Transglucosidation of methyl and ethyl d-glucofuranosides by alcoholysis

Abstract

The acid catalyzed ethanolysis of methyl 5-O-methyl-alpha- and -beta-D-glucofuranoside and the analogous methanolysis of ethyl 5-O-methyl-alpha- and -beta-D-glucofuranoside have been investigated. For all four reactions, the primarily formed transglycosylation product was a single glycoside that had the opposite anomeric configuration to the starting material. This strongly indicates that a D-glucose methyl ethyl acetal is first formed and is then ring closed by a nucleophilic attack by HO-4, giving either the starting material or a transglycosylation product with the opposite anomeric configuration. Low percentages of the methyl ethyl acetals and of dimethyl acetals were also observed in the reaction product during the methanolysis reactions.

Mechanisms of glycopyranosyl and 5-thioglycopyranosyl transfer reactions in solution

“…42, 43 These mechanistic conclusions were further refined by two recent studies on the acid-catalysed alcoholysis of methyl and ethyl 4-O-methyl-α-and β--glucopyranosides, 44 and of methyl and ethyl 5-O-methyl-α-and β--glucofuranosides. 45 In these two reports, Konradsson and co-workers show that the isomeric glycosides solvolyse by different mechanisms: pyranosides react via a D N ϩ A N exocyclic C-O cleavage, while the At the moment, it appears that the 2-hydroxy group is an important determinant for the mechanism of anomerisation. 46 Specifically, the results from a recent study of the acid-catalysed methanolysis of methyl 2-deoxy-β--arabino-hexopyranoside 47 suggest that for 2-deoxyglycopyranosides both exo-and endocyclic C-O bond cleavage mechanisms occur simultaneously, 46 although at the present time it is not clear whether the ringopening pathway occurs with or without nucleophilic solvent participation.…”

Transition-State Structure for the Quintessential SN2 Reaction of a Carbohydrate: Reaction of α-Glucopyranosyl Fluoride with Azide Ion in Water

“…Nucleophilic substitution reactions for most acetals (R′O–CHR–OR′′), which contain few electron-withdrawing groups, generally proceed via discrete oxacarbenium ion intermediates (S N 1 mechanisms) due to stabilization of positively charged species by the second oxygen atom. − In contrast, nucleophilic substitution reactions on carbohydrate acetal linkages straddle the mechanistic border between S N 1 and S N 2 reactions by virtue of the inherently short lifetimes of oxacarbenium ion intermediates; as a result, reactions via ion-pairs become important . For instance, in water the glucopyranosylium ion has an estimated lifetime of approximately 1–3 ps. , In addition, studies have shown that, during alcoholysis of alkyl furanosides, cyclic oxacarbenium ions are not intermediates (these reactions involve a concerted S N 2 ring-opening mechanism). − …”

Liberation of recalcitrant cell wall sugars from oak barrels into bourbon whiskey during aging

“…Acidification of an ethanol solution produces a reaction environment that allows the esterification of cellulose. The esterification of cellulose with ethanol can produce an ethyl-glucoside product that first degrades to become 5-hydroxymethylfurfural and then further degrades to myriad other compounds in an ethanol environment 31 , 32 , 44 , 45 . However, this reaction has not been shown to occur in a whiskey solution.…”