Some aspects of stereochemistry and hydrogen bonding of carbohydrates related to polysaccharide conformations

Abstract

Some general rules governing hydrogen bonding at the ring oxygens of furanosides, pyranosides, and bridge oxygens of glycosides have been formulated from existing data on crystal structures of carbohydrates. Ring oxygens of the majority of the glycopyranosides in the hemiacetal or acetal form are involved in hydrogen bonding such that the hydrogen bond direction is usually equatorial to the ring plane and not axial. In contrast, there are no known examples of ring oxygens of glycofuranosides and methyl‐glycopyranosides displaying hydrogen bonding in the crystal. Also, the bridge oxygens of glycosides are not involved in hydrogen bonding. The observed shortening in the exocyclic and endocyclic anomeric C(1)O bonds and the geminal CO bonds indicate that compounds with two oxygen atoms attached to the same saturated carbon atom may participate in double‐bond‐no‐bond resonance interaction in the same manner as difluoromethane. It is also possible that under these circumstances the carbon atom exhibits greater than tetracovalency. The “anomeric effect” may also be related to (a) the differences in the “double bonding” or bond shortening in the anomeric CO bonds of the anomeric glycopyranosides, (b) the shorter intramolecular O(1)…︁O(5) non‐bonded interaction, and (c) the smaller O(1)C(1)O(5) valence angle in the equatorial anomer compared to the axial anomer. An analysis has been made of the energetically preferred conformations about the glycosyl and glycosidic bonds of 1,4‐ and 1,3‐polysuc‐charides. In the 1 a, 4 e‐glycopyranosides the projected angle ϕ 1 [O(5)C(1)OR, where R = C or H] is positive, while it is negative in the 1 e, 4 e‐glycopyranosides. Angle ϕ 2 [C(1)OC(4′)C(3′)] is positive in both the 1,4‐anomeric polyglycosides. 1 e, 4 e‐ and 1 a, 4 e ‐polysaccharides are stabilized by intramolecular O(5)…︁HO(3′) and O(2′)…︁O(3′) hydrogen bonding, respectively, and generate linear and helical (cyclic) structures, respectively. 1 e, 3 e‐ and 1 a, 3 e‐polysaccharides may be stablized by one of two possible intramolecular hydrogen‐bonding schemes such that the 1 a, 3 e ‐polysaccharides generate helical structures while the 1 a, 3 e‐polysaccharides generate nonhelical structures. The conformation about the C(5)C(6) bond in the pyranosides falls into two groups where the angle ϕ00 [O(5)C(5)C(6)O(6)] is either positive, ∼+60 ± 30°, or negative, ∼–60 ± 30°, the former conformation being found more frequently. In the furanosides the latter conformation is preferred.