Synthesis and pharmacological evaluation of 7-substituted 1-ethyl-3 , 4 , 10-trimethyl-1 , 10-dihydro-11 H-pyrazolo [ 3 , 4c ] [ 1 , 6 ] benzodiazocin-11-one . A new ring system

Derivatives of the title ring system of type 10 were obtained in good yield by fusion of the intermediates 12. Attempt to cyclize the acetylamino derivative 9 under Bischler-Napieralski conditions failed because of the insufficient electronic density in the position 4 of the pyrazole ring created by the adjacent carbonyl moiety. The derivatives of the new ring system, assayed as anxiolytic agents, showed no significant activity.


Introduction
2][3] Also annelated benzodiazocines have shown interesting activities.Thus dibenzodiazocines showed to be effective inhibitors of the enzyme thromboxane A2 (TxA2) synthase; 4 or to have intercalating capability between DNA base pairs 5 whilst pyrazolo-benzodiazocines of type 1 exhibited activity on CNS. 6 In connection with our researches on pyrazole-fused heterocycles with potential pharmaceutical activity we have recently synthesized derivatives of the new ring system dipyrazolo [3,4-b:4',3'-f] [1,5]diazocine 2. 7 Unfortunately, such derivatives assayed to evidence CNS activities showed to be inactive.Continuing our studies in the same field, we became interested in the synthesis of the new ring system pyrazolo [3,4-c] [1,6]benzodiazocine of type 3, with the aim to verify whether both the replacement of the pyrazole ring with a benzene nucleus and the shift of the nitrogen of the diazocine moiety from the 5 to the 6 position would produce compounds with some CNS activity.

Results and Discussion
We started our synthesis reacting 5-pyrazolecarbonyl chloride 4 with the substituted 2nitroanilines 5a-c to give the corresponding 5-pyrazolecarboxamides 6a-c in acceptable yields (50-65%).Reaction of these latter with methyl iodide in alkaline medium afforded the methyl derivatives 7a-c in good yields (70-85%).Catalytic hydrogenation on Raney-nickel of compound 7a led to the corresponding amine derivative 8 (75%) which was quantitatively converted into 9 by reaction with acetic anhydride.
The attempt to cyclize of the acetamido derivative 9 was carried out in refluxing phosphorous oxychloride.However under the Bischler-Napieralski reaction conditions the expected new ring system pyrazolo [3,4-c][1,6]benzodiazocine was not obtained and a very complex reaction mixture was formed from which it was only possible to isolate 1-ethyl-3-methylpyrazole 5carboxylic acid.The failure of the cyclization involving electrophilic attack on the position 4 of the pyrazole ring, has probably to be ascribed to the presence of the carbonyl group in the position 5 that decreases the electronic density on the reaction centre.In fact, in compound 9, between positions 4 and 5 of the pyrazole nucleus, there is a larger transmission of electronic effects (hyper ortho) than between the positions 3 and 4 (hypo ortho) due to the high "bond fixation" which give rise to a C-4⎯C-5 bond with a high π bond order as already pointed out in pyrrole and thiophene rings. 8,9e therefore undertook a different synthetic approach to the pyrazolobenzodiazocine ring system.Thus, reaction of acetic anhydride on derivatives 7 led, in excellent yields, to the corresponding acetyl derivatives 11 which were reduced with stannous chloride in hydrochloric acid to the amino derivatives 12 (70-75%).Such amino derivative heated at their melting temperatures for one hour gave the expected new tricyclic ring system pyrazolo [3,4-c][1,6]  benzodiazocine in good yield (57-70%).
The structure of compounds 6-12 was confirmed by spectroscopic data as well as elemental analysis (see Tables 1 -3).
In the case of compounds 11 and 12 1 H NMR spectra provided evidence for the presence of a rotational isomerism because of the hindered rotation due both to the partial double bond character of the amide C-N bond, and to the steric hindrance of the substituents in position 4 and 2'.In fact two set of signals due to N-methyl, the pyrazole and phenyl substituents were observed.The relative abundance of the more stable conformer was 75-87%.The signals belonging to both conformers are reported in Table 2.It is interesting to note that, in compounds 11 and 12, the chemical shifts of the N-methyl signals of the predominant conformers are always upfield to the same signals related to the minor conformers.In accordance with the literature, the downfield resonance should be assigned to the conformer bearing the methyl group anti to the carbonyl. 10Therefore, in our case, the predominant conformer should be the structure A bearing the methyl syn to the carbonyl.This seems unlikely since two bulky groups would lie on the same side creating a remarkable steric hindrance.However, in our opinion, the predominant conformer is the form B bearing the methyl anti to the carbonyl and the unusual upfield chemical shift can be justified by the fact that the methyl protons fall inside the conical shielding region of the carbonyl anisotropic field as shown by the Figure 2 that reports the 3D-structure of the two conformers of compound 11a fully optimized in vacuo with semiempirical calculations performed with the VAMP (V 6.5) software, supplied by Oxford Molecular-Accellrys, using the Hamiltonian method PM3.

A B Figure 2
The 1 H and 13 C NMR data for the derivatives of the new ring system 10 are reported in Table 3.For these compounds, two different signals for the N-methylene protons were detected.This behaviour was observed in strictly correlated 1,5-diazocine series. 7,11t is interesting to note that compounds 10 undergo ring opening to give compounds 12 at room temperature upon silica gel acid catalysis.Compounds 10 were evaluated as anxiolytic agents utilizing chlordiazepoxide as reference drug, but none of them showed significant activity.

Experimental Section
General Procedures.Melting points were measured in open capillary tubes using a Buchi-Tottoli immersion apparatus, and are uncorrected.The IR spectra were recorded on a Perkin-Elmer Infracord 137 spectrophotometer as nujol mulls.The 1 H NMR spectra were recorded on

General procedure for the synthesis of l-ethyl-3-methyl-N-(4'-R-2'-nitrophenyl)-1Hpyrazole-5-carboxamide (6a-c)
A solution of 4 (5 mmol), aniline derivatives 5a-c (5 mmol), and triethylamine (5 mmol) in toluene (50 mL) was refluxed for 5 h.The solvent was then evaporated under reduced pressure and the residue was recrystallized from ethanol.Reaction of phosphorus oxychloride with compound 9.A mixture of 9 (2 mmol) and phosphorus oxychloride (30 mL) was refluxed at 165-175°C for 24 h..The excess of phosphorus oxychloride was evaporated under reduced pressure and the mixture was poured onto crushed ice, the solution was adjusted to pH 3.5 with solid sodium bicarbonate and extracted with chloroform (2x50 mL).The organic layers were washed with water, dried with magnesium sulfate and evaporated to give a solid which was identified as 1-ethyl-3-methylpyrazole 5carboxylic acid.Subsequently, the aqueous mother liquor was adjusted to pH 8.3 with solid sodium bicarbonate, extracted with chloroform (2x50 mL) to give an intractable material.

General procedure for the synthesis of 1-ethyl-N,3-dimethyl-4-acetyl-N-(4'-R-2'aminophenyl)-1H-pyrazole-5-carboxamide (12a-c)
Compounds 11a-c (10 mmol) were added to a magnetically stirred suspension of finely powdered stannous chloride (30 mmol) in hydrochloric acid (36%, 5 mL) at such a rate so that the temperature of the slurry was maintained below 5°C.After the complete addition of the nitro compounds, the mixture was allowed to stir for 24 hours.The white slurry thus obtained was diluted with cold water and aqueous sodium hydroxide (40%) was added till the salts of tin were dissolved.The solution was extracted with ethyl acetate (3x50 mL), the extracts were dried (magnesium sulphate) and evaporated under reduced pressure to give a residue which was recrystallized from ethanol.

General procedure for the synthesis of 7 -
Compounds 12a-c were heated for one hour at their melting temperature.After cooling the crude solid was recrystallized from dioxane or, in the case of 10b, purified by sublimation at 165°C/1mmHg.Compound 10a (R = H): yield 57%; mp: 100-101°C (white needles); IR: 1660 (CO) cm -l ; ms: m/z 282 (M +
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Table 3 .
13C NMR and ( 1 H NMR) chemical shifts of compounds 10: δ H [ppm] (3 mmol), acetic anhydride (30 mL) and a few drops of sulfuric acid was refluxed for 6 hours.After cooling, the reaction mixture was poured into crushed ice, neutralized with solid sodium bicarbonate and extracted with ethyl ether (2x50 mL).The organic layers were washed with water, dried with magnesium sulfate and concentrated to dryness under reduced pressure to give a residue which was purified by flash chromatography on column of silica gel (elution with petroleum ether(40-70)-ethyl acetate, 80:20) to give the desired products which were recrystallized from ethanol.