Anomeric forms

The optical rotations just cited for each isomer are those measured immediately after each one is dissolved m water On standing the rotation of the solution containing the a isomer decreases from +112 2° to +52 5° the rotation of the solution of the p isomer increases from +18 7° to the same value of +52 5° This phenomenon is called mutarotation What is happening is that each solution initially containing only one anomeric form undergoes equilibration to the same mixture of a and p pyranose forms The open chain form is an intermediate m the process... 

The a-anomeric form of a 2 -deoxyribose, which has the base inverted with respect to the natural P-anomeric form, can be synthesized by using the phosphoramidite method sugar modification renders the derivatives nuclease-resistant. These analogues form parallel duplexes with complementary RNA... 

In extending this direct method of synthesis, we next investigated the possibility of preparing similarly constituted halides from 2-deoxy-D-arabino-hexose (2-deoxy-D-glucose) (21). The hexose was subjected to a partial anomerization procedure described by Bergmann and co-workers (1). The solid material obtained by this procedure is a mixture of the anomeric forms of 2-deoxy-D-arabino-hexose low temperature p-nitro-benzoylation of the latter in pyridine resulted in a mixture of crystalline, anomeric tetrakis-p-nitrobenzoates in a ratio of approximately 1 1. They were readily separable by fractional recrystallization, and treatment of either with an excess of hydrogen bromide in dichloromethane, or with... 

Conformations of acyclic chains 2-Carb-6. Anomeric forms use of a and (3... 

As may be seen from the comparisons of the chemical-shift data for compounds 59 and 60 with those for compounds 61 and 62, two carbon atoms, C-l and C-4, could be monitored in order to distinguish between the two tautomeric and anomeric forms of arabinose. There is an 6-p.p.m. difference between the C-4 signal of arabinose in the pyra-nose and the furanose form. Determination of the chemical shift of the anomeric-carbon atom is, however, not that straightforward, as there is an overlap between the C-l chemical-shifts of arabino-furanosyl and -pyranosyl derivatives (compare the data for compounds 60,61, and 62). The 13C chemical-shift data for C-l, in conjunction with the 13C chemical-shift data for C-4, allow the detection and assignment of arabinose in either tautomeric and either anomeric form. 

Reeves and Goebel72 have shown that hydrolysis of the reduced methylated capsular polysaccharide of Type III pneumococcus yields 2,3,6-trimethyl-D-glucose and the anomeric forms of methyl 2,4-dimethyl-D-glucoside. The cellobiuronic acid units in the polysaccharide are thus linked through position 3 of the D-glucuronic acid residue, probably by /3-D-linkages. That is, the polysaccharide contains alternate 1,3-and l,4-/3-D-linkages. 

Figure 9.6 Anomeric forms of D-glucose and D-fructose. The alpha and beta anomers are named with reference to the configuration of the glycosidic hydroxy] group associated with the oxygen bridge.

Figure 9.8 Haworth projection formulae. The ring is considered to be planar with the substituent groups projecting above or below the plane. The thickened lines represent the portion of the ring that is directed out of the paper towards the reader. The alpha and beta anomeric forms are shown with the hydroxyl group at carbon 1 below or above the plane of the ring respectively.

Enzymic methods for the quantitation of monosaccharides are employed when a higher degree of specificity is required than can be achieved by the majority of the chemical methods. They often enable the quantitation of one stereoisomer in the presence of others and can often differentiate between the a and 0 anomeric forms. 

The cyclic form of glucose is termed glucopyra-nose, since the new ring system is a reduced form of the oxygen heterocycle pyran. Nucleophilic attack onto the planar carbonyl may occur from either of its two faces, generating two different stereochemistries at this new chiral centre, designated as a or p. This new chiral centre is termed the anomeric centre. Since there are other chiral centres in the molecule, the mixture of a- and -anomeric forms is not a racemate, but a mixture of diastereoisomers (see Section 3.4.4). The mixture does not contain 50% of each anomer (see below). Although both forms are produced, the form with the equatorial hydroxyl is thermodynamically favoured (see Section 3.3.2). 

Further, the two forms can also equilibrate via the open-chain carbonyl form of the sugar, so that the single isomers in solution are rapidly transformed into the equilibrium mixture (see Box 7.1). Since there are two anomeric forms, and these are often in equilibrium via the acyclic carbonyl compound. 

It is possible to separate the two anomeric forms of glucose by careful recrystallization from water. The two forms have different specific optical rotations (see Section 3.4.1), [aJo + 112° for a-o-glucopyranose, and [ajo + 18.7°... 

The two anomeric forms are called a- and P-ribofuranose. Again, in solution there exists an equilibrium between the open chain carbonyl form and the two anomeric hemiacetal forms. Ribose also forms six-membered pyranose anomers, and... 

One of the consequences of forming a cyclic hemi-acetal or hemiketal is that the nucleophilic hydroxyl adds to the carbonyl group and forms a new hydroxyl. This new group is susceptible to many normal chemical reactions of hydroxyls, e.g. esterification, and this type of reaction effectively freezes the carbohydrate into one anomeric form, since the ringopening and equilibration can now no longer take place. Consider esterification of glucose with acetic anhydride (see Section 7.9.1). P-o-Glucose will be... 

Wegen der grofien Bedeutung der N-Glykoside gibt es zahlreiche Uber-sichten iiber ihre Darstellung Oft sind die Struktur, d.h., RinggroBe und anomere Form, nicht mit Sicherheit bekannt. 

In addition, the reduction of a-11 and (3-11 was s tudied in dimethyl sulfoxide as solvent, in the presence of pyridiniump-toluensulfonate. This medium makes mutarota-tion slower than the redox process.73 The two anomeric forms could reduce Cr(VI) and Cr(V) by formation of a Cr(VI) and Cr(V) ester intermediate. The equilibrium constant for this step and the rate of the redox step were different for each anomer for a-11, the equilibrium constant for ester formation is higher than for (3-11, but the redox process within this complex is faster for the latter anomer. These differences can be explained by the better chelating capacity of the 1,2-cri-diol moiety of a-11. Room-temperature CW-EPR spectra of these mixtures revealed for the a anomer several five-coordinated Cr(V)-bischelates (giso= 1.9820 [crilso 15.9xl(r4cm 1 (=47.7MHz)],... 

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