Although caffeic acid phenethyl ester (CAPE), an active flavonoid, plays an important role in the antioxidant activity of honeybee propolis, the isolation of CAPE from honeybee propolis is time-consuming due to wide variety of impurities present. Therefore, biochemical method to synthesize CAPE was investigated in this study. Since ionic liquids (ILs) possess some unique characteristics as appreciated alternatives to conventional solvents for certain biotransformation, the effect of ILs as reaction media for enzymatic synthesis of CAPE was assessed. Several factors including substrate molar ratio, and reaction temperature affecting the conversion yield of lipase-catalyzed CAPE synthesis were also investigated. Reaction yields were significantly higher in hydrophobic ILs than in hydrophilic ILs (almost zero). Among nine hydrophobic ILs tested, the highest conversion of synthetic reaction was obtained in 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Emim][Tf(2)N]). A reaction temperature of 70 °C was found to give high conversion. In addition, optimal substrate molar ratio between phenethyl alcohol and caffeic acid (CA) was decreased significantly from 92:1 to 30:1 when ILs were used instead of isooctane.

Lipase-catalyzed caffeic acid phenethyl ester (CAPE) synthesis in ionic liquid, 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Emim][Tf(2)N]), was investigated in this study. The effects of several reaction conditions, including reaction time, reaction temperature, substrate molar ratio of phenethyl alcohol to caffeic acid (CA), and weight ratio of enzyme to CA, on CAPE yield were examined. In a single parameter study, the highest CAPE yield in [Emim][Tf(2)N] was obtained at 70 °C with a substrate molar ratio of 30:1 and weight ratio of enzyme to CA of 15:1. Based on these results, response surface methodology (RSM) with a 3-level-4-factor central composite rotatable design (CCRD) was adopted to evaluate enzymatic synthesis of CAPE in [Emim][Tf(2)N]. The four major factors were reaction time (36-60 h), reaction temperature (65-75 °C), substrate molar ratio of phenethyl alcohol to CA (20:1-40:1), and weight ratio of enzyme to CA (10:1-20:1). A quadratic equation model was used to analyze the experimental data at a 95 % confidence level (p < 0.05). A maximum conversion yield of 99.8 % was obtained under the optimized reaction conditions [60 h, 73.7 °C, substrate molar ratio of phenethyl alcohol to CA (27.1:1), and weight ratio of enzyme to CA (17.8:1)] established by our statistical method, whereas the experimental conversion yield was 96.6 ± 2 %.