Synthesis of Benzofuro [ 3 , 2e ]-1 , 4-diazepines of Pharmacological Interest

The title compounds (4a,b) were prepared by chloroacetylation of 2-acyl-3-aminobenzofuran(1a,b) and subsequent treatment with hexamethylene tetramine in ethanol via the complex salts (3a,b). Similar reaction with ethyl 3-aminobenzofuran-2 carboxylate (5) produced 3Hbenzofuro[3,2-e]-1,4-diazepin2,5(1H,4H)-dione (7) in good yield. All the newly synthesized compounds are characterized by elemental analysis and spectral studies, and evaluated for antimicrobial and anticonvulsant activities.


Introduction
The synthesis of condensed diazepine heterocycles has been explored to a maximum extent owing to their association with wide spectrum of pharmacological activities such as sedatives, anxiolytics, hypnotics, anticonvulsant, antipsychotic, muscle relaxants etc., in recent years [1][2][3][4][5][6] .Attempts have been made to build 1, 4-diazepine moiety on other biologically potent heterocycles in order to obtain drugs with more efficacy [7][8][9][10][11] .Encouraged by these facts and in continuation of our research program on synthesis of pharmacologically interesting furan derivatives [12][13][14][15][16] , we report in this paper annulation of 1,4-diazepine on benzo[2,1-b]furan moiety and screening the compounds for antimicrobial and anticonvulsant activities.The strategy adopted for the synthesis of these new condensed tricyclic heterocyclic compounds involved successive building up of 1, 4-diazepine ring on benzofuran 17 .

Experimental
Melting points were determined in open capillary and are uncorrected.Purity of the compounds was checked by TLC in Silica gel G. PMR spectra were recorded on various A-60, FT-80, FT-270 and XL-100 spectrometer using TMS as internal reference.Chemical shifts are expressed in terms of δ ppm through out.IR spectra were recorded (nujol) on Perkin-Elmer 277 spectrophotometer.Wave number is expressed in cm -1 .

Hexamethylenetetramine salt of compound (3a, b)
A mixture of compound 2a, b (0.002 mol), hexamethylenetetramine (0.6 g, 0.0047 mol) and potassium iodide (0.2 g) in chloroform (10 mL) was refluxed on steam bath for 10 h, the residual solid obtained, after removal of solvent, was thoroughly washed with chloroform and then with water to remove excess of HMTA.The product 3a,b on recrystallization from ethanol was obtained as colourless needles.

Method B
A mixture of 2a, b and hexamethylenetetramine (0.6 g, 0.0047 mol) in ethanol (10 mL) was heated under reflux for 12 h.the reaction mixture was cooled and the solid separated was filtered and washed with ethanol.The pure compound was obtained by recrystalised with ethanol.The mixed melting points with the compounds obtained by method A were not depressed.

Results and Discussion
The starting material 2-acyl-3-aminobenzofurans (1a, b) 18 were chloro acetylated to yield compounds 2a,b and were subjected to cyclisation using ammonia.But compound 2a failed to react with ammonia, it did not even produce the expected intermediate 3-aminoacetamido-2-acetylbenzofuran.However, the desired ring closure was accomplished by employing hexamethylene tetramine (HMTA).Thus, compound 2a on reaction with HMTA in chloroform produced a complex salt 3a, which on heating with ethanol, underwent cyclization to 5-methyl-1,3-dihydro-2H-benzofuro[3,2-e]-1,4-diazepin-2-one(4a).The direct conversion of 2a to 4a was also possible by heating 2a with HMTA in ethanol.Similar observations have been made earlier while synthesizing 1, 4-diazepine derivatives 19 .The IR, PMR and mass spectra of 4a were recorded to substantiate the structure assigned.The IR spectrum showed absorption bands at 1680 and 1625 cm -1 due to C=O and C=N also a broad band at 3440 cm -1 due NH stretching frequency.The PMR spectrum of 4a exhibited a singlet at 2.63, integrating for three of -CH 3 group, another singlet at 4.26, integrating for two protons of -CH 2 -group, a multiplet at 7.25-8.50due to four aromatic protons and a broad singlet (D 2 O exchangeable) at 11.04 due to NH proton.It's mass spectrum showed molecular ion peak at 314 m/z, corresponding to its molecular weight and fragmentation pattern also confirmed the structure assigned.
The physical data of the newly synthesized compound is presented in Table 1.

Antimicrobial activity
The newly synthesized diazepine compounds have been evaluated for antibacterial activity against Staphylococcus aureus and Klebsiella pneumoniae and antifungal activity against Aspergillus niger and Candida albicans by using ciprofloxacin and ciclopirox olamine as standards for comparison for antibacterial and antifungal activity respectively, by cup-plate method 23 .The results indicate that the compounds were either weakly active or inactive against all the four microorganisms.

Anti convulsant activity
The compounds were screened for anticonvulsant activity by maximal electroshock (MES) induced convulsant method 24 on albino rats (Wistar strain) using diazepam at the dose of 25 mg/kg body weight, as standard.After carrying out dose fixation studies, by stair-case method, the compounds were at the dose of 80 mg/kg body weight.A current of 150 ma was applied to the rats for 0.2 seconds, through corneal electrodes and the time spent by the rats in different stages of convulsions were noted.The diazepine 4b was found to possess considerable anticonvulsant activity, since it reduced the extensor phase of the MES convulsions to a greater extent than 4a and comparable with that of standard drug.Electron with drawing phenyl group present on diazepine ring may be responsible for its enhanced activity as compared with 4a which contains electron donating group in the same position.

Table 1 .
Characterization data of compounds