Synthesis and antimicrobial activity of novel naphtho [ 2 , 1-b ] furo-5 H-[ 3 , 2-d ] [ 1 , 3 , 4 ] thiadiazolo [ 3 , 2-a ] pyrimidin-5-ones

Ethyl 3-aminonaphtho[2,1-b]furan-2-carboxylate 1 was converted into 3-amino-2mercaptonaphthofuro[3,2-d]pyrimidin-4(3H)-one 4 by reacting it with carbon disulphide followed by methylation and condensation with hydrazine hydrate. The compound 4 on treatment with aryl isothiocyanates produced 2-arylaminoanilinonaphtho[2,1-b]furo-5H-[3,2d][1,3,4]thiadiazolo[3,2-a]pyrimidin-5-ones 5(a-h). The title compounds 6(a-h) were obtained also by reacting 4 with aromatic acids in presence of phosphorus oxychloride. The structures of the newly synthesized compounds were confirmed by IR, H NMR, C NMR and mass spectral studies and elemental analysis. All the synthesized compounds have been screened for antimicrobial activity.


Introduction
In the family of heterocyclic compounds, nitrogen-containing heterocycles with a sulfur atom are an important class of compounds in medicinal chemistry.There has been considerable interest in the development of preparative methods for the production of pyrimidines.This seems to be because pyrimidines represent one of the most active classes of compounds, possessing a wide spectrum of biological activity [1][2][3] .Pyrimidines and their ring-fused derivatives have a broad spectrum of biological activity; best known as the heterocyclic core of the nucleic acid bases.These ring systems are often incorporated into drugs designed for anticancer 4,5 , antiviral 6 , antihypertensive 7 , analgesic 8 , antipyretic 9 , antiinflammetry 10 , antipsoriasis 11 agents.Some of them are active on the blood circulatory system 12 and can stimulate the skin preparative regeneration and increase the efficacy of antibiotic therapy of Staphylococcus and Proteus infected wounds 13 .Similarly, derivatives of naphthofurans have attracted the attention of many organic chemists owing to their well pronounced activities such as anticancer 14 , antifungal and cytotoxic 15 and in the treatment of metabolic disorders 16 .Hence, keeping these reports in view and continuation of our search for more potent naphthofuran derivatives [17][18][19][20] , we report in this paper the synthesis of novel condensed heterocyclic compounds encompassing bioactive molecules, i.e., pyrimidine, naphthofuran and also thiadiazole.[1,3,4]thiadiazolo [3,2-a]pyrimidin-5-ones.

Results and Discussion
Amongst various methods used for construction of pyridinine [21][22][23][24] ring the synthetic strategy involving o-amino ester as starting material 25 was adopted in the present investigation.Thus treatment of ethyl 3-aminonaphtho[2,1-b]furan-2-carboxylate 26 1 with carbon disulphide and aqueous sodium hydroxide in dimethyl sulphoxide yielded a dithiocarbamate, ethyl 1-{[(sodiumthio)carbonothioyl]amino} naphtho[2,1-b]furan-2-carboxylate 2 as a salt, which (without isolating) was then methylated with dimethyl sulphate to afford a dithiocarbamate methyl ester, ethyl The structure of 3 was confirmed by IR, 1 H NMR and mass spectral studies.The IR spectrum of 3 exhibited an absorption band at 1710 cm -1 due to the -C=O for an ester carbonyl group and 3055 cm -1 due to the -NH group.In the 1 H NMR spectrum, a triplet at δ 1.4 due to three protons of the -CH 3 of an ester, a singlet at δ 2.35 integrating for the three protons of -SCH 3, a quartet at δ 4.4 integrating for the two protons of -CH 2 of ester , multiplet at δ 7.4-8.2for six aromatic protons and a broad singlet at δ 9.2 integrating for -NH (D 2 O exchangeable) proton were observed.The structure of 3 was further confirmed by mass spectral analysis.It exhibited a molecular ion peak at m/z 346 corresponding to its molecular weight.Compound 3 on reaction with hydrazine hydrate yielded the desired 3-amino-2-mercaptonaphthofuro [3,2-d]pyrimidin-4(3H)-one 4 in excellent yield.The IR spectrum of 4 exhibited an absorption band at 1673 cm -1 due to the -C=O and 3118 cm -1 due to the -NH 2 group.Additional support for the structure of 4 was obtained by recording its 1 H NMR spectrum, which exhibited a singlet at δ 2.8 due to the -SH proton, another broad singlet at δ 5.8 due to -NH 2 (D 2 O exchangeable) protons and a multiplet at δ 7.5-8.5 for six aromatic protons.The 13 C NMR spectrum showed peaks at δ 169.39, 154.66 due to the -C=O and -C-S carbon atoms respectively.The peaks at δ 133.29,   130.25, 129.79, 129.43, 128.93, 128.44, 127.89, 127.51, 125.25, 124.80, 123.93 and 122.40 were attributed to the twelve aromatic ring carbon atoms.Final proof for the structure was obtained by recording its mass spectrum, which exhibited a molecular ion peak at m/z 282, corresponding to its molecular weight.
Several synthetic routes are available for ring closure to obtain thiazolopyridines [27][28][29][30][31] .One of the methods involves the reaction of compounds possessing o-aminothiazol functionality as in the case of compound 4, with aryl isothiocyates.Thus compound 4 on reaction with various aryl ones 5(a-h).The structure of the compound 5a was elucidated from spectroscopic data.The IR spectrum of 5a exhibited an absorption band at 1676 cm -1 due to the -C=O group and at 3059 cm -1 due to -NH group.The 1 H NMR spectrum of 5a exhibited a multiplet at δ 7.54-8.55for ten aromatic protons and a broad singlet at δ 9.38 due to -NH (D 2 O exchangable) proton.The molecular ion peak at m/z 384 in its mass spectrum confirmed the structure 5a.
The compounds having o-aminomercapto functionality are also known to undergo cyclization on treatment with aromatic acids in presence of phosphorus oxychloride.Thus reaction of 4, having similar functionality, on treatment with different aromatic acids in phosphrus oxychloride led to the formation of 2-aryl naphtho [1,3,4]thiadiazolo [3,2-a]pyrimidin-5-ones 6(a-d).The IR spectrum of 6a exhibited an absorption band at 1663 cm -1 due to the -C=O group.In the 1 H NMR spectrum, only a multiplet at δ 7.5-8.1 for eleven aromatic protons was observed.The structure of 6a was further confirmed by mass spectral analysis.It exhibited a molecular ion peak at m/z 369 corresponding to its molecular weight.Similarly all the compounds were purified by column chromatography and characterized by spectroscopic studies.The reaction pathway is depicted in Scheme 1.

Evaluation of antimicrobial activity
The in vitro antimicrobial activity was carried out by cup-plate method. 32All the synthesized compounds were screened for antibacterial activity against Escherichia coli, Micrococus luteus and Staphylococcus aureus using Chloramphenicol (0.001 mole/ml) as standard.The antifungal activity was investigated against Aspergellius flavus, Aspergillus niger and Curvuliaria lunata using Flucanazole (0.001 mole/ml) as standard.Inhibition was recorded by measuring the diameter of the inhibition zone at the end of 24 hr for bacteria and 48 hr for fungi.Each experiment was repeated thrice and the average of the three independent determinations was recorded.The results are summarized in Table 1.Amongst the compounds tested for antibacterial activity, the compound 6b was found to display considerable activity against all the bacteria, whereas compound 4 was found to exhibit promising activity against E. coli and M. luteus.The compound 5c showed more antifungal activity than the standard flucanazole and the compound 5h exhibited almost equipotent activity against A. flaves and A. niger and was found to be more active then the standard against C. lunata.

Experimental Section
General Procedures.Melting points were determined in open capillary tubes and are uncorrected.IR spectra were recorded in KBr on FT-IR (Research Spectrophotometer Series) and Perkin-Elmer FT-IR (Spectrum 1000); 1 H NMR and 13 C NMR spectra on a Bruker AMX (400 MHz) Spectrophotometer using DMSO-d 6 as solvent and TMS as an internal standard (chemical shifts in δ) and mass spectra on a LC-MS instrument.Compounds were checked for their purity by TLC on silica gel G plates and spots were located by iodine vapors.Satisfactory C, H, N analyses were obtained for all the compounds.