Mutarotation mechanism

Muscalure, structure of, 287 Mutarotation, 985-986 glucose and, 985-986 mechanism of. 986 Mycomycin, stereochemistry of, 330 Mylar, structure of, 819 n yo-Inositol, structure of, 135 Myrcene. structure of, 202 Mvristic acid, catabolism of, 1137 structure of. 1062... 

The reversible reactions are initiated by an equilibrium between neutral and ionized forms of the monosaccharides (see Fig. 6). The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization via an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar, the three reducing sugars are in equilibrium via the enediol intermediate. The mechanism of isomerization, known as the Lobry de Bruyn-... 

Lowry is best known to chemistry students through the tradition of eponymony, since the proton theory of acidity is known as the "Bronsted/Lowry theory" of proton donors. His most important experimental investigation likely was a long series of studies on optical rotatory dispersion.49 For our purposes, there is special interest in his discovery of mutarotation in camphor derivatives and his theory of dynamic tautomerism, which led him to an ionic theory of organic reaction mechanisms. 

Faulkner (1923 1927) reported that mutarotation occurred much more readily in a mixture of pyridine and cresol than in either by itself. Swain and Brown (1952) extended this work and found that a-pyridone, where an acidic and a basic group are incorporated in the same molecule, is an excellent catalyst, 0 05 M concentration giving rise to a reaction 50 times faster in benzene solution than a solution of 0 05 M phenol and 0-05 M pyridine. A 0-001 M solution of a-pyridone was 7000 times more effective than equivalent concentrations of phenol and pyridine. A concerted mechanism was proposed [4]. 

To accommodate these facts, the earliest mechanisms proposed for degradation of D-fructose assumed that it was present in the furanose form, and that the ring remained intact. It was assumed that the initial reaction was the elimination of water, to form the 1,2-enolic form of 2,5-anhydro-D-mannose, and that further dehydration resulted in 2-furaldehyde. The necessity for D-glucose to isomerize to D-fructose was assumed to account for the much lower reaction-rate of D-glucose. This mechanism does not account for the observation that 2,5-anhydro-D-mannose is less reactive than D-fructose, nor is there any evidence that 2,5-anhydro-D-mannose is present in reacting D-fructose solutions. Nevertheless, similar mechanisms have since been proposed.13-16 Because of the ease of mutarotation of D-fructose... 

A scheme for the imidazole catalysis of the mutarotation of glucose, similar to the concerted mechanism proposed by Swain and Brown (13), is shown below, in which a proton is transferred from the D-glucose to imidazole or benzimidazole and from the H20 (represented as an acid) to the D-glucose in the rate-determining step. 

The catalysis of mutarotation of glucosamine hydrochloride involves an intramolecular mechanism, and so the catalytic coefficient of glucosamine. fcc,iNH2> must have the dimension, min.-1, instead of liters/mole min. The total rate is then... 

A third possible type of catalysis requires that a base and an acid act synchronously to effect the breaking and formation of bonds in a single step. Thus, tetramethyl-glucose mutarotates very slowly in benzene containing either pyridine (a base) or phenol (an acid). However, when both pyridine and phenol are present, mutarota-tion is rapid. This suggested to Swain and Brown132 a concerted mechanism (Eq. 9-92) in which both an acid and a base participate. 

Computer Modeling of the Kinetics of Tautomerization (Mutarotation) of Aldoses Implications for the Mechanism of the Process... 

If this reaction is observed polarimetrically, the equilibrium rate constant obtained is equal to the sum of the tautomerization rate constants (Ki -f- K. i) for the overall reaction, as described above (20). Therefore, no matter what the path or the mechanism of the mutarotation phenomenon, the data obtained polarimetrically are completely explained by the above overall configuration change. 

Although the crystalline forms of a- and /3-D-glucose are quite stable, in solution each form slowly changes into an equilibrium mixture of both. The process can be observed as a decrease in the optical rotation of the a anomer (+112°) or an increase for the /3 anomer (+18.7°) to the equilibrium value of 52.5°. The phenomenon is known as mutarotation and commonly is observed for reducing sugars. Both acids and bases catalyze mutarotation the mechanism, Equation 20-1, is virtually the same as described for acid- and base-catalyzed hemiacetal and hemiketal equilibria of aldehydes and ketones (see Section 15-4E) ... 

In other studies, analysis of the products of reaction between formaldehyde and guanosine at moderate pH shows a new adduct—formed by condensing two molecules of each reactant—which has implications for the mechanism of DNA cross-linking by formaldehyde,17 while the kinetics of the mutarotation of N-(/ -chlorophcnyl)-//-D-glucopyranosylamine have been measured in methanolic benzoate buffers.18 For a stereoselective aldol reaction of a ketene acetal, see the next section. 

The results obtained are shown in Table III (see p. 26). The mutarotation of the product indicated, in each case, that the /3-D-modification predominated for the acetyl sugars formed from reaction with water. The data show that the reaction was only slightly affected by change of solvent or of temperature, and was free of orthoester formation. These facts, and the almost exclusive formation of 1,2-trans products from the 1,2-cis-bromide, was interpreted as evidence that the reaction proceeds by way of the Sw2 mechanism. It was suggested that the increase of O-acetyl-a-D-glucoside formed at 50° (over that at 20°) may be due to racemization through carbonium-ion formation. 

It was generally believed that the characteristic non-linear dependence of the rate constant on n was associated with the existence of the preequilibrium (Bell, 1941). For the mutarotation of glucose a linear variation of rate constant with n had been reported (Hamill and La Mer, 1936c) and cited as an example of a mechanism of acid catalysis without pre-equilibrium. 

Mechanism 23-1 Formation of a Cyclic Hemiacetal 1108 23-7 Anomers of Monosaccharides Mutarotation 1112 23-8 Reactions of Monosaccharides Side Reactions in Base 1114... 

Isbell, Horace S., and Pigman, Ward, Mutarotation of Sugars in Solution Part II, Catalytic Processes, Isotope Effects, Reaction Mechanisms, and Biochemical Aspects, 24, 13-65 Isbell, Horace S. See also, Pigman, Ward. 

The mechanism generally accepted for mutarotation involves conversion to the open-chain form for example, XXXVI —> XL, for the acid-catalyzed mutarotation of a- and jS-D-glucose. 

Los and Simpson reported that the catalytic constant for the mutarotation of D-glucose is strongly dependent on the D-glucose concentration, and they suggested that this effect is occasioned by the incursion of a concerted mechanism for the formation of the open-chain form (see XLI — XLII). Such a mechanism was proposed by Swain and Brown ... 

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