Anomerization mechanism

The situation is completely different in the presence of 150 mol % LiNTf2- Indeed, whether isomer 110 or 111 is used, the formation of a anomer 110 predominates (a /i = >99 <1). It has been proposed that the intermediate oxocarbenium ion 1-3 is involved in the anomerization mechanism. In the presence of the trityl cation, the equilibrium between anomers 110 and 111 might shift to the more thermodynamically stable anomer 111. The significant role of perchlorate and bis(sulfonimide) anions has been consistently claimed. Thus, intermediate 1-3 would be efficiently stabilized by these anions located at the sterically less-encumbered / side of anomeric centers, achieving nucleophilic attack from the a side. It is also suggested that the Li cation is likely to effect the selective stabilization of product a anomer 110 by forming coordination complexes as exemplified by putative chelated structures C-1 and C-2 (Fig. 10). 

Pyridone also catalyzes epimerization of the anomeric position of the tetramethyl ether of glucose. The mechanism involves two double proton transfers. The first leads to a ring-opened intermediate, and the second results in ring closure to the isomerized product ... 

Anomeric effect of monosaccharides and their derivatives insights from the new QVBMM (quantized valence bond molecular mechanisms) molecular mechanics force field 98H(48)2389. 

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-... 

The lack of glycosyl transfer reaction is the class of pectinolytic hydrolases is in agreement with the observed inversion of the anomeric configuration of the newly formed reducing ends of the products. All three polygalacturonases studied here utilize the single displacement mechanism of hydrolysis. 

Fernandez, B., M. A. Rios, and L. Carballeira. 1991. Molecular Mechanics (MM2) and Conformational Analysis of Compounds with N-C-O Units. Parametrization of the Force Field and Anomeric Effect. J. Comput. Chem. 12, 78-90. 

The mechanical, nonbonded, and electrostatic expressions described above are not sufficient to describe some structural and spectroscopic effects. Three specific structural and spectroscopic phenomena have been incorporated into MM3. They are the electronegativity, anomeric, and Bohlmann effects, which essentially can be traced to molecular orbital origins. These chemical effects are a part of the MM3 program.74,75... 

The decrease of the anomeric effect in polar solvents was also supported by quantum mechanics calculations.13 Nevertheless further studies on the anomeric effect demonstrated the limitations of the electrostatic model. In particular, Juaristi et al.14 demonstrated that, at low temperature, the dependence of conformational equilibria of 2-carbomethoxy-l,3-dithiane upon solvent shows an opposite trend to the stronger anomeric effect in less polar media observed at 25 °C (Table 4). 

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