Polar co-solvents in tertiary alcohols effect initial reaction rates and regio-isomeric ratio ranging from 1.2 to 2.2 in a lipase catalysed synthesis of 6- O- and 6′- O-stearoyl sucrose

Abstract

The effect of polar, aprotic solvents in tertiary alcohols on the regio-selectivity in a Candida antarctica lipase B catalysed synthesis of sucrose stearate was investigated. The solubility of sucrose, initial formation velocities and final concentrations of 6- O- and 6′- O-stearoyl sucrose were affected by addition of polar co-solvents. Without co-solvents 6- O-stearoyl sucrose appeared as the first product followed by 6′- O-stearoyl sucrose. In the presence of polar co-solvents both regio-isomers were synthesized from the beginning of the reaction. The highest initial formation velocities of 6- O- and 6′- O-stearoyl sucrose were 26 nmol min −1 and 17 nmol min −1, respectively, with a sucrose conversion of 86.7% obtained with 45% pyridine in 2-methyl-2-butanol. In the corresponding reaction medium with 2-methyl-2-propanol as tertiary alcohol a sucrose conversion of 86.3% was achieved. In the tertiary alcohols studied the overall regio-selectivity gradually decreased by adding polar co-solvent in the following order: no co-solvent > 45% pyridine ≥ 55% pyridine > 20% DMSO. Thus the regio-isomeric distribution expressing the steady state molar ratio of the 6- O-regio-isomer to the 6′- O-regio-isomer changed from 2:1 in absence of co-solvent to 1:1 in presence of DMSO. This change could be explained kinetically by the ratio between the initial formation velocities of the two regio-isomers, defined as the regio-isomeric ratio ( R = v 0 6 - O - / v 0 6 ′ - O - ) , which changed from R = 2.2 in absence of co-solvent to R = 1.2 in presence of DMSO.