A-D-Anomers

Low diastcreoselectivities were observed in the reaction of 1 with nonanal (54 46) and ben-zaldchyde (2 1). In both cases a-D-anomers were formed predominantly8. 

On account of the more favorable anomeric effect,12 as well as of the larger nonbonding interactions of vicinal, synclinal substituents in furanoses as compared to pyranoses, jS-D-glucofuranose,13 /3-D-glucofuranurono-6,3-lactone, and their derivatives are of higher thermodynamic stability than their respective a-D anomers. 

Specifically, in reactions of 4 with acetic anhydride in the presence of zinc chloride58 or boron trifluoride17,35 as the catalyst, 1,2,5-tri-O-acetyl-/J-D-glucofuranurono-6,3-lactone (12) is the preponderant product in the presence of pyridine,58 however, acetylation of 4 by acetic anhydride leads to the favored formation of the corresponding a-D anomer. In contrast to acetylations, in benzoylation reactions, the ratio of anomers apparently changes. Thus, on treatment of 4 with benzoyl chloride in pyridine, Momose and coworkers17 isolated... 

An elegant, stereospecific synthesis of two 2-chloro-2-deoxy-pentoses was achieved during the course of a study of the anomerization of chlorosulfated glycosyl chlorides.40 It had been found37 that attempts to anomerize /3-D-xylopyranosyl chloride 2,3,4-tri(chloro-sulfate) (14) with 0.25 mol-equivalent of aluminum chloride afforded the crystalline a-D anomer only in low yield formed also was another compound whose structure has been established40 to be that of 2-chloro-2-deoxy-a-D-lyxopyranosyl chloride 3,4-di(chlorosulfate) (16). The dichloro derivative 16 became the exclusive product when 1.5 mol-equivalents of aluminum chloride were used treatment of 16 with sodium iodide yielded crystalline 2-chloro-2-deoxy-D-lyxose. Similarly, a-D-lyxopyranosyl chloride 2,3,4-tri(chlorosulfate) (15) afforded 2-chloro-2-deoxy-a-D-xylopyranosyl chloride 3,4-di(chlorosuI-fate) (17). 

The formation of the tetraacetate of the a-D anomer (25a) (no inversion) has been postulated to take place by way of a four-center transition state ... 

The p anomers of sugars are generally oxidized more rapidly than the a anomers (see later), a similar pattern is seen in the faster oxidation of /1-glycosides. The different rates of oxidation of /1-D-glucopyranosides and their a-D anomers have been attributed to the equatorial orientation of the anomeric hydroxyl group in the 4Ci conformation of the former. Other compounds have shown a similar behavior.10 Thus, the relative rate of oxidation of cw-2-tert-butylcyclohexanol (HO-axial) with respect to the trans isomer (HO-equatorial) is approximately 5 1. 

A purified D-glucose dehydrogenase, isolated from animal liver, catalyzes the oxidation of D-glucose to D-gluconic acid.202 A coenzyme (either nicotinamide adenine dinucleotide or its 2 -phosphate) is necessary for the oxidation. The reaction is reversible, and is specific for /J-D-glucopyranose it does not proceed with the a-D anomer, and it occurs only slowly with D-xylose. 

The group 29-e polysaccharide is composed of an alternating sequence of 3-deoxy-/3-D-manno-2-oetulosylonic acid and 2-acetamido-2-deoxy-a-D-glucopyranosyl residues linkage is to 0-7 of the former and to 0-3 of the latter. O-Acetyl substituents were also located on both 0-4 and 0-5 of the KDO residues.26 It is of interest that, whereas sialic acid has only been found in bacterial polysaccharides, and elsewhere in Nature, as its a-D anomer, there is strong evidence to suggest that KDO probably exists in bacterial polysaccharides in both of its an-omeric forms.28,40... 

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