The acid-catalyzed hydrolysis of β-d-xylofuranosides

Pyranosides with 2,3-trans carbamate groups: exocyclic or endocyclic cleavage reaction?

Pyranosides with 2,3-trans carbamate groups exhibit high 1,2-cis selectivity in glycosylation reactions. Using glycosyl donors with N-benzyl 2,3-trans carbamate groups, an anti-Helicobacter pylori oligosaccharide was synthesized in an efficient manner. Moreover, pyranosides with 2,3-trans carbamate groups readily undergo anomerization from the β to the α configuration under mild acidic conditions via endocyclic cleavage. Acyclic cations generated during the endocyclic cleavage reaction were captured using reduction and intramolecular Friedel-Crafts reaction. By exploiting this anomerization, multiply aligned 1,2-trans glycosyl bonds can be transformed to 1,2-cis glycosyl bonds in a single operation.

Evidence for endocyclic cleavage of conformationally restricted glycopyranosides

2,3-trans-Carbamate- and -carbonate-carrying pyranosides were very easily anomerised from the beta to the alpha direction in the presence of a Lewis acid compared to other pyranosides. This reaction is caused by endocyclic cleavage of the pyranosides. Evidence for endocyclic cleavage of conformationally restricted pyranosides in the chair form was obtained by intra- and intermolecular Friedel-Crafts reactions, chloride addition, and reduction of the generated cation. On the other hand, pyranosides with the distorted conformation were never cleaved in an endocyclic manner.

A kinetic isotope effect study on the hydrolysis reactions of methyl xylopyranosides and methyl 5-thioxylopyranosides: oxygen versus sulfur stabilization of carbenium ions

The following kinetic isotope effects, KIEs (k(light)/k(heavy)), have been measured for the hydrolyses of methyl alpha- and beta-xylopyranosides, respectively, in aqueous HClO(4) (mu = 1.0 M, NaClO(4)) at 80 degrees C: alpha-D, 1.128 +/- 0.004, 1.098 +/- 0.005; beta-D, 1.088 +/- 0.008, 1.042 +/- 0.004; gamma-D(2), (C5) 0.986 +/- 0.001, 0.967 +/- 0.003; leaving-group (18)O, 1.023 +/- 0.002, 1.023 +/- 0.003; ring (18)O, 0.983 +/- 0.001, 0.978 +/- 0.001; anomeric (13)C, 1.006 +/- 0.001, 1.006 +/- 0.003; and solvent, 0.434 +/- 0.017, 0.446 +/- 0.012. In conjunction with the reported (J. Am. Chem. Soc. 1986, 108, 7287-7294) KIEs for the acid-catalyzed hydrolysis of methyl alpha- and beta-glucopyranosides, it is possible to conclude that at the transition state for xylopyranoside hydrolysis resonance stabilization of the developing carbenium ion by the ring oxygen atom is coupled to exocyclic C-O bond cleavage, and the corresponding methyl glucopyranosides hydrolyze via transition states in which charge delocalization lags behind aglycon departure. In the analogous hydrolysis reactions of methyl 5-thioxylopyranosides, the measured KIEs in aqueous HClO(4) (mu = 1.0 M, NaClO(4)) at 80 degrees C for the alpha- and beta-anomers were, respectively, alpha-D, 1.142 +/- 0.010, 1.094 +/- 0.002; beta-D 1.061 +/- 0.003, 1.018(5) +/- 0.001; gamma-D(2), (C5) 0.999 +/- 0.001, 0.986 +/- 0.002; leaving-group (18)O, 1.027 +/- 0.001, 1.035 +/- 0.001; anomeric (13)C, 1.031 +/- 0.002, 1.028 +/- 0.002; solvent, 0.423 +/- 0.015, 0.380 +/- 0.014. The acid-catalyzed hydrolyses of methyl 5-thio-alpha- and beta-xylopyranosides, which occur faster than methyl alpha- and beta-xylopyranosides by factors of 13.6 and 18.5, respectively, proceed via reversibly formed O-protonated conjugate acids that undergo slow, rate-determining exocyclic C-O bond cleavage. These hydrolysis reactions do not have a nucleophilic solvent component as a feature of the thiacarbenium ion-like transition states.