Alditols acetates, 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. 

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],... 

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. 

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