Lipase B from Candida antarctica has been shown to be a very versatile enzyme that is capable of performing many different types of reactions aside from the typical hydrolytic reactions of lipases. Such unusual reactions include: epoxidation of olefins and fatty acids; the Baeyer-Villiger oxidation of ketones to form lactones and esters, and of aldehydes to form formats; and the epoxidation of carbon-carbon double bonds in conjugation with aromatic rings followed by the scission of the carbon-carbon bond and formation of an aldehyde. Furthermore, lipase B is capable of catalyzing the formation of linalool oxides from linalool, as has been reported recently.
The intermediate in all these reactions is a well-defined peracid which, according to the original literature, is produced by the lipase directly from a long chain carboxylic acid and hydrogen peroxide. The peracid reacts with olefinic bonds in any secondary substrate molecules present, thereby forming epoxides, which can then undergo further reactions.
Application of this enzyme system to convert menthofuran into menthofurolactone has been published recently, and the process was compared with the normal synthetic method using m-chloroperbenzoic acid as an oxidant (F-1). This latter procedure apparently gives poor yields of the desired product due to competing polymerization and isomerization reactions. The reproducibility of the reaction was described as being poor, and the proportions of the various lactones formed consequently varied quite considerably. However, when peroctanoic acid was used as oxidant, menthofurolactone was obtained in a much higher yield, which was presumably due to the milder reaction conditions that prevailed.
Over the past few years, we have developed a very viable, alternative reaction procedure utilizing ethyl acetate, which acts both as the initial reaction solvent and the source of the oxidant, peracetic acid, which is formed from the acetic acid produced by the lipolysis of the solvent. As part of a project designed to explore the synthetic possibilities of this enzyme, it was decided to determine whether peracetic acid is also suitable for the reported conversion of menthofuran to menthofurolactone, and also to study this reaction with other unsaturated, cyclic ethers as substrate.