Monosaccharides conformational analysis

Aldonolactones are useful starting materials for the synthesis of modified sugars. They have also been used as chiral templates in synthesis of natural products. Some of them are inexpensive, commercially available products or they may be obtained readily from the respective monosaccharides. The purpose of this chapter is to survey the main reactions of aldonolactones. Previous reviews on the subject include articles on gulono-1,4-lactones (1) and D-ribonolactone (2). Methods of synthesis, conformational analysis, and biological properties are not discussed in this chapter. 

Application of mass spectrometry to stereochemical analysis in the monosaccharide series is limited by the lack of experimental studies, by the absence of a generalizing, theoretical concept based upon modem conformational analysis, and by lack of precise knowledge of the mechanism and stereochemistry of the reactions taking place in the mass spectrometer. The only investigation in which a thorough study has been made of the effect of the stereochemical features of carbohydrates upon their mass spectra is that of Heyns and Scharmann, who measured the mass spectra of a number of permcthylated methyl pentopyranosides, namely, methyl tri-0-methyl-j8-D-ribo-(13), -arabino-(14), -xylo-(lS), and -lyxo-pyrano-sides (16), and of methyl tri-Omethyl-a-D-lyxopyranoside (17). 

According to accepted criteria and experimental results of conformational analysis of monosaccharides,88 the pyranose ring of the Xyl, Gal, GalNAc, and GlcNAc... 

The conformational analysis of monosaccharides, disaccharides, and ohgosaccharides is reviewed. Conformational terms are introduced through examination of the conformations of cyclohexane and cyclopentane then applied to the pyranose, furanose, and septanose rings. Concepts such as the anomeric effect are discussed. Topics of current interest, such as hydroxymethyl group and hydroxyl group rotation and disaccharide conformations are summarized. Physical methods for studying conformation are outlined. 

Finally, considerable progress has been made with regard to gas-phase conformational analysis of monosaccharides and their hydrated complexes by comparison of their IR spectra with that predicted by ab initio calculations.76 78 Many of the tools are therefore available to place a sugar in the gas phase, cleave the glycosidic bond, isolate the oxocarbenium ion, and then have the opportunity to study the kinetics for nucleophilic capture by direct kinetic techniques. Experiments such as competitive KIEs for capture of the oxocarbenium ion also seem possible. Such work may allow direct experimental determination of oxocarbenium ion lifetimes, barriers for capture, and transition state structures. Differences observed between gas-phase results and those in solution may reveal the role that solvent plays in the reaction. 

A complete structural determination of chitin/chitosan, subsequent to its isolation and purification, involves the determination (the constituent of the two monosaccharides the molecular size the molecular size distribution the linkage positions between the monomer units their sequence and degree of heterogeneity and any preferred conformation of the molecule(s) present). NMR spectroscopy can be used for structural characterization and conformational analysis of oligosaccharides in solution (van Halbeek 1996, Chaplin 2000). Conformational changes accompanied with metal-binding were examined by means of C NMR spectroscopy (Saito et al. 1987a,b). 

The product ratios of the radical addition of sodium hydrogen [ S]sulphite to eight different methyl 6-deoxyhex-5-enopyranosides have been determined. Conformational analysis of the results enabled a direct comparison of diastereo-isomeric monosaccharides for the first time for one set of products p-allo and h-talo), the agreement between the experimental results and theoretical predictions was improved if a value of 0,45 kcal mol" was assigned to the ring oxygen-Oaa,-3 interaction (182). 

About the same time, a new international research journal, Carbohydrate Research, appeared (volume 1) in 1964. It has become the principal journal for the publication of all aspects of carbohydrate research, such as chemical synthesis and modification enzymatic synthesis organic and enzymatic mechanisms involving carbohydrates carbohydrate metabolism conformational analysis isolation of carbohydrates from natural sources analytical methods chemistry of monosaccharides, oligosaccharides, and polysaccharides biological function of carbohydrates and physical properties. Approximately four to five volumes are published each year. 

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