Mannose ring structure

The base-catalyzed, / -elimination reaction of D-mannose phenyl-hydrazone is consistent with the acyclic structure for the phenylhydra-zone in solution. However, the small proportion of a nitroxide radical observed on treatment of the phenylhydrazone with a strong base may indicate the existence also of a fractional proportion in a cyclic structure in equilibrium with the open-ring structure, as was suggested by Blair and Roberts (43). The hydrazino moiety required for nitroxide-radical formation could be derived from the cyclic form of D-mannose phenylhydrazone in solution. 

Ring Structure and Optical Relationships in the Mannose-Rhamnose-Lyxose Series of Sugars. Isolation of a New Form of Lyxose, W. N. Haworth and E. L. Hirst, J. Chem Soc., 1221 (1928). 

From a structural point of view, the carbohydrate template can have either furan or pyran rings although in some cases open chain structures can be formed. A large variety of aldopentoses (e.g. d- and L-arabinose, D-ribose, D-xylose), aldohexoses (e.g. D-glucose, D-mannose, D-galactose) as well as ketohexoses (e.g. D-fructose, L-sorbose) can be used as scaffolds. 

The crystal structure of a-D and a-L-mannose has been solved,22 but full details are not yet available. With calcium chloride, D-raan-nose forms a crystalline compound (C6H,206 CaCl2 4HzO) which appears to contain the furanose ring.23 The crystal structure is of interest because of the furanose ring and the nature of the presumed complex. In the complex are three heavy atoms (one calcium and two chloride ions) which could help in solving the structure. 

The ring-chain equilibrium 46A 46B was revealed by H- and 13C-NMR spectroscopy (93ZOR278) in solutions of D-glucose, D-mannose, d-galactose, and D-arabinose thiosemicarbazones. The equilibrium state depends rather unpredictably on the structures of the initial monosaccharide and of the substituents R2 and R3 in the thiosemicarbazone moiety. In solutions of D-fructose thiosemicarbazones, pyranose and furanose tautomers participate in the equilibrium. 

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