Process [17]. Firstly, we tested the enzymatic inhibition rate of many naturally occurring naphthoquinones (L-type calcium channel Inhibitor drug juglone 2, 7-methyl juglone 16, lawsone 7, plumbagin 17 and shikonin 1), 9,10-anthraquinones (emodin 18, rhein 19 and aloe emodin 20) plus the synthetic vitamin K3 (three) in the very first library of compounds against SARS-CoV-2 Mpro at the concentration of 10 mM. The outcomes from main screening indicated that most of the natural quinones have been ineffective, with the inhibition rate of much less than 10 at ten mM (Table S1, Supplementary Details). Vitamin K3 (three) with the inhibition price of 12.7 was also inactive. The organic naphthoquinone shikonin, which had been identified as 1 of theFig. 1. The chemical structure of shikonin (1), juglone (2) and menadione (three).J. Cui and J. JiaEuropean Journal of Medicinal Chemistry 225 (2021)Scheme 1. Reagents and CB1 Antagonist medchemexpress conditions: a) CH3COOH, H2SO4, H2O2, 80 C, 3 h; b) (CH3CO)2O, H2SO4, ten C, eight h; c) CH3ONa, CH3OH, 5 C; then conc. HCl; d) CrO3, CH3COOH, 40 C for 30 min, then 65 C for 20 min.Scheme two. Reagents and conditions: e) (CH3CO)2O, H2O2, 40e60 C, 1 h; f) (CH3)2SO4, NaOH, Na2S2O4, Et2O/H2O, 5 C, overnight; g) CAN, DCM-ACN (three:1), five C; h) (CH3CH2CO)2O, Cat. Conc. H2SO4, five C, four h; i) (CH3CO)2O, Cat. Conc. H2SO4, 5 C, 4 h; j) Na2S2O4, Et2O/H2O, r.t., 2 h; then CH3I, K2CO3, DMF, 10 C, overnight; k) BF3eEt2O, 60 C, 0.five h; l) CAN, DCMCAN (3:1), five C, 0.5 h; m) NBS, ACN, 0 C, overnight; then CH3ONa, CuI, CH3OH-DMF, reflux, 48 h.sturdy Mpro inhibitors in earlier studies (IC50 15.75 eight.22 mM) [17], was employed as the constructive handle. It demonstrated moderate inhibitory effects towards the target enzyme at the concentration of 10 mM. Within the first library of naphthoquinones, juglone (two) and 7-methyl juglone (16) exhibited the strongest inhibition together with the entirely loss in the hydrolytic efficacy of Mpro. The two organic naphthoquinones have been employed because the lead compounds for additional structural modifications. In the second library, the derivatives of juglone (2) and 7-methyl juglone (16) were made by the addition of some groups on their naphthoquinone scaffold and modifications around the phenolic hydroxyl group around the B-ring. The enzyme inhibition price of compounds in the second library was displayed in Table S2. The results implied that practically all of the derivatives within the second library maintained the high inhibitory potency of juglone beneath concentrations of each ten mM and 1 mM. In the concentration of 0.1 mM, a few analogues exhibited significantly higher potency as compared with the parent compounds (two and 16). Then, the compounds with an enzymatic inhibition price of more than 25 in the concentration of0.1 mM entered the IC50 worth screening (Table S3). As shown in Table S3, within the tested synthetic 1,4naphthoquinones as sturdy Mpro inhibitors, 2-acetyl-8-methoxy1,4-naphthoquinone (15) was characterized because the most potent inhibitor against the target enzyme with its IC50 worth of 72.07 4.84 nM, which was comparable towards the not too long ago reported IC50 value of a short peptide as SARS-CoV-2 Mpro inhibitor (IC50 53 five nM) [17]. The 1,4-naphthoquinone (5) and propionyl juglone (11) have also been identified as potent inhibitors with IC50 worth of 110.13 7.04 and 129.77 0.45 nM, respectively. 7-Methyl juglone ethyl acetate (23) and its benzyl ether (25) exhibited a lot greater IC50 values than propionyl juglone did. Structure-activity relationship research. Inside the 1st library of compounds (Table S1),.
