Asymmetric Baeyer-Villiger Reaction with Hydrogen Peroxide Catalyzed by a Novel Planar-Chiral Bisflavin

Desymmetrization of Prochiral Cyclobutanones via Nitrogen Insertion: A Concise Route to Chiral γ-Lactams

Asymmetric access to γ-lactams is achieved via a cyclobutanone ring expansion using widely available (1S,2R)-1-amino-2-indanol for chiral induction. Mechanistic analysis of the key N,O-ketal rearrangement reveals a Curtin-Hammett scenario, which enables a downstream stereoinduction (up to 88:12 dr) and is corroborated by spectroscopic, crystallographic, and computational studies. In combination with an easy deprotection protocol, this operationally simple sequence allows the synthesis of a range of optically pure γ-lactams, including those bearing all-carbon quaternary stereocenters. In addition, the formal synthesis of drug molecules baclofen, brivaracetam, and pregabalin further demonstrates the synthetic utility and highlights the general applicability of the presented method.

Synthetic C6-Functionalized Aminoflavin Catalysts Enable Aerobic Bromination of Oxidation-Prone Substrates

Flavoenzymes catalyze oxidations via hydroperoxide intermediates that result from activation of molecular O₂. These reactions—such as hydroxylation and halogenation—depend on the additional catalytic activity of functional groups in the peptide environment of the flavin cofactor. We report synthetic flavin catalysts that contain C6 amino modifications at the isoalloxazine core and are consequently capable of mediating halogenations outside the peptide surrounding. The catalysts are competent in the selective, biomimetic bromination of oxidation‐prone phenols, flavones, and flavanones using a halide salt in combination with 2,6‐lutidinium oxalate as a flavin reductant under visible‐light irradiation. Our studies show the beneficial effect of stacked bisflavins as well as the catalytic activity of the flavin modifications. The designed flavin catalysts outperform isolated natural (−)‐riboflavin and contribute to the continuing search for tailored flavins in oxidation reactions.

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.

Access to a Key Building Block for the Prostaglandin Family via Stereocontrolled Organocatalytic Baeyer-Villiger Oxidation

A new protocol for the construction of a crucial bicyclic lactone of prostaglandins using a stereocontrolled organocatalytic Baeyer-Villiger (B-V) oxidation was developed. The key B-V oxidation of a racemic cyclobutanone derivative with aqueous hydrogen peroxide has enabled an early-stage construction of a bicyclic lactone skeleton in high enantiomeric excess (up to 95 %). The generated bicyclic lactone is fully primed with two desired stereocenters and enabled the synthesis of the entire family of prostaglandins according to Corey's route. Furthermore, the reactivity and enantioselectivity of B-V oxidation of racemic bicyclic cyclobutanones were evaluated and 90-99 % ee was obtained, representing one of the most efficient routes to chiral lactones. This study further facilitates the synthesis of prostaglandins and chiral lactone-containing natural products to promote drug discovery.

Flavinium and Alkali-Metal Assembly on Sulfated Chitin: A Heterogeneous Supramolecular Catalyst for H2 O2 -Mediated Oxidation

Heterogeneous multiple-catalyst assemblies were developed in which the flavinium cation and Na or Li cations were easily immobilized on a chitin-derived anionic polymeric scaffold through noncovalent ionic interactions. The supramolecular flavinium catalysts were successfully employed in the environmentally friendly heterogeneous Baeyer-Villiger oxidation and sulfoxidation by H₂ O₂ . Owing to the cooperative catalytic effect of flavinium, alkali metal, and sulfated chitin, the supramolecular flavinium assembly showed higher catalytic activity for the Baeyer-Villiger oxidation of cyclic ketones than the corresponding homogeneous flavinium catalyst. Because the ionic assembly was stable under the reaction conditions, the catalyst could be readily recovered by simple filtration and reused.

Aerobic Baeyer-Villiger Oxidation Catalyzed by a Flavin-Containing Enzyme Mimic in Water

Direct incorporation of molecular oxygen into small organic molecules has attracted much attention for the development of new environmentally friendly oxidation processes. In line with this approach, bioinspired systems mimicking enzyme activities are of particular interest since they may perform catalysis in aqueous media. Demonstrated herein is the incorporation of a natural flavin cofactor (FMN) into the specific microenvironment of a water-soluble polymer which allows the efficient reduction of the FMN by NADH in aqueous solution. Once reduced, this artificial flavoenzyme can then activate molecular dioxygen under aerobic conditions and result in the Baeyer-Villiger reaction at room temperature in water.

Self-assembled ion-pair organocatalysis--asymmetric Baeyer-Villiger oxidation mediated by flavinium-cinchona alkaloid dimer

An ion-pair catalyst generated by assembly of a chiral flavinium and a cinchona alkaloid dimer for use in asymmetric Baeyer-Villiger oxidation is presented. Ion-pair formation is essential for enhancing the catalytic activity and stereoselectivity. The catalyst is applicable to structurally diverse 3-substituted cyclobutanones, providing good to excellent enantioselectivities (up to 98 : 2 e.r.). This study provides the first example of self-assembly of a flavin derivative and a base to form a chiral reaction site that enables a highly stereoselective reaction to occur.

A Synergistic Combinatorial and Chiroptical Study of Peptide Catalysts for Asymmetric Baeyer-Villiger Oxidation

We report an approach to the asymmetric Baeyer-Villiger oxidation utilizing bioinformatics-inspired combinatorial screening for catalyst discovery. Scaled-up validation of our on-bead efforts with a circular dichroism-based assay of alcohols derived from the products of solution-phase reactions established the absolute configuration of lactone products; this assay proved equivalent to HPLC in its ability to evaluate catalyst performance, but was far superior in its speed of analysis. Further solution-phase screening of a focused library suggested a mode of asymmetric induction that draws distinct parallels with the mechanism of Baeyer-Villiger monooxygenases.

Theoretical Studies on the Asymmetric Baeyer-Villiger Oxidation Reaction of 4-Phenylcyclohexanone with m-Chloroperoxobenzoic Acid Catalyzed by Chiral Scandium(III)-N,N'-Dioxide Complexes

The mechanism and enantioselectivity of the asymmetric Baeyer-Villiger oxidation reaction between 4-phenylcyclohexanone and m-chloroperoxobenzoic acid (m-CPBA) catalyzed by Sc(III) -N,N'-dioxide complexes were investigated theoretically. The calculations indicated that the first step, corresponding to the addition of m-CPBA to the carbonyl group of 4-phenylcyclohexanone, is the rate-determining step (RDS) for all the pathways studied. The activation barrier of the RDS for the uncatalyzed reaction was predicted to be 189.8 kJ mol(-1) . The combination of an Sc(III) -N,N'-dioxide complex and the m-CBA molecule can construct a bifunctional catalyst in which the Lewis acidic Sc(III) center activates the carbonyl group of 4-phenylcyclohexanone while m-CBA transfers a proton, which lowers the activation barrier of the addition step (RDS) to 86.7 kJ mol(-1) . The repulsion between the m-chlorophenyl group of m-CPBA and the 2,4,6-iPr3 C6 H2 group of the N,N'-dioxide ligand, as well as the steric hindrance between the phenyl group of 4-phenylcyclohexanone and the amino acid skeleton of the N,N'-dioxide ligand, play important roles in the control of the enantioselectivity.

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.

Baeyer-Villiger oxidation and oxidative cascade reactions with aqueous hydrogen peroxide catalyzed by lipophilic Li[B(C6F5)4] and Ca[B(C6F5)4]2

Efficient and selective: Two lipophilic catalysts were used for Baeyer-Villiger (BV) oxidations to give lactones in high yields. Cascade reactions involving this BV oxidation were used to selectively obtain either unsaturated carboxylic acids or hydroxylactones in high yields from β-silyl cyclohexanones.

Synthesis of rigidified flavin-guanidinium ion conjugates and investigation of their photocatalytic properties

Flavin chromophores can mediate redox reactions upon irradiation by blue light. In an attempt to increase their catalytic efficacy, flavin derivatives bearing a guanidinium ion as oxoanion binding site were prepared. Chromophore and substrate binding site are linked by a rigid Kemp's acid structure. The molecular structure of the new flavins was confirmed by an X-ray structure analysis and their photocatalytic activity was investigated in benzyl ester cleavage, nitroarene reduction and a Diels-Alder reaction. The modified flavins photocatalyze the reactions, but the introduced substrate binding site does not enhance their performance.