Water enables diastereodivergency in bispidine-based chiral amine-catalyzed asymmetric Mannich reaction of cyclic N-sulfonyl ketimines with ketones

Substrate-Regulated Divergent Addition of N-Sulfonyl Ketimines to Bicyclo[1.1.0]butanes Enabled by Photoinduced Energy Transfer

A substrate-regulated divergent addition of N-sulfonyl ketimines to bicyclo[1.1.0]butanes (BCBs) was developed using a photoinduced energy transfer strategy. The [2π+2σ] cycloaddition of BCBs with saccharin-derived cyclic ketimines yields benzosultam-fused aza-BCHs with good yields and excellent diastereoselectivity. In contrast, reactions of chain N-sulfonyl ketimines with BCBs produce 1,3-fully substituted cyclobutanes via energy-transfer-induced N-S bond homolysis. The ease of large-scale synthesis and derivatizations of products highlight their application potentials.

Bispidine coordination chemistry

Bispidines are extremely rigid ligands, easy to prepare in a large variety, with denticities of four to ten, various donor sets and charges, for mono- and oligonuclear transition metal, main group and rare earth complexes. In the last approx. 20 years significantly more than 50 new bispidine based ligands were prepared and their coordination chemistry studied. Biological probes and medicinal applications is one main area in bispidine coordination chemistry, where fast complex formation, high stability, metal ion selectivity and inertness are of utmost importance. Oxygen activation and oxidation catalysis is another main focus in bispidine coordination chemistry, with catalyst efficiency and stability as well as product selectivity as important requirements. Particularly successful applications in these areas are presented and discussed in detail, in addition to fundamental principles that show the importance of ligand rigidity, cavity size and shape as overarching fundamental properties.

Efficient and easible biocatalysts: Strategies for enzyme improvement. A review

Owing to the environmental friendliness and vast advantages that enzymes offer in the biotechnology and industry fields, biocatalysts are a prolific investigation field. However, the low catalytic activity, stability, and specific selectivity of the enzyme limit the range of the reaction enzymes involved in. A comprehensive understanding of the protein structure and dynamics in terms of molecular details enables us to tackle these limitations effectively and enhance the catalytic activity by enzyme engineering or modifying the supports and solvents. Along with different strategies including computational, enzyme engineering based on DNA recombination, enzyme immobilization, additives, chemical modification, and physicochemical modification approaches can be promising for the wide spread of industrial enzyme usage. This is attributed to the successful application of biocatalysts in industrial and synthetic processes requires a system that exhibits stability, activity, and reusability in a continuous flow process, thereby reducing the production cost. The main goal of this review is to display relevant approaches for improving enzyme characteristics to overcome their industrial application.

Synergistically activating nucleophile strategy enabled organocatalytic asymmetric P-addition of cyclic imines

Herein, we present an attractive organocatalytic asymmetric addition of P-nucleophiles to five-membered cyclic N-sulfonyl imines facilitated by phosphonium salt catalysis, enabling the highly enantioselective synthesis of tri- and tetra-substituted cyclic phosphorus-containing benzosultams. With this protocol, various cyclic α-aminophosphonates were efficiently synthesized with high yields and exceptional enantioselectivities (up to >99% ee) under mild reaction conditions. The utility and practicality of this method were demonstrated through gram-scale reactions and straightforward elaborations. Notably, the success of this approach relies on the deliberate selection of a synergistic organocatalytic system, which helps circumvent foreseeable side effects while handling secondary phosphine oxides (SPOs). Systematic mechanistic studies, incorporating experiments and DFT calculations, have revealed the critical importance of judiciously selecting bifunctional phosphonium salt catalysts for effectively activating P-nucleophiles while stereoselectively controlling the P-attack process.

Stable Axially Chiral Cyclohexylidenes from Catalytic Asymmetric Knoevenagel Condensation

Axially chiral cycloalkylidenes are interesting but less developed axially chiral molecules. Here, a bispidine-based chiral amine catalytic system was developed to promote efficiently the asymmetric Knoevenagel condensation of N-protected oxindoles and benzofuranones with 4-substituted cyclohexanones. A variety of alkylidenecycloalkanes with stable axial chirality were obtained in good yields and fairly good er (enantiomeric ratio). Based on the absolute configuration determination of product and DFT calculations, a possible mechanism of stereoselective induction was proposed.

Sparteine Thiourea: The Synthesis of an N Chiral Bispidine-Quinolizidine-Derived Organocatalyst and Applications in Asymmetric Synthesis of Dihydropyrano[c]chromenes

Bispidine, a bridged bicyclic diamine, has been widely utilized as a rigid scaffold in chiral chelating ligands in asymmetric synthesis. In particular, a chiral bispidine-quinolizidine hybrid, such as sparteine, was utilized in asymmetric synthesis involving a metal, exhibiting superior catalytic activity. In this study, we report the design and synthesis of a series of sparteine-derived organocatalysts and the utilization of these catalysts in tandem Michael addition-cyclization reactions. These catalysts have shown excellent catalytic reactivity and enantioselectivity, and the corresponding dihydropyrano[c]chromenes have been prepared in ≤99% yield and ≤99% ee with a low catalyst loading. The recycled catalysts maintain a good catalytic performance even after four cycles, and a gram-scale reaction with a 1% catalyst loading is also performed, providing the product in 96% yield and 98% ee.

Water-enabling strategies for asymmetric catalysis

Water possesses unique advantages, including abundance, environmental friendliness and mild effects. Undoubtedly, it is an ideal solvent or reagent in chemical syntheses. Water also shows unique abilities in catalytic asymmetric synthesis. It can accelerate reaction rates, improve diastereo- or enantioselectivities, initiate reactions, diversify chemo, diastereo- or enantioselectivities through various effects (hydrophobic, hydrogen bonding, protonation). Several reviews have demonstrated the positive effects of water in asymmetric synthesis. In this review, we summarize water-enabling strategies in the last decade, and focus on advances which reveal how water affects a reaction.

Chiral N,N'-Dioxide Ligands Tune Diastereoselectivity in Mg(II)-Catalyzed Asymmetric Ring-Opening Desymmetrization of Azetidiniums

A diastereodivergent asymmetric desymmetrization of azetidinium salts with benzothiazoleamides as carbon nucleophiles through a chiral N,N'-dioxide/Mg(II) complex-promoted ring-opening reaction is realized by tuning ligands. Both syn- and anti-chiral δ-amino acid derivatives bearing benzothiazole structure were obtained in moderate to good yields and dr and ee values. DFT calculations indicated that the diastereodivergency stems from the different size of the chiral pocket formed by variable substructures of the ligands, leading to the opposite attack direction of the nucleophiles.

Structurally divergent enantioselective synthesis of benzofuran fused azocine derivatives and spiro-cyclopentanone benzofurans enabled by sequential catalysis

An important objective in organic synthesis and medicinal chemistry is the capacity to access structurally varied and complex molecules rapidly and affordably from easily available starting materials. Herein, a protocol for the structurally divergent synthesis of benzofuran fused azocine derivatives and spiro-cyclopentanone benzofurans has been developed via chiral bifunctional urea catalyzed reaction between aurone-derived α,β-unsaturated imine and ynone followed by switchable divergent annulation reactions by Lewis base catalysts (DBU and PPh3) with concomitant epimerization. The skeletally diversified products were formed in high yields with high diastereo- and enantioselectivities. Computational analysis with DFT and accurate DLPNO-CCSD(T) has been employed to gain deeper insights into mechanistic intricacies and investigate the role of chiral and Lewis base catalysts in skeletal diversity.