Cyclohexanone monooxygenases CHMO

Most biochemical and biocatalytic studies have been performed with type I B VMOs. This is partly because of the fact that they represent relatively uncomplicated monooxygenase systems. These monooxygenases are typically soluble and composed of only one polypeptide chain. Expression systems have been developed for a number of type I BVMOs while no recombinant expression has been reported for a type II BVMO. Cyclohexanone monooxygenase (CHMO) from an Acinetobacter sp. NCIMB9871 was the only recombinant available BVMO... 

Figure 3 Illustration of overlapping substrate specificities of four BVMOs. Cyclohexanone monooxygenase (CHMO), cyclopentanone monooxygenase (CPMO), 4-hydroxyacetophenone monooxygenase (HAPMO) and phenylacetone monooxygenase (PAMO). For each enzyme, several typical substrates are...

The Baeyer-Villiger enzyme, cyclohexanone monooxygenase (CHMO), has been applied to the oxidative ring expansion of m-2,6-dialkylperhydropyrans to afford 28 with very high yields and ee s, when R = methyl or ethyl (Scheme 11) <2001JM0349, 2003SL1973>. 

Baeyer-ViUiger oxidation involves NADPH and flavin (FAD) as cofactors and was originally proposed by Walsh et al. based on data obtained from cyclohexanone monooxygenase (CHMO) from Acinetobacter calcoaceticus (Fig. 24) [156]. In a first step, enzyme-bound flavin is reduced, followed by the addition of oxygen yielding a hydroperoxide anion. Reaction with the ketone substrate gives a Criegee intermediate, which is then converted into the product under dissociation of water. The cofactor FAD is recovered via oxidation with NADP+. 

The enzymatic oxidation by cyclohexanone monooxygenase (CHMO) from Acinetobacter of 1,3,2-dioxathiane 2-one derivatives 84 to the corresponding 2,2-dioxides 85, which could serve as an alternative path to other sulfates of special interest, has been published <1998CC415> (Scheme 13). The oxygen transfer at sulfur is enantioselective, and, moreover, the diastereomeric 2-oxides 84a and 84b can be separated easily by flash chromatography. The resulting 2,2-dioxides 85 are obtainable in satisfactory chemical yield. 

O Scheme 18.2 Conversion of cyclohexanone to hexane-6-lactone catalyzed by cyclohexanone monooxygenase (CHMO). 

More recently, Colonna and coworkers reported that cyclohexanone monooxygenase (CHMO) from Actinobacter calcoaceticus NCIMB9871 catalyzed the aerobic oxidation of amines [128]. 

The isolation and characterization of cyclohexanone monooxygenase (CHMO) from Acinetohacter sp. NCIB 9871 was reported in 1976 [68]. In addition to cyclohexanone and cydopentanone, CHMO was shown to be able to catalyze the oxidation of a variety of cyclic ketones to the corresponding lactones [69]. This attracted the attention of several other groups, which led to investigations of its mechanism [70, 71] as well as sequencing and doning [72]. It is by far the most extensively studied BVMO, and it has been used as a model system for up-scaling BVMO-mediated biocatalysis. 

Reetz and co-workers have demonstrated that the methods of directed evolution can be applied successfully to the creation of enantioselective cyclohexanone monooxygenases (CHMOs) as catalysts in Baeyer-Villiger reactions of several different substrates, for which the enantioselectivity ranges between 90-99% [100]. Ketone 5 gives a very poor enantioselectivety (9% ee, R-selective) with the wild-type CHMO. The enantioselectivety for 5 was significantly improved by directed evolution, and an S-selective variant gave 79% ee (Scheme 10.1). 

Scheme 10.1 Directed evolution of cyclohexanone monooxygenases (CHMOs) improves enantioselectivety in Baeyer-Villiger reactions.

Worthy of mention is that catalysts based on Co, Cu, Pd, P 20,60 and Al are all active towards meso or chiral cyclobutanone substrates, which are intrinsically much more reactive than larger cychc ketones. Some representative examples are reported in Scheme 23.29. However, only biocatal3dic BV oxidations performed with isolated enzymes, or whole cells containing cyclohexanone monooxygenases (CHMOs) or BV monooxygenases (BVMOs) showed good conversions as well as enantioselectivity above 95% ee with cyclohexanone as the substrate. " ... 

The Baeyer-Villiger oxidation reaction is especially interesting for the oxidation of cyclic ketones into their corresponding lactones. One of the best-characterized BVMO is the cyclohexanone monooxygenase (CHMO) firom Acinetobacter calcoaceticus. The recombinant expression of this enzyme was successful in Saccharomyces cerevisiae (Stewart et al. 1998) and Escherichia coli (Doig et al. 2001 Mihovilovic et al. 2001). A major limitation of the industrial implementation of BVMOs is the need for efficient recycling of the cofactor NADPH (Baldwin and Woodley 2006). Therefore, the process development has been... 

Fig. 6 Cyclohexanone monooxygenase (CHMO)-mediated regiodivergent oxidation of bicyclo...

Until recently only cyclohexanone monooxygenase (CHMO) had been cloned and overexpressed, but new developments have made a number of other Bayer- 511iger monooxygenases available. 

---