Oxidation ketone monooxygenases

Apart from the asymmetric metal catalysis, enantioselective Baeyer-Villiger oxidations mediated by enzymes have been known for some time [32,33,34]. Both whole-cell cultures and isolated enzymes, usually flavin-dependent monooxygenases, can be used to oxidize ketones enantioselectively. For future improvements in the asymmetric Baeyer-VilHger oxidation the use of chiral Lewis acids in combination with an appropriate oxidant seems worthy of intensive investigation. 

Owing to the lack of stability of the monooxygenase, the hydroxylation step was achieved by applying resting cells of P. monteilii, providing access to the corresponding benzylic alcohol as an intermediate. Subsequent alcohol oxidation was performed by a cell-free extract of an ADH from I. kefir using the auxiliary co-substrate acetone to push the equilibrium of the oxidative ketone formation. The optimized process displayed an overall performance of up to 87% yield and a total turnover number (TTN) of 4200. 

Various peroxidases and monooxygenases have been used as biocatalysts for the oxidation of sulfides to sulfoxides [58, 59]. Haloperoxidases have been studied in the oxidations of sulfides and these reactions work with hydrogen peroxide as the oxidant. Baeyer-Villiger monooxygenases, the natural role of which is to oxidize ketones to esters, are NAD(P)H-dependent flavoproteins that have been used for sulfoxidations. 

The November 2001 issue of the Journal of Chemical Education ipp 1533-1534) describes an introductory biochemistry laboratory ex periment involving cycio hexanone monooxygenase oxidation of cyclic ketones... 

Although a maj ority of research activities were dedicated to cycloketone converting BVMOs, the recently discovered novel MOs also enable stereoselective oxidation of noncyclic ketones to esters. An aliphatic open-chain monooxygenase (AOCMO) from Pseudomonas Jluorescens DSM 50106 displays stereoselective biooxidation of terminal acyl-groups in proximity to hydroxyls (Scheme 9.23). The biooxidation gives acetic... 

The assessment of clearance is complicated by the numerous mechanisms by which compounds may be cleared from the body. These mechanisms include oxidative metabolism, most commonly by CYP enzymes, but also in some cases by other enzymes including but not limited to monoamine oxidases (MAO), flavin-containing monooxygenases (FMO), and aldehyde oxidase [45, 46], Non-oxidative metabolism such as conjugation or hydrolysis may be effected by enzymes such as glucuronyl transferases (UGT), glutathione transferases (GST), amidases, esterases, or ketone reductases, as well as other enzymes [47, 48], In addition to metabolic pathways, parent compound may be excreted directly via passive or active transport processes, most commonly into the urine or bile. 

Alcohols are oxidized to aldehydes by the liver enzyme alcohol dehydrogenase, and aldehydes to carboxylic acids by aldehyde dehydrogenase. In mammals, monooxygenases can be induced by plant secondary metabolites such as a-pinene, caffeine, or isobornyl acetate. Reduction is less common and plays a role with ketones that cannot be further oxidized. Hydrolysis, the degradation of a compound with addition of water, is also less common than oxidation. 

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+. 

Various alkane oxidations are catalyzed by iron complexes. Such reactions are important in view of the action of non-heme iron enzymes, such as methane monooxygenase, in hydrocarbon oxidations in biological systems. For example, the oxo-bridged complex [Fe2(TPA)2(ju,-0)(ju.-0Ac)]3+ [TPA = tris(2-pyridylmethyl)-amine] catalyzes the oxidation of cyclohexane with Bu OOH. Related complexes with an Fein2(/i-0)(/i-0Ac)2 core oxidize cyclohexane or adamantane to give a mixture of alcohols and ketones.159 Less well defined systems, e.g., FeCl3-6H20/ aldehyde/AcOH/02 are similarly active.160... 

Oxidative ring expansion by an additional carbon atom is performed by expan-dases different from the Baeyer-VilUger monooxygenases capable of introdudng oxygen into cycUc ketones. The Fe(II)- and 2-oxoglutarate-dependent oxygenase... 

Biological systems overcome the inherent unreactive character of 02 by means of metalloproteins (enzymes) that activate dioxygen for selective reaction with organic substrates. For example, the cytochrome P-450 proteins (thiolated protoporphyrin IX catalytic centers) facihtate the epoxidation of alkenes, the demethylation of Al-methylamines (via formation of formaldehyde), the oxidative cleavage of a-diols to aldehydes and ketones, and the monooxygenation of aliphatic and aromatic hydrocarbons (RH) (equation 104). The methane monooxygenase proteins (MMO, dinuclear nonheme iron centers) catalyze similar oxygenation of saturated hydrocarbons (equation 105). ... 

Several types of flavoprotein monooxygenases exist. One group catalyzes electrophilic aromatic substitution or heteroatom oxidation reactions, whereas the other group catalyzes Baeyer-Villiger-type oxidations of ketones (Fig. 2) (13, 17). 

Oxidations catalyzed by monooxygenases Oxygenations of C-H and C=C bonds forming alcohols, epoxides enantioselective Baeyer-Villiger oxidations of ketones to chiral lactones and of sulfides to chiral sulfoxides nuclear and side-chain hydroxylation of aromatic compounds. 

Aliphatic alcohols can be oxidized to the corresponding carboxylic acid with Acetobacter aceti as long as the amount of substrate is kept below the inhibiting level.162 The Baeyer-Villiger reaction of cyclic ketones can be carried out with monooxygenases from P. putida (9.13) or A. calcoaceticus, the former being preferred, because the latter is pathogenic.163... 

Together with enantioselective hydrolysis/acylation reactions, enantioselective ketone reductions dominate biocatalytic reactions in the pharma industry [10], In addition, oxidases [11] have found synthetic applications, such as in enantioselective Baeyer-Villiger reactions [12] catalyzed by, for example, cyclohexanone monooxygenase (EC 1.14.13) or in the TEMPO-mediated oxidation of primary alcohols to aldehydes, catalyzed by laccases [13]. Hence, the class of oxidoreductases is receiving increased attention in the field of biocatalysis. Traditionally they have been perceived as difficult due to cofactor requirements etc, but recent examples with immobilization and cofactor regeneration seem to prove the opposite. 

A Baeyer-Villiger monooxygenase was applied to oxidize cyclic ketones produced in situ by HLADH with concomitant regeneration of NAD4 (Fig. 16.2-8) 153L Even though yields and enantiomeric excesses are moderate, this concept has synthetic significance and should be optimized in future. 

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