Synthesis of Tryptoline-3-Carboxylic Acid Derivatives A Novel Antidiabetic Agent

The compounds, 2-(methylsulfonyl)-1,2,3,4-tetrahydro-9H-pyrido[3,4-b]indole-3-carboxylic acid (DM3), 2-(phenylsulfonyl)-1,2,3,4-tetrahydro-9H-pyrido[3,4-b]indole-3-carboxylic acid (DM4), and 2-(p-toluenesulfonyl)-1,2,3,4-tetrahydro-9H-pyrido[3,4-b]indole-3-carboxylic acid (DM5) were synthesized by coupling of 1,2,3,4-tetrahydro-9H-pyrido[3,4-b]indole-3-carboxylic acid (DM2) with methanesulfonyl chloride, benzenesulfonyl chloride, and toluenesulfonyl chloride, which in turn, was synthesized by dissolving dilute aqueous ammonia with 2-(N-hydroxy methyl amino)-indol-3-yl-propanoic acid (DM1) which is the reaction product of l-tryptophan and formalin. All the intermediates and title compounds were characterized by physical, chemical, analytical, and spectral data. All the title compounds have been screened for in vivo antidiabetic activity in streptozotocin-induced diabetic rats, and serum glucose was estimated spectrophotometrically at 505 nm by glucose oxidase/peroxidase method. Compound DM5 showed potent antidiabetic activity.

the class of thioglitazone available in the market are showing severe adverse effects. [9,10] A survey of literature revealed that replacement of the thiazolidine-2,4-dione ring by various acidic groups such as α-heteroatom-substituted carboxylic acids and α-carbonsubstituted carboxylic acids can results in qualitative and quantitative changes in the activity, [11][12][13] which prompted us to undertake the synthesis of various new derivatives with the aim of having improved activity and less toxicity.

MATERIALS AND METHODS
The melting points were recorded in open sulphuric acid or oil bath using thermometer and were uncorrected. IR spectra were recorded using Hitachi 270-30 infrared and Bruker Vector 22 spectrophotometers using KBr pellet techniques. 1 H-NMR spectrum was recorded using DMSO-d 6 on Bruker Avance DPX-200 at 300 MHz, and their chemical shifts are recorded in δ (parts per million) units with respect to tetramethyl silane (TMS) as an internal standard. Atmospheric pressure ionization (API) mass spectra were recorded on Bruker Ion Trap Esquire 3000 spectrometer with the ionization potential 3000 V. Progress of the reactions was monitored using TLC, performed on aluminum plates precoated with silica gel-G, using chloroformmethanol (92:8) as the solvent systems, and the spots were visualized by exposure to iodine vapors. The physical constants of the title compounds are reported in Table 1.

Procedure for the preparation of 2-(N-hydroxymethyl amino)-indol-3-yl-propanoic acid (DM 1 )
In a 250-mL conical flask, 5 g (0.0245 M) of l-tryptophan was dissolved in 120 mL of water. To this, 20 mL formalin was added and the mixture was incubated at 38°C for 6 h. The colorless, crystalline, but rather granular solid that had then separated was collected, washed with cold water, and dried in vacuum. The crude product was recrystallized from hot water.

Procedure for the preparation of 1,2,3,4-tetrahydro-9H-pyrido [3,4-b] indole-3-carboxylic acid (DM 2 ):
In a 250-mL conical flask, 150 mL of dilute aqueous ammonia was placed and 5 g (0.0214 M) of α-hydroxymethylaminoβ-3-indolylpropionic acid (DM 1 ) was added and dissolved. Then, this solution was refluxed for 3 h and concentrated to a small volume. The white-to-cream colored crystals obtained were given charcoal treatment and filtered by suction, washed with cold water and dried at 80°C. The product was obtained in colorless leaflets and was purified by recrystallization from hot water.

Procedure for the preparation of 2-(methylsulfonyl)-1,2,3,4-tetrahydro-9H-pyrido [3,4-b]indole-3carboxylic acid (DM 3 )
Treated 3 g (0.01389 M) of the amine (DM 2 ) with 50 mL of 10% sodium hydroxide solution and 2 mL (3.5 g, 0.0303 M) of methanesulfonyl chloride was added in small proportions. The reaction mixture was stirred vigorously for 30 min and warmed it slightly with continued shaking. When the odor of methanesulfonyl chloride was dissipated, the mixture was cooled in an ice bath. As oil separated, crystallization was induced by rubbing the inner wall of the beaker with the glass rod in an ice bath. Any solid produced should be separated by filtration; and the crude product was recrystallized from dilute ethanol to give pure DM 3 as light brown crystals.

Procedure for the preparation of 2-(phenylsulfonyl)-1,2,3,4-tetrahydro-9H-pyrido [3,4-b] indole-3carboxylic acid (DM 4 )
To a 100-mL round bottom flask was added 3 g (0.01389 M) of amine (DM 2 ), 20 mL of dry pyridine, and 2 mL (2.75 g, 0.01559 M) of benzene sulfonylchloride. The reaction mixture was refluxed for 2 h on heating mantle and poured into crushed ice to give a light-brownish solid. As oil separated, crystallization was induced by rubbing the inner wall of the beaker with the glass rod in an ice bath. Any solid produced should be separated by filtration, and the crude product was recrystallized from dilute ethanol to give pure DM 4 as light brown crystals.

Procedure for the preparation of 2-(p-toluenesulfonyl)-1,2,3,4-tetrahydro-9H-pyrido[3,4-b]indole-3carboxylic acid (DM 5 )
Treated 3g (0.01389 M) of the amine (DM 2 ) with 50 mL of 10% sodium hydroxide solution and 3 g (0.01574 M) of p-toluenesulfonyl chloride was added in small proportions. The reaction mixture was stirred vigorously for 30 min and warmed it slightly and shaking was continued. When the odor of p-toluenesulfonyl chloride was dissipated, the mixture was cooled in an ice bath. As oil started separating, crystallization was induced by rubbing the inner wall of the beaker with the glass rod in an ice bath. Any solid produced should be separated by filtration, and the crude product was recrystallized from dilute ethanol to give pure DM 5 as light brown crystals.

Evaluation of antidiabetic activity
The synthesized compounds were screened for in vivo antidiabetic activity in streptozotocin-induced diabetic rats. After administration of standard drug (pioglitazone) and synthesized compounds (DM 1-5 ), serum glucose was estimated spectrophotometrically at 505 nm by glucose oxidase/peroxidase method using a commercially available kit (Span Diagnostic Ltd, Surat, India).
Concentration of glucose (mg/dL) = Optical density of test/Optical density of std. × 100.
The results are summarized in Table 2.