Anomers, definition

Since the observations of the ICD were on equilibrium mixtures between anomers, definitive interpretations must await for other physicochemical analyses such as NMR. 

From these results, it was concluded that the configuration of the molecule has a definite effect upon the direction of attack on the carbonyl group. Thus, the hydride attacks the keto group of the a anomer, trans to the purine, producing an axial OH-2 (91), whereas, in the case of the ft anomer, this attack occurs52 cis, leading to an equatorial OH-2 (90a and 90b). 

It is not definitely known whether the a or the /3 anomer of AT-acetyl-n-glucosamine 1-phosphate is involved in this isomerization of A -acetyl-D-glucosamine 1-phosphate. However, an extremely acid-labile A -acetyl-n-glucosamine phosphate did contaminate some of the A -acetyl-... 

As seen from Table I, the 01 to C-l bonds, which join the glycosidic oxygen to the aglyconic carbon, have normal G-0 bond lengths for both the anomers. The similar bonds at the other end of the acetal grouping namely, the C-5 to 05 bonds appear somewhat shorter than normal. The 06 to C-l bonds are definitely shorter... 

Perlin, but a definite structure was not given. During the same work, both anomers of ethyl n-fructofuranoside were treated with chlorotriphenyl-methane. Only the /J-d anomer gave a crystalline 1,6-di-O-trityl derivative. The positions of the ether groups were determined by periodic acid oxidation, and, in both cases, one mole was consumed per mole. 

This study is of interest because Davoll and Lythgoe observed that the more deaiirorotatory anomer of the acylated theophylline nucleosides yielded the more Uvo-rotatory anomer of the free nucleoside. Because of this anomaly, these investigators suggested that the isorotation rules of Hudson have only hmited validity in the case of nucleosides of 2-deoxy-D-ribose and were unable to assign a definite configuration to the anomers. 

Of the pyranoid conformations for which Reeves rules do not permit clearcut decisions, none have been studied examples include the a anomers of D-allose, D-altrose, D-idose, and D-gulose. It would be of interest to examine some of these, particularly those for which Lemieux was able, definitely, to assign the conformation of the polyacetates in nonpolar solvents, and those for which Tipson and Isbell have studied the in-... 

To answer this problem, one must know the structures of the molecules in question and a couple of definitions. By definition, epimers are a pair of molecules that differ from each other only in their configuration at a single asymmetric center. Anomers are special epimers that differ only in their configuration at a carbonyl carbon hence, they are usually acetals or hemiacetals. An aldose-ketose pair is obvious. Inspection of Fischer representations of the molecular pairs leads to the conclusion that (a), (c), and (e) are aldose-ketose pairs (b) and (f) are epimers and (e) are anomers. 

The most widely-used catalysts for acetylation are sodium acetate and zinc chloride. With sodium acetate, the product is predominantly in the /J-form, except in the case of d- and L-arabinose, where the a-anomer is obtained. (By definition of the terms ot and / , -d- and a-L-arabinose are enantiomorphs.) Zinc chloride usually gives the a-acetyl derivative of the sugar, but D-glucosamine and D-galactosamine yield the jS-form with this catalyst. Of the other catalysts commonly employed, perchloric acid behaves like zinc chloride, whereas pyridine tends to give a mixture of the - and jS-poly-O-acetates. 

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