| dc.description.abstract |
In this work, the chemoenzymatic epoxidation of terpenes citronellol and â-caryophyllene is reported. Some experimental conditions, such as the use of different lipases and mycelia, oxidizing agents (H2O2 or UHP), reaction time, acyl donor type, temperature (15-45°C) and the influence of the organic medium, including the use of ionic liquids (ILs) were evaluated. In the epoxidation of citronellol two products were obtained; namely citronellol oxide 43 and/or citronellol epoxide ester 47. Depending on the reaction conditions, the product 43 was obtained in conv. >99%. CAL-B was the most efficient catalyst. The highest conversions were achieved for the epoxide 43 at 20 and 25 °C, with conv. of 80% and 77%. The organic medium was one of the most important parameters in this reaction. Using chloroform or dichloromethane, product 43 was obtained in conversions >99% after 24 h. Using mixtures of organic solvents and ILs, the results were dependent on both the organic solvent and the counter ion type in IL. These parameters were also evaluated when â-caryophyllene was used as the substrate. Using this substrate two products, the mono-epoxide 44 and the di-epoxide 45, were formed. The use of several lipases from different sources was also evaluated, and CAL-B was the most efficient catalyst (conv. >99% in 44). By the use of commercial lipases (Amano PS-C I, F-AP15, Amano PS-C SD, Amano PS, Amano AY 30, Amano PS IM, M Amano 10, 20 Amano AK, Amano PS-C II, A Amano 12, Lipozyme RM IM, Lipozyme IM), the conversions in product 44 were of 16-27%; by using the native lipases LAN and LRO the conversions were 20-23% and using the mycelia UEA_06 and UEA_53, the conversions were 16 and 21%, in 24 h. The best oxidizing agent was H2O2 as compared to UHP, considering that the reaction occurred in a shorter time and only a small amount of the reagent was necessary to obtain 44 or 45 as unique products in high conv. (60 ->99%). Using 2.0 mmol of H2O2, CAL-B could be reused up to 5 times with minimal loss in the conversion degrees to 44 (93-90%). The organic solvents were also selective. By using n-hexane, the preferential formation of 44 was observed with conv. >99% after 8 and 24 h reaction. When ethyl acetate and toluene were used, the conversions were of >99% on 45 after 8 and 24h. It is worth of mentioning that the chemoenzymatic methodology used mild conditions and presented selectivity, as depending on the reaction conditions only one product could be obtained in good conversion degrees. |
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