Hemiacetal carbohydrate anomers

Because six membered rings are normally less strained than five membered ones pyranose forms are usually present m greater amounts than furanose forms at equilib rium and the concentration of the open chain form is quite small The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and NMR spectroscopy In aqueous solution for example d ribose is found to contain the various a and p furanose and pyranose forms m the amounts shown m Figure 25 5 The concentration of the open chain form at equilibrium is too small to measure directly Nevertheless it occupies a central position m that mterconversions of a and p anomers and furanose and pyranose forms take place by way of the open chain form as an inter mediate As will be seen later certain chemical reactions also proceed by way of the open chain form... 

Arts. The p- does not refer to the linkage between glucose units. The convention is that the type of 1,4-linkage in a carbohydrate does not appear in its name, it being understood whether the linkage in a particular carbohydrate is a-1,4- or /3-l,4-. The p- in the name -o-maltose refers to the fact that the structure drawn is that of the -anomer of o-maltose. C-1 of the far right glucose unit of maltose is a hemiacetal carbon, which means there are three forms of maltose—an open-chain, the a-anomer of the cyclic form, and the /3-anomer of the cyclic form. 

Section 23.8 Hemiacetal forms of carbohydrates are interconvertible in water. The equilibrium mixture can contain a and p anomers of furanose and pyranose forms. The change from one form to the equilibrium mixture is accompanied by a change in optical rotation called mutarotation. For D-glucose, mutarotation can be described in terms of the interconversion of a-pyranose and P-pyranose forms by way of the open-chain form. 

We saw in Chapter 2 that glucose can cyclize to form either the a ox hemiacetal. All of the substituents on the tetrahydropyran ring of glucose are equatorial except for the hemiacetal hydroxyl group, which may be either axial (for the a anomer) or equatorial (for the 6 anomer). If the conformational preferences of the tetrahydropyran ring are similar to those of cyclohexane, then an A value for OH of 0.87 in a protic solvent would predict that the axial anomer should comprise no more than 10% of the mixture. Experimentally, it has been determined that the a anomer is present to the extent of 34% of the equilibrium population of conformers. In the case of o-marmose, the a anomer (10) is the major isomer, and the )3 anomer (11) comprises only 32% of the equilibrium mixture. This preference for axial conformer in carbohydrates has been termed the anomeric effect. This term is also used for any system R-Y-C-X, where X is an electronegative atom and Y is an atom with at least one lone pair. 

Anomeric carbon (Section 22.2C) The hemiacetal or acetal carbon in the cyclic form of a carbohydrate. The anomeric carbon can have either the a or /3 stereochemical configuration (using carbohydrate nomenclature), resulting in diastereomeric forms of the carbohydrate called anomers (a-anomers and /3-anomers). Anomers differ on/y in the stereochemistry at the anomeric carbon. 

Anomers (Section 22.2C) A term used in carbohydrate chemistry. Anomers are diastereomers that differ only in configuration at the acetal or hemiacetal carbon of a sugar in its cyclic form. 

D-Glucose, the most important carbohydrate in mammalian metabolism, exists as a six-membered cyclic hemiacetal form as both a and j8 anomers. 

Usually, acetylated glycomonomers are easily obtained from the glyco-sylation of commercially available or readily accessible peraeetylated carbohydrates with a nucleophilic donor such as 2-hydro qrethyl methaciylate, as shown in Figure 2.4. The combined influence of the ehemistry of the hemiacetal and the reaction conditions affect the regio- and stereoselectivity of the reaction and, in most cases, can be used to selectively generate the a or p anomer. 

In carbohydrate chemistry diastereomers differing only at the hemiacetal or acetal carbon are called anomers, and the hemiacetal or acetal carbon atom is called the anomeric carbon atom. 

The imine (Formula 4.52) formed by the reaction of glucose with the amine is easily converted to the cyclic hemiaminal, a- and P-glucosylamine. However, N-glycosides of this type are relatively instable because they very easily mutarotate, i. e., they are easily hydrolyzed via the open-chain imine or are converted to the respective a- and P-anomer. However, the Amadori rearrangement leads to furanose, which as a hemiacetal, has a stability to mutarotation comparable with that of carbohydrates. 

The keto forms of carbohydrates exist in eqnilibrinm with the corresponding five-membered (furanoses) or six-membered (pyranoses) cyclic hemiacetals. The new stereocenter formed by cyclization is called the anomeric carbon, and the two anomers are designated a and /3. 

Carbohydrate trichloroacetimidates have become very useful derivatives for the activation of the carbohydrate moiety. The carbohydrate can be readily transferred to form many types of glycosides [78]. Glycosides are readily formed by reaction of the hemiacetal hydroxyl with trichloroacetonitrile under base catalysis conditions. The p-anomer is rapidly and preferentially formed first as a favored kinetic product. Under base catalyzed conditions, however, it is slowly anomerized to give the a-anomer, which is the more stable thermodynamic product, due to the anomeric effect (reaction 4.63). Thus, either the a- or the P-anomer can be isolated in pure form and in high yield, depending on the length of time that the reaction is allowed to proceed. 

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