Glucopyranosides, hydrolyses

A 4-nitrophenyl a-D-glucopyranoside-hydrolysing a-D-glucosidase of a thermophile, Bacillus thermoglucosidius, has been purified to an electrophoreti-cally homogeneous state (see p. 458). " ... 

A neighbouring carboxyl group in o-carboxyphenyl-2-acetamido- 3-d-glucopyranoside [89] gives further rate enhancement, hydrolysing 8 1 X 10" times faster than expected from a Hammett ap plot... 

A. J. Bennet and M. L. Sinnott, Complete kinetic isotope effect description of transition states for acid-catalyzed hydrolyses of methyl a- and jS-glucopyranosides, J. Am. Chem. Soc., 108 (1986) 7287-7294. 

Chlorotriphenylmethane (PhsCCl) reacts under more moderate conditions on the sugar dissolved in pyridine. At room temperature, if treatment is not unduly prolonged, only the primary alcohol function is etherified. For example, ether 5.1 is obtained with methyl a-D-glucopyranoside. These ethers are hydrolysed under very mild acidic conditions, for example in aqueous acetic acid at 80°C. There is also the risk that hydrolysis will occur when they are purified by chromatography on silica gel finally they are cleaved by hydrogenolysis under the same conditions as benzyl ethers. 

Intramolecular general acid catalysis is normally detected in the first instance by a horizontal portion on the plot of logiokobs versus pH, governed by the add dissociation constant of the substrate. Thus, the hydrolyses of various salicyl acetals (including the P-D-glucopyranoside below pH 10, " where a base-catalysed process occurs) obey the rate law of eqn (3.7), where ko is the first-order rate constant for hydrolysis of the neutral molecule and ka is the second-order rate constant for the acid-catalysed hydrolysis of the neutral molecule ... 

Figure 3.32 Base-catalysed hydrolyses of aryl a-glucopyranosides and aryl a- and P-mannopyranosides.

Hydrolyses of the glycosidic linkages of phenyl a- and 3-D-glucopyranoside were studied at different alkali concentrations, and the reaction rates increased rapidly above 2M. Mechanisms involving direct hydrolysis and the intermediacy of l,6-anhydro- 8-D-glucose and, with the j3-anomer, the l,2-a-anhydride were proposed. Analogous work with p-acetylphenyl 3- and 5-0-methyl- 3-D-xylo-furanoside led to the conclusion that the 2-oxyanions (and presumably 1,2-anhydrides) were involved as intermediates. ... 

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