Mutarotation nuclear magnetic resonance

Carbohydrates in nature are optically active and polarimetry is widely used in establishing their structure. Measurement of the specific rotation gives information about the linkage type (a or (3 form) and is also used to follow mutarotation. Nuclear magnetic resonance spectroscopy (NMR) can be used to differentiate between the anomeric protons in the a- or /3-pyranose and furanose anomers and their proportions can be measured from the respective peak areas. 

Because the mutarotation reaction in methyl sulfoxide is slow, nuclear magnetic resonance measurements in this solvent are par-... 

In some instances, nuclear magnetic resonance techniques employing trimethylsilyl ethers have provided information on the composition of sugar solutions that could not be obtained by classical methods. Thus, the formation of D-galactofuranose during mutarotation was shown by gas chromatography of the trimethylsilyl derivatives of D-galactose (from the mutarotation mixture). The products were... 

Studies, by means of nuclear magnetic resonance measurements with pertrimethylsilyl ethers, of the behavior in solution of glycol-aldehyde and related substances formed by periodate oxidation gave striking results. Products formed under mild conditions showed that the original material was composed of dimers having p-dioxane structures products separated after complete mutarotation showed eight dimeric forms. ... 

At pH 4.4, 5-thio-D-xylopyranose (215) shows a slow mutarotation, namely, [a]p +202 - +178° (half-time, 10 hours). At pH 6.6, however, the half-time is only about 10 minutes. The direction of the rotation shows that 215 crystallizes in the a-D form. The nuclear magnetic resonance spectrum of 215 in deuterium oxide shows the presence of the H-1 proton of both anomers. The diaxial coupling of 8.2 Hz for the H-1 proton at the higher field (t 5.25) corresponds to the )8-D anomer in the CJ(d) conformation of the molecule having a sulfur-containing ring. 

Nitriles, a-amino, mutarotation of, 13 Nitrogen compounds, of cyclic monosaccharides, 115 — 232 Nojirimycin, 116,132,133 Nomenclature, of enzymes, 306 Nuclear magnetic resonance anomer determination by, 43 characterization of anhydrodeoxyaldi-tolsby, 74,77,79... 

Lemieux and coworkers296 investigated the mutarotation of 2-deoxy-/3-D-eryt/zro-pentopyranose in deuterium oxide at 0° by nuclear magnetic resonance measurements and optical rotations. The optical mutarotation had a rapid and a slow phase. The components at equilibrium were the /3-pyranose (44%), the a-pyranose (41%), and the a-and /3-furanoses (15%). The rate of formation of each component was determined. 

The enol ether (23) has been obtained from 2-0-methyl-D-glucose and from 2,3-di-O-methyl-D-glucose. Klemer, Lukowski, and Zerhusen isolated a crystalline form of (23) with C ]d + 16° no mutarotation was mentioned. They assumed their compound to be pyranoid by analogy with (24). The nuclear magnetic resonance spectrum of the reaction mixture from 2,3-di-O-methyl-D-glucose showed the presence of equimolar amounts... 

Some of the important differences in the nuclear magnetic resonance spectra of the /3-n-furanose form in various solvents are shown in Table I. As expected, the chemical shifts are somewhat altered on changing the solvent however, new bands appear with pyridine. In methyl sulfoxide, the mutarotation is very slow, and only one form is seen soon after dissolution. In anhydrous pyridine, the mutarotation is a little faster, so that, in a few minutes, the a- and /S-n-furanose forms are present in equal amounts. With the addition of deuterium oxide, equilibrium between the three forms is rapidly reached. This last spectrum is essentially the same as that of the crude product mentioned earlier. The small values for Jz 4 indicate that all three forms are in fixed conformations and are, therefore, not acyclic. The spectrum of the a-D-pyranose form is closely related to that of (24). The two furanose forms show, overall, a similar spectrum, except that the proton at C-1 of the )3-d anomer shows a relatively large coupling, J = 3.25, of unknown origin. The spectra of many five-membered heterocyclic compounds are anomalous, and not yet fuUy understood. 

The above equilibria are confirmed by the complex, positive mutarotation in ethanol. First, a slow 3-D-furanose interconversion to equilibrium occurs, and then a very slow interconversion to equilibrium with the -D-pyranose form. The crystalline compound is, therefore, a /3-d anomer. This compound is interesting in that the identity of the forms taking part in the mutarotation can be ascertained, and the relative amounts of each determined, by nuclear magnetic resonance spectroscopy. Furthermore, the compound is unusual in existing in solution as the furanose forms, mostly. 

Chapman and co-workers followed the nuclear magnetic resonance spectra of osazones in methyl sulfoxide during the process of mutarotation, and observed that the spectra become constant when the optical rotation reaches the equilibrium values. They found that, during the course of mutarotation, new NH resonances appear in the region of the unchelated... 

The hydrazones of sugars are capable of existing in various cyclic forms, whose presence is apparent from their nuclear magnetic resonance (NMR) spectra and from the complex mutarotation curves they exhibit110,111117118 (which seldom follow first-order kinetics). The principal structures encountered in saccharide hydrazones are the acyclic, Schiff base-type true hydrazones and the cyclic hydrazino forms, namely glycopyranosyl- and gly-cofuranosylhydrazines. For example, three isomeric forms of D-glucose phenylhydrazone have been isolated.119 The Schiff base derivatives can be... 

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