Synthesis of (-)-menthol fatty acid esters in and from (-)-menthol and fatty acids - novel concept for lipase catalyzed esterification based on eutectic solvents

Deep eutectic solvents (DESs) based on (-)-menthol and fatty acids (octanoic, decanoic and dodecanoic acid) were investigated as reaction media for the lipase catalyzed esterification of the DES compounds itself to synthesize (-)-menthol fatty acid esters. The DES acts as reaction medium and substrate pool simultaneously without the need of adding any solvent. Candida rugosa lipase was active in the neat (-)-menthol:fatty acid DESs to synthesize (-)-menthol fatty acid esters. This example shows for the first time that a valuable product can be enzymatically produced using both components of a DES without any co-solvent. The addition of water to the DESs enhanced the reaction outcome likely due to interfacial activation of the enzyme. In biphasic reaction systems with an addition of 10 wt% of water to the DES phase, the conversion (7 d) of octanoic, decanoic and dodecanoic acid reached 50%, 83% and 71%, respectively. This corresponds to a batch productivity of 133 g L-1 d(-1) (24 h) and a final (-)-menthyl dodecanoate concentration of 957 mm (7 d) in the (-)-menthol:-dodecanoic acid DES. Closer investigation of this DES reaction system revealed that water addition and stirring speed are interacting parameters to optimize the process. The developed DES reaction systems represent neat reactant mixtures enabling the lipase catalyzed esterification under solvent-free conditions.