Benzothiopyran-4-one based reversible inhibitors of the human cytomegalovirus (HCMV) protease

Screening of the Pandemic Response Box identifies anti-microsporidia compounds

Microsporidia are fungal obligate intracellular pathogens, which infect most animals and cause microsporidiosis. Despite the serious threat that microsporidia pose to humans and agricultural animals, few drugs are available for the treatment and control of microsporidia. To identify novel inhibitors, we took advantage of the model organism Caenorhabditis elegans infected with its natural microsporidian Nematocida parisii. We used this system to screen the Pandemic Response Box, a collection of 400 diverse compounds with known antimicrobial activity. After testing these compounds in a 96-well format at high (100 μM) and low (40 μM) concentrations, we identified four inhibitors that restored the ability of C. elegans to produce progeny in the presence of N. parisii. All four compounds reduced the pathogen load of both N. parisii and Pancytospora epiphaga, a C. elegans-infecting microsporidia related to human-infecting species. One of these compounds, a known inhibitor of a viral protease, MMV1006203, inhibited invasion and prevented the firing of spores. A bis-indole derivative, MMV1593539, decreased spore viability. An albendazole analog, MMV1782387, inhibited proliferation of N. parisii. We tested albendazole as well as 5 other analogs and observed that MMV1782387 was amongst the strongest inhibitors of N. parisii and displayed the least host toxicity. Our study further demonstrates the effectiveness of the C. elegans-N. parisii system for discovering microsporidia inhibitors and the compounds we identified provide potential scaffolds for anti-microsporidia drug development.

Progress on the Cu-Catalyzed 1,4-Conjugate Addition to Thiochromones

Carbon-carbon bond formation is one of the most important tools in synthetic organic chemists' toolbox. It is a fundamental transformation that allows synthetic chemists to synthesize the carbon framework of complex molecules from inexpensive simple starting materials. Among the many synthetic methodologies developed for the construction of carbon-carbon bonds, organocopper reagents are one of the most reliable organometallic reagents for this purpose. The versatility of organocuprate reagents or the reactions catalyzed by organocopper reagents were demonstrated by their applications in a variety of synthetic transformations including the 1,4-conjugate addition reactions. Sulfur-containing heterocyclic compounds are a much less studied area compared to oxygen-containing heterocycles but have gained more and more attention in recent years due to their rich biological activities and widespread applications in pharmaceuticals, agrochemicals, and material science. This paper will provide a brief review on recent progress on the synthesis of an important class of sulfur-heterocycles-2-alkylthiochroman-4-ones and thioflavanones via the conjugate additions of Grignard reagents to thiochromones catalyzed by copper catalysts. Recent progress on the synthesis of 2-substituted thiochroman-4-ones via alkynylation and alkenylation of thiochromones will also be covered in this review.

Nickel-Catalyzed One-Pot Carbonylative Synthesis of 2-Mono- and 2,3-Disubstituted Thiochromenones from 2-Bromobenzenesulfonyl Chlorides and Alkynes

A nickel-catalyzed one-pot carbonylation reaction of 2-bromobenzenesulfonyl chlorides with alkynes for the synthesis of thiochromenones has been established. Both terminal and internal alkynes were suitable substrates in this carbonylative transformation, and a broad range of 2-mono- and 2,3-disubstituted thiochromenone products were obtained in moderate to good yields with quite high functional group compatibility. Notably, this procedure presents the first example of nickel-catalyzed carbonylative synthesis of thiochromenones with 2-bromobenzenesulfonyl chlorides as a promising sulfur precursor.

Conjugate Addition of Grignard Reagents to Thiochromones Catalyzed by Copper Salts: A Unified Approach to Both 2-Alkylthiochroman-4-One and Thioflavanone

Grignard reagents undergo conjugate addition to thiochromones catalyzed by copper salts to afford 2-substituted-thiochroman-4-ones, both 2-alkylthiochroman-4-ones and thioflavanones (2-arylthiochroman-4-ones), in good yields with trimethylsilyl chloride (TMSCl) as an additive. The best yields of 1,4-adducts can be attained with CuCN∙2LiCl as the copper source. Excellent yields of 2-alkyl-substituted thiochroman-4-ones and thioflavanones (2-aryl substituted) are attained with a broad range of Grignard reagents. This approach works well with both alkyl and aromatic Grignard reagents, thus providing a unified synthetic approach to privileged 2-substituted thiochroman-4-ones and a potential valuable precursor for further synthetic applications towards many pharmaceutically active molecules. The use of commercially available and/or readily prepared Grignard reagents will expedite the synthesis of a large library of both 2-alkyl substituted thiochroman-4-ones and thioflavanones for additional synthetic applications.

Transition-metal-free sulfonylations of methylthiolated alkynones to synthesize 3-sulfonylated thioflavones

A set of transition-metal-free NaI/TBHP-mediated sulfonylation cyclization reactions of methylthiolated alkynones with sulfonyl hydrazides was developed, by which various 3-sulfonylated thioflavones were prepared under mild reaction conditions.

Development of Conjugate Addition of Lithium Dialkylcuprates to Thiochromones: Synthesis of 2-Alkylthiochroman-4-ones and Additional Synthetic Applications

Lithium dialkylcuprates undergo conjugate addition to thiochromones to afford 2-alkylthiochroman-4-ones in good yields. This approach provide an efficient and general synthetic approach to privileged sulfur-containing structural motifs and valuable precursors for many pharmaceuticals, starting from common substrates-thiochromones. Good yields of 2-alkyl-substituted thiochroman-4-ones are attained with lithium dialkylcuprates, lithium alkylcyanocuprates or substoichiometric amount of copper salts. The use of commercially available inexpensive alkyllithium reagents will expedite the synthesis of a large library of 2-alkyl substituted thiochroman-4-ones for additional synthetic applications.

Human Cytomegalovirus Protease: Why is the Dimer Required for Catalytic Activity?

Human cytomegalovirus (HCMV) is a pathogenic agent responsible for morbidity and mortality in immunocompromised and immunosuppressed individuals. HCMV encodes a serine protease that is essential for the production of infectious virions. In this work, we applied molecular dynamics (MD) simulations on HCMV protease models in order to investigate the experimentally observed (i) catalytic activity of the enzyme homodimer and (ii) induced-fit mechanism upon the binding of substrates and peptidyl inhibitors. Long and stable trajectories were obtained for models of the monomeric and dimeric states, free in solution and bound covalently and noncovalently to a peptidyl-activated carbonyl inhibitor, with very good agreement between theoretical and experimental results. The MD results suggest that HCMV protease indeed operates by an induced-fit mechanism. Also, our analysis indicates that the catalytic activity of the dimer is a result of more favorable interactions between the oxyanion in the covalently bound state and the backbone nitrogen of Arg165, resulting in a reaction that is 7.0 kcal/mol more exergonic and a more significant thermodynamic driving force. The incipient oxyanion in the transition state should also benefit from the stronger interactions with Arg165, reducing in this manner the intrinsic activation barrier for the reaction in the dimeric state.

Microwave-assisted three-component coupling-addition-S(N)Ar (CASNAR) sequences to annelated 4H-thiopyran-4-ones

A whole family of annelated 4H-thiopyran-4-ones as the core structural unit was readily synthesized in good yields by a microwave-assisted coupling-addition-S(N)Ar (CASNAR) sequence starting from readily available (het)aroyl chlorides, alkynes and sodium sulfide nonahydrate in a consecutive one-pot three-component reaction. All representatives display a pronounced halochromicity of the absorption bands upon protonation. According to DFT calculations, the electronic ground state of the annelated 4H-thiopyran-4-ones possess a considerable zwitterionic character.

Heterocyclic analogs of thioflavones: synthesis and NMR spectroscopic investigations

The synthesis of several hitherto unknown heterocyclic ring systems derived from thioflavone is described. Coupling of various o-haloheteroarenecarbonyl chlorides with phenylacetylene gives 1-(o-haloheteroaryl)-3-phenylprop-2-yn-1-ones, which were treated with NaSH in refluxing ethanol to yield the corresponding bi- and tricyclic annelated 2-phenylthiopyran-4-ones. Detailed NMR spectroscopic investigations of the ring systems and their precursors are presented.

Design and Syntheses of 1,6-Naphthalene Derivatives as Selective HCMV Protease Inhibitors

Through high throughput screening of various libraries, substituted styryl naphthalene 6 was identified as an HCMV protease inhibitor. Optimization of various regions of the lead molecule using parallel synthesis resulted in 1,6-substituted naphthalenes 19d-i. These compounds displayed good potency and were selective over elastase, trypsin, and chymotrypsin. The optimization approach on lead compound 6 in three different regions of the molecule using parallel solution-phase synthesis and the corresponding SAR are discussed in detail.

Investigation of the induced-fit mechanism and catalytic activity of the human cytomegalovirus protease homodimer via molecular dynamics simulations

Human cytomegalovirus (HCMV) is a highly species-specific DNA virus infecting up to 80% of the general population. The viral genome contains the open reading frame UL80, which encodes the full-length 80 kDa HCMV serine protease and its substrate. Full-length HCMV protease is composed of an N-terminal 256-amino-acid proteolytic domain, called assemblin, a linker region, and a C-terminal structural domain, the assembly protein precursor. Biochemical studies have shown that dimerization activates assemblin because of an induced stabilization of the oxyanion hole (Arg166). Thus, we performed here molecular dynamics (MD) simulations on HCMV protease models to study the induced-fit mechanism of the enzyme upon the binding of substrates and peptidyl inhibitors, and structural and energetic factors that are responsible for the catalytic activity of the enzyme dimer. Long and stable trajectories were obtained for the models of the monomeric and dimeric states, free in solution and bound to a peptidyl-activated carbonyl inhibitor, with very good agreement between theoretical and experimental results. Our results suggest that HCMV protease is indeed a novel example of serine protease that operates by an induced-fit mechanism. Also, in agreement with mutagenesis studies, our MD simulations suggest that the dimeric form is necessary to activate the enzyme because of an induced stabilization of the oxyanion hole.

Recent strategies in the development of new human cytomegalovirus inhibitors

Human cytomegalovirus (HCMV) is one of the most common opportunistic infections in immunucompromised individuals, such as AIDS patients and organ transplant recipients, and is the most frequent congenital viral infection in humans. Despite a reduction of the incidence of AIDS-related opportunistic infections in patients under highly active antiretroviral treatment, attention should be paid to the HCMV risk factor in these individuals. Furthermore, HCMV may have an important role in atherosclerosis. Existing antiviral treatments for the HCMV infection suffer from poor bioavailability, toxicity, and limited effectiveness, mainly due to the development of drug resistance. Fortunately there are novel and potentially very effective new compounds undergoing pre-clinical and clinical evaluation. This review provides an overview in the last five years of new HCMV inhibitors (chemical structures, SAR, and new mechanisms of action) with the aim to provide new clues for the development of future drugs against this opportunistic virus.

Chemo- and diastereoselectivity in the dimethyldioxirane oxidation of 2,3-dihydro-4H-1-benzothiopyran-4-ones and 4H-1-benzothiopyran-4-ones. Unusual reactivity of 4H-1-benzothiopyran-4-one 1-oxides

The oxidation of the 1-thiochromanones 1-3 by dimethyldioxirane (DMD) produced the corresponding sulfoxides 4-6 or sulfones 7-9; their relative amounts depended on the amount of oxidant used. A low diastereoselectivity was observed in the sulfoxidation of the 2-substituted 1-thiochromanones 2 and 3, due to the small steric differentiation during the DMD attack. An unusual reactivity pattern was found in the DMD oxidation of the 1-thiochromones 10-12, in that the sulfoxides 13-15 were more reactive toward the electrophilic oxidizing agent than the corresponding sulfides. The observed anomaly may be explained in terms of transannular stabilization of the transition structure (TS) for the sulfone formation, promoted through favorable conformational effects in the sulfoxide. Higher sulfoxide/sulfone ratios were found in solvents of greater hydrogen bond donor capacity, which is in accordance with the postulated stabilizing effect.

Selective nonpeptidic inhibitors of herpes simplex virus type 1 and human cytomegalovirus proteases

The proteases encoded by herpesviruses including herpes simplex virus type 1 (HSV-1) and human cytomegalovirus (HCMV) are attractive targets for antiviral drug development because of their important roles in viral replication. We randomly screened a chemical compound library for inhibitory activity against HSV-1 protease. 1,4-Dihydroxynaphthalene and three naphthoquinones were found to be potent inhibitors of HSV-1 protease with IC50 values of 6.4 to 16.9 microM. Inhibitory mode analysis of the compounds against HSV-1 protease suggested that, in spite of structural similarities, only 1,4-dihydroxynaphthalene was a competitive inhibitor, whereas the three naphthoquinones were noncompetitive inhibitors. Among all assayed dihydroxynaphthalene derivatives in the chemical compound library, 1,4-dihydroxynaphthalene proved to be the most potent inhibitor of HSV-1 protease. Therefore, the two hydroxyl groups located at positions 1 and 4 on the naphthalene structure seemed essential for exertion of a potent inhibitory activity against HSV-1 protease. In addition, we have found that these compounds are also potent inhibitors of HCMV protease with extremely low micromolar IC50 values. This differed from the results of inhibitory mode analysis of HSV-1 protease, 1,4-dihydroxynaphthalene was a noncompetitive inhibitor of HCMV protease, and three naphthoquinones were competitive inhibitors. These compounds showed no effective inhibitory activity against several mammalian serine proteases (trypsin, chymotrypsin, kallikrein, plasmin, thrombin and Factor Xa) at 100 microM. These results suggest that 1,4-dihydroxynaphthalene and three naphthoquinones may be useful in the development of nonpeptidic antiherpesvirus agents.

Metal mediated protease inhibition: design and synthesis of inhibitors of the human cytomegalovirus (hCMV) protease

A versatile synthetic route to a novel series of bis-imidazolemethanes designed to inhibit the hCMV protease has been developed and a series of potential metal binding inhibitors has been identified. In selectivity assays, the compounds were highly specific for CMV protease and showed no inhibition (IC50 > 100 microM) of other prototypical serine proteases such as trypsin, elastase, and chymotrypsin. Although the presence of free zinc ions was found to be an absolute requirement for the in vitro biological activity of this class of inhibitor, the potency of the inhibitors could not be improved beyond the micromolar level.