Oxidative Kinetic Resolution of Cyclic Benzylic Ethers

Synthesis of Chiral Sulfoxides by A Cyclic Oxidation-Reduction Multi-Enzymatic Cascade Biocatalysis

Optically pure sulfoxides are valuable organosulfur compounds extensively employed in medicinal and organic synthesis. In this study, we present a biocatalytic oxidation-reduction cascade system designed for the preparation of enantiopure sulfoxides. The system involves the cooperation of a low-enantioselective chimeric oxidase SMO (styrene monooxygenase) with a high-enantioselective reductase MsrA (methionine sulfoxide reductase A), facilitating "non-selective oxidation and selective reduction" cycles for prochiral sulfide oxidation. The regeneration of requisite cofactors for MsrA and SMO was achieved via a cascade catalysis process involving three auxiliary enzymes, sustained by cost-effective D-glucose. Under the optimal reaction conditions, a series of heteroaryl alkyl, aryl alkyl and dialkyl sulfoxides in R configuration were synthesized through this "one-pot, one step" cascade reaction. The obtained compounds exhibited high yields of >90 % and demonstrated enantiomeric excess (ee) values exceeding 90 %. This study represents an unconventional and efficient biocatalytic way in utilizing the low-enantioselective oxidase for the synthesis of enantiopure sulfoxides.

Harnessing Dpp-Imine as a Powerful Achiral Cocatalyst to Dramatically Increase the Efficiency and Stereoselectivity in a Magnesium-Mediated Oxa-Michael Reaction

Dpp-imines are classic model substrates for synthetic method studies. Here, we disclose their powerful use as achiral coligands in metal-catalyzed reactions. It is highly interesting to find that the Dpp-imine can not only act as powerful ligand to create excellent chiral pockets with magnesium complexes but also, more importantly, this coligand can dramatically enhance the catalytic ability of the metal catalyst. The underlying reaction mechanism was extensively explored by conducting a series of experiments, including 31P NMR studies of the coordination complex between the Dpp-imine coligand and magnesium complexes, ESI capture results, multiple control experiments, studies and comparison of different coligands, 1H NMR studies on the relationship between the substrate and Dpp-imine coligand, as well as the relationship between the substrate and the full complexes. Furthermore, DFT calculation provided valuable insights in the role of the imine additive and demonstrated that adding the Dpp-imine coligand in the magnesium catalyst can switch the deprotonation/nucleophilic addition steps from a stepwise mechanism to a concerted process during the oxa-cyclization reaction. The crucial factors responsible for the excellent enantioselectivity and enhanced reaction efficiency brought by Dpp-imine have been extracted from the calculation model. These mechanistic experiments and DFT calculation data clearly disclose and prove the powerful and interesting functions of the Dpp-imine coligand, which also direct a novel application of this type of active imine as useful ligands in metal-catalyzed asymmetric reactions.

C-H Bonds as Functional Groups: Simultaneous Generation of Multiple Stereocenters by Enantioselective Hydroxylation at Unactivated Tertiary C-H Bonds

Enantioselective C-H oxidation is a standing chemical challenge foreseen as a powerful tool to transform readily available organic molecules into precious oxygenated building blocks. Here, we describe a catalytic enantioselective hydroxylation of tertiary C-H bonds in cyclohexane scaffolds with H2O2, an evolved manganese catalyst that provides structural complementary to the substrate similarly to the lock-and-key recognition operating in enzymatic active sites. Theoretical calculations unveil that enantioselectivity is governed by the precise fitting of the substrate scaffold into the catalytic site, through a network of complementary weak non-covalent interactions. Stereoretentive C(sp3)-H hydroxylation results in a single-step generation of multiple stereogenic centers (up to 4) that can be orthogonally manipulated by conventional methods providing rapid access, from a single precursor to a variety of chiral scaffolds.

Palladium/TY-Phos-Catalyzed Asymmetric Heck/Tsuji-Trost Reaction of o-Bromophenols with 1,3-Dienes

2,3-Dihydrobenzofurans are crucial building blocks in the synthesis of natural products and pharmaceutical molecules. However, their asymmetric synthesis has been a long-standing formidable challenge so far. In this work, we developed a highly enantioselective Pd/TY-Phos-catalyzed Heck/Tsuji-Trost reaction of o-bromophenols with various 1,3-dienes, allowing expedient access to chiral substituted 2,3-dihydrobenzofurans. This reaction features excellent regio- and enantiocontrol, high functional group tolerance, and easy scalability. More importantly, the demonstration of this method as a highly valuable tool for the construction of optically pure natural products (R)-tremetone and fomannoxin is highlighted.

Concerted Enantioselective [2+2] Cycloaddition Reaction of Imines Mediated by a Magnesium Catalyst

Enantioselective [2 + 2] cyclization between an imine and a carbon-carbon double bond is a versatile strategy to build chiral azetidines. However, α-branched allenoates have never been successfully applied in [2 + 2] cyclization reactions with imines, as they always undergo Kwon's [4 + 2] annulation in previous catalytic methods. Herein, a simple in situ generated magnesium catalyst was employed to successfully achieve the enantioselective [2 + 2] cyclization reaction of DPP-imines and α-branched allenoates for the first time. Insightful experiments including KIE experiments, controlled experiments, Hammett plot analysis, and ³¹P NMR studies of initial intermediates indicate that the current [2 + 2] cyclization of imine most likely involves an asynchronous concerted transition state. Further mechanistic investigations by combining kinetic studies, ESI experiments, ³¹P NMR studies of coordination complexes, and controlled experiments on reaction rates under different catalyst loading amounts provided the coordination details for this [2 + 2] cyclization reaction between DPP-imines and α-branched allenoates. This new approach was applied to the synthesis of various chiral aza-heterocycles, including the enantioselective synthesis of the key intermediate of a lipid-lowering agent Ezetimibe.

Site- and Enantioselective Manganese-Catalyzed Benzylic C-H Azidation of Indolines

A manganese-catalyzed highly site- and enantioselective benzylic C-H azidation of indolines has been described. The practical method is applicable for azidation of a tertiary benzylic C-H bond with good functional group tolerance, allowing facile access to structurally diverse tertiary azide-containing indolines in high efficiency with excellent site-, chemo-, and enantioselectivity. The generality of the method was further demonstrated by site- and enantioselective azidation of the secondary benzylic C-H bond for a range of secondary azide-containing indolines. The benzylic C-H azidation method allows to straightforwardly and enantioselectively install a variety of nitrogen-based functional groups and diverse bioactive molecules at the C³ position of indoline frameworks through post-azidation manipulations. Gram-scale synthesis was also demonstrated, further highlighting the synthetic potential of the method. Mechanistic studies by combined experiments and computations elucidated the reaction mechanism and origins of stereoselectivity.

Multienzyme Redox System with Cofactor Regeneration for Cyclic Deracemization of Sulfoxides

Optically pure sulfoxides are noteworthy compounds applied in a wide range of industrial fields; however, the biocatalytic deracemization of racemic sulfoxides is challenging. Herein, a high-enantioselective methionine sulfoxide reductase A (MsrA) was combined with a low-enantioselective styrene monooxygenase (SMO) for the cyclic deracemization of sulfoxides. Enantiopure sulfoxides were obtained in >90% yield and with >90% enantiomeric excess ( ee ) through dynamic "selective reduction and non-selective oxidation" cycles. The cofactors of MsrA and SMO were subsequently regenerated by the cascade catalysis of three auxiliary enzymes through the consumption of low-cost D-glucose. Moreover, this "one-pot, one-step" cyclic deracemization strategy exhibited a wide substrate scope toward various aromatic, heteroaromatic, alkyl and thio-alkyl sulfoxides. This system proposed an efficient strategy for the green synthesis of chiral sulfoxide .

Kinetic resolution of cyclic benzylic azides enabled by site- and enantioselective C(sp3)-H oxidation

Catalytic nonenzymatic kinetic resolution (KR) of racemates remains one of the most powerful tools to prepare enantiopure compounds, which dominantly relies on the manipulation of reactive functional groups. Moreover, catalytic KR of organic azides represents a formidable challenge due to the small size and instability of the azido group. Here, an effective KR of cyclic benzylic azides through site- and enantioselective C(sp³)-H oxidation is described. The manganese catalyzed oxidative KR reaction exhibits good functional group tolerance, and is applicable to a range of tetrahydroquinoline- and indoline-based organic azides with excellent site- and enantio-discrimination. Computational studies elucidate that the effective chiral recognition is derived from hydrogen bonding interaction between substrate and catalyst.

Ligand-Enabled C-H Olefination and Lactonization of Benzoic Acids and Phenylacetic Acids via Palladium Catalyst

A novel ligand propan-2-one O-(p-tolylcarbamoyl) oxime (L7) has been developed to promote C(sp²)-H olefination of benzoic acids and phenylacetic acids via a palladium catalyst. With the subsequent lactonization of the olefinated products through 1,4-addition, highly monoselective cyclic lactone products of benzofuranones and benzopyrones were obtained in moderate to excellent yields. The DFT calculation demonstrated that the novel ligand propan-2-one O-(p-tolylcarbamoyl) oxime (L7) could improve the C-H activation reaction to give cyclic lactone products elegantly.

Organocatalytic cascade reactions for multi-functionalized chiral cyclic ethers through vinylidene ortho-quinone methides

An organocatalytic approach to installing various alcohols into the carbonyl of α,β-unsaturated ketones mediated by VQM intermediates was achieved, followed by dearomatization to provide the stereo-defined cyclic ethers via a cascade process. Along with the transformations, this strategy affords efficient access to the underexplored chiral cyclic ether chemospace.

Site and Enantioselective Aliphatic C-H Oxidation with Bioinspired Chiral Complexes

Selective oxidation of aliphatic C-H bonds stands as an unsolved problem in organic synthesis, with the potential to offer novel paths for preparing molecules of biological interest. The quest for reagents that can perform this class of reactions finds oxygenases and their mechanisms of action as inspiration motifs. Among the numerous families of synthetic catalysts that have been explored, complexes with linear tetraazadentate ligands combining two aliphatic amines and two aromatic amine heterocycles display a structural versatility proven instrumental in the design of C-H oxidation reactions showing site and enantioselectivities, not accessible by conventional oxidants. This manuscript makes a review of recent advances in the field.

Kinetic resolution of indolines by asymmetric hydroxylamine formation

Catalytic kinetic resolution of amines represents a longstanding challenge in chemical synthesis. Here, we described a kinetic resolution of secondary amines through oxygenation to produce enantiopure hydroxylamines involving N–O bond formation. The economic and practical titanium-catalyzed asymmetric oxygenation with environmentally benign hydrogen peroxide as oxidant is applicable to a range of racemic indolines with multiple stereocenters and diverse substituent patterns in high efficiency with efficient chemoselectivity and enantio-discrimination. Late-stage asymmetric oxygenation of bioactive molecules that are otherwise difficult to synthesize was also explored.

Catalytic kinetic resolution of amines is a longstanding challenge in chemical synthesis. Here, the authors report on titanium‐catalysed asymmetric oxygenation with hydrogen peroxide for kinetic resolution of secondary amines through oxygenation to produce enantiopure hydroxylamines involving N–O bond formation.