Acid-catalysed and metal-ion-catalysed hydrolysis of 8-quinolyl β-D-glucopyranoside

Abstract

The hydrolysis of 8-quinolyl β-D-glucopyranoside has been studied over a wide range of acidity. In the pH region 1·0–5·2, hydrolysis of the free base from and of the N-protonated glucoside occurs. There is little evidence for intramolecular general-acid catalysis in the hydrolysis of the N-protonated glucoside. In more strongly acidic media (1·79–5·02M-HCl) at 70°C, only the specific-acid-catalysed hydrolysis of the N-protonated glucoside occurs. Application of various mechanistic criteria [ϕ=+0·49; k(D₂O)/k(H₂O)= 1·1; ΔS₃₃₃^‡=+3·0 cal k^–1 mol^–1] suggest that the reaction proceeds by a predominantly A-2, rather than the expected A-1 pathway. The hydrolysis of the glucoside is the glucoside is susceptible to metal ion catalysis in the pH range 5·5–6·2. Copper(II), nickel(II), and cobalt(II) catalysis has been studied. Copper(II) is a particularly effective catalyst and it is estimated that the Cu^II complex is hydrolysed ca. 10⁵–10⁶ times faster than the uncomplexed glycoside in the pH range 5·5–6·2.