Design and synthesis of novel main protease inhibitors of COVID-19: quinoxalino[2,1-b]quinazolin-12-ones

The COVID-19 pandemic represents a substantial global challenge, being a significant cause of mortality in numerous countries. Thus, it is imperative to conduct research to develop effective therapies to combat COVID-19. The primary aim of this study is to employ a two-step tandem reaction involving 2,3-dichloroquinoxaline and 2-amino-N-substituted benzamides in alkaline media/DMF at an elevated temperature to design and synthesize a series of polycyclic derivatives endowed with quinoxalino[2,1-b]quinazolin-12-one framework. Following synthesis, the newly synthesized heterocycles were evaluated for their potential as inhibitors of the main protease of SARS-CoV-2 by means of molecular docking and dynamic simulation techniques. The in silico investigation demonstrated that all tested compounds effectively establish stable binding interactions, primarily through multiple hydrogen bonding and hydrophobic interactions, at the active site of the enzyme. These findings offer crucial structural insights that can be employed in future endeavors toward designing potent inhibitors targeting the main protease (Mpro). Among the investigated compounds, the p-tolylamino-substituted quinoxalino[2,1-b]quinazolinone derivative exhibited the most promise as an inhibitor of the main protease in COVID-19. Consequently, it warrants further investigation both in vitro and in vivo to identify it as a prospective candidate for anti-SARS-CoV-2 drug development.

Domino protocol for the synthesis of diversely functionalized derivatives of a novel fused pentacyclic antioxidant/anticancer fluorescent scaffold: Pyrazolo[5'',1'':2',3']pyrimido[4',5':5,6][1,4]thiazino[2,3-b]quinoxaline

Rising to the challenge of formidable multi-step reaction needed for the synthesis of polycyclic compounds, an efficient one-pot two-step procedure for the synthesis of densely functionalized novel pyrazolo[5″,1'':2',3']pyrimido[4',5':5,6] [1,4]thiazino[2,3-b]quinoxalines from synthetically accessible starting materials 6-bromo-7-chloro-3-cyano-2-(ethylthio)-5-methylpyrazolo[1,5-a]pyrimidine, 3-aminoquinoxaline-2-thiol and some readily accessible alkyl halides was established. The domino reaction pathway involves cyclocondensation/N-alkylation sequence in K₂CO₃/N,N-dimethyl formamide under heating condition. DPPH free radical scavenging activity of all synthesized pyrazolo[5″,1'':2',3']pyrimido[4',5':5,6][1,4]thiazino[2,3-b]quinoxalines was evaluated to determine their antioxidant potentials. IC₅₀ values were recorded in the range of 29-71 μM. N-benzyl substituted derivative represented the most effective antioxidant activity as well as antiproliferative activity against MCF-7 cells. Moreover, fluorescence in solution for these compounds exhibited strong red emission in the visible region (λ_flu. = 536-558 nm) with good to excellent quantum yields (61-95%). Due to their interesting fluorescence properties, these novel pentacyclic fluorophores can be used as fluorescent markers and probes for studies in biochemistry and pharmacology.

Synthesis, molecular docking and dynamics studies of pyridazino[4,5-b]quinoxalin-1(2H)-ones as targeting main protease of COVID-19

The COVID-19 pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has created a crisis in public health. Because, the 3CLpro, the main protease of SARS-CoV-2, possesses a critical role in coronavirus replication, many efforts have been devoted to developing various inhibitors to prevent the fast spread of COVID-19. In the current work, a number of various pyridazino[4,5-b]quinoxalin-1(2H)-one derivatives bearing thiadiazine and thiadiazole fragments has been prepared via a straightforward and practical strategy involving the reaction of 2-(ethoxycarbonyl)-3-formylquinoxaline 1,4-dioxide with thiocarbohydrazide under reflux conditions. To determine the bioavailability of pyridazino[4,5-b]quinoxalin-1(2H)-one derivatives, Lipinski's rule of five has been carried out. Regarding this rule, none of the synthesized compounds exhibit any deviation from Lipinski's rule of five. Furthermore, molecular docking and molecular dynamics approaches have been implemented to figure out the potential interactions of products with SARS-CoV-2 main protease. The outcomes of molecular docking studies demonstrate that the phenyl and nitrophenyl substituted pyridazino[4,5-b]quinoxalin-1(2H)-one show the lowest binding affinity among the other compounds, indicating a favorable orientation in the active site of the chymotrypsin-like cysteine protease. In addition, the MD simulation performed to evaluate the stability of the protein-ligand complex represents that the average binding energy of the nitrophenyl complex is less than that of the phenyl complex. Therefore, according to the in silico results, the inhibitory effect of the nitrophenyl complex is more significant than the phenyl complex.Communicated by Ramaswamy H. Sarma.

Pyrido[1,2-e]purine: Design and Synthesis of Appropriate Inhibitory Candidates against the Main Protease of COVID-19

A series of tricyclic and polycyclic pyrido[1,2-e]purine derivatives were designed and synthesized via a two-step, one-pot reaction of 2,4-dichloro-5-amino-6-methylpyrimidine with pyridine under reflux conditions. Various derivatives of pyrido[1,2-e]purine were also synthesized by substituting the chlorine atom with secondary amines. After careful physiochemical and pharmacokinetic predictions, the inhibitory effects of the synthesized compounds against the main protease of SARS-CoV-2 have been evaluated by molecular docking and molecular dynamics approaches. The in silico results revealed that among all of the studied compounds, the morpholine/piperidine-substituted pyrido[1,2-e]purine derivatives are the best candidates as effective inhibitors of SARS-CoV-2.

A Dimroth rearrangement approach for the synthesis of selenopheno[2,3-e][1,2,4]triazolo[1,5-c]pyrimidines with cytotoxic activity on breast cancer cells

New selenopheno[2,3-e][1,2,4]triazolo[1,5-c]pyrimidine derivatives have been synthesized via Dimroth rearrangement by cyclocondensation of 7-cyano-4-hydrazinyl-6-(pyrrolidin-1-yl)selenopheno[3,2-d]pyrimidine with electrophilic carbons of either orthoesters in acetic acid or carbon disulfide in pyridine followed by S-alkylation. All the newly synthesized products have been structurally elucidated. The in vitro anticancer screening of the tricyclic Se-containing heterocycles was accomplished against human breast carcinoma MCF-7 cancerous cell line and L929 cells. Anticancer results revealed that the S-hexyl-substituted compound with an IC₅₀ value of 158.9 µM in 72 h was foremost among others in cytotoxic potency. In the following order, S-pentyl and S-ethyl-substituted derivatives with IC₅₀ values of 216.1 and 396.5 µM were second and third efficient compounds as in anticancer activity, respectively. The inhibitory effects of the mentioned compounds were less on the growth of L929 cells.

Dimroth rearrangement-based synthesis of novel derivatives of [1,3]selenazolo[5,4-e][1,2,4]triazolo[1,5-c]pyrimidine as a new class of selenium-containing heterocyclic architecture

As a part of our ongoing endeavor towards developing novel heterocyclic architectures, a number of novel Se-containing tricyclic heterocycles of the type [1,3]selenazolo[5,4-e][1,2,4]triazolo[1,5-c]pyrimidine have been synthesized through heteroannulation of a newly produced hydrazino derivative of selenazolo[4,5-d]pyrimidine with either orthoesters or carbon disulfide in pyridine followed by S-alkylation. Moreover, the multistep protocol employed in this investigation provides a new insight into the Dimroth rearrangement in both acidic and basic media as a means for the cyclocondensation of triazole on the selenazolopyrimidine framework leading to selenazolotriazolopyrimidines. The synthesis of new derivatives of novel selenazolotriazolopyrimidines via Dimroth rearrangement in both acidic and basic media is presented.

A New Family of Fused Azolo[1,5-a]pteridines and Azolo[5,1-b]purines

The nitration of azolo[1,5-a]pyrimidin-7-amines with several nitration agents (such as acetic nitric anhydride, nitronium tetrafluoroborate, and a mixture of concentrated nitric acid and sulfuric acid) has been investigated. It has been shown that, depending on the conditions, the nitration of pyrazolopyrimidin-7-amines bearing electron-withdrawing groups in the pyrazole ring leads to nitration products in the pyrimidine and/or pyrazole ring. The nitration of triazolo[1,5-a]pyrimidin-7-amines with "nitrating mixture" has been optimized, thus allowing us to obtain a series of 6-nitro[1,2,4]triazolo[1,5-a]pyrimidin-7-amines, followed by their reduction into the corresponding [1,2,4]triazolo[1,5-a]pyrimidin-6,7-diamines (yields 86-89%). The latter have been subjected to heterocyclization by a variety of electrophilic compounds (such as CS₂, glyoxal, triethyl orthoformate) with the formation of five- or six-membered annulated cycles.