tetrahydropyrans. The phenomenon is known as the anomeric ect, because it involves a substituent at the anomeric position in carbohydrate pyranose rings. Scheme 3.1 lists... 

FIGURE 7.9 (a) Chair and boat conformations of a pyranose sugar, (b) Two possible chair conformations of /3-D-glncose. 

Just as certain pyranose sugars can give rise to bis-acetal or bis-ketal derivatives which constitute linearly fused 5 6 6 systems (cf. Section 12.17.2.1.7), another set of bis-acetals and bis-ketals - in many cases derived from the same sugars - correspond to angularly fused 5 6 6 systems. These, like their linearly fused analogues, serve to protect, selectively, four hydroxyl groups of the parent sugars, and cyclic carbonates (l,3-dioxolan-2-ones) may fulfill similar functions. 

A versatile protocol for the generation and cyclization of secondary radicals from hexo-pyranose sugars is shown in Scheme 3 [10], The Wittig reaction of reducing sugars with two eq of an alkylidene phosphorane readily provide hex-5-ene-l-ols, which were converted into hex-5-enyl radicals by the l-ff-imidazole-l-carbothioate. The cyclization reaction is carried out in refluxing benzene or toluene with tributyltin hydride and AIBN, according to... 

Three-center bonds can be symmetrical with r3 r2, 0 02 and asymmetrical, in which rt and r2 differ by as much as 1.0 A, 0t is close to 180° and 02 is close to 90°. Examples of these have been well established by the neutron diffraction studies of the amino acids [60, 74] and the pyranose sugars [58]. A case where the decision between two- and three-centered bonding is difficult is illustrated from the neutron analysis of erythritol [77], shown in Fig. 2.3. Although one bond is much longer than the other, both are primarily electrostatic. It is difficult to refer to one as a hydrogen bond and the other as an electrostatic attraction. In this monograph, we call these the major and minor component of a three-center bond. 

In the pyranose sugar crystal structures, intramolecular hydrogen bonds are formed between C(n)OH and C(n+2)OH groups when the molecular conformation is such that the C-OH bonds are in the syndiaxial orientation, i.e. ... 

Since this topic was last reviewed in this series,much additional information about the scission of epoxy derivatives of pyranose sugars has become available. Only this type of epoxide will be discussed here. 

Other epoxy derivatives of pyranose sugars do not show any regularity in the direction of scission. Methyl 2,3-anhydro-4,6-di-0-methyl-/3-n-mannopyranoside reacts with ammonia to afford predominantly the product with the D-altro configuration, but with sodium methoxide equal amounts of T>-altro and D-gluco isomers are obtained.No useful rules could be formulated to predict the direction of opening of epoxides by acidic reagents. ... 

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