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The carbonate catalyzed anomerization of protected 2, 4-dinitrophenyl glucopyranosides : a mechanistic study Berven, Leise Ann


The mechanism of the carbonate catalyzed conversion of 2,4-dinitrophenyl 2 ,3 ,4,6-tetra-0-acetyl-β-D-glucopyranoside in DMSO to an equilibrated mixture of the α- and β-glycosides has been investigated using a variety of techniques. Pseudo-first-order rate constants (k) measured for the anomerization of the parent substrate and the 1-deuterio substrate indicated a secondary deuterium isotope effect of kH/kD = 1.09 ± 0.06. Pseudo-first-order rate constants measured for several deoxy and deoxyfluoro derivatives of the parent sugar showed that the deoxyfluoro sugars react at least as fast as the parent sugar whereas the deoxy sugars reacted more slowly. In addition to the 2,4-dinitro-phenyl glucoside, 2,6-dinitrophenyl glucoside also was found to anomerize, yet attempts to exchange the 2,4-dinitrophenolate groups of the glucoside with added 2,6-dinitrophenolate anion and vice versa were unsuccessful. Exchange of the proton at the anomeric carbon with a deuteron also does not occur when the anomerization is performed in the presence of a deuteron source (and vice versa). Exchange of the glucosyl residue was observed, however, when the 1-deuterio substrate was anomerized in the presence of non-deuterated 2,3,4,6-tetra-O-acetyl-D-glucopyranose. ¹H-n.m.r. of the 2,4-dinitrophenyl α-glucoside isolated from this reaction indicated that the a-glucoside possessed only 50% of the deuterium label, at the anomeric center. These results along with the observation of a Meisenheimer intermediate indicate that the anomerization proceeds via nucleophilic aromatic substitution and as such is novel mechanism for glycoside anomerization.

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