Aldopyranoses conformational analysis

In 1937 Isbell published an important paper on the conformational analysis of aldopyranoses, in which several of the forms were depicted (see Fig. 2). They comprise a 4C, chair (I), afl03 boat (II), two half-chairs (III and IV), and a coplanar pyranose (V). He correctly favored the chair form (I) and predicted that, in the case of saccharides, the chair would tend to assume a somewhat flatter conformation than that of carbocycles such as cyclohexane because of the smaller bond angles of oxygen as compared to those of carbon (105° instead of 109.5°). We now know that such coplanarity is strongly avoided because of the strain that would be produced and because of the repulsive forces between the substituents. 

Angyal SJ (1968) Conformational analysis in carbohydrate chemistry. I. Conformational free energies. The conformation and a / ratios of aldopyranoses in aqueous solution. Aust J Chem 21 2737-2746. Angew Chem Intern Edit 8 157-166 (1969)... 

Figure 6 Three point binding model for prediction of stereoselectivity in NeuA-catalyzed reactions, based on a conformational analysis of C-3 epimeric aldopyranoses as acceptor substrates [22].

A notable exception has been the application of i.r. spectroscopy by Isbell and Tipson to the analysis of the conformations of methyl aldopyranosides," acetylated methyl aldopyranosides," " and fully acetylated aldopyranoses in the solid state. Their approach was based on (J) the assumption that the pyranoid sugar derivatives predicted by Reeves to exist in only one of the chair conformations in solution would adopt that same conformation in the crystalline state, and (2) the observation that axially and equatorially attached groups at the anomeric carbon atom give rise to different vibrations. 

---