An Investigation of Intermediates in the Hydrolysis of Ortho Esters Derived from D-Glucose and D-Mannose

Article abstract of DOI:10.1021/jo00014a018

The hydrolysis of a series of 1,2-ortho esters derived from α-D-glucopyranose and β-D-mannopyranose have been investigated by NMR and UV spectroscopy.When the hydrolysis of 1,2-O-(1-exo-ethoxyethylidene)-α-D-glucopyranose (6) in CD3CN (97.2 v percent) and D2O (2.8 v percent) containing DCl (2.8 x 10^-4M) was followed by 1H-NMR spectroscopy, an intermediate was detected that may be the corresponding hemi ortho ester.Evidence was also obtained for the incursion of a hemi ortho ester in the hydrolysis of 1,2-O-(α-exo,4-dimethoxybenzylidene)-α-D-glucopyranose (14) under similar conditions.The proportions of the hydrolysis products of 6,1-O-acetyl-α-D-glucopyranose (13), and 2-O-acetyl-α-D-glucopyranose (12) depend on acid concentration with more of the former being formed at the higher acid concentrations.When the hydrolysis of 14 was studied at higher acid concentrations (DCl, 0.17 M) the intermediate cation 15 was detected.Evidence was obtained by 18O-labeled studies for decomposition of this by attack of water at C-1 of the glucopyranose ring and by an attack at the pro-acyl carbon of the dioxolanylium ion depending on the reaction conditions.In the hydrolysis of the ortho esters derived from β-D-mannopyranose, tricyclic 1,2,6-ortho esters were detected in solvents of low water content and when the concentration of DCl was 0.33 M, the intermediate cation was also detected.The kinetics of hydrolysis of the two series of ortho esters were studied by UV spectroscometry, and evidence was obtained that the 1,2-O-(α-exo-alkoxy-4-methoxybenzylidene)-α-D-glucopyranoses reacted with rate-limiting breakdown of intermediate hemi ortho ester at high acid concentrations.Evidence for similar behavior was obtained for the hydrolysis of the analogous glucose orthobenzoate esters and mannose 4-methoxyorthobenzoate esters, but the other compounds studied showed no evidence for a change in the rate-determining step of their hydrolyses or else showed complex kinetics on hydrolysis indicative that formation and breakdown of the hemi ortho ester were proceeding at comparable rates.