Orientations of hydroxyl groups. Conformational studies on 2,3:5,6-di-O-isopropylidene-α-D-mannofuranose by nuclear magnetic resonance spectroscopy

Abstract

Various nmr spectroscopic techniques have been used to determine the orientation of the anomeric hydroxyl group of 2,3:5,6-di-O-isopropylidene-α-D-mannofuranose (1) in dimethylsulfoxide solution, as well as other conformational features of themolecule. 13C and 1H coupling constants indicate that in its most probable orientation the O—H bond is anti with respect tocarbon-2, and forms an angle of ~40° with the C-1—H-1 bond. Spin-lattice relaxation rates (R1) and nuclear Overhauser enhancements (nOe) have been measured for the pertinent protons and used, in conjunction with 13C relaxation times, as additional criteria for characterizing the geometry of the anomeric alcohol group of 1. These results are in good agreement with the coupling constant data. The conformations of the mannofuranose ring, the 2,3-O-isopropylidene ring, and the C-4 to C-6 region of the molecule are also described.