Some aspects of glycoside and acetal hydrolysis.

2015-11-19T08:45:36Z (GMT) by John Martin. Williams
The evidence leading to the various mechanisms for acetal and glycoside hydrolysis is reviewed. A series of fourteen aryl di-O-acetyl-beta-D-gluco-furanosiduronolactones was prepared. The beta-naphthyl, phenyl,beta-cresyl and beta-methoxyphenyl compounds yielded the corresponding beta-D-glucofuranoside on reduction with lithium aluminium hydride. The attempted preparation of m- and beta-chlorophenyl beta-D-glucofuranoside by this method afforded the unsubstituted phenyl glucoside via removal of the halogen atom. beta-Nitrophenyl tetra-O-acetyl-beta-D-glucofuranoside, prepared from a mixture of the penta-O-acetyl-D-glucofuranoses, decomposed during deacetylation attempts. a-Nitrophenyl di-O-acetyl-beta-D-glucofuranosidurono lactone also failed to yield beta-nitrophenyl beta-D-glucofuranoside on reduction with lithium aluminium hydride and sodium borohydride. Phenyl beta-D-galactofuranoside was prepared from phenyl tetra-O-acetyl-beta-D-galactofuranoside. The hydrolyses of the aryl beta-D-glucofuranosides and phenyl beta-D-galactofuranoside were studied in aqueous perchloric acid solutions. The positive entropies of activation measured in 1.00M acid are consistent with a unimolecular process. The variation of rate constant with acid concentration is interpretable in terms of the Hammett-Zucker hypothesis for an A-1 process. The hydrolysis of phenyl beta-D-glucofuranoside and of phenyl 2-deoxy-alpha-D-glucopyranoside were studied in a series of monochloroacetate and phosphate buffers. The results are consistent with a specific-acid catalysed mechanism, as are the solvent isotope effects for five glycosides studied. Series of 2-aryloxytetrahydropyrans and tetra-hydrofurans were prepared and their hydrolysis studied in aqueous acid and in aqueous buffered solutions of acetic, formic and monochloroacetic acids. A general-acid term was observed in the rate-law for the hydrolysis of 2-phenoxy-tetrahydrofuran in acetate buffers, 2-(D-nitrophenoxy)-tetrahydropyran and 2-(D-nitrophenoxy)-tetrahydrofuran in all three buffers but not for 2-phenoxytetrahydropyran in acetate buffers. Possible A-S E2 mechanisms are discussed and a concerted mechanism favoured. The catalytic coefficients are greater for the tetrahydrofurans than for the corresponding tetrahydropyrans, and an explanation of this is advanced. The hydronium-ion catalysed reactions for both series yielded small negative rho values and the deuterium solvent isotope effect decreased with basicity of the acetal oxygen. The 2-p-nitrophenoxy---compounds but not the 2-phenoxy---undergo a spontaneous hydrolysis.

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