First catalytic enantioselective synthesis of P-stereogenic phosphoramides via kinetic resolution promoted by a chiral bicyclic imidazole nucleophilic catalyst

Nickel-Catalyzed Asymmetric Synthesis of Ambiphilic Secondary Phosphine Oxides

The synthesis of ambiphilic compounds, which possess both strong nucleophilic and electrophilic functional groups, presents a significant challenge due to their propensity to self-react, forming oligomers or polymers. We have successfully achieved the nickel-catalyzed asymmetric synthesis of ambiphilic P-stereogenic alkenyl secondary phosphine oxides from a tailored primary phosphine oxide by leveraging their controversial stability and reactivity. This method demonstrates remarkable tolerance toward a wide range of unactivated alkynes, including those derived from natural products and medicinally relevant molecules, thus providing a universal synthon for P-stereogenic phosphines with high enantioselectivity and regioselectivity. The ambiphilic product exhibits interesting orthogonal reactivities with both nucleophiles and electrophiles and can be easily converted to a variety of medicinally relevant P-stereogenic compounds.

Stereogenic P(V) Synthesis via Catalytic Continuous Substitutions

Phosphorus(V) stereocenters that are fully substituted by heteroatoms play important roles in bioactive molecules and organocatalysts. Existing methods to achieve such motifs rely almost entirely on resolution or diastereocontrol, and prefunctionalized substrates are usually required to generate specific P(V) stereocenters. In contrast, related catalytic methods are rare, and no generally applicable method is described. Here, we report a modular strategy to access a broad variety of stereogenic-at-phosphorus skeletons, including ProTide analogs, alkoxylphosphoramidates, phosphates, phosphorothioates, and phosphonamidates, through designed enantioselective continuous substitutions of simple P(V) precursors. The nucleophilic substitution sequence readily determined the stereoconfiguration of the products. Concise synthesis of ProTide analogs and drug molecules demonstrated the practical value of the protocol. Experimental and computational studies unveiled a unique π-π stacking effect and chalcogen bonding interaction between the catalyst and substrate as the origin of stereoselectivity.

From Cortisone to Enlicitide: A Journey of Synthetic Chemistry Innovations at Merck

Necessity is the mother of invention. Most synthetic chemistry innovations are driven by our desire to make molecules. In the first half of the 20th century, much of this work was inspired by natural products, but as we started to understand the impact that specific molecules could have on biology and human health, a new stimulus for invention appeared. The pharmaceutical industry first brought mass production and formulation of natural products for medicinal purposes but quickly started tinkering with molecular structure to modify compounds' properties, eventually designing molecules from scratch. This necessity for invention of new molecules to improve human health and to manufacture them on large scale is an excellent stimulus for synthetic chemistry innovations. In this Perspective, examples from Merck's chemistry groups are used to highlight the types of innovations that can arise from these endeavors.

Catalytic Enantioselective Nucleophilic Desymmetrization at Phosphorus(V): A Three-Phase Strategy for Modular Preparation of Phosphoramidates

Chiral phosphoramidates characterized by at least a P-N bond without a P-C bond demonstrate a significant applicative value within nucleoside phosphoramidate prodrugs. Despite the availability of methodologies for the selective construction of diverse chiral organophosphorus entities, achieving P-stereocenters solely substituted by heteroatoms often relies on diastereomeric synthesis. Here, we present a catalytic enantioselective desymmetrization strategy using an electrophilic phosphorus reagent with three leaving groups as a substrate, enabling a three-phase nucleophilic attack with various alcohols and amines. By generating a broad range of possible substituent combinations around phosphorus atoms, this synthetic strategy may expedite the synthesis and screening of biologically active phosphoramidates.

Rational Design of Bifunctional Imidazoles as Acyl Transfer Catalysts: Dynamic Kinetic Resolution of 5-Hydroxy-furanones/3-Hydroxy-phthalides

A new class of chiral bifunctional imidazole catalysts has been designed and synthesized, utilizing economical amino alcohols as precursors, significantly expanding the diversity of N-1 position catalysts. These catalysts exhibit excellent substrate activation and stereoselectivity control and have been successfully employed in the asymmetric acylation of 5-hydroxy-furanones/3-hydroxy-phthalides through dynamic kinetic resolution, producing a series of chiral furanone and phthalide analogues featuring a quaternary stereocenter. This asymmetric acylation reaction exhibits excellent reactivity and enantioselectivity, has a wide range of applicability, requires a low catalyst loading, and can be readily converted into valuable building blocks. Moreover, DFT calculations revealed the detailed reaction mechanism and demonstrated that the weak N-H···O and C-H···O interactions between the catalyst and substrate are the key factors affecting the stereoselectivity.

Synthesis of P(V)-Stereogenic Phosphorus Compounds via Organocatalytic Asymmetric Condensation

Enantioenriched phosphorus(V)-stereogenic compounds, featuring a pentavalent phosphorus atom as the stereogenic center, are crucial in various natural products, drugs, bioactive molecules, and catalysts/ligands. While a handful of stereoselective synthetic approaches have been developed, achieving direct stereocontrol at the phosphorus atom through catalytic generation of phosphorus(V)-heteroatom bonds continues to be a formidable challenge. Here, we disclose an organocatalytic asymmetric condensation strategy that employs a novel activation mode of stable feedstock phosphinic acids by the formation of mixed phosphinic anhydride as the reactive species to facilitate further catalyst-controlled asymmetric P-O bond formations, involving a dynamic kinetic asymmetric transformation (DYKAT) process with alcohol nucleophiles via a cinchonidine-derived bifunctional catalyst. The resulting H-phosphinate intermediates allow further stereospecific derivatizations, affording modular access to a diverse library of chiral phosphonates and phosphonamidates with notable antibacterial activity. Furthermore, this synthetic platform facilitates P-O/N coupling with various natural products and drugs, presenting a valuable tool for medicine and agrochemical discovery.

Ligand-Enabled Cu-Catalyzed Stereoselective Synthesis of P-Stereogenic ProTides

Nucleoside analogues have seen significant advancements in treating viral infections and cancer through ProTide technology, leading to a series of FDA-approved drugs such as sofosbuvir, tenofovir alafenamide, and remdesivir. The stereochemical configuration at the phosphorus center of ProTides significantly influences their pharmacological properties, necessitating efficient stereoselective synthesis. Traditional methods using chiral auxiliaries or nonracemic phosphorylating agents are labor-intensive and inefficient, while recent organocatalytic approaches, despite their promise, still face limitations. Herein, we present a novel approach employing chiral metal complexes for the stereoselective assembly of P-stereogenic ProTides via asymmetric P-O bond formation. This approach leverages a chiral metal catalyst to activate the electrophilic phosphorylating reagent, facilitating a base-promoted nucleophilic replacement pathway. Our protocol, featuring mild reaction conditions and broad applicability, enables the highly stereoselective synthesis of previously inaccessible (S,RP) and (R,SP)-ProTide derivatives. The practical utility of this method is demonstrated through the preparation of pharmaceutically relevant ProTide targets and mechanistic studies were conducted to elucidate the reaction pathway, offering significant advancements for drug development and pharmaceutical research.

Synthesis of P-Stereogenic Phosphinamides via Nickel-Catalyzed Kinetic Resolution of H-Phosphinamides by Alkenylation and Arylation

A nickel-catalyzed enantioselective cross-coupling for the synthesis of P-stereogenic phosphinamides has been developed. The asymmetric alkenylation and arylation of racemic H-phosphinamides using alkenyl and aryl bromides resulted in the formation of P-stereogenic N-phosphinyl compounds with good yields and high enantioselectivities. This method tolerates a variety of functional groups, and its applications are explored through scale-up reactions and product transformations.

Enantioselective construction of stereogenic-at-sulfur(IV) centres via catalytic acyl transfer sulfinylation

Chiral sulfur pharmacophores are crucial for drug discovery in bioscience and medicinal chemistry. While the catalytic asymmetric synthesis of sulfoxides and sulfinate esters with stereogenic-at-sulfur(IV) centres is well developed, the synthesis of chiral sulfinamides remains challenging, which has primarily been attributed to the high nucleophilicity and competing reactions of amines. In this study, we have developed an efficient methodology for the catalytic asymmetric synthesis of chiral sulfinamides and sulfinate esters by the sulfinylation of diverse nucleophiles, including aromatic amines and alcohols, using our bifunctional chiral 4-arylpyridine N-oxides as catalysts. The remarkable results are a testament to the efficiency, versatility and broad applicability of the developed synthetic approach, serving as a valuable tool for the synthesis of sulfur pharmacophores. Mechanistic experiments and density functional theory calculations revealed that the initiation and stereocontrol of this reaction are induced by an acyl transfer catalyst. Our research provides an efficient approach for the construction of optically pure sulfur(IV) centres.

Catalytic enantioselective nucleophilic desymmetrization of phosphonate esters

Molecules that contain a stereogenic phosphorus atom are crucial to medicine, agrochemistry and catalysis. While methods are available for the selective construction of various chiral organophosphorus compounds, catalytic enantioselective approaches for their synthesis are far less common. Given the vastness of possible substituent combinations around a phosphorus atom, protocols for their preparation should also be divergent, providing facile access not only to one but to many classes of phosphorus compounds. Here we introduce a catalytic and enantioselective strategy for the preparation of an enantioenriched phosphorus(V) centre that can be diversified enantiospecifically to a wide range of biologically relevant phosphorus(V) compounds. The process, which involves an enantioselective nucleophilic substitution catalysed by a superbasic bifunctional iminophosphorane catalyst, can accommodate a wide range of carbon substituents at phosphorus. The resulting stable, yet versatile, synthetic intermediates can be combined with a multitude of medicinally relevant O-, N- and S-based nucleophiles.

Organocatalytic asymmetric synthesis of P-stereogenic molecules

P-chirality broadly appears in natural and synthetic functional molecules. The catalytic synthesis of organophosphorus compounds bearing P-stereogenic centers is still challenging, due to the lack of efficient catalytic systems. This review summarizes the key achievements in organocatalytic methodologies for the synthesis of P-stereogenic molecules. Different catalytic systems are emphasized for each strategy class (desymmetrization, kinetic resolution, and dynamic kinetic resolution) with examples cited to illustrate the potential applications of the accessed P-stereogenic organophosphorus compounds.

Di-, tetra-, and perhydropyrrolo[1,2-a]imidazoles: The Methods of Synthesis and Some Aspects of Application

The review summarizes and systematizes the literature data on the synthesis and some aspects of application of pyrrolo[1,2-a]imidazoles. Synthetic approaches are grouped according to the degree of saturation of the product pyrroloimidazole ring. The bibliography of the review includes 110 sources over the last 15 years.

Design, Synthesis, and Application of Chiral Bicyclic Imidazole Catalysts

Asymmetric organocatalysis has been considered to be an efficient and reliable strategy for the stereoselective preparation of optically active chemicals. In particular, chiral tertiary amines as Lewis base organocatalysts bearing core structures including quinuclidine, dimethylaminopyridine (DMAP), N-methylimidazole (NMI), amidine, etc. have provided new and powerful tools for various chemical transformations. However, due to the limitations in structural complexity, synthetic difficulty, low catalytic efficiency, and high cost, the industrial application of such catalysts is still far from being widely adopted. Therefore, the development of new chiral tertiary amine catalysts with higher activity and selectivity is greatly desired.In order to address the contradiction between activity and selectivity caused by the ortho group, a bicyclic imidazole structure bearing a relatively large bond angle ∠θ was designed as the skeleton of our new catalysts. 6,7-Dihydro-5H-pyrrolo[1,2-a]imidazole (abbreviated as DPI) and 5,6,7,8-tetrahydroimidazo[1,2-a]pyridine (abbreviated as TIP) are two of the utilized skeletons. In addition to obtaining satisfactory catalytic activity, excellent enantioselectivity would also be expected because the stereocontrol R group is neither far nor close to the catalytic active site (sp²-N atom) and is adjustable. Based on this skeleton, a family of chiral bicyclic imidazole catalysts were easily prepared and successfully applied in several enantioselective reactions for the synthesis of a variety of valuable chiral compounds.6,7-Dihydro-5H-pyrrolo[1,2-a]imidazole (abbreviated as DPI) is the predominantly utilized skeleton. First, HO-DPI, the key intermediate of the designed chiral bicyclic imidazole catalysts, could be efficiently synthesized from imidazole and acrolein, then separated by kinetic resolution or optical resolution. Second, Alkoxy-DPI, the alkyloxy-substituted chiral bicyclic imidazole catalysts, were synthesized by a one-step alkylation from HO-DPI. This type of catalyst has been successfully applied in asymmetric Steglich rearrangement (C-acylation rearrangement of O-acylated azlactones), asymmetric phosphorylation of lactams, and a sequential four-step acylation reaction. Third, Acyloxy-DPI, the acyloxy-substituted chiral bicyclic imidazole catalysts, were synthesized with a one-step acetylative kinetic resolution from racemic HO-DPI or acylation from enantiopure HO-DPI. The catalyst AcO-DPI has been successfully applied in enantioselective Black rearrangement and in direct enantioselective C-acylation of 3-substituted benzofuran-2(3H)-ones and 2-oxindoles. Fourth, Alkyl-DPI was synthesized via a two-step reaction from racemic HO-DPI and separated easily by resolution. The catalyst Cy-DPI has been successfully applied in dynamic kinetic resolution of 3-hydroxyphthalides through enantioselective O-acylation. Cy-PDPI was synthesized through a Cu-catalyzed amidation from Cy-DPI and successfully applied in the kinetic resolution of secondary alcohols with good to excellent enantioselectivities. Finally, the carbamate type chiral bicyclic imidazole catalysts, Carbamate-DPI, were readily synthesized from HO-DPI, and the catalyst Ad-DPI bearing a bulky adamantyl group was successfully applied in the synthesis of the anti-COVID-19 drug remdesivir via asymmetric phosphorylation. Alongside our initial work, this Account also introduces four elegant studies by other groups concerning asymmetric phosphorylation utilizing chiral bicyclic imidazole catalysts.In summary, this Account focuses on the chiral bicyclic imidazole catalysts developed in our group and provides an overview on their design, synthesis, and application that will serve as inspiration for the exploration of new organocatalysts and related reactions.

Chemical Synthesis of the Anti-COVID-19 Drug Remdesivir

Remdesivir has become an important compound for the treatment of COVID‐19. Here, we describe the catalytic asymmetric synthesis of this anti‐COVID‐19 drug. First, the P‐racemic phosphoryl chloride is synthesized in a facile procedure. Then, it is possible to obtain the protected remdesivir via the organocatalytic asymmetric phosphorylation of protected nucleoside GS441524 with P‐racemic phosphoryl chloride catalyzed by chiral bicyclic imidazole. Finally, remdesivir is easily prepared by deprotection. © 2021 Wiley Periodicals LLC.

Basic Protocol 1: Synthesis of 2‐ethylbutyl (chloro(phenoxy)phosphoryl)‐l‐alaninate rac‐4

Basic Protocol 2: Synthesis of chiral bicyclic imidazole Ad‐DPI

Basic Protocol 3: Synthesis of remdesivir

Ni-Catalyzed Enantioselective Allylic Alkylation of H-Phosphinates

The asymmetric synthesis of P-stereogenic phosphinates through allylic alkylation of H-phosphinates has been developed. With H-phosphinates and allylic acetates as the starting materials, a variety of allylic P-chiral phosphinates were accessed in high enantioselectivities of up to 92% ee and generally high yields. In addition, a further study demonstrated the applicability of this protocol, including the scale-up synthesis and facile transformation of chiral products from phosphinates to phosphine oxides with organolithium reagents under mild reaction conditions.

Nickel-Catalyzed Asymmetric Synthesis of P-Stereogenic Vinyl Phosphines

Addition reaction to alkynes is an efficient strategy for constructing valuable alkenyl compounds. However, the elusive regioselectivity has been a persistent challenge. In the context of hydrophosphination reaction which could afford valuable P-stereogenic phosphines, the control of enantioselectivity as well as regioselectivity were especially tricky. Here, we highlighted our recent work on the nickel-catalyzed regio- and enantioselective hydrophosphination of unactivated alkynes with in situ generated secondary phosphines.

1 Introduction

2 Hydrophosphination of Alkynes

3 Derivatization Reactions

4 Mechanism Research

5 Summary and Outlook

Palladium/Xiao-Phos-Catalyzed Kinetic Resolution of sec-Phosphine Oxides by P-Benzylation

P-stereogenic tert- and sec-phosphines have wide applications in asymmetric catalysis, materials, and pharmaceutical chemistry, however, their practical synthesis still constitutes a significant challenge. Herein, a successful kinetic resolution of rac-secondary phosphine oxides via the enantioselective P-benzylation process catalyzed by the palladium/Xiao-Phos was designed. Both tert- and sec-phosphine oxides were delivered in good yield and excellent enantiopurity (selectivity factor up to 226.1). The appealing synthetic utilities are further demonstrated by the facile preparation of several valuable P-chiral compounds, precursors of bidentate ligands, as well as transition metal complexes.

Ni-Catalyzed Asymmetric Hydrophosphination of Unactivated Alkynes

The practical synthesis of P-stereogenic tertiary phosphines, which have wide applications in asymmetric catalysis, materials, and pharmaceutical chemistry, represents a significant challenge. A regio- and enantioselective hydrophosphination using cheap and ubiquitous alkynes catalyzed by a nickel complex was designed, in which the toxic and air-sensitive secondary phosphines were prepared in situ from bench-stable secondary phosphine oxides. This methodology has been demonstrated with unprecedented substrate scope and functional group compatibility to afford electronically and structurally diversified P(III) compounds. The products could be easily converted into various precursors of bidentate ligands and organocatalysts, as well as a variety of transition-metal complexes containing both P- and metal-stereogenic centers.

Practical Remdesivir Synthesis through One-Pot Organocatalyzed Asymmetric (S)-P-Phosphoramidation

Remdesivir, an inhibitor of RNA-dependent RNA polymerase developed by Gilead Sciences, has been used for the treatment of COVID-19. The synthesis of remdesivir is, however, challenging, and the overall cost is relatively high. Particularly, the stereoselective assembly of the P-chirogenic center requires recrystallization of a 1:1 isomeric p-nitrophenylphosphoramidate mixture several times to obtain the desired diastereoisomer (39%) for further coupling with the d-ribose-derived 5-alcohol. To address this problem, a variety of chiral bicyclic imidazoles were synthesized as organocatalysts for stereoselective (S)-P-phosphoramidation employing a 1:1 diastereomeric mixture of phosphoramidoyl chloridates as the coupling reagent to avoid a waste of the other diastereomer. Through a systematic study of different catalysts at different temperatures and concentrations, a mixture of the (S)- and (R)-P-phosphoramidates was obtained in 97% yield with a 96.1/3.9 ratio when 20 mol % of the chiral imidazole-cinnamaldehyde-derived carbamate was utilized in the reaction at -20 °C. A 10-g scale one-pot synthesis via a combination of (S)-P-phosphoramidation and protecting group removal followed by one-step recrystallization gave remdesivir in 70% yield and 99.3/0.7 d.r. The organocatalyst was recovered in 83% yield for reuse, and similar results were obtained. This one-pot process offers an excellent opportunity for industrial production of remdesivir.

Access to P-stereogenic compounds via desymmetrizing enantioselective bromination

A novel and efficient desymmetrizing asymmetric ortho-selective mono-bromination of bisphenol phosphine oxides under chiral squaramide catalysis was reported. Using this asymmetric ortho-bromination strategy, a wide range of chiral bisphenol phosphine oxides and bisphenol phosphinates were obtained with good to excellent yields (up to 92%) and enantioselectivities (up to 98.5 : 1.5 e.r.). The reaction could be scaled up, and the synthetic utility of the desired P-stereogenic compounds was proved by transformations and application in an asymmetric reaction.

Catalytic Asymmetric Synthesis of the anti-COVID-19 Drug Remdesivir

The catalytic asymmetric synthesis of the anti-COVID-19 drug Remdesivir has been realized by the coupling of the P-racemic phosphoryl chloride with protected nucleoside GS441524. The chiral bicyclic imidazole catalyst used is crucial for the dynamic kinetic asymmetric transformation (DyKAT) to proceed smoothly with high reactivity and excellent stereoselectivity (96 % conv., 22:1 S_P :R_P ). Mechanistic studies showed that this DyKAT is a first-order visual kinetic reaction dependent on the catalyst concentration. The unique chiral bicyclic imidazole skeleton and carbamate substituent of the catalyst are both required for the racemization process, involving the phosphoryl chloride, and subsequent stereodiscriminating step. A 10 gram scale reaction was also conducted with comparably excellent results, showing its potential for industrial application.

Treating a Global Health Crisis with a Dose of Synthetic Chemistry

The SARS-CoV-2 pandemic has prompted scientists from many disciplines to work collaboratively toward an effective response. As academic synthetic chemists, we examine how best to contribute to this ongoing effort.

A step-economic and one-pot access to chiral Cα-tetrasubstituted α-amino acid derivatives via a bicyclic imidazole-catalyzed direct enantioselective C-acylation

C^α-Tetrasubstituted α-amino acids are ubiquitous and unique structural units in bioactive natural products and pharmaceutical compounds. The asymmetric synthesis of these molecules has attracted a lot of attention, but a more efficient method is still greatly desired. Here we describe the first sequential four-step acylation reaction for the efficient synthesis of chiral C^α-tetrasubstituted α-amino acid derivatives from simple N-acylated amino acids via an auto-tandem catalysis using a single nucleophilic catalyst. The synthetic efficiency is improved via a direct enantioselective C-acylation; the methodology affords the corresponding C^α-tetrasubstituted α-amino acid derivatives with excellent enantioselectivities (up to 99% ee). This step-economic, one-pot, and auto-tandem strategy provides facile access to important chiral building blocks, such as peptides, serines, and oxazolines, which are often used in medicinal and synthetic chemistry.

The first four-step sequential reaction for the synthesis of C^α-tetrasubstituted chiral α-amino acid derivatives via auto-tandem catalysis has been developed.

Development of a new bicyclic imidazole nucleophilic organocatalyst for direct enantioselective C-acylation

A new chiral nucleophilic organocatalyst bearing a 5,6,7,8-tetrahydroimidazo[1,2-a]pyridine skeleton has been developed.

A multifunctional catalyst that stereoselectively assembles prodrugs

The catalytic stereoselective synthesis of compounds with chiral phosphorus centers remains an unsolved problem. State-of-the-art methods rely on resolution or stoichiometric chiral auxiliaries. Phosphoramidate prodrugs are a critical component of pronucleotide (ProTide) therapies used in the treatment of viral disease and cancer. Here we describe the development of a catalytic stereoselective method for the installation of phosphorus-stereogenic phosphoramidates to nucleosides through a dynamic stereoselective process. Detailed mechanistic studies and computational modeling led to the rational design of a multifunctional catalyst that enables stereoselectivity as high as 99:1.

4-Dimethylaminopyridine-catalyzed dynamic kinetic resolution in asymmetric synthesis of P-chirogenic 1,3,2-oxazaphospholidine-2-oxides

Excellent diastereoselectivity was achieved in DMAP-catalyzed P–N bonding in the synthesis of P-chirogenic organophosphines through dynamic kinetic resolution.

Enantioselective Black rearrangement catalyzed by chiral bicyclic imidazole

A newly developed chiral imidazole nucleophilic catalyst, , was readily prepared and successfully applied to the enantioselective Black rearrangement with up to 88% ee for a wide range of substrates possessing different substituted groups. A plausible mechanism for the high enantioselectivity was proposed.

Palladium-catalyzed 1,4-addition of secondary alkylphenylphosphines to α,β-unsaturated carbonyl compounds for the synthesis of phosphorus- and carbon-stereogenic compounds

Asymmetric 1,4-addition of alkylphenylphosphines to α,β-unsaturated carbonyl compounds catalyzed by pincer–Pd complexes is described to synthesize P-chiral compounds in high ee.