Alditols conformation

Like the polymethylene hydrocarbons, acyclic carbohydrates and alditols tend to adopt the planar, zigzag conformation wherein all of the carbon atoms lie in the same plane. " However, non-bonded interactions between parallel hydroxyl groups in the 1,3 positions (sy/i-axial interaction) are the most highly destabilizing, and, in xylitol and ribitol, this... 

With due consideration of the explanations just presented for the observed, relative stabilities of cyclic acetals derived from polyols, in terms of their constitution and conformation, nearly all of the following observations on the selective hydrolysis of cyclic acetals of alditols and dialkyl dithioacetals may be readily understood. 

Several reviews have already been published on the subject, for example, the acetala-tion of alditols [4], of aldoses and aldosides [5,6], and of ketoses [7]. Some aspects of the stereochemistry of cyclic acetals have been discussed in a review dealing with cyclic derivatives of carbohydrates [8], also in a general article [9] and, more recently, in a chapter of a monograph devoted to the stereochemistry and the conformational analysis of sugars [10], Aspects on predicting reactions patterns of alditol-aldehyde reactions are reviewed within a general series of books on carbohydrates [11]. The formation and migration of cyclic acetals of carbohydrates have also been reviewed [12,13],... 

L-fucitol by it conforms with this extended interpretation of the Hann, Tilden and Hudson rule. Further studies of the action, or lack of action, of Acetobacter suboxydans on other w-deoxy-alditols are being reported169 and so far they indicate the general applicability of the rule under the extended interpretation that a terminal methyl group is to be regarded as qualitatively equivalent to a hydrogen atom, insofar as the specificity of the oxidation by Acetobacter suboxydans is concerned. 

A simple, two-fold axis of symmetry (C2), and, hence, equivalence of the protons interchanged by this symmetry operation, is quite common in specific conformations of inositols and their derivatives, for example, the half-chair conformation (60) of myo-inosose-2 phe-nylosotriazole derivatives,187 and in derivatives of alditols having an even number of chain carbon atoms,188 for example, 2,3 4,5-dianhydro-D-iditol and its 1,6-diacetate and -benzoate189 (61). The 2,5-O-meth-... 

The alditol crystal structures are conformationally homogeneous. The cocrystallization of different conformers of the same molecule is not common, but sufficient examples are known, especially in hydrogen-bonded crystal structures, for it to be a phenomenon with which crystallographers are familiar. It occurs when there is a population of conformers in solution, all of which have similar energies. A classical example is that of 2,3-dimethyl-2,3-butanediol (pinacol)... 

It would be of interest to study the crystal structures of alditol-cation complexes, because, in them, the conformation would differ from that of the free alditol. However, no suitable example is known. Attempts to prepare crystalline complexes from D-glucitol and various salts have not been successful. ... 

The relationship between configuration and conformation observed for the unsubstituted alditols in the crystalline state can be used to predict, with significant confidence, the favored conformations of other acyclic sugar derivatives whose structures have not yet been determined experimentally. 

For molecules in the solid state, it is possible that lattice forces may profoundly affect the process of adoption of a particular conformation, and thus the favored conformation of a molecule in the crystalline state is not necessarily the same as that adopted when the molecule is surrounded by a sheath of solvent molecules. Nevertheless, there is a high degree of correlation between the results of X-ray crystallography of alditols and their derivatives (see p. 69) and the conformations adopted by various types of acyclic sugar derivatives in solution. 

The configurations of a series of 2- and 5-substituted derivatives of 1,4 3,6-dianhydro-hexitols (4) have been established from their C n.m.r. spectra. Crystal structure measurements have shown thatl,5-anhydro-2,3,4-tri-0-benzoyl-xylitol and -ribitol both adopt the expected Ci(d) conformation, whereas l,2,3,4-tetra-0-benzoyl-/3-D-xylopyranoside adopts the alternative C4 form, emphasizing the importance of the anomeric effect in the latter case. Unsaturated alditol derivatives are mentioned in Chapter 12. 

The relative stereochemistry of alditols can be deduced by formation of borate complexes, acetylation, partial hydrolysis to the polyol acetates, and then n.m.r. analysis. Borate complexes form preferentially in the sequence l,2-syn> 1,3-syn > 1,2-anti, 1,3-anti, and terminal diols (in staggered conformation). ... 

Formation and migration of cyclic acetals of carbohydrates has been reviewed. Molecular mechanical calculations have been used to calculate the energies of various conformations of bicyclic acetals of C4-C6 alditols with formaldehyde. The thermodynamic stabilities of the [4.4.0] products were predicted to be higher than for the [5,3.0] products in the gas phase. Discrepancies with experimentally observed data were ascribed to solvent effects. 

The regioselective complexation of D-mannitol, -galactitol, -xylitol, -ribitol and -erythritol with bis(phosphlne)platinum(II) carbonate complexes has been studied, and conformational analyses of the products by H n.m.r have been performed. Also a study on the preferred conformations of hex-l,S-dien-3,4-diols, which includes reference to hydroxylation reactions, suggesting an approach to hexitol and higher alditol syntheses has been reported. ... 

Volumetric specific heats and densities have been measured for several alditols in water and in salt solutions. " The lower apparent molal heat capacities of d-glucitol and xylitol suggest that alditols which adopt a bent conformation in solution are relatively more disruptive of the solvent structure than those which adopt a planar zig-zag conformation. 

The distribution of substituents in ethylcellulose has been determined by hydrolysis and conversion to the alditol acetates which were determined quantitatively by g.l.c. ° Reference ethyl ethers of D-glucose were synthesized and isolated from a hydrolysate of ethylcellulose of high degree of substitution, and the conformation of these derivatives determined by mass spectrometry. The ratio of ethylation on 0-2,0-3, and 0-6 was found to be 7.45 1 1.5. 

Angyal SJ, Greeves D, Mills JA (1974) Complexes of carbohydrates with metal cations. 111. Conformations of alditols in aqueous solution. Aust J Chem 27 1447-1456. doi 10.1071/ CH9741447... 

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