Part III. Direct enzymatic esterification of lactic acid with fatty acids.
Lipase catalyzed esterification reactions between lactic acid and several fatty acids have been studied. Difficulties arise in esterifying lactic acid because of the potential for this substance to act both as an acyl donor and as a nucleophile. These difficulties were minimized via strategies which greatly increased the yield of the desired ester. Use of the companion fatty acid in excess with respect to lactic acid in an apolar solvent (n-hexane) in which the lactic is not completely dissolved has been employed to minimize the potential for lactic acid to act as an acyl donor in a self-polymerization reaction.Beneficial and sinergistic effects of both silica gel and molecular sieves on conversion to the desired product are described. However, careful control of the amount of molecular sieves used is required. This fact is a consequence of two opposing effects of this material: i.e. adsorption of both lactic acid and water from the reaction mixture. For reaction between caprylic and lactic acids, use of an excessive amount of enzyme reduces the extent of conversion to 2-O-caproyl-lactic acid.A very pure ester of the L-enantiomer (optical rotation of [alpha]D(25) = -23.5) can be prepared in n-hexane using a four fold excess of caprylic acid and Candida antarctica lipase. Optimum reaction conditions lead to 35% yield of 2-O-caproyl-lactic acid, a result which is close to the maximum yield that can be enantioselectively obtained from commercial grade lactic acid (68 mole per cent monomer).
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Instituto de Catálisis, CSIC. Cantoblanco 28049, Madrid, Spain
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