Synthesis
Fe
3O
4@SiO
2-SnCl
4 as an efficient nano magnetic catalyst is a readily available catalyst. This catalyst is removed from the reaction medium by an external magnet. In our ongoing research for new synthetic methods for the development of efficient and environmentally friendly protocols for the synthesis of biologically important heterocyclic products, Pyrimido [4,5-
b]quinolin derivatives were synthesized. These heterocyclic compounds were synthesized by 6-amino-2-(methylthio)pyrimidin-4(3
H)-one (1), aryl aldehydes (2), dimedone or 1,3-cyclohexadione (3) and H
2O (5 mL) in the presence of a nanocatalytic of Fe
3O
4@SiO
2-SnCl
4 under ultrasound irradiation (
Scheme 1 and
Table 1). The products were characterized by their melting points using FT-IR,
1H, and
13C NMR spectroscopy.
The reusability of Fe3O4@SiO2-SnCl4 was examined for four times without any considerable decrease in its efficiency.
The mechanism of the reaction and formation of compound 4a can be explained by the condensation, addition, cyclization, and dehydration reactions. The plausible mechanism for the synthesis of pyrimido [4,5-
b]quinolones in the presence of Fe
3O
4@SiO
2-SnCl
4, which can act as Lewis acid catalyst is depicted in
scheme 2.
The carbonyl oxygen of aldehyde coordinates with the Lewis acid moiety increasing the electrophilicity of the carbonyl carbon and thereby making it possible to carry out the reaction in short time. In a plausible mechanism, it is assumed that the reaction may proceed initially through the Knoevenagel condensation between aryl aldehydes and dimedone or 1,3-cyclohexadione to form intermediate (I). Next, Michael addition of 6-aminouracil to intermediate (I) affords (II). Intermediate (II) converts to (III) after tautomerization (Fe
3O
4@SiO
2-SnCl
4 can also act as a mild base for the deprotonation of an acidic proton of Intermediate (II). Then, intermediate (III) converts to (IV)
via cyclization. Finally, the desired product (V) is obtained after dehydration of (IV) (
Scheme 2).
Antifungal activities of the synthetic compounds
In this study, compounds (
D1-D16) evaluated against fungi (
Table 2). None of the compounds have any effect on bacteria. Considering our results showed that compound (
D13) had the most antifungal activity against
C. dubliniensis, C. Albicans,
C. Tropicalis, and
C. Neoformance at concentrations ranging (MIC90) from 1-4 μg/mL. Compounds (
D9), (
D10), (
D14), and (
D15) had significant inhibitory activities against
C. dubliniensis at concentrations ranging (MIC90) from 4-8 μg/mL, respectively. In comparison of the antifungal activities of the synthetic compounds based on variation of substitutions on 2,3, and 4-position of phenyl ring, we found that the compound
D13 with OH residue in meta and para positions of phenyl ring exhibited a better antifungal activities against the tested fungi than the other compounds. Compounds (
D9) and (
D10), had significant inhibitory activities against
C. Dubliniensis, respectively. we found that the compound (
D9) with Br residue in para position of phenyl ring exhibited a better antifungal activity against the
C. Dubliniensis than the compound (
D10) with CH
3 residue in para position of phenyl ring.
Synthesis of pyrimido[4,5-b]quinolones in the presence of Fe3O4@SiO2
A proposed mechanism for preparation of pyrimido [4,5-b]quinolones in the presence of Fe3O4@SiO2-SnCl4
(a) The docked configuration of Fluconazole in the binding site of lanosterol 14α-demethylase (CYP51); (b) the docked configuration of D9; (c) the docked configuration of D10; and (d) the docked configuration of D13 in the binding site. Hydrogen bonds are shown as purple dotted lines
| Yield (%)a | Time (min) | Product | R | Ar | Entry |
|---|
| 98 | 75 | 4l | Me | 4-OMeC6H4 | D1 |
| 97 | 80 | 4m | Me | 3,4-OMe2C6H3 | D2 |
| 99 | 55 | 4c | Me | 2,4-Cl2C6H3 | D3 |
| 99 | 60 | 4b | Me | 4-NO2C6H4 | D4 |
| 96 | 90 | 4j | Me | C6H5 | D5 |
| 97 | 75 | 4i | Me | 4-OH-3-OMeC6H3 | D6 |
| 98 | 65 | 4g | Me | 4-CO2MeC6H4 | D7 |
| 99 | 50 | 4d | Me | 3,4,5-F3C6H2 | D8 |
| 98 | 65 | 4e | Me | 3-BrC6H4 | D9 |
| 97 | 80 | 4k | Me | 4-MeC6H4 | D10 |
| 99 | 60 | 4a | Me | 4-ClC6H4 | D11 |
| 98 | 60 | 4f | Me | 3-NO2C6H4 | D12 |
| 97 | 80 | 4h | Me | 3,4-OH2C6H3 | D13 |
| 99 | 70 | 4p | H | 4-NMe2C6H4 | D14 |
| 98 | 65 | 4o | H | 4-CO2MeC6H4 | D15 |
| 98 | 60 | 4n | H | 2-ClC6H4 | D16 |
| Compounds | concentration | Fungi |
|---|
| D16 | D15 | D14 | D13 | D12 | D11 | D10 | D9 | D8 | D7 | D6 | D5 | D4 | D3 | D2 | D1 |
|---|
| - | - | - | 4 | - | - | - | - | - | - | - | - | - | - | - | - | MIC90 | C.albicans |
| - | - | - | 16 | - | - | - | - | - | - | - | - | - | - | - | - | MFC |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MIC90 | C.glabrata |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MFC |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MIC90 | C.krusei |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MFC |
| - | - | - | 1 | - | - | - | - | - | - | - | - | - | - | - | - | MIC90 | C.tropicalis |
| - | - | - | G | - | - | - | - | - | - | - | - | - | - | - | - | MFC |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MIC90 | C.parapsilosis |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MFC |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MIC90 | Exophilia |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MFC |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MIC90 | A.clavatus |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MFC |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MIC90 | A.fumigatus |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MFC |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MIC90 | A.flavus |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MFC |
| - | - | - | 2 | - | - | - | - | - | - | - | - | - | - | - | - | MIC90 | C.neoformans |
| - | - | - | 2 | - | - | - | - | - | - | - | - | - | - | - | - | MFC |
| - | 32 | 8 | 4 | - | - | 4 | 2 | - | - | - | - | - | - | - | - | MIC90 | C.dubliniensis |
| - | G | 8 | 4 | - | - | 4 | 2 | - | - | - | - | - | - | - | - | MFC |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MIC90 | S.aurous |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MFC |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MIC90 | E.coli |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MFC |
| - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | - | MIC90 | E.fecalis |
| Compound | Binding Energy (Kcal/mol)* |
|---|
| D9 | -8.04 |
| D10 | -7.59 |
| D13 | -8.86 |
| Fluconazol | -6.47 |
Docking study of active compounds
Docking is a method, frequently used to find the binding orientation of small molecule drug candidates to their protein targets in order to predict and interpret the affinity and activity of the small molecule. We performed molecular docking studies on
D9, D10, D13, and
Fluconazole to find, compare and validate their binding site, binding modes, and their best direction according to their binding energy (
Figures 1). Once the docking procedure was completed, to find the type of interactions, the protein–ligand complex was studied. All the docking protocols were done on validated structures, with RMSD values below 2 Å. The conformation with the lowest ones was considered as the best docking result. Docking binding energies of these active compounds were summarized in
Table 3. Our results indicated that overall there is a good correlation between experimental pIC50 and docking binding energy (
Figure 1). As displayed in
Table 3, all investigated complexes showed better docking binding energies than the co-crystal ligands (fluconazole)
. Consistent with our biological results,
D13 was observed to have the best docking binding energies with the receptor through polar interaction with phe 255, His 259, and Thr 260 which responds to the key ineractions of pharmacophoric elements in the ligands (
Figures 1).
The docking model also indicated the specific alignment of fused three ring system to the pocket of heme iron of CYP51 in each ligand with similar positioning of triazole ring of fluconazole. This orientation with respect to the hydrogen bonding and hydrophobic interactions may be in favor of antifungal activity.