Enzymatic Kinetic Resolution by Addition of Oxygen

Diastereo- and Enantioselective Catalytic Kinetic Resolution of 2-Substituted 3-Nitro-2H-chromenes

An unprecedented catalytic kinetic resolution of 2-substituted 3-nitro-2H-chromenes via Rh-catalyzed asymmetric hydrogenation has been developed to simultaneously produce a variety of syn-chiral 2-substituted 3-nitro-2H-chromanes containing vicinal stereogenic centers with excellent diastero- and enantioselectivities [diastereomeric ratio (dr) of >99:1 and up to 99.9% enantiomeric excess (ee)] and recovered chiral 2-substituted 3-nitro-2H-chromenes with 90-99.9% ee in high yields, respectively, achieving excellent kinetic selectivity factors of up to 1057. Moreover, in addition to syn-chiral 2-substituted 3-nitro-2H-chromanes, this new approach provides concise, atom-economical, and straightforward access to anti-chiral 2-substituted 3-nitro-2H-chromanes and a mammalian target of rapamycin (mTOR) inhibitor, (R)-DHM25.

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.

Synthesis of furan-modified cationic cellulose for stereo-specific imprinting and separation of S-indacrinone via Diels-Alder reaction

This study introduces a novel approach for the separation of indacrinone (IC) enantiomers, crucial in treating edema, hypertension, and hyperuricemia. A cationic biopolymer from furan-2-ylmethylhydrazine-cellulose (FUH-CE), derived from cyanoethyl cellulose (CEC), serving as a substrate in molecular imprinting. A key innovation is the use of the Diels-Alder reaction for efficient cross-linking with bis(maleimido)ethane (BME). This chemical strategy resulted in molecularly imprinted microparticles with high selectivity for the S-IC enantiomer, which can be eluted by adjusting the solution's pH. Extensive characterization confirmed the chemical modifications and selective binding efficacy of these biopolymers. Utilizing separation columns, our method achieved an impressive chiral resolution of (±)-IC, with an enantiomeric excess (ee) of 95 % for R-IC during the loading phase and 97 % for S-IC during elution. Under optimized conditions, the biopolymer demonstrated a maximum binding capacity of 131 mg/g at pH 6. This advanced approach represents a significant advancement in chiral separation technology, offering a robust and efficient technique for the selective isolation of enantiomers. This method not only enhances potential targeted therapeutic applications but also provides a scalable solution for industrial chiral separations.

Dinuclear Titanium(III)-Catalyzed Radical-Type Kinetic Resolution of Epoxides for the Enantioselective Synthesis of cis-Glycidic Esters

Glycidic esters represent pivotal constituents in synthetic chemistry, offering enhanced versatility for tailoring toward a diverse array of molecular targets in comparison with simple epoxides. While considerable progress has been made in the asymmetric synthesis of trans- and trisubstituted glycidic esters, achieving enantioselective preparation of cis-glycidic esters has remained a long-standing challenge. Here, we demonstrate a selectivity-predictable modular platform for the asymmetric synthesis of cis-glycidic esters via a novel dinuclear (salen)titanium(III)-catalyzed radical-type kinetic resolution (KR) approach. This radical KR protocol operates under mild conditions and demonstrates a wide substrate scope, facilitating the synthesis of alkyl- and aryl-substituted cis-glycidic esters with high levels of regioselectivity and enantioselectivity, along with hydroxy ester byproducts representing synthetically valuable motifs as well. This study presents a unique exploration of radical-type KR applied to epoxides, effectively overcoming the steric challenges inherent in conventional nucleophilic-type methodologies typically employed in epoxide chemistry.

Choose Your Own Adventure: A Comprehensive Database of Reactions Catalyzed by Cytochrome P450 BM3 Variants

Cytochrome P450 BM3 monooxygenase is the topic of extensive research as many researchers have evolved this enzyme to generate a variety of products. However, the abundance of information on increasingly diversified variants of P450 BM3 that catalyze a broad array of chemistry is not in a format that enables easy extraction and interpretation. We present a database that categorizes variants by their catalyzed reactions and includes details about substrates to provide reaction context. This database of >1500 P450 BM3 variants is downloadable and machine-readable and includes instructions to maximize ease of gathering information. The database allows rapid identification of commonly reported substitutions, aiding researchers who are unfamiliar with the enzyme in identifying starting points for enzyme engineering. For those actively engaged in engineering P450 BM3, the database, along with this review, provides a powerful and user-friendly platform to understand, predict, and identify the attributes of P450 BM3 variants, encouraging the further engineering of this enzyme.

An Enantiospecific Synthesis of 5-epi-α-Bulnesene

As a result of its unique fragrance and wider role in traditional medicine, agarwood produced in Aquilaria spp. and certain other trees has been harvested to near extinction as a natural phenomenon. Artificially induced agarwood production in Aquilaria plantations has sated some of the demand although the product quality is variable. Synthetic chemistry may have a role to play in providing sustainable routes to many of the fragrant components identified in agarwood and its smoke when burnt as incense. In this work, we report efforts towards a total synthesis of the guaiane sesquiterpene α-bulnesene, which is found, along with its more fragrant oxidised derivatives, in agarwood. Following the ring-expansion of (R)-carvone using reported procedures, α-butenylation gave a substrate for samarium diiodide mediated reductive cyclisation, the two butenyl epimers of the substrate each leading to a single bicyclic alcohol (24 and 25). Overall homoconjugate hydride reduction of one of these alcohols was achieved by Lewis acid-mediated ionisation and then hydride transfer from triethylsilane to complete an overall seven-step synthesis of 5-epi-α-bulnesene. This new synthesis paves the way for short routes to both α-bulnesene enantiomers and a study of their aerial and enzymatic oxidation products.

Open Nitrogen Site-Induced Kinetic Resolution and Catalysis of a Gold Nanocluster

The emerging metal nanocluster provides a platform for the investigation of structural features, unique properties, and structure-property correlation of nanomaterials at the atomic level. Construction of open sites on the surface of the metal nanocluster is a long-pursued but challenging goal. Herein, we realized the construction of "open organic sites" in a metal nanocluster for the first time. Specifically, we introduce the PNP (2,6-bis(diphenylphosphinomethyl)pyridine) pincer ligand in the synthesis of the gold nanocluster, enabling the construction of a structurally precise Au₈(PNP)₄ nanocluster. The rigidity and the unique bonding mode of PNP lead to open nitrogen sites on the surface of the Au₈(PNP)₄ nanocluster, which have been utilized as multifunctional sites in this work for efficient kinetic resolution and catalysis. The gold pincer nanocluster and the open nitrogen site-induced performance will be enlightening for the construction of multifunctional metal nanoclusters.

Effects of Silver Carbonate and p-Nitrobenzoic Acid for Accelerating Palladium-Catalyzed Allylic C-H Acyloxylation

An allylic C-H acyloxylation of terminal alkenes with 4-nitrobenzoic acid was assisted by a bidentate-sulfoxide-ligated palladium catalyst combined with 1,4-benzoquinone and Ag₂CO₃ under mild reaction conditions. The catalytic activity was remarkably enhanced by Ag₂CO₃ as an additive and 4-nitrobenzoic acid as a carboxylate source; both components were essential to exhibiting high catalytic activity, high branch selectivity, and a wide substrate scope with low loading of the palladium catalyst. Branch-selective allylic acyloxylation of ethyl 7-octenoate (1a) gave the product which was led to ethyl 6,8-dihydroxyoctanoate (5), a useful synthetic intermediate of (R)-α-lipoic acid.

Enzymatic strategies for asymmetric synthesis

Enzymes, at the turn of the 21st century, are gaining a momentum. Especially in the field of synthetic organic chemistry, a broad variety of biocatalysts are being applied in an increasing number of processes running at up to industrial scale. In addition to the advantages of employing enzymes under environmentally friendly reaction conditions, synthetic chemists are recognizing the value of enzymes connected to the exquisite selectivity of these natural (or engineered) catalysts. The use of hydrolases in enantioselective protocols paved the way to the application of enzymes in asymmetric synthesis, in particular in the context of biocatalytic (dynamic) kinetic resolutions. After two decades of impressive development, the field is now mature to propose a panel of catalytically diverse enzymes for (i) stereoselective reactions with prochiral compounds, such as double bond reduction and bond forming reactions, (ii) formal enantioselective replacement of one of two enantiotopic groups of prochiral substrates, as well as (iii) atroposelective reactions with noncentrally chiral compounds. In this review, the major enzymatic strategies broadly applicable in the asymmetric synthesis of optically pure chiral compounds are presented, with a focus on the reactions developed within the past decade.

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.