Synthesis and antimicrobial activity of novel 2-( heteryl-carboxamido )-2 , 3-dihydro-1 H-1 , 2 , 5-oxadiazolo [ 3 , 4c ] [ 1 , 3 , 2 ] diazaphosphole-2-oxides

Some novel 2-(heterylcarboxamido)-2,3-dihydro-1H-1,2,5-oxadiazolo[3,4-c][1,3,2]diazaphosphole-2-oxides (4-13) have been synthesized from reaction of 3,4-diamino-1,2,5-oxadiazole (3) with various dichlorophosphinylcarboxamides (2), which were prepared by C-acylation of electron-rich heterocyclic compounds with dichloro isocyanato phosphine oxide (Kirsanov isocyanate) (1) in dry octane, in the presence of triethylamine at 50-60 °C. Their structures were determined by IR, H, C, P NMR and mass spectral (MS) studies. They were screened for antifungal and antibacterial activity against Curvularia lunata / Aspergillus niger and Staphylococcus aureus / Escherichia coli, respectively. Most of these compounds exhibited moderate activity in the assays.


Introduction
In a study of new pharmaceuticals and agrochemicals, the application of heterocycles is warranted to improve their biological activity.Diaminofurazan has been identified as an urea equivalent for histamine H 2 -receptor antagonists, [1][2][3] and its derivatives have found increased pharmaceutical and medicinal applications. 4][7][8][9][10][11][12][13][14][15] As a part of our ongoing programme, aimed at searching for novel antibacterial and antiviral agents with high activity and low toxicity, a synthetic route has been developed to the title compounds.
The synthetic route (Chart 1) involved the C-acylation of various electron-rich heterocycles (Het _ H) with Kirsanov isocyanate (1) [16][17] at 15-30 °C in dry octane, under inert and anhydrous conditions to furnished N-dichlorophosphorylheterylcarboxamides (2). 18Cyclocondensation of 2 in situ with 3,4-diamino-1,2,5-oxadiazole (3) 19 in the presence of triethylamine at 50-60 °C in octane, yielded the title compounds (4-13).Thin layer chromatography (TLC) was employed to follow the progress of the reaction and purity of the products.The target compounds were obtained by filtering off triethylamine hydrochloride, evaporation of the filterate, washing the residue with water and recrystallization of the resulting solids, using suitable solvents.All the title compounds (4-13) were readily soluble in polar organic solvents and melted in the range 160-216 °C.Their chemical structures were established by IR, 1 H, 13 C, 31 P NMR and MS data.The presence of characteristic bands at 3222-3394 cm⎯ 1 (P-N-H), 20 2948-3204 cm⎯ 1 (P-N-H-CO), 20 1635-1697 cm⎯ 1 (C=O), 21 and 1216-1248 cm ⎯ 1 (P=O) [22][23][24][25] in the IR spectra of 4-13 showed that cyclisation of 3 with 2 had occurred to form the furazanodiaza-phosphole. 1 H-NMR data agreed well with the proposed structures for 4-13.However, it is observed that the exocyclic protons of the carboxamide group (NH-CO) of 4 and 5 resonated downfield (δ 10.69-10.89),when compared to others (δ 6.43-9.00).Endocyclic protons of N 1 and N 3 exhibited signals as doublets at δ  8.61-9.38 (J = 8.2-9.3Hz) and 8.42-9.13(J = 8.5-8.9Hz) in all compounds 4-13, [26][27] due to their coupling with phosphorus.The chemical shifts of other protons of the carboxamide moiety appeared in the expected regions. 18It is of further interest to observe that the protons of the carboxamide function resonated downfield, when compared to the corresponding protons in the free heterocycles.The NH proton signals were confirmed by D 2 O exchange experiments.
The 13 C NMR chemical shifts of 4-13 were interpreted on the basis of additivity rules.The nitrogen bonded C-4 and C-5 exhibited signals at 156-165 ppm. 19The carbonyl carbon of the carboxamide function resonated in the range δ 88-207.The other carbon chemical shifts of the carboxamide function appeared downfield (-10 ppm), when compared to the corresponding carbon chemical shifts in the respective free heterocycles.The resonances of other carbon atoms of the carboxamide moiety appeared in the expected regions. 31P NMR signals of these compounds (4-13) appeared in the range -10.59 to 16.81 ppm.The striking appearance of two or three 31 P NMR signals in the spectra of 4, 5, 7, 8, 9, 10 and 12 suggests that they may exist as two or three conformers in the solution state. 30he mass spectra fragmentation patterns for 4-13 are rationalized in Chart

Antimicrobial activity
Compounds 4-13 were screened for their antibacterial activity against Staphylococcus aureus and Escherichia coli (10 6 cell/mL) by the disc-diffusion method 28 in nutrient agar medium, at various concentrations (250, 500 µg/disc) in dimethylformamide (DMF).These solutions were added to each filter disc and DMF was used as the control.The plates were incubated at 35 °C and examined for zone of inhibition around each disc after 24 h.The results were compared with the activity of the standard antibiotics like Penicillin and Tetracyclin (250 µg/disc).Their antifungal activity 29 was evaluated against Curvularia lunata and Aspergillus niger, at concentrations of 250 and 500 µg/disc.Griseofulvin was used as the reference compound.Fungal cultures were grown on potato dextrose broth at 25 °C and finally spore suspension was adjusted to 10 5 spores/mL.Most of the compounds showed significant activity against both bacteria and fungi.P).Mass spectral data was recorded on GC-MS instrument at 70 eV with a direct inlet system.
2. Appearance of M +• at the appropriate molecular weight, [M-(Het)] + at m/z 187, [M-(NH-CO-Het)] + at m/z 145, [M-(H 2 N-CO-Het)] + at m/z 144, [M-(NH-CO-Het and H 2 O)] +. at m/z 127 and [M-(CO-Het)] +at m/z 160 with the furazanodiazaphosphole-2-oxide moiety conclusively establishes their proposed molecular structure.These daughter ions being characteristic of these molecules could be used as diagnostic ions of these compounds for their monitoring in bio and eco systems.

Table 1 .
Antimicrobial The melting points were determined on a Mel.-Temp apparatus and were left uncorrected.Elemental analyses were performed at the Central Drug Research Institute, Lucknow, India.IR spectra were recorded in KBr pellets and Nujol mulls on a Perkin -Elmer 283 unit. 1 H,13C and 31 P NMR spectra were taken on a AMX 400 MHz spectrometer operating at 400 MHz for 1 H, 100 MHz for13C and 161.9 MHz for 31 P.The compounds were dissolved in DMSO-d 6 or CDCl 3 , and chemical shifts were referenced to TMS ( 1 H and 13 C) and 85% H 3 PO 4( 31