Anomers 3-anomeric form

The final compound was isolated after peracetylation no trace of P-anomer was formed during the process. As in their first report, these authors also investigated the MPEG-DOX linkage of the first sugar residue via hydroxyl groups other than the anomeric one (Scheme 4.16). 

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.

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). 

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... 

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)],... 

Anomers of Sucrose Although lactose exists in two anomeric forms, no anomeric forms of sucrose have been reported. Why ... 

In all cases, the rate of anomerization is very high, relative to the rate of conversion into pyranosides this rate of anomerization is the major factor in determining the relative yields of the ct- and /8-furanosides from a reaction mixture, and not the relative proportions of these two anomers initially formed from the free sugar. The ratio of anomers was found to be approximately constant throughout the course of the reaction (1.7 1 for the 0- and a-n-xylofuranosides).42... 

The majority of the many methods used to study the composition of equilibrium solutions of carbohydrates examine the mixture without separating the individual components. With the discovery that the anomeric forms of sugars could be readily separated by gas chromatography of their tri-methylsilyl ethers, a new approach to the problem was found. A protocol was developed for the direct gas chromatographic analysis of the amount of each anomer present in an aqueous solution. The protocol can be used on the micro scale and can be used in enzyme assays such as that for mutarotase. The method has been made more effective by combining gas chromatography with mass spectrometry. It is shown how mass spectral intensity ratios can be used to discriminate anomers one from another. The application of these methods to the study of complex mutarotations is discussed. 

Analysis of Freshly Cyclized 2,3-di-O-methyl-D-arabinose. The system 3,4-di-O-methyl-D-mannitol (2)—sodium metaperiodate meets the foregoing specifications. 3,4-Di-O-methylmannitol was rapidly cleaved by periodate to 2,3-di-O-methyl-D-arabinose (3), which is resistant to further oxidation (19). We prepared the crystalline / -anomer of this sugar (the a-anomer had previously been crystallized (20)) and showed that on a suitable GLC column the trimethylsilyl derivatives of the four ring forms could be separated (Figure 9). The individual peaks were characterized by preparative GLC and mass spectrometry. This allowed the analysis of the mixtures of anomeric forms generated in kinetic experiments (see p. 34). 

Free energies, enthalpies, and entropies of ionization for a number of reducing sugars have been determined by Christensen and co-workers (13). Their data, however, are for equilibrium mixtures of anomers rather than for individual anomeric forms. 

Whatever the site of the enzyme may be, Keston et al. have recently produced fairly conclusive evidence that glucose, which is reabsorbed by the kidney, is exposed to mutarotase at some stage of the process (117). Glucose infused into the renal artery spills into urine when the renal threshold is exceeded in the same anomeric form as that administered, whereas reabsorbed glucose in the renal vein is mutarotated. Hill has also shown that the anomer infused in excess is excreted in excess (73). 

There is an important question as to which one of the two anomeric forms of a sugar should be designated as a and which one as 8. The convention is simple the a anomer is the one that has the same configuration of the OH at the anomeric carbon as the carbon which determines the configuration of the sugar itself ... 

As compared to the corresponding agalacto compound 6, the presence of Gal residues 6 and 6 introduces a change in the chemical shift of the H-l atoms of the peripheral GlcNAc residues 5 and 5 from 8 4.558 to 4.582. However, this extension has virtually no effect upon the chemical shifts of the H-l atoms of Man residues 3, 4, and 4. Interestingly, the H-l atom of Man-4 and also that of Man-4 each yields two separated doublets which must be ascribed to the two anomeric forms of 7. From the relative intensities of these doublets, it may be concluded that the anomers of this oetasaccharide occur in the ratio of ot /3 = 2 1. Apparently, the chain length does not affect the anomeric-equilibrium constant. 

Equation I is a simplified one, as it does not take into account the interchange between the enzymically active a anomer of D-glucose and the /3 anomer. The same reservation applies to D-fructose, which exists in four anomeric forms in solution, and which of the four is the... 

Free 4-acetamido-4,5-dideoxy-D (or L)-xylose is a syrup, and its infrared spectrum shows negligibly small bands for the imino and carbonyl groups. The syrup consists of two components, present in about equal amounts, - that are chromatographically separable. They yield the same (2-benzyl-2-phenyl)hydrazone, and are interconvertible by acid catalysis. Their optical rotations and their nuclear magnetic resonance spectra show that they are anomeric forms - of 195. Their nuclear magnetic resonance spectra are differentiated by the positions of the signals for their H-1 protons (t 4.72 and 4.84). The spectrum of each anomer shows, on acidification, a rapid attainment of anomeric equilibrium. A... 

In a study of the reaction of the anomeric forms of penta-O-acetyl-D-glucopyranose with titanium chloride in chloroform solution, Lemieux and Brice have found that, at 40°, the a-D anomer reacts but slowly, whereas the /3-d anomer reacts extremely rapidly, with the initial formation of (unstable) tetra-0-acetyl- S-D-glucopyranosyl chloride which then rearranges relatively slowly to (stable) tetra-O-acetyl-a-D-glucopyranosyl chloride. 

It was observed by Jackson and Hudson that the products from an anomeric pair of methyl pentopyranosides have equal, but opposite rotations. Since they believed that the products were dialdehydes, this was reasonable, as the anomeric carbon atom was the only asymmetric center. However, it is now known that these products exist as hemialdals, and, since the optical rotations are high, it is certain that they are in the same form in solution. The hemialdal form has three asymmetric centers and, therefore, the two anomeric forms would be expected to have different rotations. It is the author s opinion that a conformational change occurs in the a-D anomer (assuming a conformation), such that the final hemi-... 

---