Part III. Direct enzymatic esterification of lactic acid with fatty acids

Lipase catalyzed esterification reactions between lactic acid and several f atty acids have been studied. Difficulties arise in esterifying lactic acid because of the potential for this substance to act both as an acyl donor a nd as a nucleophile. These difficulties were minimized via strategies which greatly increased the yield of the desired ester. Use of the companion fat ty acid in excess with respect to lactic acid in an apolar solvent (n-hexan e) in which the lactic is not completely dissolved has been employed to min imize the potential for lactic acid to act as an acyl donor in a self-polym erization reaction. Beneficial and sinergistic effects of both silica gel and molecular sieves on conversion to the desired product are described. However, careful contro l of the amount of molecular sieves used is required. This fact is a conseq uence of two opposing effects of this material: i.e. adsorption of both lac tic 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% yie ld 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). (C) 2001 Elsevier Science Inc. All rights reser ved.