Rotamers arising from restricted motions and electromc interactions of acetoxymethyl groups in carbohydrates

Abstract

The two dimensional conformational spaces available for acetoxymethyl groups in carbohydrates are reported. The studies were performed on two molecules differing in their C-4 configurations, i.e. gluco and galacto. The acetoxymethyl group was assigned a mean geometry derived from the computation of 19 equivalent groups as found in pertinent acetylated carbohydrates. The internal energy calculations were performed by including the partitioned contributions arising from the non-bonded energies, electrostatic interactions, and torsion energy. An instability rating of 3 kcal/mol was assigned to conformations involving peri interactions, i.e., trans–gauche and gauche–gauche forms in gluco and galacto carbohydrates, respectively. The results which are best described in term of topology show that several conformations corresponding to the trans and gauche states of the C—O single bond are preferred. However, significant shifts from perfect staggering are observed. The predicted regions of low energy are found to be in good agreement with experimental data from crystal structures of acetylated carbohydrates.

Crystal and molecular structure of .beta.-maltose octaacetate. A model in retrospect for amylose triacetate?

“…A recent publication analyzes the conformation of these molecules and uses semiempirical potential energy calculations in order to rationalize the observed conformations. 13 The survey clearly establishes that the conformation of the acetyl group in carbohydrate acetates is * Author to whom correspondence should be addressed at Université de Montréal. II Author to whom correspondence should be addressed at the Xerox Research Centre of Canada, Mississango, Ontario, L5L 1J9.…”

Structures of the Common Cyclodextrins and Their Larger AnaloguesBeyond the Doughnut

“…A reason for this behavior could be that in the trans orientation, adverse steric interactions might occur between O6−H and atoms of the adjacent glucose unit in the ring. In addition, the gauche effect, , operative in O−C−C−O systems, should also destabilize the trans arrangement.

2 Glucose 4 C 1 chair conformation with O6−H in (−)gauche (left) and (+)gauche (right) orientations.

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Conformations of the hydroxymethyl group in crystalline aldohexopyranoses

“…To quantify this statement, a conformational analysis of the rotation about the exo-cyclic bond has been performed using an empirical energy approximation, the details of which have been presented elsewhere. 16 This procedure expressing the steric interactions is widely used in the field of polysaccharide conformational analysis. In this study both acetoxy and hydroxymethyl groups are considered interchangeably, since the statistical distribution of the rotations around the C(5)-C(6) bond for the subgroups in questionI6 are similar to those obtained independently of the nature of the substituent at O(6).…”