Design, synthesis, characterization and antitubercular activity of some 2-heterocycle-substituted phenothiazines

Some novel 2-heterocycle-substituted phenothiazines having a pyrazolo[3,4-d ]pyrimidine nucleus have been synthesized by using the Biginelli multi-component cyclocondensation reaction. The products were characterized by FT-IR, 1 H NMR, 13 C NMR, mass spectra and elemental analysis. The products were evaluated for their antitubercular activity against Mycobacterium tuberculosis H 37 Rv


Introduction
The chemistry of nitrogen-sulfur heteroatom containing aromatic compounds is becoming more popular as an area of research.Phenothiazines and related compounds have shown diverse biological activities including as tranquilizers, 1 anti-inflammatory, 2 antimalarial, 3 antipsychotropic, 4 antimicrobial, 5 antitubercular, 6,7 antitumour [8][9][10] and stimulation of the penetration of anticancer agents via the blood-brain barrier.They bind to physiological targets or receptors, producing many possible mechanisms of actions.However, solid cancers of the brain and stomach are generally resistant to chemotherapeutic agents. 11Phenothiazines are inexpensive and widely available, and therefore have been examined as anticancer drugs.
A slight variation in the substitution pattern on the phenothiazine nucleus often causes a marked difference in activities and therefore phenothiazines with various substituents are being synthesized and tested for activities in search of better medicinal agents.It has been reported 12 that some phenothiazines inhibit intracellular replication of viruses including human immunodeficiency viruses (HIV).Furthermore, some of these derivatives have been reported to exhibit significant anticancer activities 13,14 and great interest has arisen in the design and synthesis of new phenothiazines to explore their anticancer activities.The pyrimidine nucleus, which has a useful structure for further molecular exploration for the development of new derivatives with different biological activities, has received much attention in recent years. 15yrimidine derivatives are of interest because of their pharmacological properties 15-26 including antiviral, 16 antitumour, 19 antibacterial [20][21][22][23][24] and antihypertensive 18 effects.8][29][30][31][32][33] Most of these are based on modification of the classical one-pot Biginelli reaction 24,[28][29][30][31][32] and in some cases on more complex multi-step processes, 33,34 which may involve the use of some expensive and commercially non-available materials.Owing to the versatility of pyrimidines and as a continuation of our previous work, 35 we have extended the convenient Biginelli reaction to include some pyrimidine derivatives containing a phenothiazine nucleus.

Chemistry
The classical three-component Biginelli condensation is usually carried out in alcoholic solution containing a few drops of concentrated hydrochloric or sulfuric acid as catalyst, although other systems such as THF/HCl, dioxane/HCl, or acetic acid/HCl have also been employed. 36One major drawback of the classical Biginelli protocol is the low yield that is frequently encountered when using sterically more demanding aldehydes or 1,3-dicarbonyl compounds. 36n order to promote conditions that would favor higher yields of products, we have recently performed Biginelli condensations using different catalysts such as PPA, AlCl 3 , BF 3 etc.We found that using phosphorus pentoxide as a catalyst in the Biginelli one-pot protocol, gave a significant increase in the yields of DHPMs, especially for systems that give only moderate yields using traditional Biginelli conditions.

Conclusions
In conclusion, we have developed a simple and efficient method for the synthesis of pyrimidines having a phenothiazine nucleus.We also believe that the procedural simplicity, the efficiency and the easy accessibility of the reaction partners gives access to a wide array of heterocyclic frameworks equipped with a pendant phenothiazine unit.Other compounds of this group are presently under investigation.

Experimental Section
General Procedures.All chemicals were purchased from Aldrich Chemicals (Mumbai, India) and were used without further purification.Melting points were determined in open capillary tubes and are uncorrected.Formation of the compounds was routinely checked by TLC using Silica G and the spots were exposed to iodine vapour for visualization. 1 H NMR spectra were obtained in CDCl 3 solution on a Bruker DPX 300 MHz spectrometer. 13C-NMR (75 and 125 MHz) spectra were measured on a Bruker AC 200, DPX 300 and ARX 500, at 25 °C, in CDCl 3 .IR spectra were recorded on a Shimadzu 8400 spectrometer in KBr (γ in cm -1 ).Elemental analyses of the newly synthesized compounds were carried out on Carlo Erba 1108 analyzer.

Table 1 .
The antitubercular activity of the compounds was assessed at the Tuberculosis Antimicrobial Acquisition and Co-ordination Facility (TAACF), U.S.A.Primary screening of the compounds was conducted at >6.25 µg/ml against Mycobacterium tuberculosis H 37 Rv in BECTEC 12B medium using the BACTEC 460 radiometric system.The antitubercular activities are represented in Table1.Antitubercular activity of 3a-h By visualizing the antitubercular data, it could be observed that all the compounds displayed mild to moderate activity.Compounds 3c, 3d and 3e were found to be particularly active against Mycobacterium tuberculosis H 37 Rv strain.