Enzymatic Acylation of Natural Polyhydroxylated Compounds

In order to investigate the efficiency and selectivity of the enzymahc acylation of various polyhydroxylated natural compounds in ionic liquid media the transesterification of sahcin, hehcin, escuHn, and naringin, as well as sUybin catalyzed by immobilized CALB was investigated using vinyl butyrate as the acyl donor. Two ionic Hquids were used, the water-miscible 1-butyl-3-methyHmidazolium tetrafluo-roborate ([bmim]BF4) and l-butyl-3-methylimidazoHum hexafluorophosphate ([bmimJPFs), in which the solubility of water is hmited. 

As can be seen, immobilized CALB efficiently catalyzes the acylation of aU polyhydroxylated compounds in the ionic liquid media used, leading to high conversion yields. The reaction rates for the enzymatic acylations are summarized in Table 9.3. Higher reaction rates were obtained for aU polyhydroxylated compounds tested when [bmim]BF4 was used as the reaction medium. It is interesting to note that the solubilities of esculin, salicin, helicin, naringin, and silybin at 60 C were approximately 68, 23,40,100, and 82 mM, respectively, in [bmim]BF4. On the other hand, in [bmim]PFs and acetone, in which the reactions rates were lower, the solubilities of the aforenamed compounds were 20, 5.5,19.5,1.5, and 7.2 mM and 6.5, 

Substrates Initial reaction rates (mmol/h/g of biocatalyst) (mol% of monoester produced )  

respectively. For the enzymatic acylation of all polyhydroxylated compounds studied in the present work, higher reaction rates were observed in ionic liquid media where the solubility of the substrates was higher, which is in accordance with the results reported by Park and Kazlauskas [16] for the enzymatic acetylation of glucose. 

The monoacylated derivative (6 -0-butyrate) was detected as the major product by H PLC analysis for all enzymatic reactions, while a diacylated derivative was also detected and confirmed by MS analysis [17]. As it can be seen in Table 9.3, higher regioselectivity of the process was observed in [bmim]BF4 than in [bmim]PF,5, while the selectivity was even lower in acetone. The lower regioselectivity of the enzymatic acylation of polyhydroxylated compounds in [bmim]PF,5 as well as in the organic solvent could be related to the lower solubility of unmodified phenolic substrates in these media, compared to that of their monoacylated derivatives [5, 16, 17]. On the other hand, the enhanced solubility of phenoHc substrates in [bmim]BF4 could explain the increased regioselectivity observed in this ionic liquid. 

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The enzymatic acylation of natural polyhydroxylated compounds in ionic liquids ([bmim]BF4 and [bmimJPpG), and organic solvents was carried out in stirred flasks according to the procedure reported elsewhere [5, 6, 17]. 

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