Cysteine and C-terminal cysteine peptides are selectively S-acylated at 0-20 degrees C by N-(Pg-alpha-aminoacyl)benzotriazoles to give N-Pg-S-acyl-isodi-, -isotri-, and -isotetra-peptides isolated in good yields. N-Fmoc-S-acyl-isopeptides are Fmoc deprotected to afford free S-acyl-isopeptides isolated in high yields. S-Acyl-isodi-, S-acyl-isotetra-, and S-acyl-isopentapeptides undergo chemical ligation; migration of the cysteine S-acyl groups to the N-terminal amino acids via 5-, 11-, and 14-membered transition states giving the corresponding native di-, tetra-, and penta-peptides. By contrast, the Sacyl-isotripeptide prefers intermolecular acylation from one molecule to another over an 8-membered intramolecular transition state. The developed methodology allows convenient isolation of stable, unprotected S-acyl cysteine peptides including the first isolation of S-acyl-isopeptides, which should facilitate the investigation of ligation by physical organic chemistry techniques.

N-(Acyl)-1H-benzotriazoles 6a-f react with l-cysteine 5 at 20 degrees C to give exclusively (i) N-acyl-l-cysteines 8a-e in the presence of triethylamine in CH(3)CN-H(2)O (3:1), but (ii) S-acyl-l-cysteines 7a-e in CH(3)CN-H(2)O (5:1) in the absence of base. Structures 7b, 7d and 8b, 8d are supported by 2D NMR spectroscopic methods including gDQCOSY, gHMQC, gHMBC, and (1)H-(15)N CIGAR-gHMBC experiments. The structure of compound 8d was also supported by single-crystal X-ray diffraction.

Graphical abstract A series of quinolone-triazole conjugates synthesized as potential antiviral agents for SARS-CoV2. Some of the conjugates are more potent than the standards.

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A series of new fluoroquinolone conjugates 8a-g and 9a-f were synthesized via benzotriazole-mediated synthetic approach with good yield and purity. Some of the synthesized analogs exhibited significant antibacterial properties against Escherichia coli and Staphylococcus aureus with potency higher than that of the parent drugs through in vitro standard bioassay procedure (conjugates 8c and 8d reveal antimicrobial properties with potency 1.9, 61.9, 20.7 and 2.4, 37.1, 8.3 folds relative to the parent antibiotic 6 against E. coli, S. aureus, and Enterococcus faecalis, respectively). The observed experimental data were supported by enzymatic DNA gyrase inhibitory property. Developed BMLR-QSAR model validates the observed experimental data and recognizes the parameters responsible for the enhanced antibacterial properties. K E Y W O R D S antibacterial, dichloroacetic acid, DNA gyrase, fluoroquinolone, hybrid conjugates F I G U R E 1 Examples of different class of antibiotics 250 | SELIEM Et aL.to microwave irradiation (Discover mode; run time: 60 s; Power Max-cooling mode). N-(Boc-aminoacyl)benzotriazoles 10a-g (Panda, Hall, et al., 2014; Panda, Naumov, et al., 2014) Compounds 10a-g were synthesized by irradiating an equimolar amount of Boc-protected amino acid with 1-(methylsulfonyl)-1H-benzo[d] [1,2,3]triazole (BtSO 2 Me) in the presence of 2.0 eq. of triethylamine for 2 min run time and 60 min hold time at 70°C and 50 W irradiation power. Completion of the reaction was monitored by TLC. After completion of the reaction, the mixture was quenched with water. The product obtained was extracted with ethyl acetate and then washed with a saturated solution of sodium carbonate and water to afford compound 10a-g. | General procedure for the preparation of