ACID AND ALKALINE HYDROLYSIS OF GLYCOPYRANOSYL FLUORIDES

Hydrolyses of α- and β-D-glucopyranosyl, α-D-galactopyranosyl, α-D-xylopyranosyl, and β-L-arabinopyranosyl fluorides with perchloric acid and sodium hydroxide have been investigated over a range of temperatures and concentrations. All acid hydrolyses show pseudo-first-order kinetics and, except for β-D-glucopyranosyl fluoride, a dependence of log10(rate) on HO with slopes of -0.76 to -0.85, and have positive entropies of activation, suggesting an Al mechanism. The relative rates of hydrolysis are similar to those of the corresponding methyl glycopyranosides and suggest the same cyclic carbonium-ion intermediate. β-D-Glucopyranosyl fluoride, in which the hydroxyl at C-2 is trans to the fluorine, has a negative entropy of activation, and hydrolysis may involve an intramolecular A2 mechanism. The alkaline hydrolyses present three different cases. α-D-Xylopyranosyl and β-L-arabinopyranosyl fluorides react initially with pseudo-first-order kinetics to give the free sugar as the only primary product. A plot of log10(rate) against pH gave a slope close to 1.0, and the fluorides appear to react with nucleophilic attack of hydroxyl ion. α-D-Glucopyranosyl and α-D-galactopyranosyl fluorides also give pseudo-first-order kinetics initially, and show a fair dependence of log10(rate) on H_with slope 1.0 over the range 1.0 to 5.0N. These fluorides give a mixture of free sugar and 1,6-anhydro-β-D sugar as the products, and two simultaneous reactions probably occur, one involving direct nucleophilic attack by hydroxyl ion at C-1, and the other involving equilibrium ionization of the hydroxyl group at C-6 followed by intramolecular attack. β-D-Glucopyranosyl fluoride reacts 5000 times faster than the α-anomer, necessitating the use of second-order kinetics. The product is 1,6-anhydro-β-D-glucose, and the reaction must proceed through a 1,2-epoxide. All of the alkaline hydrolyses studied showed a negative entropy of activation. © 1969.