Exploring Antimicrobial Features for New Imidazo[4,5-b]pyridine Derivatives Based on Experimental and Theoretical Study.

5-bromopyridine-2,3-diamine reacted with benzaldehyde to afford the corresponding 6-Bromo-2-phenyl-3H-imidazo[4,5-b]pyridine (1). The reaction of the latter compound (1) with a series of halogenated derivatives under conditions of phase transfer catalysis solid-liquid (CTP) allows the isolation of the expected regioisomers compounds (2-8). The alkylation reaction of (1) gives, each time, two regioisomers, N3 and N4; in the case of ethyl bromoactate, the reaction gives, at the same time, the three N1, N3 and N4 regioisomers. The structures of synthesized compounds were elucidated on the basis of different spectral data (1H NMR, 13C NMR), X-Ray diffraction and theoretical study using the DFT method, and confirmed for each compound. Hirshfeld surface analysis was used to determine the intermolecular interactions responsible for the stabilization of the molecule. Density functional theory was used to optimize the compounds, and the HOMO-LUMO energy gap was calculated, which was used to examine the inter/intra molecular charge transfer. The molecular electrostatic potential map was calculated to investigate the reactive sites that were present in the molecule. In order to determine the potential mode of interactions with DHFR active sites, the three N1, N3 and N4 regioisomers were further subjected to molecular docking study. The results confirmed that these analogs adopted numerous important interactions, with the amino acid of the enzyme being targeted. Thus, the most docking efficient molecules, 2 and 4, were tested in vitro for their antibacterial activity against Gram-positive bacteria (Bacillus cereus) and Gram-negative bacteria (Escherichia coli). Gram-positive bacteria were more sensitive to the action of these compounds compared to the Gram-negative, which were much more resistant.

Methyl 2-(4a,8-Dimethyl-7-oxodeca-hydro-naphthalen-2-yl)acrylate.

The title compound, C(16)H(24)O(3), was isolated from the aerial part of Inula Viscosa- (L) Aiton [or Dittrichia Viscosa- (L) Greuter]. The mol-ecule contains two fused (trans) six-membered rings which both exibit a chair conformation. In the crystal, mol-ecules are linked into chains along [100] by weak C-H⋯O hydrogen bonds involving the methyl and carbonyl groups.

Methyl 2-(1a,4a-dimethyl-2,8-dioxo-2,3,4,4a,5,6,7,8-octa-hydro-1aH-1-oxacyclo-propa[d]naphthalen-7-yl)acrylate.

The title compound, C(16)H(20)O(5), was synthesized from ilicic acid [2-(8-hy-droxy-4a,8-dimethyl-deca-hydro-naphthalen-2-yl)acrylic acid], which was isolated from the chloro-form extract of the aerial part of Inula viscose (L) Aiton [or Dittrichia viscosa- (L) Greuter]. The molecule is built up from two fused six-membered rings, the epoxidized six-membered ring adopts a half-chair conformation while the other ring displays a perfect chair conformation. The crystal structure features C-H⋯O hydrogen bonds.

Methyl 2-(8a-hy-droxy-4a-methyl-8-methyl-enedeca-hydro-naphthalen-2-yl)acrylate.

The title compound, C(16)H(24)O(3), was synthesized from ilicic acid which was isolated from the aerial part of Inula Viscosa- (L) Aiton [or Dittrichia Viscosa- (L) Greuter]. The mol-ecule contains two fused six-membered rings both in chair conformations. In the crystal, mol-ecules are linked into chains running parallel to the a axis by O-H⋯O hydrogen bonds.

Methyl 2-(3a,8a-dimethyl-4-oxodeca-hydro-azulen-6-yl)acrylate.

The title compound, C(16)H(24)O(3), was synthesized from ilicic acid, which was isolated from the aerial part of Inula viscosa- (L) Aiton [or Dittrichia viscosa- (L) Greuter]. The asymmetric unit contains two independent mol-ecules, in each of which the seven-membered ring shows a chair conformation, whereas the five-membered ring presents disorder. In the two molecules, three C atoms in the five-membered ring are disordered over two positions with site-occupancy factors of 0.53/0.47 and 0.83/0.17. The dihedral angle between the two rings is different in the two mol-ecules [31.7 (3) and 47.7 (7)°]. The crystal structure is stabilized by weak inter-molecular C-H⋯O hydrogen-bond inter-actions.

1-(4a,8-Dimethyl-1,2,3,4,4a,5,6,8a-octa-hydro-naphthalen-2-yl)-3-phenyl-prop-2-en-1-one.

The title compound, C(21)H(26)O, was semisynthesized from isocostic acid, isolated from the aerial part of Inula Viscosa- (L) Aiton [or Dittrichia Viscosa- (L) Greuter]. The cyclo-hexene ring has a half-chair conformation, whereas the cyclo-hexane ring displays a chair conformation.

3-(4-Chloro-phen-yl)-1-[(2R,4aR,8aR)-4a,8-dimethyl-1,2,3,4,4a,5,6,8a-octa-hydro-naphthalen-2-yl]prop-2-en-1-one.

The title compound, C(21)H(25)ClO, was semi-synthesized from isocostic acid, isolated from the aerial part of Inula Viscosa- (L) Aiton [or Dittrichia Viscosa- (L) Greuter]. The cyclo-hexene ring has a half-chair conformation, whereas the cyclo-hexane ring displays a chair conformation.

1-(4a,8-Dimethyl-1,2,3,4,4a,5,6,8a-octa-hydro-naphthalen-2-yl)-3-(4-methyl-phen-yl)prop-2-en-1-one.

The title compound, C(22)H(28)O, was isolated from the aerial part of Inula viscosa- (L) Aiton [or Dittrichia viscosa- (L) Greuter]. The cyclo-hexene ring has a half-chair conformation, whereas the cyclo-hexane ring displays a chair conformation being substituted at position 2 by a 3-(4-methyl-phen-yl)prop-2-enoyl group. In the crystal, weak inter-molecular C-H⋯O hydrogen bonds link mol-ecules into chains in the [010] direction.

1-[(2R,4aR,8R,8aR)-8-Hy-droxy-4a,8-di-methyl-perhydronaphthalen-2-yl]ethan-1-one.

The title compound, C(14)H(24)O(2), was synthesized from ilicic acid, which was isolated from the aerial part of Inula Viscosa- (L) Aiton [or Dittrichia Viscosa- (L) Greuter]. The mol-ecule contains two fused six-membered rings, which both display a chair conformation. In the crystal, mol-ecules are linked into chains propagating along the b axis by inter-molecular O-H⋯O hydrogen bonds.

α-Costic anhydride.

THE TITLE COMPOUND [SYSTEMATIC NAME: 2-(4a,8-dimethyl-1,2,3,4,4a,5,6,8a-octa-hydro-naphthalen-2-yl)acrylic acid anhydride], C(30)H(42)O(3), is a new isocostic anhydride which was synthesized from the aerial part of Inula Viscosa- (L) Aiton [or Dittrichia Viscosa- (L) Greuter]. The mol-ecule adopts an essentially linear shape with two terminal fused-rings bridged by the anhydride group. The external rings have the same conformation (half-chair) while each of the two inner rings has an almost ideal chair conformation. In the crystal, inter-molecular C-H⋯O inter-actions link the mol-ecules into a two-dimensional array in the bc plane.

3-[4-(Dimethyl-amino)-phen-yl]-1-(4a,8-dimethyl-1,2,3,4,4a,5,6,8a-octa-hydro-naphthalen-2-yl)prop-2-en-1-one.

The title compound, C(23)H(31)NO, was semisynthesized from isocostic acid, isolated from the aerial part of Inula Viscosa- (L) Aiton [or Dittrichia Viscosa- (L) Greuter]. The cyclo-hexene ring has a half-chair conformation, whereas the cyclo-hexane ring displays a chair conformation. The dihedral angle between the latter ring and its substituent is 83.6 (7)°.