Advances in Chemocatalytic Asymmetric Baeyer–Villiger Oxidations

Seven novel sesquiterpenes with hemostasis activities from leaves of Callicarpa nudiflora

A pair of epimer with an unprecedented 4/7/6 tricyclic skeleton sesquiterpene (1 and 2) and five novel sesquiterpene (3-7) were isolated from Callicarpa nudiflora Hook. et Arn. Their structures were established by extensive spectroscopic analyses, single crystal X-ray diffraction, experimental electronic circular dichroism (ECD) spectra comparison and DP4+ probability analysis. In addition, the plausible biosynthetic pathways of 1 and 2 were proposed. Biological tests indicated that sesquiterpenes derived from the caryophyllene skeleton displayed hemostasis activities in vivo and in vitro, also possessed certain anti-inflammatory ability simultaneously.

Triflic Anhydride-Mediated Synthesis of Anthraquinones and Anthrones via Unusual Intramolecular Rearrangement

Triflic anhydride (Tf2O) is a potent electrophilic activator for carbonyl groups, facilitating the formation of versatile triflate intermediates that undergo nucleophilic trapping to generate a broad array of valuable compounds. In this work, we have developed a novel method that utilizes Tf2O as a mediator for carbonyl activation and subsequent intramolecular rearrangement of 3,3-diarylbenzofuranones into anthraquinones 2 and anthrones 3. Both anthraquinones and anthrones are important molecules in the fields of natural products research, pharmaceuticals, and traditional Chinese medicines. A plausible reaction mechanism is proposed involving Tf2O-mediated activation, carbocation formation, nucleophilic addition, and intramolecular cyclization, ultimately leading to the formation of anthraquinones and anthrones. The developed methodology is both operationally simple and convenient, without the need for an inert atmosphere. This study broadens the scope of Tf2O utility and extends the practical application of intriguing intramolecular rearrangement to the synthesis of biologically active compounds, pharmaceuticals, and functional materials.

Synthesis of Ester-Substituted Polycyclic N-Heteroaromatics via Photocatalyzed Alkoxycarbonylation/Tricyclization Reactions of Enediyne in Continuous Flow Conditions

For the first time, a photoredox-catalyzed alkoxycarbonylation/tricyclization reaction was developed by employing readily accessible enediynes and alkyloxalyl chlorides as starting materials, enabling the synthesis of ester-substituted polycyclic N-heteroaromatics under mild conditions through a radical-mediated mechanism. This photocatalytic strategy is notable for its exceptional compatibility with diverse functional groups, scalability, and efficiency in the formation of rings and chemical bonds. Notably, continuous flow photocatalysis technology markedly improved these reactions compared to the equivalent batch reactions, efficiently decreasing the reaction times to merely 40 min.

FeCl2-Mediated Rearrangement of Aminoperoxides into Functionalized Tetrahydrofurans: Dynamic Non-innocence of O-Ligands at an Fe Center Coordinates a Radical Cascade

The selective reaction of cyclic aminoperoxides with FeCl2 proceeds through a sequence of O-O and C-C bond cleavages, followed by intramolecular cyclization, yielding functionalized tetrahydrofurans in 44-82% yields. Replacing the peroxyacetal group in the peroxide structure with a peroxyaminal fragment fundamentally alters the reaction pathway. Instead of producing linear functionalized ketones, this modification leads to the formation of hard-to-access substituted tetrahydrofurans. Although the aminoperoxide cores undergo multiple bond scissions, this cascade is atom-economical. Computational analysis shows that the O-ligands at the Fe center have enough radical character to promote C-C bond fragmentation and subsequent cyclization. The stereoelectronic flexibility of oxygen, combined with iron's capacity to stabilize multiple reactive intermediates during the multistep cascade, explains the efficiency of this new atom-economic peroxide rearrangement.

Ru Metalloligands Participate in the Construction of POM@MOF for Enhancing the Visible Photoinduced Baeyer-Villiger Oxidation Reaction

Directed synthesis of high-efficiency visible photoinduced Baeyer-Villiger oxidation catalysts is of primary significance. Here, the isopolymolybdate anion [β-Mo8O26]4- is for the first time encapsulated with the photosensitive metalloligand [Ru(bpy)2(H2dcbpy)]2+ (bpy = 2,2'-bipyridine; H2dcbpy = 2,2'-bipyridine-5,5'-dicarboxylic acid) to synthesize polyoxometalate@metal-organic frameworks, {(CdDMF)2[Ru(bpy)2(dcbpy)]3([β-Mo8O26])}·5DMF (Ru-Mo8). The composite photocatalyst Ru-Mo8 not only has a light absorption of 700 nm but also shortens the photogenerated electron transfer distances and accelerates charge and proton transfer. Ru-Mo8 can perform the Baeyer-Villiger oxidation with high selectivity and up to 96.7% yield under visible light (λ > 400 nm) irradiation. The turnover number and turnover frequency of the reaction were computed to be 967 and 548 h-1, respectively, and the apparent quantum yield was 6.84% by 425 nm. Simultaneously, the radical mechanism of Baeyer-Villiger oxidation of Ru-Mo8 in the O2/benzaldehyde system under visible light (λ > 400 nm) irradiation was proposed.

Stereodivergent Total Synthesis of Tacaman Alkaloids

This paper describes a concise, asymmetric and stereodivergent total synthesis of tacaman alkaloids. A key step in this synthesis is the biocatalytic Baeyer-Villiger oxidation of cyclohexanone, which was developed to produce seven-membered lactones and establish the required stereochemistry at the C14 position (92 % yield, 99 % ee, 500 mg scale). Cis- and trans-tetracyclic indoloquinolizidine scaffolds were rapidly synthesized through an acid-triggered, tunable acyl-Pictet-Spengler type cyclization cascade, serving as the pivotal reaction for building the alkaloid skeleton. Computational results revealed that hydrogen bonding was crucial in stabilizing intermediates and inducing different addition reactions during the acyl-Pictet-Spengler cyclization cascade. By strategically using these two reactions and the late-stage diversification of the functionalized indoloquinolizidine core, the asymmetric total syntheses of eight tacaman alkaloids were achieved. This study may potentially advance research related to the medicinal chemistry of tacaman alkaloids.

Molecular Editing of Ketones through N-Heterocyclic Carbene and Photo Dual Catalysis

The molecular editing of ketones represents an appealing strategy due to its ability to maximize the structural diversity of ketone compounds in a straightforward manner. However, developing efficient methods for the arbitrary modification of ketonic molecules, particularly those integrated within complex skeletons, remains a significant challenge. Herein, we present a unique strategy for ketone recasting that involves radical acylation of pre-functionalized ketones facilitated by N-heterocyclic carbene and photo dual catalysis. This protocol features excellent substrate tolerance and can be applied to the convergent synthesis and late-stage functionalization of structurally complex bioactive ketones. Mechanistic investigations, including experimental studies and density functional theory (DFT) calculations, shed light on the reaction mechanism and elucidate the basis of the regioselectivity.

Catalytic Desymmetrizing Baeyer-Villiger Oxidation of Quaternary Carbon-Containing Cyclobutane-1,3-diones

The first highly enantioselective Baeyer-Villiger oxidation of quaternary carbon-containing cyclobutane-1,3-diones using chiral phosphoric acid catalysis and commercially available oxidants was reported. According to the structure of the substrates, two optimized reaction conditions were developed to afford the corresponding chiral tetronic acid products in ≤93% and ≤95% ee values. This reaction offers the first catalytic asymmetric approach to chiral 5,5-disubstituted tetronic acid derivatives. The synthetic potential of this method has been demonstrated by the formal asymmetric synthesis of (-)-vertinolide and the first catalytic asymmetric total synthesis of plakinidone B.

Stereospecific Nitrogen Insertion Using Amino Diphenylphosphinates: An Aza-Baeyer-Villiger Rearrangement

Amino diphenylphosphinates, which are commercially available or easily prepared from hydroxylamine, undergo ring expansion of cyclobutanones toward γ-lactams under mild conditions. A reaction pathway profoundly different from the common Beckmann reaction is achieved through the ambivalent character of the aminating agent. Thus, rearrangement occurs from a Criegee-like intermediate prior to the formation of the oxime species, which is corroborated by mechanistic experiments. Based on this observation, the migrating aptitude of the adjacent groups is analyzed and found to be in line with the parent Baeyer-Villiger reaction rendering a regioselective (up to >99:1 rr), stereospecific (>99% enantiospecificity), and chemoselective (>99%) insertion process possible. The method thus qualifies for late-stage skeletal editing as showcased by the synthesis of Rolipram and its N-alkylated analogs.

Redesigning Enzymes for Biocatalysis: Exploiting Structural Understanding for Improved Selectivity

The discovery of new enzymes, alongside the push to make chemical processes more sustainable, has resulted in increased industrial interest in the use of biocatalytic processes to produce high-value and chiral precursor chemicals. Huge strides in protein engineering methodology and in silico tools have facilitated significant progress in the discovery and production of enzymes for biocatalytic processes. However, there are significant gaps in our knowledge of the relationship between enzyme structure and function. This has demonstrated the need for improved computational methods to model mechanisms and understand structure dynamics. Here, we explore efforts to rationally modify enzymes toward changing aspects of their catalyzed chemistry. We highlight examples of enzymes where links between enzyme function and structure have been made, thus enabling rational changes to the enzyme structure to give predictable chemical outcomes. We look at future directions the field could take and the technologies that will enable it.

Immobilization of Baeyer-Villiger monooxygenase from acetone grown Fusarium sp

OBJECTIVE

A novel biocatalyst for Baeyer-Villiger oxidations is necessary for pharmaceutical and chemical industries, so this study aims to find a Baeyer-Villiger monooxygenase (BVMO) and to improve its stability by immobilization.

RESULTS

Acetone, the simplest ketone, was selected as the only carbon source for the screening of microorganisms with a BVMO. A eukaryote, Fusarium sp. NBRC 109816, with a BVMO (FBVMO), was isolated from a soil sample. FBVMO was overexpressed in E. coli and successfully immobilized by the organic-inorganic nanocrystal formation method. The immobilization improved the thermostability of FBVMO. Substrate specificity investigation revealed that both free and immobilized FBVMO were found to show catalytic activities not only for Baeyer-Villiger oxidation of ketones to esters but also for oxidation of sulfides to sulfoxides. Furthermore, a preparative scale reaction using immobilized FBVMO was successfully conducted.

CONCLUSIONS

FBVMO was discovered from an environmental sample, overexpressed in E. coli, and immobilized by the organic-inorganic nanocrystal formation method. The immobilization successfully improved its thermostability.

Synthesis of Ester-Substituted Indolo[2,1-a]isoquinolines via Photocatalyzed Alkoxycarbonylation/Cyclization Reactions

A direct alkoxycarbonylation/cyclization reaction is accomplished under visible light-induced photoredox catalysis. With this approach, a variety of ester-substituted indolo[2,1-a]isoquinolines are prepared in good to excellent yields. It is worth noting that this method not only can afford the synthesis of indolo[2,1-a]isoquinolines but also can provide an alternative route for generating complex target structures bearing carboxylic esters.

Cu(II)/SPDO complex-catalyzed asymmetric Baeyer–Villiger oxidation of 2-arylcyclobutanones and its application for the total synthesis of eupomatilones 5 and 6

A novel classical kinetic resolution of 2-aryl-substituted or 2,3-disubstituted cyclobutanones of Baeyer–Villiger oxidation catalyzed by a Cu(ii)/SPDO complex is reported for the first time, producing normal lactones in excellent enantioselectivities (up to 96% ee) and regioselectivities (up to >20/1), along with unreacted ketones in excellent enantioselectivities (up to 99% ee). The current transformation features a wide substrate scope. Moreover, catalytic asymmetric total syntheses of natural eupomatilones 5 and 6 are achieved in nine steps from commercially available 3-methylcyclobutan-1-one.

A novel classical kinetic resolution of Baeyer–Villiger oxidation catalyzed by a Cu(ii)/SPDO complex with excellent enantioselectivity, regioselectivity and wide substrate scope is reported for the first time and explore the synthetic application.

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.

Enzymatic Kinetic Resolution by Addition of Oxygen

Kinetic resolution using biocatalysis has proven to be an excellent complementary technique to traditional asymmetric catalysis for the production of enantioenriched compounds. Resolution using oxidative enzymes produces valuable oxygenated structures for use in synthetic route development. This Minireview focuses on enzymes which catalyse the insertion of an oxygen atom into the substrate and, in so doing, can achieve oxidative kinetic resolution. The Baeyer-Villiger rearrangement, epoxidation, and hydroxylation are included, and biological advancements in enzyme development, and applications of these key enantioenriched intermediates in natural product synthesis are discussed.

Formation of δ-Lactones by Cyanide Catalyzed Rearrangement of α-Hydroxy-β-oxoesters

δ-Valerolactone derivatives are formed by cyanide-catalyzed ring-transformation of cyclic α-hydroxy-β-oxoesters. This unprecedented reaction defines a new synthetic methodology, and the products are obtained in up to quantitative yields. Several alkyl substitutions as well as different ester residues are tolerated. Furthermore, benzo- and heteroarene-annulated starting materials are converted without problems. As an additional benefit, the starting materials are straightforwardly accessed by cerium-catalyzed aerobic α-hydroxylation of readily available β-oxoesters.