Glycosyl pyridinium salts

Sinay, P., clearly formulated this possible reaction course in a Euchem Conference, Galway, Ireland, July 1976. We had expected that the a anomer of the more reactive onium salts would not form, although both anomeric glycosyl pyridinium salts are known. Cf. Lemieux, R. U. and Morgan, A. R., J. Am. Chem. Soc., (1963), 85, 1889-1890. 

The key piece of evidence for the rationalisation of the anomeric effect in terms of classical electrostatic dipole interactions has been the apparent existence of a reverse anomeric effect. Reversal of the sense of the dipole of the C-X bond, so that X now carries a net positive rather than negative charge, should, on the electrostatic picture, result in X having a more than ordinary tendency to be equatorial. The effect was first observed in glycosyl pyridinium salts and, in what at first sight seemed to be an elegant experiment, the steric demand of the bulky pyridinium salt was apparently eliminated as the source of the effect by the observation that tri-O-acetyl-ot-D-xylopyranosylimidazole changed from 65% to > 95% equatorial on addition of excess acid. Protonation of imidazole will have only a minor effect on its steric requirements, since protonation takes place at a remote site . ... 

The preparation, X-ray crystal structure and ab initio calculation of the charge distribution of lV-(2-acetamido-3,4,6-tri-0-acetyl-2-deoxy-p-D-glucopyranosyl)-pyridinium chloride have been reported. The anomeric iV-(2,3,4,6-tetra-0-acetyl-D-glucopyranosyl)-4-dodecylpyridinium bromides were prepared as novel cationic surfactants. The panel of five substituted-pyridinium salts 15 were synthesized from the peracetylated glycosyl chloride of p-D-lV-acetyl-neuraminic acid and used to probe the mechanisms of enzymatic and non-enzymatic hydrolysis of neuraminides. " Detailed analysis of the solvolysis of Af-(2-deoxy-a- and p-D-glucopyranosyl)-4-bromoisoquinolinium tetra-fluoroborate in aqueous alcoholic solvents indicated Sjv2-like transition states. 

Several carbohydrate applications have been included in a review of micro-wave-assisted organic reactions, and a survey of chemical reagents in photo-aflinity labelling included the role of various base-labelled azido- and thio-nucleosides, sugar azide derivatives and p-benzoylbenzoate esters of nucleosides. A further review intitled Reverse Anomeric Effect Fact or Fiction refers to early evidence based on studies of glycosyl pyridinium and imidazolium salts. It concludes that its origin as an electronic effect is not supported by theory or experimental results. ... 

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