Enzyme-mediated catalytic asymmetric oxidations

Fully Biocatalytic Rearrangement of Furans to Spirolactones

A multienzymatic pathway enables the preparation of optically pure spirolactone building blocks. In a streamlined one-pot reaction cascade, the combination of chloroperoxidase, an oxidase, and an alcohol dehydrogenase renders an efficient reaction cascade for the conversion of hydroxy-functionalized furans to the spirocyclic products. The fully biocatalytic method is successfully employed in the total synthesis of the bioactive natural product (+)-crassalactone D, and as the key module in a chemoenzymatic route yielding lanceolactone A.

Bovine serum albumin-cobalt(ii) Schiff base complex hybrid: an efficient artificial metalloenzyme for enantioselective sulfoxidation using hydrogen peroxide

An artificial metalloenzyme (BSA-CoL) based on the incorporation of a cobalt(ii) Schiff base complex {CoL, H2L = 2,2'-[(1,2-ethanediyl)bis(nitrilopropylidyne)]bisphenol} with bovine serum albumin (BSA) has been synthesized and characterized. Attention is focused on the catalytic activity of this artificial metalloenzyme for enantioselective oxidation of a variety of sulfides with H2O2. The influences of parameters such as pH, temperature, and the concentration of catalyst and oxidant on thioanisole as a model are investigated. Under optimum conditions, BSA-CoL as a hybrid biocatalyst is efficient for the enantioselective oxidation of a series of sulfides, producing the corresponding sulfoxides with excellent conversion (up to 100%), chemoselectivity (up to 100%) and good enantiomeric purity (up to 87% ee) in certain cases.

A Click Chemistry Approach towards Flavin-Cyclodextrin Conjugates-Bioinspired Sulfoxidation Catalysts

A click chemistry approach based on the reaction between alkynylflavins and mono(6-azido-6-deoxy)-β-cyclodextrin has proven to be a useful tool for the synthesis of flavin-cyclodextrin conjugates studied as monooxygenase mimics in enantioselective sulfoxidations.

Enzyme-catalysed oxidation of 1,2-disulfides to yield chiral thiosulfinate, sulfoxide and cis-dihydrodiol metabolites

Oxidative biotransformations of 1,2-disulfides using bacterial whole cells and pure enzymes yielded an unexpectedly wide range of products including thiolactones and chiral thiosulfinate, sulfoxide, cis-diol and metabolites.

Resolution of racemic sulfoxides with high productivity and enantioselectivity by a Rhodococcus sp. strain as an alternative to biooxidation of prochiral sulfides for efficient production of enantiopure sulfoxides

Whole cells of Rhodococcus sp. ECU0066 were used a catalyst for resolution of racemic sulfoxides, as an alternative to asymmetric oxidation of sulfides for efficient production of enantiopure sulfoxides. Racemic sulfoxides were excellent substrates for biotransformation because of their lower biotoxicity compared to sulfides. Determination of apparent kinetic parameters indicated that phenyl methyl sulfide (PMS), but not racemic phenyl methyl sulfoxide (rac-PMSO) caused substrate inhibition. (S)-PMSO was formed at a higher concentration and good enantiomeric excess (37.8 mM and 93.7% ee(S)) in a fed-batch reaction, than by an asymmetric oxidation of PMS (10 mM and 80% eeP (S)). The bacterium also displayed fairly good activity (yields, 22.7-43.2%; within 1-8 h) and enantioselectivity (ee(S)>99.0%) towards para-substituted (methyl and chloro) phenyl methyl sulfoxides and ethyl phenyl sulfoxide, indicating it could be a promising agent for synthetic applications.

Protein engineering of toluene monooxygenases for synthesis of chiral sulfoxides

Enantiopure sulfoxides are valuable asymmetric starting materials and are important chiral auxiliaries in organic synthesis. Toluene monooxygenases (TMOs) have been shown previously to catalyze regioselective hydroxylation of substituted benzenes and phenols. Here we show that TMOs are also capable of performing enantioselective oxidation reactions of aromatic sulfides. Mutagenesis of position V106 in the alpha-hydroxylase subunit of toluene ortho-monooxygenase (TOM) of Burkholderia cepacia G4 and the analogous position I100 in toluene 4-monooxygenase (T4MO) of Pseudomonas mendocina KR1 improved both rate and enantioselectivity. Variant TomA3 V106M of TOM oxidized methyl phenyl sulfide to the corresponding sulfoxide at a rate of 3.0 nmol/min/mg protein compared with 1.6 for the wild-type enzyme, and the enantiomeric excess (pro-S) increased from 51% for the wild type to 88% for this mutant. Similarly, T4MO variant TmoA I100G increased the wild-type oxidation rate by 1.7-fold, and the enantiomeric excess rose from 86% to 98% (pro-S). Both wild-type enzymes showed lower activity with methyl para-tolyl sulfide as a substrate, but the improvement in the activity and enantioselectivity of the mutants was more dramatic. For example, T4MO variant TmoA I100G oxidized methyl para-tolyl sulfide 11 times faster than the wild type did and changed the selectivity from 41% pro-R to 77% pro-S. A correlation between regioselectivity and enantioselectivity was shown for TMOs studied in this work. Using in silico homology modeling, it is shown that residue I100 in T4MO aids in steering the substrate into the active site at the end of the long entrance channel. It is further hypothesized that the main function of V106 in TOM is the proper positioning or docking of the substrate with respect to the diiron atoms. The results from this work suggest that when the substrate is not aligned correctly in the active site, the oxidation rate is decreased and enantioselectivity is impaired, resulting in products with both chiral configurations.

Recent advances in oxygenase-catalyzed biotransformations

Oxygenases continue to be widely studied for selective biooxidation of organic compounds. Protein engineering has resulted in heme and flavin monooxygenases with widely altered substrate specificities, and attempts have been reported to scale-up reactions catalyzed by these enzymes. Cofactor regeneration is still a key issue in these developments. Protein engineering contributed to understanding of structure versus function in dioxygenases.

Opposite enantioselectivities of two phenotypically and genotypically similar strains of Pseudomonas frederiksbergensis in bacterial whole-cell sulfoxidation

Soil samples were screened to select microorganisms with the capability to oxidize organic sulfides into the corresponding sulfoxides with differential enantioselectivities. Several bacterial strains that preferentially produced the S-configured sulfoxide enantiomer were isolated. Surprisingly, one bacterial strain, genotypically and phenotypically characterized as Pseudomonas frederiksbergensis, selectively gave the R enantiomer. The finding that two apparently identical organisms displayed opposite enantioselectivities is novel for non-genetically modified organisms.

[Cp*Rh(bpy)(H2O)]2+ as a coenzyme substitute in enzymatic oxidations catalyzed by Baeyer–Villiger monooxygenases

[Cp*Rh(bpy)(H2O)]2+ was applied as a flavin regenerating reagent in BVMO catalyzed oxidations of organic sulfides to chiral sulfoxides.