New Triazole-Isoxazole Hybrids as Antibacterial Agents: Design, Synthesis, Characterization, In Vitro, and In Silico Studies.

Novel isoxazole-triazole conjugates have been efficiently synthesized using 3-formylchromone as starting material according to a multi-step synthetic approach. The structures of the target conjugates and intermediate products were characterized by standard spectroscopic techniques (1H NMR and 13C NMR) and confirmed by mass spectrometry (MS). The all-synthesized compounds were screened for their antibacterial activity against three ATCC reference strains, namely Staphylococcus aureus ATCC 25923, Staphylococcus aureus ATCC BAA-44, and Escherichia coli ATCC 25922 as well as one strain isolated from the hospital environment Pseudomonas aeruginosa. The findings indicate that conjugate 7b exhibits a stronger antibacterial response against the tested Escherichia coli ATCC 25922 and Pseudomonas aeruginosa pathogenic strains compared to the standard antibiotics. Furthermore, hybrid compound 7b proved to have a bactericidal action on the Escherichia coli ATCC 25922 strain, as evidenced by the results of the MBC determination. Moreover, the ADMET pharmacokinetic characteristics revealed a favorable profile for the examined compound, as well as a good level of oral bioavailability. Molecular docking and molecular dynamics simulations were performed to explore the inhibition mechanism and binding energies of conjugate 7b with the proteins of Escherichia coli and Pseudomonas aeruginosa bacterial strains. The in silico results corroborated the data observed in the in vitro evaluation for compound 7b.

Chromone-isoxazole hybrids molecules: synthesis, spectroscopic, MEDT, ELF, antibacterial, ADME-Tox, molecular docking and MD simulation investigations.

A mechanistic study was performed within the molecular electron density theory at the B3LYP/6-311G (d,p) computational level to explain the regioselectivity observed. An electron localization function analysis was also performed, and the results confirm the zwitterionic-type (zw-type) mechanism of the cycloaddition reactions between nitrile oxide and alkylated 4H-chromene-2-carboxylate derivatives and shed more light on the obtained regioselectivity experimentally. In silico studies on the pharmacokinetics, ADME and toxicity tests of the compounds were also performed, and it was projected that compounds 5a, 5b, 5c and 5d are pharmacokinetic and have favorable ADME profiles. Moreover, docking and molecular dynamics investigations were conducted to evaluate the interactions, orientation and conformation of the target compounds on the active sites of four distinct enzymes. The results of this investigation showed that two compounds, 5a and 5c, interacted effectively with the S. aureus active site while maintaining acceptable binding energy.Communicated by Ramaswamy H. Sarma.

Crystal structure of 3,4'-diphenyl-3'-p-tolyl-4'H-spiro-[indan-2,5'-[1,2]oxazol]-1-one.

In the title compound, C30H23NO2, the five-membered rings are both in envelope conformations with the same spiro C atom as the flap. The benzene ring and the two phenyl rings are inclined to the mean plane of the indene ring system by 83.98 (8), 81.46 (8) and 72.31 (7)°. In the crystal, mol-ecules are linked by pairs of C-H⋯O hydrogen bonds into inversion dimers. The dimers are further connected by C-H⋯N inter-actions, forming layers parallel to (10-1).

Crystal structure of (E)-4-ethyl-2-(4-meth-oxy-benzyl-idene)-3,4-di-hydro-naphthalen-1(2H)-one.

In the title compound, C20H20O2, the exocyclic C=C double bond has an E conformation. The ethyl substituent on the cyclo-hexa-none ring is in an axial orientation. The cyclo-hexa-none ring adopts a screw-boat conformation, with the methyl-ene C atom and the C atom bearing the 4-meth-oxy-benzyl-idene group displaced from the other atoms by 0.812 (1) and 0.334 (1) Å, respectively. The dihedral angle between the planes of the benzene rings is 42.20 (8)°. In the crystal, no directional inter-actions beyond van der Waals contacts are observed.

Crystal structure of 4-(4-meth-oxy-phen-yl)-4',4'-dimethyl-3-p-tolyl-3',4'-di-hydro-1'H,3H-spiro-[isoxazole-5,2'-naphthalen]-1'-one.

In the title compound, C28H27NO3, the cyclo-hexa-none and isoxazole rings have envelope conformations, with the methyl-ene and spiro C atoms as the flaps, respectively. The mean plane of the isoxazole ring is inclined slightly to the p-tolyl ring, making a dihedral angle of 14.20 (9)°, and is nearly perpendicular to the mean plane through the tetra-lone moiety and to the meth-oxy-phenyl ring [dihedral angles = 83.41 (8) and 72.12 (9)°, respectively]. The crystal packing is stabilized mainly by van der Waals forces.

Ethyl 2-amino-4-(3-nitro-phen-yl)-4H-1-benzothieno[3,2-b]pyran-3-carboxyl-ate.

The mol-ecule of the title compound, C20H16N2O5S, is built up by one fused five-membered and two fused six-membered rings linked to eth-oxy-carbonyl and 3-nitro-phenyl groups. The benzothieno-pyran ring system is nearly planar (r.m.s deviation = 0.0392 Å) and forms a dihedral angle of 86.90 (6)° with the aromatic ring of the nitro-benzene group. In the crystal, mol-ecules are linked by N-H⋯O hydrogen bonds and by π-π inter-actions between the phenyl ring and the six-membered heterocyle [inter-centroid distance = 3.5819 (8) Å], forming a three-dimensional network.

(2E)-2-Benzyl-idene-4-ethyl-3,4-dihydro-naphthalen-1(2H)-one.

In the title compound, C(19)H(18)O, the exocyclic C=C double bond has an E configuration. The ethyl substituent on the cyclo-hexa-none ring is in an axial position. The cyclo-hexa-none ring adopts a half-chair conformation, presumably due to conjugation in the benzene ring.

(2E)-4-tert-Butyl-2-(4-meth-oxy-benzyl-idene)-3,4-dihydro-naphthalen-1(2H)-one.

In the title compound C(22)H(24)O(2), the exocyclic C=C double bond is in an E configuration and the tert-butyl group is in an axial position on the cyclo-hexa-none ring. The cyclo-hexa-none ring in the dihydro-naphthalene fused-ring system adopts a half-chair conformation in both independent two mol-ecules in the asymetric unit. The benzene ring system is oriented angles of 43.97 (12) and 39.24 (12)° with respect to the naphthyl ring system in the two independent mol-ecules. In the crystal, mol-ecules are linked via C-H⋯O hydrogen bonds and C-H⋯π inter-actions.

4-tert-Butyl-3',4'-bis-(4-methyl-phen-yl)-3,4-dihydro-1H,4'H-spiro-[naphthalene-2,5'-[1,2]oxazol]-1-one.

In the title compound, C(30)H(31)NO(2), the cyclo-hexa-none ring in the naphthalene fused-ring system adopts a half-chair conformation, presumably due to conjugation of the benzene ring. The naphthalene ring system makes dihedral angles of 86.63 (7), 65.15 (8) and 63.18 (8)° with respect to the two methyl-benzene planes and the 1,2-oxazole ring system. Inter-molecular C-H⋯O and C-H⋯N hydrogen bonding and C-H⋯π inter-actions stabilize the crystal structure. The H atoms of the two methyl groups of the methyl-phenyl groups are disordered over two positions with equal occupancies.

4-tert-Butyl-4'-(4-meth-oxy-phen-yl)-3'-(4-methyl-phen-yl)-1,2,3,4-tetra-hydro-spiro-[naphthalene-2,5'(4'H)-1,2-oxazol]-1-one.

In the title compound, C(30)H(31)NO(3), the tolyl ring is almost coplanar with the isoxazole ring [dihedral angle = 12.51 (7)°], whereas the meth-oxy-phenyl ring is almost perpendicular to the isoxazole ring [dihedral angle = 89.77 (5)°]. In the crystal, mol-ecules are connected through C-H⋯O hydrogen bonds, forming chains running along the a axis.

2-(4-Meth-oxy-benzyl-idene)-4,4-dimethyl-3,4-dihydro-naphthalen-1(2H)-one.

The title compound C(20)H(20)O(2), has the exocyclic C=C double bond in an E configuration. The two benzene rings form a dihedral angle of 72.92 (6)°.