Pyranose–furanose and Anomeric Equilibria: Influence of Solvent and of Partial Methylation

Sugars possessing the arabino (2,3,4-trans,cis) configuration exist as furanoses to a greater extent in dimethyl sulfoxide than in water. Their 2,3-di-O-methyl derivatives show an even stronger preference for a five membered ring structure in both solvents. This is most marked for 2,3-di-O-methyl-D-arabinose and 2,3-di-O-methyl-D-altrose, which are 65% and 80% furanose, respectively, in dimethyl sulfoxide. Compounds in the xylo (2,3,4-trans,trans) or lyxo (2,3,4-cis,trans) series show little tendency to be furanoses in either solvent. However, α-pyranoses in the lyxo series are relatively more stable in dimethyl sulfoxide than in water, whereas the anomeric composition for members of the xylo series is the same in both solvents. Some of these variations in the equilibria are attributed to the preferential stabilization of pyranose forms in water. D-Lyxose and D-ribose show nuclear magnetic resonance spectral differences in the two solvents that appear to be due to conformational, as well as tautomeric, equilibrium changes.An equatorial anomeric hydroxyl proton of a given pair in dimethyl sulfoxide shows a larger spacing (6.5–8.0 c.p.s.) than an axial anomeric hydroxyl proton (4–5 c.p.s.). The signal for the latter proton occurs at higher field than an equatorial OH-1, except when OH-2 is axial.