Unexpected Single-Ligand Occupancy and Negative Cooperativity in the SARS-CoV-2 Main Protease.

Many homodimeric enzymes tune their functions by exploiting either negative or positive cooperativity between subunits. In the SARS-CoV-2 Main protease (Mpro) homodimer, the latter has been suggested by symmetry in most of the 500 reported protease/ligand complex structures solved by macromolecular crystallography (MX). Here we apply the latter to both covalent and noncovalent ligands in complex with Mpro. Strikingly, our experiments show that the occupation of both active sites of the dimer originates from an excess of ligands. Indeed, cocrystals obtained using a 1:1 ligand/protomer stoichiometry lead to single occupation only. The empty binding site exhibits a catalytically inactive geometry in solution, as suggested by molecular dynamics simulations. Thus, Mpro operates through negative cooperativity with the asymmetric activity of the catalytic sites. This allows it to function with a wide range of substrate concentrations, making it resistant to saturation and potentially difficult to shut down, all properties advantageous for the virus' adaptability and resistance.

Synthesis of o-Sulfinylanilines from N-Alkyl Sulfoximines and Arynes.

N-Alkyl sulfoximines react with arynes generated in situ under mild conditions providing o-sulfinylanilines in good yields. The transformation is characterized by a broad substrate scope and a good functional group tolerance. The structure of a reaction product was confirmed by single-crystal X-ray diffraction.

Organocatalytic Asymmetric Synthesis of trans-γ-Lactams.

An N-heterocyclic carbene (NHC)-catalyzed highly diastereo- and enantioselective formal [3+2] reaction of o-hydroxy aromatic aldimines and cinnamaldehydes for the preparation of enantioenriched trans-γ-lactams was developed. An internal hydrogen bond between the o-hydroxy and the imine function was crucial for the reactivity and chemical selectivity. Trans-γ-lactam 3 d was easily oxidized to multifunctional 1,4-benzoquinone 8, which could further be transformed to biaryl 9 in the presence of a phosphoric acid. Finally, preliminary results for a kinetic resolution of (±)-trans-γ-lactam 3 d under asymmetric NHC catalysis are reported.

Organocatalytic Kinetic Resolution of Sulfoximines.

An efficient kinetic resolution of sulfoximines with enals was realized using chiral N-heterocyclic carbene (NHC) catalysts. The stereoselective amidation proceeds without additional acyl transfer agent. Both enantiomers of the sulfoximines can be obtained with excellent ee values (up to 99% ee and -97% ee, respectively). Performing the catalysis on a gram scale allowed using the recovered sulfoximine (+)-1j in an asymmetric synthesis of FXa inhibitor F.

Asymmetric Copper-Catalyzed Vinylogous Mukaiyama Michael Addition of Cyclic Dienol Silanes to Unsaturated α-Keto Phosphonates.

A highly stereoselective vinylogous Mukaiyama Michael reaction (VMMR) leading to α-keto phosphonate-containing γ-butenolides with two stereogenic centers is described. The presented transformation is catalyzed by a combination of a commercially available C2 -symmetric bisoxazoline (BOX) ligand and a copper salt and tolerates a variety of nucleophiles and electrophiles. The stereoselectivities of the reactions are good to excellent and the products are obtained in moderate to high yields.

Iron-catalyzed imidative kinetic resolution of racemic sulfoxides.

Kinetic resolution of racemic sulfoxides requires either custom substrates or shows moderate enantioselectivity, leading to achiral coproducts (such as sulfones) as an intrinsic part of the process. A new strategy is demonstrated that allows the resolution of racemic sulfoxides through catalytic asymmetric nitrene-transfer reactions. This approach gives rise to both optically active sulfoxides and highly enantioenriched sulfoximines. By using a chiral iron catalyst and a readily available iodinane reagent, high selectivity factors have been achieved under very practical reaction conditions. With respect to the substrate scope, it is noteworthy that this unprecedented imidative kinetic resolution of racemic sulfoxides provides access to both aryl-alkyl and dialkyl sulfoximines in highly enantioenriched forms.

Catalytic, asymmetric synthesis of phosphonic γ-(hydroxyalkyl)butenolides with contiguous quaternary and tertiary stereogenic centers.

A procedure that enables high yielding access to phosphonic γ-(hydroxyalkyl)butenolides with excellent regio-, diastereo- and enantiocontrol is reported. The simultaneous construction of up to two adjacent quaternary stereogenic centers by a catalytic asymmetric vinylogous Mukaiyama aldol reaction unites biologically and medicinally relevant entities, namely α-hydroxy phosphonates and γ-(hydroxyalkyl)butenolides. This is achieved by utilizing a readily available chiral copper-sulfoximine catalyst showing a broad functional group tolerance for both the electrophilic and nucleophilic reactants. A discussion about potential factors affecting the observed level of enantioselectivity, which stems from the enantiopure sulfoximine ligand, is also included.

Synthesis of chiral sulfoximine-based thioureas and their application in asymmetric organocatalysis.

For the first time, chiral sulfoximine derivatives have been applied as asymmetric organocatalysts. In combination with a thiourea-type backbone the sulfonimidoyl moiety leads to organocatalysts showing good reactivity in the catalytic desymmetrization of a cyclic meso-anhydride and moderate enantioselectivity in the catalytic asymmetric Biginelli reaction. Straightforward synthetic routes provide the newly designed thiourea-sulfoximine catalysts in high overall yields without affecting the stereohomogeneity of the sulfur-containing core fragment.

Evaluation of Chiral Organosulfur Compounds on Their Activity against the Malaria Parasite Plasmodium falciparum.

Malaria is one of the deadliest tropical diseases, especially causing havoc in children under the age of five in Africa. Although the disease is treatable, the rapid development of drug resistant parasites against frontline drugs requires the search for novel antimalarials. In this study, we tested a series of organosulfur compounds from our internal library for their antiplasmodial effect against Plasmodium falciparum asexual and sexual blood stages. Some active compounds were also obtained in enantiomerically pure form and tested individually against asexual blood stages of the parasite to compare their activity. Out of the 23 tested compounds, 7 compounds (1, 2, 5, 9, 15, 16, and 17) exhibited high antimalarial activity, with IC50 values in the range from 2.2 ± 0.64 to 5.2 ± 1.95 µM, while the other compounds showed moderate to very low activity. The most active compounds also exhibited high activity against the chloroquine-resistant strain, reduced gametocyte development and were not toxic to non-infected red blood cells and Hela cells, as well as the hematopoietic HEL cell line at concentrations below 50 µM. To determine if the enantiomers of the active compounds display different antimalarial activity, enantiomers of two of the active compounds were separated and their antimalarial activity compared. The results show a higher activity of the (-) enantiomers as compared to their (+) counterparts. Our combined data indicate that organosulfur compounds could be exploited as antimalarial drugs and enantiomers of the active compounds may represent a good starting point for the design of novel drugs to target malaria.

Photochemistry of N-Phenyl Dibenzothiophene Sulfoximine.

Sulfoximines are popular scaffolds in drug discovery due to their hydrogen bonding properties and chemical stability. In recent years, the role of reactive intermediates such as nitrenes has been studied in the synthesis and degradation of sulfoximines. In this work, the photochemistry of N-phenyl dibenzothiophene sulfoximine [5-(phenylimino)-5H-5λ4 -dibenzo[b,d]thiophene S-oxide] was analyzed. The structure resembles a combination of N-phenyl iminodibenzothiophene and dibenzothiophene S-oxide, which generate nitrene and O(3 P) upon UV-A irradiation, respectively. The photochemistry of N-phenyl dibenzothiophene sulfoximine was explored by monitoring the formation of azobenzene, a photoproduct of triplet nitrene, using direct irradiation and sensitized experiments. The reactivity profile was further studied through direct irradiation experiments in the presence of diethylamine (DEA) as a nucleophile. The studies demonstrated that N-phenyl dibenzothiophene sulfoximine underwent S-N photocleavage to release singlet phenyl nitrene which formed a mixture of azepines in the presence of DEA and generated moderate amounts of azobenzene in the absence of DEA to indicate formation of triplet phenyl nitrene.

C1-symmetric aminosulfoximines in copper-catalyzed asymmetric vinylogous Mukaiyama aldol reactions.

Vinylogous Mukaiyama-type aldol reactions have been catalyzed by a combination of Cu(OTf)2 and readily available C1-symmetric aminosulfoximines. After a fine-tuning of the reaction conditions and an optimization of the modularly assembled ligand structure, high stereoselectivities and excellent yields have been achieved in catalyzed reactions involving various electrophile/nucleophile combinations. The relative and absolute configurations of two products were assigned by X-ray single crystal structure analysis and a comparison of calculated and experimental CD spectra.

Chiral Analogues of PFI-1 as BET Inhibitors and Their Functional Role in Myeloid Malignancies.

Structural analogues of PFI-1 varying at the sulfur core were prepared, and their activities as BET inhibitors in myeloid cell lines and primary cells from patients with acute myeloid leukemia were studied. Docking calculations followed by molecular dynamics simulations revealed the binding mode of the newly prepared inhibitors, suggesting explanations for the observed high enantiospecificity of the inhibitory activity.

Catalyzed vinylogous Mukaiyama aldol reactions with controlled enantio- and diastereoselectivities.

In control: A new catalytic vinylogous Mukaiyama aldol reaction provides products with high diastereo- and enantioselectivities (up to 99 % de and ee; see scheme). The relative and absolute stereochemistry of a representative product was rigorously assigned by NMR and CD spectroscopies (measured and calculated), X-ray diffraction, and quantum-chemical calculations.

Organocatalytic Synthesis of Sulfoximidoyl-Containing Carbamates from Sulfoximines and Morita-Baylis-Hillman Carbonates.

An unexpected substitution reaction providing sulfoximidoyl-containing carbamates was observed when Morita-Baylis-Hillman carbonates and NH-sulfoximines were mixed in acetonitrile at elevated temperature. Key ingredients were triethylamine and ortho-hydroxybenzoic acid, which both had to be present in catalytic quantities.

Organocatalytic Asymmetric Allylic Alkylations of Sulfoximines.

An enantio- and regioselective allylic alkylation of sulfoximines with Morita-Baylis-Hillman carbonates was developed. The asymmetric reaction is directed by a quinidine-derived organocatalyst providing a range of optically active α-methylene ß-sulfoximidoyl esters in high yields (up to 93%) with good to excellent enantiomeric excesses (up to 95%) under mild reaction conditions.

Sulfoximines from a Medicinal Chemist's Perspective: Physicochemical and in vitro Parameters Relevant for Drug Discovery.

Sulfoximines, sulfondiimides and sulfonimidamides are fascinating but not yet fully explored variants of the common sulfone or sulfonamide motif. In this study, we report the physicochemical and in vitro properties of sulfoximines and compare them with related analogs and isosteres. Furthermore, we present a matched molecular pair analysis of compounds from drug discovery projects within Boehringer Ingelheim. We demonstrate that the sulfoximine moiety is a chemically stable, comparatively polar and weakly basic functional group, often leading to favorable aqueous solubility, permeability and metabolic stability. Moreover, their additional vectors at nitrogen enable simple chemical modifications and thus facilitate exploration and fine-tuning of the molecular properties. We conclude that sulfoximines and their congeners do not exhibit any intrinsic flaw but significantly enrich the toolbox of medicinal chemists.

Halocyclizations of Unsaturated Sulfoximines.

A method for halocyclizations of S-alkenylsulfoximines is reported. When unsaturated NH-sulfoximines are treated with a combination of iodobenzene diacetate and potassium iodide, a transformation to the corresponding five- and six-membered cyclic products occurs providing S-oxides of dihydro isothiazoles and tetrahydro-1,2-thiazines, respectively, in moderate to high yields with good diastereoselectivities and excellent regioselectivities.

Copper-catalyzed N-Alkynylations of sulfoximines with bromoacetylenes.

N-Alkynylated sulfoximines have been obtained by copper-catalyzed cross-coupling reactions starting from NH-sulfoximines and bromoacetylenes in moderate to good yields. The reaction conditions are mild, and the substrate scope is wide.

Enantioselective synthesis of highly functionalised amides by copper-catalysed vinylogous Mukaiyama aldol reaction.

Amides with quaternary stereogenic centers have been synthesised by catalytic asymmetric vinylogous Mukaiyama aldol reactions. The chiral copper-sulfoximine catalyst gives rise to products with moderate to good yields and up to 92% ee.