Application of Organic Solvent System for Lipase-catalyzed Regioselective Benzoylation of 1-β-D-arabinofuranosylcytosine

In this paper, enzymatic regioselective acylation of 1-β-D-arabinofuranosylcytosine (ara-C) with vinyl benzoate (VB) using immobilized Candida antarctica lipase B in binary organic solvents was explored. It was found that the lipase showed high regioselectivity (greater than 99%) towards the 5'-OH of ara-C in the representative organic solvent mixture (hexane-pyridine). To understand the enzymatic processes and provide a fair comparison of hexane-pyridine with C₄MImㆍPF∧6-pyridine (the representative ionic liquid-containing system), the effect of each process variable on the reactions in hexane-pyridine was investigated. The results indicate that the optimum hexane content, initial a∧w, molar ratio of VB to ara-C, and temperature were 28% (v/v), 0.11, 15, and 40℃, respectively. Under optimized conditions, the initial reaction rate in hexane-pyridine (44.4 mM/h) was much higher than that in C₄MImㆍPF∧6-pyridine (29.4 mM/h) for each case. The maximum conversion yield, however, was increased when the reaction system was shifted from hexane-pyridine to C₄MImㆍPF∧6-pyridine. Further study revealed that the presence of an acidic by-product (benzoate acid, released during the acylation process) may cause rapid inactivation of the enzyme in hexane-pyridine, leading to a lower conversion rate, whereas the ionic liquid may have coating and protecting effects on the lipase during the reaction.