THE MECHANISM OF THE DEHYDROBROMINATION OF TETRA-O-AGETYL-α-D-GLUGOPYRANOSYL BROMIDE

Abstract

The conversion of tetra-O-acetyl-α-D-glucopyranosyl bromide (I) to 2-acetoxy-D-glucal triacetate (II) was found to be catalyzed by secondary amines containing n-alkyl groups. Substitution at the α-positions of the amine reduced the catalysis. Although triethylamine and tri-n-propylamine did not serve as useful catalysts, the less hindered and strongly basic 1,4-diazabicyclo[2,2,2]octane was more effective than diethylamine. The reaction is strongly promoted by polar solvents and added salts—especially tetra-n-butylammonium bromide. The course of the reaction involves direct elimination of hydrogen bromide and this reaction is in competition with the formation of acetylated N-glucosides. A kinetic study of the reaction using a variety of amines led to the conclusion that the first stage of the reaction is dissociation of I to glycosyloxocarbonium ion strongly solvated by the amine. Depending on the structure of the amine, the second stage of the reaction may involve transfer of the 2-proton to the amine to form 11 at a faster or slower rate than the formation of the N to C1 bond of the acetylated N-glucoside. Conditions were established for the near quantitative conversion of I to II in about 1/100th the reaction time reported in the literature.