Motional behavior of 2,3:5,6-di-O-isopropylidene-α-D-mannofuranose in solution. A 13C spin-lattice relaxation study

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Abstract
13C nuclear magnetic resonance spin-lattice relaxation times (T1) have been used to probe the motional behavior of 2,3:5,6-di-O-isopropylidene-α-D-mannofuranose (1) in dimethyl sulfoxide solution. This system offers structural features well suited to the study of a variety of internal motions, i.e., ring oscillation, ring puckering interconversion, and methyl internal rotation, all of which are superimposed on an isotropic overall reorientation. Among various models examined to evaluate internal rotations, a two-sites jump model was found satisfactory for interpreting the oscillation and puckering motions of the flexible 5,6-O-isopropylidene ring, whereas a diffusional model described hindered rotation of the geminal methyl groups of the rigid 2,3-O-isopropylidene ring from 15° to 80 °C. The activation energy associated with the temperature dependence of the rate of overall molecular tumbling was found to agree with the hydrodynamical prediction of 4.57 kcal/mol associated with the temperature dependence of the ratio (η/T). In addition, an explicit treatment of the relaxation data vs. solution viscosities, as a function of temperature, indicated that 1 reorients under the "slip" boundary conditions.

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