Improving the Enantioselectivity of CHMOBrevi1 for Asymmetric Synthesis of Podophyllotoxin Precursor

(R)-β-piperonyl-γ-butyrolactones are key building blocks for the synthesis of podophyllotoxin, which have demonstrated remarkable potential in cancer treatment. Baeyer-Villiger monooxygenases (BVMOs)-mediated asymmetric oxidation is a green approach to produce chiral lactones. While several BVMOs were able to oxidize the corresponding cyclobutanone, most BVMOs gave the (S) enantiomer while Cyclohexanone monooxygenase (CHMO) from Brevibacterium sp. HCU1 gave (R) enantiomer, but with a low enantioselectivity (75 % ee). In this study, we use a strategy called "focused rational iterative site-specific mutagenesis" (FRISM) at residues ranging from 6 Å from substrate. The mutations by using a restricted set of rationally chosen amino acids allow the formation of a small mutant library. By generating and screening less than 60 variants, we achieved a high ee of 96.8 %. Coupled with the cofactor regeneration system, 9.3 mM substrate was converted completely in a 100-mL scale reaction. Therefore, our work reveals a promising synthetic method for (R)-β-piperonyl-γ-butyrolactone with the highest enantioselectivity, and provides a new opportunity for the chem-enzymatic synthesis of podophyllotoxin.

Organocatalytic aldol approach for the protecting group-free asymmetric synthesis of (7R')-parabenzlactone, (-)-hinokinin, (-)-yatein, (-)-bursehernin, (-)-pluviatolide, (+)-isostegane and allied lignans

A short and efficient catalytic asymmetric protection-free synthesis of dibenzylbutyrolactone lignans, such as (-)-hinokinin, (-)-yatein, (-)-bursehernin, (-)-pluviatolide, and their 7'-hydroxylignans - (7'R)-parabenzlactone, (7'R)-hydroxyyatein, (7'R)-hydroxybursehernin, and (7'R)-hydroxy pluviatolide, respectively, is described. The syntheses of (+)-isostegane and the formal synthesis of (-)-podophyllotoxin and bicubebins are also described. Organocatalytic aldol-reduction-lactonization and Pd/C-catalyzed hydrogenative debromination are two-pot sequential reactions for the enantioselective synthesis of hydroxybutyrolactone 13b with excellent diastereo- and enantioselectivity (dr 33 : 1 and >99% ee). The protecting group-free chemoselective α-alkylation of 13b directly led to 7'-hydroxydibenzylbutyrolactone lignans, followed by hydrogenative dehydroxylation, which led to their (deoxy) dibenzylbutyrolactone lignans, and the syntheses were completed in three to five steps from 6-bromopiperonal.

Enantioselective Desymmetrization of Cyclobutanones: A Speedway to Molecular Complexity

Cyclobutanones hold a privileged role in enantioselective desymmetrization because their inherent ring strain allows for a variety of unusual reactions to occur. Current strategies include α-functionalization, rearrangement, and C-C bond activation to directly convert cyclobutanones into a wide range of enantiomerically enriched compounds, including many biologically significant scaffolds. This Minireview provides an overview of state-of-the-art methods that generate complexity from prochiral cyclobutanones in a single operation.

Rhodococcus as A Versatile Biocatalyst in Organic Synthesis

The application of purified enzymes as well as whole-cell biocatalysts in synthetic organic chemistry is becoming more and more popular, and both academia and industry are keen on finding and developing novel enzymes capable of performing otherwise impossible or challenging reactions. The diverse genus Rhodococcus offers a multitude of promising enzymes, which therefore makes it one of the key bacterial hosts in many areas of research. This review focused on the broad utilization potential of the genus Rhodococcus in organic chemistry, thereby particularly highlighting the specific enzyme classes exploited and the reactions they catalyze. Additionally, close attention was paid to the substrate scope that each enzyme class covers. Overall, a comprehensive overview of the applicability of the genus Rhodococcus is provided, which puts this versatile microorganism in the spotlight of further research.

Cloning and characterization of the Type I Baeyer-Villiger monooxygenase from Leptospira biflexa

Baeyer–Villiger monooxygenases are recognized by their ability and high selectivity as oxidative biocatalysts for the generation of esters or lactones using ketones as starting materials. These enzymes represent valuable tools for biooxidative syntheses since they can catalyze reactions that otherwise involve strong oxidative reagents. In this work, we present a novel enzyme, the Type I Baeyer–Villiger monooxygenase from Leptospira biflexa. This protein is phylogenetically distant from other well-characterized BVMOs. In order to study this new enzyme, we cloned its gene, expressed it in Escherichia coli and characterized the substrate scope of the Baeyer–Villiger monooxygenase from L. biflexa as a whole-cell biocatalyst. For this purpose, we performed the screening of a collection of ketones with variable structures and sizes, namely acyclic ketones, aromatic ketones, cyclic ketones, and fused ketones. As a result, we observed that this biocatalyst readily oxidized linear- and branched- medium-chain ketones, alkyl levulinates and linear ketones with aromatic substituents with excellent regioselectivity. In addition, this enzyme catalyzed the oxidation of 2-substituted cycloketone derivatives but showed an unusual selection against substituents in positions 3 or 4 of the ring.

First chemo-enzymatic synthesis of the (R)-Taniguchi lactone and substrate profiles of CAMO and OTEMO, two new Baeyer-Villiger monooxygenases

ABSTRACT

This study investigates the substrate profile of cycloalkanone monooxygenase and 2-oxo-Δ³-4,5,5-trimethylcyclopentenylacetyl-coenzyme A monooxygenase, two recently discovered enzymes of the Baeyer-Villiger monooxygenase family, used as whole-cell biocatalysts. Biooxidations of a diverse set of ketones were performed on analytical scale: desymmetrization of substituted prochiral cyclobutanones and cyclohexanones, regiodivergent oxidation of terpenones and bicyclic ketones, as well as kinetic resolution of racemic cycloketones. We demonstrated the applicability of the title enzymes in the enantioselective synthesis of (R)-(-)-Taniguchi lactone, a building block for the preparation of various natural product analogs such as ent-quinine.

GRAPHICAL ABSTRACT

Regio- and enantioselective Baeyer-Villiger oxidation: kinetic resolution of racemic 2-substituted cyclopentanones

A kinetic resolution of racemic 2-substituted cyclopentanones via highly regio- and enantioselective Baeyer-Villiger oxidation has been successfully developed. The reaction could afford the normal 6-substituted δ-lactones in up to 98% ee and >19/1 regioselectivity. Meanwhile, the unreacted ketones were recovered in excellent ee values (up to 98%). It represents the best results of the kinetic resolution of racemic 2-substituted cyclopentanones via nonenzymic asymmetric BV oxidation.

Recent developments in the application of Baeyer-Villiger monooxygenases as biocatalysts

Baeyer-Villiger monooxygenases (BVMOs) represent a specific class of monooxygenases that are capable of catalyzing a variety of oxidation reactions, including Baeyer-Villiger oxidations. The recently elucidated BVMO crystal structures have provided a more detailed insight into the complex mechanism of these flavin-containing enzymes. Biocatalytic studies on a number of newly discovered BVMOs have shown that they are very potent oxidative biocatalysts. In addition to catalyzing the regio- and enantioselective Baeyer-Villiger oxidations of a wide range of carbonylic compounds, epoxidations, and enantioselective sulfoxidations have also been shown to be part of their catalytic repertoire. This review provides an overview on the recent developments in BVMO-mediated biocatalytic processes, identification of the catalytic role of these enzymes in metabolic routes and prodrug activation, as well as the efforts in developing effective biocatalytic methodologies to apply BVMOs for the synthesis of high added value compounds.

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.