Synthesis, Characterization and Biological evaluation of some novel Pyrazolo [1,5-a]Pyrimidine derivatives

A convenient synthesis of substituted Pyrazolo[1,5-a]pyrimidine was carried out by the reaction of different ketene dithioacetals with different aromatic amine in isopropanol in the presence of potassium carbonate. The newly synthesized compound were characterized by 1 H NMR, IR, Mass and screened for their antimicrobial activity against various strains of bacteria and fungi. From the synthesized different NCEs, compounds 8a, 8d and 8e are broad spectrum drug which can inhibit the growth of gram positive, gram negative bacteria and fungi.

Some pyrazolopyrimidines serve as efficient sedative-hypnotic and anxiolytic drugs like zaleplon (Sonata, hypnotic) [21], indiplon (hypnotic) [22][23][24] and ocinaplon (anxiolytic) [25][26], fasiplon (anxiolytic). These drugs are related to the class of nonbenzodiazepines, and their therapeutic effect is due to allosteric enhancement of the action of the inhibitory neurotransmitter GABA at the GABAA receptor. These examples emphasize the importance of pyrazole-fused heterobiaryls, as well as pyrazolopyridines, as key pharmacophores in bioactive small molecules.
We have synthesize pyrazolo [1,5-a] pyrimidine derivatives by refluxing different ketene dithioacetals with aromatic amine in the presence of potassium carbonate in isopropanol.
The newly synthesized compounds were characterized by IR, Mass and 1 H NMR. All the synthesized compounds were evaluated for their antimicrobial activity.

General procedure for the Preparation of 2-Cyano-N-arylacetamide (2)
To a stirred solution of different aromatic amine (110 mmol) 1 and cyanoaceticacid (121 mmol) in the presence of triethylamine (165 mmol) in dichloromethane (100 ml), EDC.HCl (121 mmol) was added lotwise at 0-10°C with 30 to 60 minutes. The reaction mixture was allowed to stir at room temperature for 2 to 3 hours. The progress of the reaction was monitored on TLC. After completion of the reaction water was added into the reaction mixture. The organic layer was separated and washed with water and dried over sodium sulphate. The solvent was distilled under vacuum to give 2-Cyano-N-arylacetamide 2 as a cream to brown color solid compound which was used for the next stage without purification (82-89 % yield).

General procedure for the Preparation of 2-cyano-3,3-bis(methylthio)-N-arylacrylamide (3)
To a stirred solution of 2-cyano-N-arylacetamide (20 mmol) 2 in N,N-dimethylformamide (25 ml), dry potassium carbonate (20 mmol) was added at room temperature. The reaction mixture was stirred at room temperature for 2 hours. Carbon disulfide (75 mmol) was added at room temperature and the reaction mixture was stirred for 2 hours. Then the reaction mixture was cooled to 0-10°C and methyl iodide (50 mmol) was added in to the reaction mixture and the reaction mixture was stirred for 3 to 4 hours at room temperature. The progress of the reaction was monitored on TLC. After completion of the reaction, water was added into the reaction mixture and the reaction mixture was stirred for 1 hour at room temperature. The precipitated solid material was filtered, washed with water and dried to give 2-Cyano-3,3-bis(methylthio)-N-arylacrylamide 3 (75-88 % yield) as a yellow solid.

General procedure for the preparation of 5-amino-N-aryl-3-(methylthio)-1H-pyrazole-4carboxamide (4)
To a stirred solution of 2-Cyano-3,3-bis(methylthio)-N-arylacrylamide (20 mmol) 3 and hydrazine hydrate (25 mmol) in isopropanol (100 ml) was heated to reflux for 2 to 3 hours. The progress of the reaction was monitored by TLC. After completion of the reaction, the reaction mixture was cooled to room temperature. The reaction mixture was poured into crushed ice and stirred for 1 hour at room temperature. The precipitated solid material was filtered, washed with water and dried to give 5-amino-N-aryl-3-(methylthio)-1H-pyrazole-4-carboxamide 4 as a yellow color solid compound in 70-90 % yield.

General procedure for the preparation of different Acetoacetanilide derivatives (6)
To a stirred solution of different aromatic amine (10 mmol) 5 and ethylacetoacetate (20 mmol) in the presence of catalytic amount potassium hydroxide (10%) in toluene (100 ml) was heated at reflux temperature for 15 to 20 hours. The progress of the reaction was monitored by TLC. After completion of the reaction the solvent was removed under vacuum and methanol was added into the residue and stirred for 1 hour at room temperature. The precipitated solid material was filtered, washed with methanol and dried to give different acetoacetanilide compounds 6 as an off white to light yellow color solid compound in 35-45 % yield.

General procedure for the preparation of different ketene dithioacetals (7)
To a stirred solution of acetoacetanilide derivatives (10 mmol) 6 in N,N-dimethylformamide (10 ml), dry potassium carbonate (11 mmol) was added at room temperature. The reaction mixture was stirred at room temperature for 2 hours. Carbon disulfide (30 mmol) was added at room temperature and the reaction mixture was stirred for 2 hours. Then the reaction mixture was cooled to 0-10°C and methyl iodide (25 mmol) was added in to the reaction mixture and the reaction mixture was stirred for 3 to 4 hours at room temperature. The progress of the reaction was monitored by TLC. After completion of the reaction, water was added into the reaction mixture and the reaction mixture was stirred for 1 hour at room temperature. The precipitated solid material was filtered, washed with water and dried to give ketene dithioacetals derivatives 7 as a yellow color solid in 70-80% yield.

International Letters of Chemistry, Physics and Astronomy Vol. 65
General procedure for the preparation of fused pyrazolopyrimidines (8) To a stirred mixture of 5-amino-N-aryl-3-(methylthio)-1Hpyrazole-4-carboxamide (5 mmol) 4, ketene dithioacetals (5 mmol) 7 potassium carbonate (10 mmol) in (10 ml) was heated to reflux temperature for 12 to 14 hours. The progress of the reaction was monitored by TLC. After completion of reaction the reaction mixture was cooled to room temperature and water was added. The reaction mixture was stirred for 1 hour at room temperature. The solid material was filtered, washed with water and dried to give fused pyrazolopyrimide derivatives. The compound was crystallized from methanol to give pure compound in 80-90% yield.

RESULT AND DISCUSSION
Chemistry: The synthesis of 7-Methyl-2,5-bis(methylthio)-N 3 ,N 6 -diarylpyrazolo[1,5-a]pyrimidine-3,6-dicarboxamide is outlined in scheme-1. Different aromatic amine is reacted with ethylacetoacetate in the presence of catalytic sodium hydroxide in toluene to give acetanilide derivatives, which is reacted with carbon disulfide the presence of base followed by methyl iodide in N,N-dimethyl formamide to give ketene dithioacetyl derivatives. Different aromatic amine is reacted with cyano acetic acid in the presence of EDC.HCl in dichloromethane to give cyano acetamide derivative, which on reaction with carbon disulfide in the presence of base followed by methyl iodide to yield ketene dithiocyanoacetyl derivatives.

CONCLUSION
An efficient method for preparing substituted pyrazolo [1,5-a]pyrimidine derivatives was described and the structure of synthesized compounds was determine by IR, 1 H NMR, and Mass spectroscopic analysis and evaluated for their in vitro antimicrobial activity by broth dilution method which shows good to poor activity against different bacterial strains.