Centhaquine

 Synthesis of Centhaquin

[0061] The synthesis of centhaquin was reported by Murthi and coworkers (Murthi et al U.S. Patent No. 3,983,121 ; Murti, Bhandari et al. 1989). In one procedure, reactants 1 and 2 were stirred at reflux for 15 hours. The resulting product was purified by evaporating the solvents to obtain an oil, which was heated in vacuo (100°C, 1 mm Hg). The remaining residue was recrystallized from ether-petroleum ether to obtain the final centhaquin product 3. The melting point reported for centhaquin was 76-77°C. In a subsequent publication (Murti, Bhandari et al. 1989), the reaction mixture was concentrated following 24 hours of reflux, diluted with water, and basified with aqueous NaOH. The basic mixture was extracted with ethyl acetate, and the ethyl acetate extracts were dried over anhydrous sodium sulfate and evaporated in vacuo to give centhaquin which was crystallized from hexane. The melting point of centhaquin (free base) obtained in this procedure was 82°C. The product obtained using either purification method is light tan in color, which is indicative of small amounts of impurities that were not completely removed using previously reported purification methods.

[0062] In accordance with the present invention, an improved purification method was found. According to the improved method, reactants 1 and 2 were stirred at reflux for 24 hours. The solvents were evaporated in vacuo and the resulting mixture was diluted with water and basified (10% NaOH). The basic mixture was extracted with ethyl acetate and the combined ethyl acetate extracts are dried over anhydrous sodium sulfate and evaporated in vacuo to obtain a residue, which was further purified with column chromatography (Si0₂, ethyl acetate). The resulting product can be decolorized using activated charcoal or directly crystallized from hot hexane to yield pure centhaquin. The resulting product is an off-white crystalline solid having a melting point of 94-95°C (free base). The product was

characterized using proton NMR, mass spectral, and elemental analysis and indicated high purity and superior quality.

[0063] Synthesis and characterization of centhaquin (free base): A mixture of 2- vinylquinoline (1) (5.0 g, 32.2 mmol, 98.5%) and 1 -(3-methylphenyl)piperazine (2) (5.68 g, 32.2 mmol, 99.0%) in absolute ethyl alcohol (150 ml) and glacial acetic acid (3.5 ml) was stirred at reflux for 24 hours in a round bottom flask. The reaction mixture was concentrated in vacuo, diluted with water (150 ml) and treated with 10% aqueous NaOH (150 ml). The residue was extracted with ethyl acetate (4 x 125 ml), dried with anhydrous Na₂S0₄, and concentrated under reduced pressure to yield a crude product which was purified by column chromatography using silica gel (100-200 mesh) with ethyl acetate as an eluent. The resulting compound was recrystallized from hot hexane and filtered, to yield centhaquin as an off- white crystalline solid (7.75 g, 23.4 mmol, 73% yield); mp. 94-95°C; i? 0.30 (100% ethyl acetate); 1H NMR (300 MHz, CDC1₃): δ 8.07 (t, J= 7.5 Hz, 2 H), 7.78 (d, J= 7.8 Hz, 1 H), 7.70 (t, J= 7.8 Hz, 1 H), 7.50 (t, J= 7.5 Hz, 1 H), 7.36 (d, J= 8.4 Hz, 1 H), 7.16 (t, J = 7.5 Hz, 1 H), 6.77 – 6.74 (m, 2 H), 6.69 (d, J= 7.2 Hz, 1 H), 3.26- 3.21 (m, 6 H), 2.97 – 2.92 (m, 2 H), 2.76 – 2.73 (m, 4 H), 2.32 (s, 3 H); HRMS (ESI) m/z 332.2121 [M+l]⁺ (calcd for C₂₂H26N₃ 332.2122); Anal. (C₂₂H₂₅N₃) C, H, N.

[0064] Preparation of centhaquin citrate: Centhaquin (free base) (5.62 g, 16.98 mmol) was treated with citric acid (3.26 g, 16.98 mmol) in a suitable solvent and converted to the citrate salt obtained as an off-white solid (7.96 g, 15.2 mmol, 90%); m.p. 94-96°C ; Anal.

10065] Figs. 1(a) and 1(b) are high resolution mass spectral analyses of centhaquin free base (Fig 1(a)) and centhaquin citrate (Fig. 1(b)). Compound samples were analyzed following ionization using electrospray ionization (ESI).

[0066J For centhaquin free base in Fig 1(a), a base peak [M+l]⁺ was observed at m z 332.2141 (theory: 332.2121) consistent with the elemental composition of protonated centhaquin (C₂₂H₂₆N₃).

[0067] For centhaquin citrate in Fig 1(b), the mass spectrum was identical to the mass spectrum obtained for the free base. An [M+l]⁺base peak was observed at m z 332.2141 (theory: 332.2121), which corresponds to the elemental composition of protonated centhaquin (C₂₂H₂₆N₃). This result is typical of salts of organic bases to yield the [M+l]⁺ of the free base as observed here with centhaquin citrate.

[0068] Mass spectrometry is one of the most sensitive analytical methods, and examination of the mass spectra of Fig. 1 indicate that the samples are devoid of any extraneous peaks and are of homogeneous purity (>99.5).