Catalytic Kinetic Resolution of Disubstituted Piperidines by Enantioselective Acylation: Synthetic Utility and Mechanistic Insights

Stereochemical editing logic powered by the epimerization of unactivated tertiary stereocenters

The stereoselective synthesis of complex targets requires the precise orchestration of chemical transformations that simultaneously establish the connectivity and spatial orientation of desired bonds. In this work, we describe a complementary paradigm for the synthesis of chiral molecules and their isomers, which tunes the three-dimensional structure of a molecule at a late stage. Key to the success of this strategy is the development of a mild and highly general photocatalytic method composed of decatungstate polyanion and disulfide cocatalysts, which enable the interconversion of unactivated tertiary stereogenic centers that were previously configurationally fixed. We showcase the versatility of this method-and the implementation of stereoediting logic-by the rapid construction of chiral scaffolds that would be challenging to access using existing tools and by the late-stage stereoediting of complex targets.

Kinetic Resolution of 2-Aryl-4-methylenepiperidines toward Enantioenriched Functionalizable Piperidine Fragments

The base n-BuLi with sparteine allows a kinetic resolution of N-Boc-2-aryl-4-methylenepiperidines. The 2,2-disubstituted products and recovered starting materials were isolated with high enantiomeric ratios. From VT-NMR spectroscopy and DFT studies, the rate of rotation of the N-Boc group is fast. Lithiation and trapping of the enantioenriched starting materials gave 2,2-disubstituted piperidines with retention of stereochemistry. Functionalization of the 4-methylene group led to a variety of 2,4-disubstituted piperidines without loss of enantiopurity that could be useful building blocks for drug discovery.

Visible-Light-Induced α,γ-C(sp3)-H Difunctionalization of Piperidines

Herein, we describe a novel protocol for visible-light-induced α,γ-C(sp³)-H difunctionalization of piperidines. This redox-neutral, atom-economical protocol, which exhibits a broad substrate scope and good functional group compatibility, constitutes a concise, practical method for constructing piperidine-containing bridged-ring molecules. Preliminary mechanistic studies indicated that highly regioselective activation of the inert γ-C(sp³)-H bond of piperidines was achieved through a 1,5-hydrogen atom transfer reaction of a nitrogen radical generated in situ.

Methane Monooxygenase Mimic Asymmetric Oxidation: Self-Assembling μ-Hydroxo, Carboxylate-Bridged Diiron(III)-Catalyzed Enantioselective Dehydrogenation

Mimicking naturally occurring metalloenzymes to enrich the diversity of catalytic asymmetric oxidation reactions is a long-standing goal for modern chemistry. Toward this end, a range of methane monooxygenase (MMO) mimic chiral carboxylate-bridged (μ-hydroxo) diiron(III) dimer complexes using salan as basal ligand and sodium aryl carboxylate as additive have been designed and synthesized. The chiral diiron complexes exhibit efficient catalytic reactivity in dehydrogenative kinetic resolution of indolines using environmentally benign hydrogen peroxide as oxidant. In particular, complex C9 bearing sterically encumbered salan ligands and a 2-naphthoate bridge is identified as the optimal catalyst in terms of chiral recognition. Further investigation reveals that this MMO mimic chiral catalyst can be readily generated by self-assembly under the dehydrogenation conditions. The self-assembling catalytic system is applicable to a series of indolines with multiple stereocenters and diverse substituent patterns in high efficiency with a high level of chiral recognition (selectivity factor up to 153). Late-stage dehydrogenative kinetic resolution of bioactive molecules is further examined.

Catalytic Kinetic Resolution and Desymmetrization of Amines

Optically active amines represent critically important subunits in bioactive natural products and pharmaceuticals, as well as key scaffolds in chiral catalysts and ligands. Kinetic resolution of racemic amines and enantioselective desymmetrization of prochiral amines have proved to be efficient methods to access enantioenriched amines, especially when the racemic or prochiral amines were easy to prepare while the chiral ones are difficult to be accessed directly. In this review, we systematically summarized the development of kinetic resolution and desymmetrization of amines through nonenzymatic asymmetric catalytic approaches in the last two decades.

Acylative Kinetic Resolution of Cyclic Hydroxamic Acids

Racemic cyclic hydroxamic acids bearing an aryl substituent adjacent to the hydroxyl group undergo effective acylative kinetic resolution promoted by benzotetramisole (BTM).

Acylative kinetic resolution of racemic methyl-substituted cyclic alkylamines with 2,5-dioxopyrrolidin-1-yl (R)-2-phenoxypropanoate

The diastereoselective acylation of a number of racemic methyl-substituted cyclic alkylamines with active esters of 2-phenoxypropanoic acid was studied in detail. The ester of (R)-2-phenoxypropanoic acid and N-hydroxysuccinimide was found to be the most selective agent. The highest stereoselectivity was observed in the kinetic resolution of racemic 2-methylpiperidine in toluene at -40 °C (selectivity factor s = 73) with the predominant formation of (R,R)-amide (93.7% de). To explain the observed stereoselectivity, DFT modelling of the transition states in the reactions of the title acylating agent with 2-methylpiperidine and 2-methylpyrrolidine was performed. The calculated values were in good agreement with experimental data. It has been demonstrated that the acylation proceeds via a concerted mechanism, in which the addition of an amine occurs simultaneously with the elimination of the hydroxysuccinimide fragment. The high stereoselectivity of the (R,R)-amide formation is largely ensured by the lower steric hindrances in the transition states as compared to the formation of (R,S)-amide.

Asymmetric Synthesis of Hydroquinazolines Bearing C4-Tetrasubstituted Stereocenters via Kinetic Resolution of α-Tertiary Amines

A novel protocol for asymmetric synthesis of hydroquinazolines bearing C4-tetrasubstituted stereocenters has been achieved through kinetic resolution of 2-amido α-tertiary benzylamines via chiral phosphoric acid catalyzed intramolecular dehydrative cyclizations. This method gave access to both α-tertiary benzylamines and hydroquinazolines with broad scope and high enantioselectivities. An intriguing restricted rotation of the C-N bond was observed for hydroquinazoline products bearing C4-tetrasubstituted stereocenters.

Kinetic resolution of N-aryl β-amino alcohols via asymmetric aminations of anilines

An efficient kinetic resolution of N-aryl β-amino alcohols has been developed via asymmetric para-aminations of anilines with azodicarboxylates enabled by chiral phosphoric acid catalysis. Broad substrate scope and high kinetic resolution performances were afforded with this method. Control experiments supported the critical roles of the NH and OH group in these reactions.

Novel (S)-Selective Hydrolase from Arthrobacter sp. K5 for Kinetic Resolution of Cyclic Amines

Chiral 2-methylpiperidine (2-MPI) is an important building block that has potential for applications in pharmaceuticals and pesticides. In this study, we observed that the hydrolase in Arthrobacter sp. K5 exhibits high (S)-selectivity toward rac-N-pivaloyl-2-MPI to yield (S)-2-MPI with 80.2% enantiomeric excess (ee) in a 38.2% conversion. The hydrolase, which was identified by analyses of partial amino acid sequences of the purified enzyme and genome sequence of Arthrobacter sp. K5, exhibited moderate homology with amidohydrolases up to 67% (molinate hydrolase from Gulosibacter molinativorax). The hydrolase gene was overexpressed in Rhodococcus erythropolis. The recombinant cells produced (S)-2-MPI with 83.5% ee in a 48.4% conversion (E = 26.3) from 100 mM rac-N-pivaloyl-2-MPI. These results suggest the possibility of an efficient preparation of chiral 2-MPI in kinetic resolution.

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.

Asymmetric Synthesis of Hydroquinolines with α,α-Disubstitution through Organocatalyzed Kinetic Resolution

The first kinetic resolution of hydroquinoline derivatives with α,α-disubstitution has been achieved through asymmetric remote aminations with azodicarboxylates enabled by chiral phosphoric acid catalysis. Mechanistic studies suggest a monomeric catalyst pathway proceeding through rate- and enantio-determining electrophilic attack promoted by a network of attractive non-covalent interactions between the substrate and catalyst. Facile subsequent removal and transformations of the newly introduced hydrazine moiety enable these protocols to serve as powerful tools for asymmetric synthesis of N-heterocycles with α,α-disubstitution.

Using the Competing Enantioselective Conversion Method to Assign the Absolute Configuration of Cyclic Amines with Bode's Acylation Reagents

The competing enantioselective conversion (CEC) method is a quick and reliable means to determine absolute configuration. Previously, Bode's chiral acylated hydroxamic acids were used to determine the stereochemistry of primary amines, as well as cyclic and acyclic secondary amines. The enantioselective acylation has been evaluated for 4-, 5-, and 6-membered cyclic secondary amines, including medicinally relevant compounds. The limitations of the method were studied through computational analysis and experimental results. Piperidines with substituents at the 2-position did not behave well unless the axial conformer was energetically accessible, which is consistent with the transition state geometries proposed by Bode and Kozlowski. Control experiments were performed to investigate the cause of degrading selectivity under the CEC reaction conditions. The present study expands the scope of the CEC method for secondary amines and provides a better understanding of the reaction profile.

A Selenourea-Thiourea Brønsted Acid Catalyst Facilitates Asymmetric Conjugate Additions of Amines to α,β-Unsaturated Esters

β-Amino esters are obtained with high levels of enantioselectivity via the conjugate addition of cyclic amines to unactivated α,β-unsaturated esters. A related strategy enables the kinetic resolution of racemic cyclic 2-arylamines, using benzyl acrylate as the resolving agent. Reactions are facilitated by an unprecedented selenourea-thiourea organocatalyst. As elucidated by DFT calculations and 13C kinetic isotope effect studies, the rate-limiting and enantiodetermining step of the reaction is the protonation of a zwitterionic intermediate by the catalyst. This represents a rare case in which a thiourea compound functions as an asymmetric Brønsted acid catalyst.

Iron-Catalyzed Aerobic Dehydrogenative Kinetic Resolution of Cyclic Secondary Amines

A nonenzymatic iron-catalyzed dehydrogenative kinetic resolution of cyclic secondary amines using air as an oxidant has been reported. The economical and practical method is applicable to a series of cyclic benzylic amines, including 5,6-dihydrophenanthridines and 1,2-dihydroquinolines, with diverse functional groups at the α position in high yields with excellent enantioselectivities. The direct dehydrogenative kinetic resolution of advanced intermediates of bioactive molecules that are difficult to access using existing catalytic asymmetric synthetic strategy was also demonstrated.

Kinetic resolution of racemic 2-substituted 1,2-dihydroquinolines via asymmetric Cu-catalyzed borylation

A highly efficient kinetic resolution of racemic 2-substituted 1,2-dihydroquinolines via asymmetric Cu-catalyzed borylation has been realized for the first time. Under mild conditions, a variety of chiral 3-boryl-1,2,3,4-tetrahydroquinolines containing two vicinal stereogenic centers as well as the recovered 2-substituted 1,2-dihydroquinolines were afforded after 30 minutes in high yields with up to 99% ee (dr > 99 : 1) and over 98% ee values, respectively, corresponding to kinetic selectivity factors of up to 569. Moreover, this protocol was successfully applied to the asymmetric synthesis of a selective estrogen receptor modulator.

A reductive aminase from Aspergillus oryzae

Reductive amination is one of the most important methods for the synthesis of chiral amines. Here we report the discovery of an NADP(H)-dependent reductive aminase from Aspergillus oryzae (AspRedAm, Uniprot code Q2TW47) that can catalyse the reductive coupling of a broad set of carbonyl compounds with a variety of primary and secondary amines with up to >98% conversion and with up to >98% enantiomeric excess. In cases where both carbonyl and amine show high reactivity, it is possible to employ a 1:1 ratio of the substrates, forming amine products with up to 94% conversion. Steady-state kinetic studies establish that the enzyme is capable of catalysing imine formation as well as reduction. Crystal structures of AspRedAm in complex with NADP(H) and also with both NADP(H) and the pharmaceutical ingredient (R)-rasagiline are reported. We also demonstrate preparative scale reductive aminations with wild-type and Q240A variant biocatalysts displaying total turnover numbers of up to 32,000 and space time yields up to 3.73 g l^-1 d^-1.

Kinetic Resolution of 2-Substituted Indolines by N-Sulfonylation using an Atropisomeric 4-DMAP-N-oxide Organocatalyst

The first catalytic kinetic resolution by N-sulfonylation is described. 2-Substituted indolines are resolved (s=2.6-19) using an atropisomeric 4-dimethylaminopyridine-N-oxide (4-DMAP-N-oxide) organocatalyst. Use of 2-isopropyl-4-nitrophenylsulfonyl chloride is critical to the stereodiscrimination and enables facile deprotection of the sulfonamide products with thioglycolic acid. A qualitative model that accounts for the stereodiscrimination is proposed.

Catalytic Kinetic Resolution of Saturated N-Heterocycles by Enantioselective Amidation with Chiral Hydroxamic Acids

The preparation of enantioenriched chiral compounds by kinetic resolution dates back to the laboratories of Louis Pasteur in the middle of the 19th century. Unlike asymmetric synthesis, this process can always deliver enantiopure material (ee > 99%) if the reactions are allowed to proceed to sufficient conversion and the selectivity of the process is not unity (s > 1). One of the most appealing and practical variants is acylative kinetic resolution, which affords easily separable reaction products, and several highly efficient enzymatic and small molecule catalysts are available. Unfortunately, this method is applicable to limited substrate classes such as alcohols and primary benzylamines. This Account focuses on our work in catalytic acylative kinetic resolution of saturated N-heterocycles, a class of molecules that has been notoriously difficult to access via asymmetric synthesis. We document the development of hydroxamic acids as suitable catalysts for enantioselective acylation of amines through relay catalysis. Alongside catalyst optimization and reaction development, we present mechanistic studies and theoretical calculation accounting for the origins of selectivity and revealing the concerted nature of many amide-bond forming reactions. Immobilization of the hydroxamic acid to form a polymer supported reagent allows simplification of the experimental setup, improvement in product purification, and extension of the substrate scope. The kinetic resolutions are operationally straight forward: reactions proceed at room temperature and open to air conditions, without generation of difficult-to-remove side products. This was utilized to achieve decagram scale resolution of antimalarial drug mefloquine to prepare more than 50 g of (+)-erythro-meflqouine (er > 99:1) from the racemate. The immobilized quasienantiomeric acyl hydroxamic acid reagents were also exploited for a rare practical implementation of parallel kinetic resolution that affords both enantiomers of the amine products in high enantiopurity. The success of this process relied on identification of two cleavable acyl groups alongside implementation of flow-chemistry techniques to ensure reusability of the resolving agents. The work discussed in this Account has laid foundations for new catalyst design as well as development of desymmetrization and dynamic kinetic resolution processes. In the meantime, as all the requisite reagents are commercially available, we hope that hydroxamic acid promoted acylative kinetic resolution will become a method of choice for preparation of saturated N-heterocycles in enantiopure form.