THE MECHANISMS OF GLUCOSE PENTAACETATE ANOMERIZATION AND LEVOGLUCOSAN FORMATION

Abstract

The stannic chloride catalyzed anomerization of the pentaacetyl-D-gluco-pyranoses in chloroform solution is specific for the C1-acetoxy group. The reactions involve complete dissociation of the C1-carbon atom to acetoxy group bond with an intermediate formation of carbonium ions. The initial step of the beta to alpha rearrangement is a rapid dissociation, involving the participation of the C2-acetoxy group, to a resonance-stabilized carbonium ion with the lactol carbon atom occupied in the α-configuration. The rate-controlling step in the reaction appears to be the rearrangement of this ion to other ions which are capable of recombining with acetate ion to yield the α-acetate. The α-acetate is highly stable, as compared to the β-anomer, and the dissociation of the C1-carbon atom to acetoxy group bond is the rate-controlling step in its rearrangement. The stability of the α-acetate toward a variety of acidic reagents which readily dissociate the β-form is pointed out. For example, although the α-acetate is highly stable toward titanium tetrachloride, the reaction of the β-anomer with this reagent, to yield tetraacetyl-β-D-glucopyranosyl chloride, is extremely fast. This product is unstable under the reaction conditions used and rearranges to the α-form at a measurable rate. 1,2,3,4-Tetraacetyl-β-D-glucopyranose with stannic chloride in chloroform solution yielded triacetyl-D-glucosan <l, 5 > β <1, 6>.The alkaline hydrolysis of triacetyl-D-glucosan <1, 5> α <1, 2> yielded D-glucosan <1, 5> β <1, 6>. The mechanisms of these reactions are discussed.