Synthesis and antibacterial screening of new N-Substituted-9H-β-carboline- 6-amine derivatives

Alivelu Samala*1,2, Srinivasa Murthy M3, Krishna Mohan Gottumukkala2 1Department of Pharmaceutical Chemistry, Holy Mary Institute of Technology and Science, Bogaram, Keesara, Hyderabad, Telangana, India 2Centre for Pharmaceutical Sciences, Inst of Science and Technology, JNT University Hyderabad, Kukatpally, Telangana, India 3Department of Pharmaceutical Chemistry, Vignan Institute of Pharmaceutical sciences, Near Ramoji Film City, Deshmukhi, Nalgonda, India

Pictet Spengler reaction is one of the most obvious ways for construction of tricyclic β-Carboline heterocyclic framework. (Cox and Cook, 1995). The different biological potential of β-Carbolines and the importance of the search for new antibacterial agents have led us to study this class of compounds.
Current work is reported with the synthesis of a title compounds from the well known Pictet Spengler reaction by taking 5-Chlorotryptamine and glyoxalic acid as starting materials, characterization by spectroscopically (FT-IR, 1 H NMR and Mass) and screened for antibacterial activity (Savariz et al., 2010).

Experimental
Melting points were determined on a capillary melting point apparatus and are uncorrected. The progress of the reactions was monitored by thinlayer chromatography on silica gel plates. Column chromatography was performed on silica gel. IR spectra were measured in KBr on Bruker spectrophotometer. 1 H-NMR spectra were recorded in a Bruker spectrometer at 400MHz (DMSO). Mass spectra were recorded in an APCI Mass spectrometer (APCI-AtmosphericPressure Chemical Ionization). All reagents were purchased from commercial suppliers.

Synthesis of 6-Chloro-9H-β-carboline (4)
Compound 3 (1mmol) was taken in a beaker and dissolved in dimethyl formamide (DMF). To this mixture Iodobenzene diacetate (2 mmol) was added. Which was stirred for 1h at RT. Completion of reaction was monitored by TLC. The resulting mixture was quenched by saturated NaHCO 3 solution and extracted with CH 3 COOC 2 H 5. All organic layers were combined and washed with brine and dried over MgSO 4 and iltered. Solvent was evaporated by rota evaporator. Obtained residue was puri ied by column chromatography using ethyl acetate and hexane as solvent system in increasing order of their polarities.

Synthesis of N-Substituted-9H-β-carbolin-6amine derivatives (5a-j)
N-Substituted amine (1mol) was added to a solution of 6-Chloro-9H-β-carboline (1mol) in anhydrous toluene (5.0ml). The above mixture was stirred at RT for 20-24h. Under reduced pressure, excess toluene was distilled off from the reaction mixture. The mixture was poured into crushed ice and further neutralized with HCl. The resultant precipitate was iltered; air dried and recrystallized using ethanol as solvent.

Anti bacterial activity
The inal derivatives were screened for their antibacterial activity against Gram +Ve bacteria (Steptococcus pyogenes, Bacillus subtilis and Staphylococcus aureus) and Gram -Ve bacteria (Escherichia coli and Pseudomonas aeruginosa) by the disc diffusion method at concentrations of 10, 20, 30 and 40µg/ml in DMSO. Amoxicillin was used as standard drug and DMSO as a control. The zone of inhibition was measured after 24h incubation at 37 0 C

Chemistry
The synthetic route for the compounds 5 a-j is outlined in Scheme 1. The Pictet Spengler condensation of 5-Chlorotryptamine (1) with glyoxalic acid (2) afforded compound 3 in 52.6% yield. 1 H NMR conirmed it, the disappearance of indole C 2 hydrogen in compound 3 which was present in compound 1 and also con irmed by mass spectrum. The Mass spectrum of compound 3 obtained the base peak coincided with the molecular ion (M + +1) at 251.
Oxidative decarboxylation of compound 3 with iodobenzene diacetate led to the 6-Chloro-9H-βcarboline (4) in 76.5% yield. By shifting of aliphatic hydrogen's ( C 1 , C 3 and C 4 ) particularly at C 1 to the aromatic region in 1 H NMR con irmed that oxidative decarboxylation has occurred, and in mass spectrum (Molecular weight: 202.63) base peak obtained at 203 (M+1).
Compound (4) was treated with different mono substituted amines gave the title compounds (5 a-J) in between 53-88% yields. All new derivatives were characterized by their spectroscopic data (FT-IR, 1 H NMR and Mass), which were given under the experimental section. The IR spectra showed absorption bands characteristic for NH indole and C=N stretching in the range between 3449-3274Cm −1 and 1197-1094Cm −1 , respectively. The NH signal at C 6 did not appear in 1 H NMR spectra. Due to it may be hidden with in the aromatic region, remaining all protons appeared at their respective regions. Mass spectra also con irmed the structures of 5a-j. All compounds showed the presence of a base peak at M+1.
For example, Compound 4 was treated with methylamine gave 5a in 64.5% yield. It was possible to con irm the structure of the 5a by proton NMR, due to the presence of the methyl group signals. It was also showed the base peak at 198 (M+1) in mass spectra. Like this, structures of the remaining all derivatives (5 b -5 j ) were con irmed.

Antibacterial activity
In the current work, a set of 10 new β-Carboline derivatives were prepared according to Scheme 1 and screened for their in vitro antibacterial activity, and the zone of inhibition was measured after 24h incubation at 37 0 C. Results were given under the experimental section. (Table 1).
The results obtained in this study revealed that all new derivatives were inactive at 10µg/ml Compounds 5h, 5i and 5j possessed good activities in growth inhibition of E-coli. Compound 5e showed moderate activity against E-coli and P-Aeruginosa. Remaining all compounds (including 5e, 5h, 5i and 5j) showed mild to moderate activity with all bacte-rial strains.

CONCLUSION
A new series of N-Substituted-9H-β-carboline-6amine derivatives were obtained by Pictet-Spengler reaction followed by oxidative decarboxylation and amination, with good yields. Final derivatives were con irmed spectroscopically and screened for antibacterial activity against various pathogenic bacterial strains by disc diffusion method. Among all tested compounds, compound 5h, 5i and 5j were active displaying good activity against E-coli while compound 5e was found to exhibit moderate antibacterial activity against E-coli and P-Aeruginosa.