THE EFFECT OF CHLORINE SUBSTITUTIONS AT THE C2-ACETOXY GROUP ON SOME PROPERTIES OF THE GLUCOSE PENTAACETATES

Abstract

The effects were studied of substituting one, two, and three chlorine atoms at the C2-acetoxy group of the α- and β-D-glucopyranose pentaacetates on the rates both of anomerization and of dissociation of the C1 to acetoxy group bond in 1:1 acetic acid – acetic anhydride 0.5 M with respect to sulphuric acid at 25 °C. In the case of the α-anomers, the rates of anomerization appeared about equal to the rates of dissociation as measured by isotopic exchange. The β-anomers dissociated more rapidly than they underwent anomerization. The difference in rate decreased rapidly, however, with the introduction of chlorine atoms indicating that the tendency for C2-acyl group participation in the dissociation is reduced by the chlorine substitutions. Evidence was obtained that only a very small fraction, if any, of the C1-acetoxy group of β-glucose pentaacetate passes into the α-anomer without becoming completely dissociated. It is pointed out that the data for the anomerizations can be rationalized on the basis of ionic mechanisms, if specific conformations are allocated to the intermediate carbonium ions.