Development of a Manufacturing Process toward the Convergent Synthesis of the COVID-19 Antiviral Ensitrelvir.

We describe the development of the practical manufacturing of Ensitrelvir, which was discovered as a SARS-CoV-2 antiviral candidate. Scalable synthetic methods of indazole, 1,2,4-triazole and 1,3,5-triazinone structures were established, and convergent couplings of these fragments enabled the development of a concise and efficient scale-up process to Ensitrelvir. In this process, introducing a meta-cresolyl moiety successfully enhanced the stability of intermediates. Compared to the initial route at the early research and development stage, the overall yield of the longest linear sequence (6 steps) was improved by approximately 7-fold. Furthermore, 9 out of the 12 isolated intermediates were crystallized directly from each reaction mixture without any extractive workup (direct isolation). This led to an efficient and environmentally friendly manufacturing process that minimizes waste of organic solvents, reagents, and processing time. This practical process for manufacturing Ensitrelvir should contribute to protection against COVID-19.

Structure-activity relationship studies of anti-bunyaviral cap-dependent endonuclease inhibitors.

Bunyaviruses, including the Lassa virus (LASV), are known to cause hemorrhagic fever and have a high fatality rate among hospitalized patients, as there are few effective treatments. We focused on the fact that bunyaviruses use cap-dependent endonuclease (CEN) for viral replication, which is similar to influenza viruses. This led us to screen carbamoyl pyridone bicycle (CAB) compounds, which compose a series of baloxavir acid (BXA) derivatives, against lymphocytic choriomeningitis virus (LCMV) and Junin virus (JUNV) among the bunyaviruses. This led to the discovery of 1c, which has potent anti-bunyaviral activities. In SAR studies, we found that a large lipophilic side chain is preferred for the 1-position of the CAB scaffold, similar to the influenza CEN inhibitor, and that a small alkyl group for the 3-position shows high activity. Moreover, the 7­carboxyl group of the scaffold is essential for anti-bunyaviral activities, and the antiviral activity is reduced by conversion to various carboxylic acid bioisosteres. The SAR results are discussed using a binding model of 9d in the active center of the known LCMV CEN crystal structure. These compounds show promise as broad-spectrum anti-bunyavirus therapeutics, given their relatively favorable metabolic stability and PK profiles.

Identification of cap-dependent endonuclease inhibitors with broad-spectrum activity against bunyaviruses.

Viral hemorrhagic fevers caused by members of the order Bunyavirales comprise endemic and emerging human infections that are significant public health concerns. Despite the disease severity, there are few therapeutic options available, and therefore effective antiviral drugs are urgently needed to reduce disease burdens. Bunyaviruses, like influenza viruses (IFVs), possess a cap-dependent endonuclease (CEN) that mediates the critical cap-snatching step of viral RNA transcription. We screened compounds from our CEN inhibitor (CENi) library and identified specific structural compounds that are 100 to 1,000 times more active in vitro than ribavirin against bunyaviruses, including Lassa virus, lymphocytic choriomeningitis virus (LCMV), and Junin virus. To investigate their inhibitory mechanism of action, drug-resistant viruses were selected in culture. Whole-genome sequencing revealed that amino acid substitutions in the CEN region of drug-resistant viruses were located in similar positions as those of the CEN α3-helix loop of IFVs derived under drug selection. Thus, our studies suggest that CENi compounds inhibit both bunyavirus and IFV replication in a mechanistically similar manner. Structural analysis revealed that the side chain of the carboxyl group at the seventh position of the main structure of the compound was essential for the high antiviral activity against bunyaviruses. In LCMV-infected mice, the compounds significantly decreased blood viral load, suppressed symptoms such as thrombocytopenia and hepatic dysfunction, and improved survival rates. These data suggest a potential broad-spectrum clinical utility of CENis for the treatment of both severe influenza and hemorrhagic diseases caused by bunyaviruses.

Discovery of S-217622, a Noncovalent Oral SARS-CoV-2 3CL Protease Inhibitor Clinical Candidate for Treating COVID-19.

The coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has resulted in millions of deaths and threatens public health and safety. Despite the rapid global spread of COVID-19 vaccines, effective oral antiviral drugs are urgently needed. Here, we describe the discovery of S-217622, the first oral noncovalent, nonpeptidic SARS-CoV-2 3CL protease inhibitor clinical candidate. S-217622 was discovered via virtual screening followed by biological screening of an in-house compound library, and optimization of the hit compound using a structure-based drug design strategy. S-217622 exhibited antiviral activity in vitro against current outbreaking SARS-CoV-2 variants and showed favorable pharmacokinetic profiles in vivo for once-daily oral dosing. Furthermore, S-217622 dose-dependently inhibited intrapulmonary replication of SARS-CoV-2 in mice, indicating that this novel noncovalent inhibitor could be a potential oral agent for treating COVID-19.

Phosphenium-Cation-Mediated Formal Cycloaddition Approach to Benzophospholes.

A phosphenium-cation-mediated formal [3+2]-cycloaddition reaction of internal alkynes is reported. The reaction proceeds under metal-free conditions to give the corresponding C-P rearranged benzophospholes regioselectively, even when ortho- and para-substituted arylphosphine oxides are starting substrates. Mechanistic studies by 31 P{1 H} NMR analysis suggest an involvement of three-membered phosphirenium cation species and C-P rearrangement prior to a ring closure for benzophosphole skeleton formation.

Metal-Free Electrophilic Phosphination/Cyclization of Alkynes.

A metal-free electrophilic phosphination reaction has been developed. Electrophilic phosphorus species generated in situ from secondary phosphine oxides and Tf2O smoothly couple with alkynes possessing pendant nucleophiles to afford the corresponding phosphinated cyclization products in good yield. Preliminary NMR studies show that phosphirenium species may be involved as intermediates of the cyclization reactions.

Regioselective Synthesis of Benzo[b]phosphole Derivatives via Direct ortho-Alkenylation and Cyclization of Arylthiophosphinamides.

A new regioselective synthetic methodology for benzo[b]phosphole derivatives has been developed. Thus, a range of functionalized benzo[b]phosphole oxides could be synthesized via Rh(III)-catalyzed C-H alkenylation of arylthiophosphinamides with alkynes followed by formal phospha-Friedel-Crafts cyclization.

1,2-Thiazines: One-Pot Syntheses Utilizing Mono and Diaza Analogs of Sulfones.

A one-pot Michael addition/cyclization/condensation reaction sequence for the regioselective synthesis of 1,2-thiazines, starting from propargyl ketones and NH-sulfoximines or NH-sulfondiimines, has been developed. Under mild and operationally simple reaction conditions previously unprecedented 1,2-thiazine 1-imide and 1-oxide derivatives are formed in good to excellent yields. The products represent heterocyclic building blocks, readily modifiable by a regioselective C-H bond functionalization, classical cross-coupling reactions, and deprotection.

Synthesis of Benzo[c]thiophenes by Rhodium(III)-Catalyzed Dehydrogenative Annulation.

The dehydrogenative annulation of thiophen-2-carboxamides with 2 equiv of alkynes proceeds efficiently in the presence of a rhodium catalyst and a copper oxidant to furnish multiply substituted benzo[c]thiophenes. Some of the synthesized benzo[c]thiophenes exhibited strong solid-state fluorescence.

Rhodium(III)-catalyzed oxidative alkenylation of 1,3-dithiane-protected arenecarbaldehydes via regioselective C-H bond cleavage.

A Rh(III)-catalyzed direct alkenylation of 2-aryl-1,3-dithiane derivatives with alkenes has been developed. The 1,3-dithiane group can serve as an effective directing group for the exclusively monoselective alkenylation under mild oxidative conditions. The directing group is readily removable after the coupling event.

Rhodium(III)-catalyzed regioselective C-H alkenylation of phenylphosphine sulfides.

The regioselective alkenylation at the ortho position of phenylphosphine sulfides using alkenes proceeds efficiently in the presence of a cationic Cp*-rhodium(III) catalyst and an appropriate oxidant. A similar rhodium catalyst also promotes the redox-neutral coupling of the phosphine sulfides with alkynes to afford ortho-alkenylated products.

An approach to benzophosphole oxides through silver- or manganese-mediated dehydrogenative annulation involving C-C and C-P bond formation.

Benzophosphole construction was achieved through the Ag(I) -mediated dehydrogenative annulation of phenylphosphine oxides with internal alkynes in a process involving CC and CP bond formation. A wide range of asymmetrical phenylacetylenes could be employed and the reactions proceeded with perfect regioselectivity. Moreover, the annulation could be conducted even at room temperature when a Mn(III) promoter was used in place of Ag(I) .

Rhodium(III)-catalyzed oxidative coupling through C-H bond cleavage directed by phosphinoxy groups.

A straightforward synthesis of phosphaisocoumarins is achieved by the rhodium-catalyzed oxidative coupling of diarylphosphinic and phenylphosphonic acid derivatives with alkynes. The P-OH groups effectively act as the key function for the regioselective C-H bond cleavage. Related oxidative coupling of phenylphosphine oxides with alkenes can also be conducted smoothly under similar conditions.

Palladium-catalyzed decarboxylative arylation of benzoylacrylic acids toward the synthesis of chalcones.

It has been found that readily available 3-benzoylacrylic acids undergo palladium-catalyzed decarboxylative arylation with arylboronic acids in the presence of a copper salt oxidant to produce chalcone derivatives. The decarboxylative arylation could also be achieved using aryl halides as the alternative aryl source to expand the applicable scope.