Studies on the sulphonyl esters of carbohydrates
The general principles of conformational analysis in the sugar series have been reviewed briefly in the introduction. The 6-tosyl esters of a number of glycopyranosides and some related compounds have been prepared by direct tosylation of the corresponding methyl glycoside or deoxy compound. A simple counter current extraction procedure was found to be a useful method of isolating the product in some cases. The kinetics of the reaction between sodium hydroxide and a 6-tosyl ester, capable of ring closing to form a 3,6-anhydro compound, have been studied. Attempts to determine directly, the expected second-order rate constant for the reaction, gave results which did not obey the accepted second-order law. Further examination of the products of the reaction, revealed that those 3,6-anhydro sugars having a 1,3-diaxial arrangement of hydroxyl groups in their most probable conformation, were appreciably acidic. Infrared measurements on a number of 3,6-anhydro compounds showed that the acidic compounds contained a very strong intramolecular hydrogen bond in the un-ionised state. The difficulties involved in the kinetic measurements were avoided by determining first-order rate constants using a large excess of sodium hydroxide for the reaction. Measurements were made at various concentrations of sodium hydroxide for each compound and a considerable increase in the derived secondorder rate constant with increasing hydroxide ion concentration was observed with those compounds giving rise to the more acidic 3,6-anhydro compounds. A kinetic scheme is described in which the reaction is assumed to occur partly via a mono ionised species and partly via a di-ion, the latter is particularly important in those compounds in which a 1,3-diaxial arrangement of hydroxyl groups is possible and it is suggested that the di-ion is stabilised by hydrogen bonding. Using the method of least squares, second-order rate constants have been derived for each compound. The rate constants so obtained have been discussed in terms of sugar conformations, and the importance of dipole interactions and hydrogen bonding in determining conformation,is pointed out.