Sugars anomeric effects

Table IV. Comparison of Anomeric Proton (Hi) Chemical Shifts for Free and Acetylated D-gluco and D-mannopyranoses With Those of Related 2-Deoxy Sugar Derivatives. Effect of Replacing an Axial or an Equatorial C2-Methylene Proton With a Hydroxy or Actetoxy Substituent...

Carba-sugars (pseudo-sugars) and their derivatives, chemistry of, 48, 21 - 90 Carbohydrate chemistry anomeric and exo-anomeric effects in, 47, 45-123... 

It is of interest to note that the anomeric effect directs the boron substituent into the a-configuration in the case of mannose, despite the considerable steric demand. NMR studies in solution indicate that the usual 4Q conformation of the pyranose is retained, with the boron substituent in the axial position. Depending on the sugar moiety, tumor uptake is observed when the compounds are given to tumor-bearing animals. Accumulation is, however, only transient, and within a fairly short time, the boron is lost from the tumor, and from other tissues. Even at... 

Two recent versions of MM2, MMP2(85) and MM2(87), automatically compensate for the anomeric effects that are important for sugars. Accomodations for carbohydrates are discussed further in the chapter in this book by French,... 

In a microwave investigation, Lopata and Kuczkowski (Lopata, A. D. Kuczkow-ski, R. L., J. Am. Chem. Soc., 1981, 103, 3304-3309) determined that the equilibrium geometry of FCH2— —CHO was as shown below. Explain the near 90° orientation of the CF bond relative to the plane of the rest of the molecule (the explanation is the same as for the anomeric effect in sugars.)... 

The only systematic study published on the influence of solvents on the solution equilibria of sugars is contained in two articles by Perlin.51-57 This work showed that, in other solvents, the a / -pyranose ratio is higher than in water (if the a-anomeric hydroxyl group is axial), and that there is a greater proportion of the furanose forms. The increase in the a-pyranoses is caused by the increased anomeric effect the possible reason for the increase in the furanose forms has been discussed in Section III, 1. The anomeric effect becomes particularly important in nonpolar solvents for example, in a chloroform solution of evernitrose... 

The formation of an a-D-glycoside (52) may be satisfactorily accounted for in terms of the "anomeric effect,MU) particularly applicable with 2-deoxy sugars, in which the absence of substitution at C-2 eliminates factory... 

The polarized-TT frontier molecular orbital (PPFMO) method has been employed to study protonation and sulphenylation of sugar-related dihydrofurans and tetrahydropy-rans. The predictions are consonant with the experimental observations62. Contrary to expectations, the proton-catalysed addition of alcohols to glycals, such as 30, has been shown by isotope labelling (2H) not to be anii-diaxial addition. This observation has been rationalized by the initial attack by deuteron from the bottom, giving ion 31, and by the anomeric effect favouring axial substituent at C-l (32)63. 

In a probe for the presence of stereoelectronic effects in nucleophilic addition to 12 sterically unbiased ketones, calculations have identified subtle bond length differences in the C-Nu bond of the diastereomeric alcohol products, where Nu- = H-or Me-.304 The calculated differences are weak (<1%) but consistent the bond is longer in the major product, acting as a fossil record of the TS. Using microscopic reversibility, the easier bond to cleave (the longer one) is the easier to form. The effect bears comparison with the kinetic anomeric effect in sugars, where such bond length differences in calculation are borne out in X-ray crystal structures. 

Fig. 7.7 Anomeric effect in sugars (left) preferentially stabilising the axial position of the anomeric C—O bond, and isomer energy difference between 1,3- and 1,4-dioxane at B3LYP/6-31 G(d) as clear evidence against steric (Pauli) repulsion arguments for the anomeric effect.

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