Of pyranose sugars

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. ... 

With his close interest in stereochemistry and appreciation of physical methods, it was fortunate that Lemieux could be involved from the outset in the application of NMR spectroscopy in the field. Ottawa, where he was on the faculty of the university, also housed the National Research Council Laboratories with the spectroscopists Harold Bernstein, William Schneider, Rudolf Kullnig (a Lemieux Ph.D. student) and a very early 40 MHz instrument (Figure 1.9). The first carbohydrate spectra were recorded there. The paper with these collaborators, reporting 40 MHz studies of pyranose sugar acetates, showed the significance of chemical shifts for determining the nature of... 

Interaction between the Oxygen p-type Lone Pair and Adjacent q-bonds The interaction between the oxygen p-type lone pair and the adjacent antibonding orbital of a carbon-halogen bond was first considered in 1959 by Lucken as a possible explanation of the abnormally low NQR frequencies of Ot-haloethers (33). Later on, Altona showed that the same phenomenon could explain the peculiarities of some bond lengths in similar compounds (2). The spectroscopic properties of pyranose sugars which we have described in the first two sections of this article seem to imply that the existence of such an interaction lies beyond all possible doubt in these molecules. It does not follow that they can explain the Anomeric Effect (34) (35). For this, we need a quantitative estimation of the stabilization introduced in a molecule by such interactions. 

The anomeric effect, still an object of active research (Kirby 1983 Juaristi and Cuevas 1992 Thatcher 1993), was first observed as a property of pyranose sugars (Edward 1955 Lemieux 1964). However, the anomeric effect emerges in a pure state on a very simple molecule, methyl choromethyl ether (CH3OCH2CI), represented as projected along the O-CH2CI bond of formula 2.22. 

The oxidative ring expansion of furyl carbinols can be accomplished with many of the same oxidants discussed earlier for routine oxidation of the furan ring. As a pyran is formed in the reaction, an obvious application would be for the synthesis of pyranose sugars (Scheme 19). 

Honda et al. [266] described the GC determination of conjugated aldehydes in the products of periodate oxidation of pyranose sugars as diethyl dithioacetals. The component dialdehyde in dialdehyde compounds formed on periodate oxidation of carbohydrates can be determined by treating the dialdehyde with ethanethiol and trifluoro-acetic acid to convert the component aldehydes into diethyldithioacetals, which can be determined directly on OV-1 columns as their trimethylsilyl derivatives. 

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. 

Sulfinylacrylate 41 has been successfully used in the enantioselective synthesis of pseudo-sugar [46, 47]. Cycloaddition of (S)-3-(2-pyridylsulfinylacrylate) (41) with furan and 3,4-dibenzyloxyfuran under Et2AlCl catalysis afforded cycloadducts 42, 43 and 44 (Equation 3.12) which were converted into pseudo-manno-pyranoses 45, 46 and 47 (Figure 3.5). 

There are two forms of carba-sugar carba-pyranoses and -furanoses. The former, especially the carba-hexopyranoses, have been extensively studied during the past two decades, ever since their derivatives were found in Nature as components of important antibiotics. However, very little is known about carba-furanoses, except for 4a-carba-)3-L-arabinofuranose ... 

Fig. 4 shows c.d.-difference spectra that are due to the presence of a hydroxymethyl group in one member of the pair. The pairs of pyranoses have the same conflguration about each asymmetric carbon atom that is part of the ring. Three related pairs have the same conflguration at the carbon atoms near the hydroxymethyl group and give very similar difference spectra. The similarity indicates that the rotameric distribution for the hydroxymethyl group is similar for each of these pairs. This is to be expected, as all three sugars have 4-hydroxyl groups oriented equatorially, but the relationship is certainly not obvious from the c.d. spectra themselves (see Fig. 3). In contrast, when the conflguration near the hydroxymethyl group...

As already shown in Sect. 4.4, ribose-2,4-diphosphate is obtained in a base-catalysed condensation of glycolaldehyde phosphate in the presence of formaldehyde (Muller et al., 1990). The phosphate group in the 4 position of the sugar prevents the formation of a 5-membered furanose ring, but a 6-membered pyranose structure can be formed. 

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. 

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