Carbohydrates gauche-effect

In addition to steric effects and electrostatic effects, the conformational analysis of carbohydrates requires two stereoelectronic effects to be taken into consideration the gauche effect and the anomeric effect, in its various manifestations. Both of these effects can be considered as aspects of no-bond resonance. 

It is very probable that the energetics favoring the left-handed helix are also effective when the molecule is in solution. These forces, therefore, would also affect the contribution of chain stiffness and form to the optical rotation. The relation between these two phenomena and the conformation revealed by x-rays remains to be explored. A preliminary attempt at relating the conformation of /3-d-(l— 4)-linked polysaccharides to simple conformational concepts in carbohydrate chemistry has focused on the carbon-oxygen bond sequence between the residues (see Fig. 22) that form the well-known molecule methylal. Measurements of dipole moments of methylal show that the conformation shown in Fig. 22A is favored here, the group moments are in partial opposition, and the O-C-4 bond is gauche, relative to C-1-0-5. This is exactly the con-... 

Figure 4.2.6 Two effects determine the configurations of open-chain carbohydrate derivatives (a) gauche confirmations are favored in 1,2-diols over anti conformations, and (b) 1,3-syn diaxial interactions between hydroxyl substituents are repulsive and often lead to distortions of the all-anti chain.

However, carbohydrates also possess additional polar substituents that combine with the anomeric effect to yield new stereoelectronic patterns. In fact, the chemistry of carbohydrates provides a great testing ground for the design of cooperative stereoelectronic patterns. In Chapter 6, we briefly discussed the competition between gauche and anomeric effects. We will expand our discussion of the interplay between stereoelectronic effects... 

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