Enzymic Baeyer–Villiger Oxidations by Flavin-Dependent Monooxygenases

There has recently been a revival of interest in microbial biochemistry. Part of this resurgence is due to the increased presence of man-made organic compounds in the biosphere and the resultant interest in microorganisms which can degrade these xenobiotic molecules. Also, advances in genetic engineering have raised the possibility of utilizing the chemical machinery of bacteria for commercial profit and social benefit. The Baeyer-Villiger reaction is a useful transformation in organic synthesis which provides chemists with a gentle means of converting ketones into esters or lactones. The reaction, however, suffers from the problems of low yield and the need, in some cases, to utilize harsh conditions. There exist bacteria, capable of growth on aliphatic molecules, that contain enzymes which can catalyze Baeyer-Villiger reactions. These enzymes, known as monooxygenases, are involved in the breakdown of acyclic and alicyclic ketones to provide simpler carbon units for further catabolism. The gamut of reactions catalyzed by some of these enzymes is remarkable. This diversity, plus their availability in pure form in quantity by genetic engineering raises the possibility that these biocatalysts can be useful as reagents in organic synthesis.     

Discovery, application and protein engineering of Baeyer-Villiger monooxygenases for organic synthesis

Baeyer-Villiger monooxygenases (BVMOs) are useful enzymes for organic synthesis as they enable the direct and highly regio- and stereoselective oxidation of ketones to esters or lactones simply with molecular oxygen. This contribution covers novel concepts such as searching in protein sequence databases using distinct motifs to discover new Baeyer-Villiger monooxygenases as well as high-throughput assays to facilitate protein engineering in order to improve BVMOs with respect to substrate range, enantioselectivity, thermostability and other properties. Recent examples for the application of BVMOs in synthetic organic synthesis illustrate the broad potential of these biocatalysts. Furthermore, methods to facilitate the more efficient use of BVMOs in organic synthesis by applying e.g. improved cofactor regeneration, substrate feed and in situ product removal or immobilization are covered in this perspective.     

Towards practical biocatalytic Baeyer-Villiger reactions: applying a thermostable enzyme in the gram-scale synthesis of optically-active lactones in a two-liquid-phase system

Baeyer-Villiger monooxygenases (BVMOs) are extremely promising catalysts useful for enantioselective oxidation reactions of ketones, but organic chemists have not used them widely due to several reasons. These include instability of the enzymes in the case of in vitro and even in vivo systems, reactant/product inhibition, problems with upscaling and the necessity of using specialized equipment. The present study shows that the thermally stable phenylacetone monooxygenase (PAMO) and recently engineered mutants can be used as a practical catalysts for enantioselective Baeyer-Villiger oxidations of several ketones on a preparative scale under in vitro conditions. For this purpose several parameters such as buffer composition, the nature of the solvent system and the co-factor regeneration system were optimized. Overall a fairly versatile and efficient catalytic system for enantioselective laboratory scale BV-oxidations of ketones was developed, which can easily be applied even by those organic chemists who are not well versed in the use of enzymes.     

Increasing the enantioselectivity of cyclopentanone monooxygenase (CPMO): profile of new CPMO mutants

A series of cyclohexanones substituted at the 4-position with a selection of hydrophobic and hydrophilic groups were used as substrates in the evaluation of six new cyclopentanone monooxygenase (CPMO) mutants. These mutants were obtained through evolutionary modifications in two specific regions of the CPMO’s putative active site. Several mutant enzymes with improved enantioselectivity were identified. Analysis of the results, in terms of a diamond model, illustrates how a family of cyclohexanone substrates may be used to explore putative active sites of Baeyer–Villiger monooxygenases (BVMOs) and to design productive mutations for specific substrates.     

Asymmetric Baeyer-Villiger oxidations of 4-mono- and 4,4-disubstituted cyclohexanones by whole cells of engineered Escherichia coli.

Whole cells of an Escherichia coli strain that overexpresses Acinetobacter sp. NCIB 9871 cyclohexanone monooxygenase have been used for the Baeyer-Villiger oxidations of a variety of 4-mono- and 4,4-disubstituted cyclohexanones. In cases where comparisons were possible, this new biocatalytic reagent provided lactones with chemical yields and optical purities that were comparable to those obtained from the purified enzyme or a strain of bakers' yeast that expresses the same enzyme. The efficient production of cyclohexanone monooxygenase in the E. coli expression system (ca. 30% of total soluble protein) allowed these oxidations to reach completion in approximately half the time required for the engineered bakers' yeast strain. Surprisingly, 4,4-disubstituted cyclohexanones were also accepted by the enzyme, and the enantioselectivities of these oxidations could be rationalized by considering the conformational energies of bound substrates along with the enzyme's intrinsic enantioselectivity. The enzyme expressed in E. coli cells also oxidized several 4-substituted cyclohexanones bearing polar substituents, often with high enantioselectivities. In the case of 4-iodocyclohexanone, the lactone was obtained in > 98% ee and its absolute configuration was assigned by X-ray crystallography. The crystal belongs to the monoclinic P2(1) space group with a = 5.7400(10), b = 6.1650(10), c = 11.377(2) A, b = 99.98(2) degrees, and Z = 2. Taken together, these results demonstrate the utility of an engineered bacterial strain in delivering useful chiral building blocks in an experimentally simple manner.     

Stereoselective Synthesis of 3-Substituted 4-(Formyloxy)-2-azetidinones by the Unusual Baeyer-Villiger Reaction of beta-Lactam Aldehydes. Scope and Synthetic Applications

The Baeyer-Villiger oxidation of 4-formyl-beta-lactams 1with m-CPBA gave 4-(formyloxy) beta-lactams 2 in a simple, efficient, and totally stereoselective process. This reaction is one of the scarce examples of the preferred migration of a carbon moiety in an aliphatic aldehyde. The influence of the substituents at N1 and C3 of the four-membered ring in the Baeyer-Villiger rearrangement has been studied. Thus, alkyl, alkenyl, aryl, and alkyloxy 3-substituted-1-(p-anisyl)-2-azetidinones 1 form exclusively 4-(formyloxy) beta-lactams 2. Amide or acetoxy substituents at C3 of the four-membered ring produce mixtures of 4-(formyloxy) beta-lactams 2and 4-carboxy beta-lactams 5. The exclusive formation of carboxy derivatives is observed sometimes for 1-alkyl-substituted-2-azetidinones 1. 4-(Formyloxy) beta-lactams 2 are suitable starting materials to prepare different 4-unsubstituted beta-lactams 9 using beta-hydroxy amides 8 as isolable intermediates. The overall transformation 4-formyl-2-azetidinone to 4-unsubstituted beta-lactam is an easy and convenient stereoselective route to these interesting types of compounds.     

New bioorganic reagents: evolved cyclohexanone monooxygenase--why is it more selective?

Four mutants of the cyclohexanone monooxygenase (CHMO) evolved as catalysts for Baeyer-Villiger oxidation of 4-hydroxycyclohexanone were investigated as catalysts for a variety of 4-substituted and 4,4-disubstituted cyclohexanones. Several excellent catalytic matches (mutant/substrate) were identified. The most important, however, is the finding that, in a number of cases, a mutant with a single exchange, Phe432Ser, was shown to be as robust and more selective as a catalyst than the wild-type CHMO. All biotransformations were performed on a laboratory scale, allowing full characterization of the products. The absolute configurations of two products were established. A model suggesting a possible role of the 432 serine residue in enantioselectivity control is proposed.     

Electrophilic activation of hydrogen peroxide: selective oxidation reactions in perfluorinated alcohol solvents

[reaction; see text] The catalytic electrophilic activation of hydrogen peroxide with transition metal compounds toward reaction with nucleophiles is a matter of very significant research and practical interest. We have now found that use of perfluorinated alcoholic solvents such as 1,1, 1,3,3,3-hexafluoro-2-propanol in the absence of catalysts allowed electrophilic activation of hydrogen peroxide toward epoxidation of alkenes and the Baeyer-Villiger oxidation of ketones.     

Novel concurrent redox cascades of (R)- and (S)-carvones enables access to carvo-lactones with distinct regio- and enantioselectivity

Within this study, we investigated a one-pot enzymatic redox cascade composed of different enoate reductases (5 EREDs from diverse bacterial origins) and various Baeyer-Villiger monooxygenases (4 BVMOs) with complementary regioselectivity that enabled access to six out of eight carvo-lactone stereoisomers starting from readily available natural carvones. Applicability of this two-step cascade was demonstrated by preparative scale experiments yielding up to 76% of the desired chiral carvolactone.     

A nonenzymatic acid/peracid catalytic cycle for the Baeyer-Villiger oxidation

The combined action of carbodiimide and hydrogen peroxide upon exposure to carboxylic acid catalysts serves to generate transient peracids that can be engaged in the Baeyer-Villiger rearrangement of ketones to lactones. Up to 35 turnovers of the catalytic cycle may be achieved. The conditions are especially useful in the context of reactive cyclohexanones, and allow the use of H2O2 as the terminal oxidant. A singular example of a chiral catalyst demonstrates, in principle, that enantioselective catalysis will be possible with this strategy for catalyst turnover.     

Synthesis of new pyrazolone-based heterocycles as inhibitors of monoamine oxidase enzymes

A series of new substituted pyrazoles embedded with a variety of function groups has been synthesized from 4-[4-(1,2-dihydro-2,3-dimethyl-1-phenyl-5-oxo-3H-pyrazol-4-yl)]-2-cyano-4-oxo-butanenitrile (4). The synthesized compounds were fully characterized and their structures were elucidated based on elemental analysis, spectral data, and alternative synthetic pathways, whenever possible. The pharmacological activities of these new compounds as inhibitors for of type A and type B monoamine oxidase (MAO) enzymes have been investigated and compared to the most common inhibitors of MAO enzymes used to treat depression and anxiety such as deprenyl (selegiline), moclobemide, and clorgyline drugs. The most potent of the synthesized compounds was 4, 11, and 53 which showed higher inhibition activity toward type A MAO enzyme and even exceeded that of deprenyl, moclobemide, and clorgyline drugs.     

(+)-菊花烯酮的Baeyer-Villiger反应

Baeyer-Villiger反应是酮在过氧酸氧化下形成酯的反应,是有机反应中合成酯的主要方法,这一反应可应用在环酮扩环形成含氧杂环内酯的反应上．由于反应是立体控制的,已广泛应用于天然产物,例如：抗生素、类固醇和信息素中间体的合成,某些用其它方法难以合成的羟基酸可由内酯水解得到。     

Sur une nouvelle interaction intramoléculaire attractive influencant les équilibres conformationnels de Σ-4 cyclohexanones

The structural study of cyclohexanones with polar substituants in the γ position

shows:(1) lack of solvent effect on the conformational equilibria, giving a result which differs from those already obtained from α halogeno cyclohexanones(2) the predominance of the axial conformation due to syn diaxial attractive interactions.This work has been done by means of model compounds with a fixed conformation, synthesised by chlorination of corresponding Σ-4 cyclohexanones in α or αα' positions; their structural study gives supplementary proof of the existence of syn diaxial attractive interactions in the studied molecules.     

Protein engineering of stereoselective baeyer-villiger monooxygenases

Baeyer-Villiger monooxygenases (BVMOs) have been used for decades as catalysts in stereoselective Baeyer-Villiger reactions, including oxidative kinetic resolution of racemic ketones and desymmetrization of prochiral substrates with high enantioselectivity. These complement catalytic BV processes based on chiral synthetic catalysts. However, as in any enzyme-catalyzed process, limitations exist due to the often observed narrow substrate scope and/or insufficient stereoselectivity. Recent protein engineering of BVMOs in the form of directed evolution and rational design have eliminated these traditional limitations, which is the subject of this Minireview. The main focus is on phenylacetone monooxygenase (PAMO); an unusually thermostable and robust BVMO, which has a very narrow substrate scope. Protein engineering of PAMO has provided a number of mutants that display relatively wide substrate scope, high stereoselectivity, and maintained thermostability.     

光催化下(-)-马鞭烯酮的重排

( )-马鞭烯酮[( )-Verbenone]是Wender研究组全合成紫杉醇(Taxol)的重要起始物．( )-马鞭烯酮可由( )-α-蒎烯在Co(Ⅱ)催化下氧化得到,并且在光照下重排得到( )-菊花烯酮[( )-Chrysanthenone]．但从( )-α-蒎烯到( )-菊花烯酮的总产率较低,约为16．5％．我们在重复由光活性α-蒎烯出发经过光活性马鞭烯酮及菊花烯酮衍生物合成紫杉醇AB环的实验中发现,由α-蒎烯合     

Diastereoselective Vinyl Phosphate/beta-Keto Phosphonate Rearrangements

Several nonracemic diols and amino alcohols have been tested for their utility as chiral auxiliaries in vinyl phosphate/beta-keto phosphonate rearrangements. When (2R,4R)- or (2S,4S)-pentane-2,4-diol were employed, the diastereomeric beta-keto phosphonates obtained from three prochiral cyclohexanones gave separate resonances in their respective (31)P NMR spectra, allowing estimation of diastereomeric excess (de) via straightforward integration. A crystalline enol form of the beta-keto phosphonate was obtained from one ketone (4-(1,1-(ethylenedioxy)ethyl)-4-methylcyclohexanone, 10), which allowed determination of its absolute stereochemistry by X-ray diffraction analysis. When LTMP was used to induce rearrangement of vinyl phosphates derived from ketone 10 and (2R,4R)- or (2S,4S)-pentane-2,4-diol, a de of 2.5:1 was obtained. In addition, de values of 2.0:1 and 1.4:1 were obtained with parallel rearrangements of 4-isopropyl- and 4-methylcyclohexanone, suggesting that this sequence may be more generally applicable to preparation of nonracemic materials from prochiral cyclohexanones.     

Development of highly stereoselective asymmetric 6pi-azaelectrocyclization of conformationally flexible linear 1-azatrienes. from determination of multifunctional chiral amines, 7-alkyl cis-1-amino-2-indanols, to application as a new synthetic strategy: formal synthesis of 20-epiuleine

The highly stereoselective asymmetric 6pi-azaelectrocyclization was achieved as a general synthetic method based on the reaction between the (E)-3-carbonyl-2,4,6-trienal compounds and the (-)-7-alkyl-cis-1-amino-2-indanol derivatives which are effective chiral amines. The 7-alkyl-substituted 2-indanol moiety of the cyclized products was efficiently removed by the novel manganese dioxide oxidation under remarkably mild conditions, and the method was successfully applied to the formal synthesis of optically active 20-epiuleine.     

Microbial Baeyer-Villiger oxidation of terpenones by recombinant whole-cell biocatalysts--formation of enantiocomplementary regioisomeric lactones

Recombinant whole-cell expression systems for Baeyer-Villiger monooxygenases of various bacterial origin were utilized in the regiodivergent biooxidation of cyclic terpenones enabling access to enantio- and regioisomeric lactones on preparative scale.     

Biooxidation of bridged cycloketones using Baeyer-Villiger monooxygenases of various bacterial origin

Bridged cycloketones were synthesized and utilized as substrates to study biooxidations mediated by Baeyer-Villiger monooxygenases (BVMO) of various bacterial origin. The required enzymes were heterologously produced by recombinant overexpression systems based on Escherichia coli to enable facile recycling of the required nicotinamide cofactors during the whole-cell biotransformations. Ketone precursors of various structural demands were chosen to evaluate steric limitations and flexibility of the active site of BVMOs. By desymmetrization of the prochiral substrates, four to six stereogenic centers were generated within a single biooxidation step. The enzyme library investigated in this study allowed access to antipodal lactone products with excellent enantioselectivity in several cases. Together with a distinct substrate acceptance profile, the recently proposed classification into two groups of cycloketone converting BVMOs was supported by the biotransformation results obtained within this study.     

Electrochemical oxidation of 4-monoalkyl-substituted 1,4-dihydropyridines

The electrochemical oxidation of 4-monoalkyl-substituted 1,4-dihydropyridines has been studied in an aprotic medium and in the presence of pyridine. In an aprotic medium the products of oxidation are both 4-alkyl-substituted and 4-unsubstituted pyridines or mixtures of them. On oxidation in acetonitrile of 4-Et-, 4-n-Pr-, and 4-i-Bu-substituted dihydropyridines, 2-methylene-1,2,3,4-tetrahydropyridines were obtained in addition to the oxidized forms. In the presence of base the products of preparative electrolysis of the studied compounds were 4-alkyl-substituted pyridines. The exception was the 4-i-Pr- substituted dihydropyridine which was dealkylated on oxidation even in the presence of base. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1829–1838, December 2008.