Synthesis and Biological Activity of Some New Pyrazoline and Pyrrolo[3,4-c]pyrazole-4,6-dione Derivatives: Reaction of Nitrilimines with Some Dipolarophiles

Several 1,3-diaryl-5-(cyano-, aminocarbonyl- and ethoxycarbonyl-)-2-pyrazoline, pyrrolo[3,4-c]pyrazole-4,6-dione and 1,3,4,5-tetraaryl-2-pyrazoline derivatives were prepared by the reaction of nitrilimine with different dipolarophilic reagents. The new compounds were characterized using IR, 1H-NMR, 13C-NMR and mass spectra. Biological screening of some compounds is reported.


Results and Discussion
The reactivity of the nitrilimines 2a,b, generated in situ by base catalyzed elimination of hydrogen bromide from hydrazonoyl bromides 1a,b, with various mono-, di-and tri-substituted dipolarophiles was examined. Thus, heating of equimolar amounts of 1a,b and each of acrylonitrile, acrylamide or ethyl acrylate in dry benzene in the presence of triethylamine gave exclusively 3-aryl-1-(4nitrophenyl)-5-substituted-2-pyrazolines 3a,b-5a,b, respectively, in good yields (Scheme 1).

Scheme 1.
The assigned 5-substituted-2-pyrazoline structures 3-5 were supported by elemental analyses and spectroscopic data (see Experimental). The chemical shifts of the methine and methylene hydrogens in the 1 H-NMR spectra of 3-5 compared favorably with those reported for the corresponding protons in 1-aryl-3-(2-naphthoyl)-5-substituted-2-pyrazolines [21]. Such similarity, while confirming the assigned structures, indicates that both substituents, the 4-fluorophenyl-and 2,4-dichlorophenyl-, have similar effects on the chemical shifts of the methylene protons at C-4 of substituted 2-pyrazoline derivatives, compared with the 2-naphthoyl-group. The structures of 3a,b were also confirmed by the absence of the nitrile absorption in the IR spectra, as observed in the case of aliphatic nitriles activated by a nitrogen or an oxygen atom in the β-position [28,29]. The IR spectra of 4a,b showed the C=O amide absorption band at υ 1680 cm -1 and NH 2 absorption bands at υ 3420 and 3300 cm -1 . The IR of 5a,b exhibited an ester carbonyl absorption band near υ 1730 cm -1 . The 13 C-NMR spectrum of 5b displays the characteristic signals of the suggested structure. The signal for the carbonyl carbon of ester group appears at δ 169.4 ppm, the signals for pyrazoline C-4 at δ 39.9, pyrazoline C-5 at δ 59.9, the ester group methylene carbon at δ 61.6 and ester group methyl carbon at δ 13.8 ppm (see Figure 1 for the carbon numbering).

Figure 1.
Treatment of each of the hydrazonoyl bromides 1a and 1b with N-arylmaleimides 6a-g in refluxing chloroform in the presence of triethylamine gave the corresponding cycloadducts 7a-n, respectively (Scheme 2).

Scheme 2.
The structure of the latter products was elucidated on the basis of their elemental analyses and the study of their spectroscopic data (see Experimental). The IR spectra of products 7 exhibit characteristic bands near υ 1720 cm -1 , assignable to a carbonyl group. The 1 H-NMR spectra of all compounds showed, in each case, two doublets near δ 5.5 and 5.8 ppm, assignable to the protons at C-3a and C-6a, respectively. The vicinal coupling constants have been shown to be diagnostic, J trans < (6 Hz), J cis (9)(10)(11)(12) [30,31], so the observed values of the coupling constants (J = 10.0-11.5 Hz) are compatible with the expected cis-configuration. On the basis of these spectroscopic data, the products 7a-n were assigned 3a,4,6,6a-tetrahydro-3,5-diaryl-1-(4-nitrophenyl)-1H,5H-pyrrolo [3,4-c]pyrazole-4,6-dione structures. The 13 C-NMR spectrum of 7m exhibited two signals at δ 171.49 and 170.59 for the two amide carbonyl carbons. The signals at δ 66.54, 53.81, 27.33 and 14.78 are assignable to C-5 pyrazoline, C-4 pyrazoline ( Figure 1) and the methylene and methyl carbons of the ethyl group respectively. The mass spectra of all prepared compounds 7a-n displayed the correct molecular ion

Scheme 3.
Also, treatment of the hydrazonoyl bromides 1a,b with the appropriate α,β-unsatruated ketones 8a-c in benzene in the presence of triethylamine afforded, in each case, one regioisomer, as evidenced by TLC analysis. The structures of the products obtained were identified as 9a-f respectively (Scheme 4). The structures of compounds 9 were assigned on the basis of their elemental analyses and spectroscopic data (see Experimental). The 1 H-NMR spectra of 9 were characterized, in each case, by the presence of two doublets (J = 3.6-6.0 Hz) near δ 5.2 and 6.5 ppm, assignable to the protons at C-4 and C-5 of the pyrazoline ring respectively [32].

Antimicrobial activity
Four of the newly synthesized compounds were screened for their antibacterial activity against the Gram -ve bacteria Escherchia coli and the Gram +ve bacteria Staphylococcus aureus, in addition to their antifungal activity against Asperagillus flavus and Candida albicans using the agar diffusion method [33,34] at a concentration 20 mg/mL using DMSO as a solvent. The results, recorded as average diameter of inhibition zone in mm, are given in Table I.

General
All melting points were measured on an Electrothermal melting point apparatus and are uncorrected. The infrared spectra were recorded in potassium bromide disks on a Pye Unicam SP 3-300 or a Shimadzu FT-IR 8101 PC infrared spectrophotometer. The 1 H-NMR (200 MHz) and 13 C-NMR (50 MHz) spectra were recorded in DMSO-d 6 on a Varian Mercury VX 200 NMR using TMS as the internal reference. Mass spectra were measured on a GCMS-QP 1000 EX spectrometer at 70 eV. Elemental analyses were carried out at the Microanalytical Centre of Cairo University, Giza, Egypt. Compounds 1a [35] and 1b [36] were prepared according to the methods reported in the literature.
To a solution of hydrazonoyl bromides 1a,b (0.005 mol) and the appropriate monosubstituted olefin (acrylonitrile, acrylamide and ethyl acrylate, 0.005 mol) in chloroform (50 mL) was added triethylamine (0.7 mL, 0.005 mol) at room temperature. The mixture was refluxed for 4-6 h and then cooled. The mixture was extracted with water and the organic layer was collected, dried (anhydrous sodium sulfate) and then filtered. The solvent was evaporated and the residue was triturated with methanol whereupon it solidified. The solid was collected and recrystallized from the indicated solvents to give the corresponding cycloadducts 3-5 in good yield.