A Concise Synthesis of Pyrrole-Based Drug Candidates from α-Hydroxyketones, 3-Oxobutanenitrile, and Anilines

A simple and concise three-component synthesis of a key pyrrole framework was developed from the reaction between α-hydroxyketones, oxoacetonitriles, and anilines. The synthesis was used to obtain several pyrrole-based drug candidates, including COX-2 selective NSAID, antituberculosis lead candidates BM212, BM521, and BM533, as well as several analogues. This route has potential to obtain diverse libraries of these pyrrole candidates in a concise manner to develop optimum lead compounds.


Introduction
The practical synthesis of drug lead compounds in a concise manner is crucial for biological evaluation and the development of structure activity relationship (SAR) studies for further optimization. This becomes even more crucial when developing an industrial synthetic route for a specific drug. Several pyrrole-based drugs have already been introduced in the market with great success [1][2][3]. Additionally, many pyrrole-based lead compounds demonstrated positive bioactivities, such as anti-inflammatory [4,5], anti-bacterial [6], and anti-cancer [7] activities. For example, pyrrole-based compounds 1a-d have demonstrated selective cyclooxygenases (COX-2) inhibition with nonsteroidal anti-inflammatory activity [4][5][6][7][8], while pyrrole-based compounds 2a-e have demonstrated antituberculosis activities [9,10] (Figure 1).

Introduction
The practical synthesis of drug lead compounds in a concise manner is crucial for biological evaluation and the development of structure activity relationship (SAR) studies for further optimization. This becomes even more crucial when developing an industrial synthetic route for a specific drug. Several pyrrole-based drugs have already been introduced in the market with great success [1][2][3]. Additionally, many pyrrole-based lead compounds demonstrated positive bioactivities, such as anti-inflammatory [4,5], anti-bacterial [6], and anti-cancer [7] activities. For example, pyrrole-based compounds 1a-d have demonstrated selective cyclooxygenases (COX-2) inhibition with nonsteroidal anti-inflammatory activity [4][5][6][7][8], while pyrrole-based compounds 2a-e have demonstrated antituberculosis activities [9,10] (Figure 1). Several procedures were reported to obtain pyrrole COX-2 selective NSAIDs. However, the reported syntheses are lengthy and gave low overall yields, making SAR studies difficult and synthesis at large scale less practical. Typical syntheses of pyrroles possessing COX-2 [4] and antituberculosis inhibitory activities [11] are shown in Scheme 1. Therefore, considering the problems associated with the current synthetic routes and the potential of pyrrole-based drugs, a practical and concise synthetic route is highly advantageous. Scheme 1. Examples of previous syntheses of pyrrole-based bioactive compounds [4,12].
Previously, we reported the selective synthesis of N-substituted 2,3,5-functionalized 3-cyanopyrroles via a one-step, three-component reaction between α-hydroxyketones, oxoacetonitriles, and primary amines [13]. The mild reaction conditions (AcOH as a catalyst, EtOH, 70 °C, 3 h), applicability on a large scale, and high atom efficiency (water is the only molecule lost during the reaction) warranted the application of this synthesis to important pyrrole-based lead drug candidates [13]. In this work, we report the synthesis of a key pyrrole framework and develop it further for the synthesis of several pyrrole lead drug candidates, including COX-2 selective inhibitor, antituberculosis lead candidates BM212 2a, BM521 2b, and BM533 2c, and several analogues. Scheme 1. Examples of previous syntheses of pyrrole-based bioactive compounds [4,12].
Previously, we reported the selective synthesis of N-substituted 2,3,5-functionalized 3-cyanopyrroles via a one-step, three-component reaction between α-hydroxyketones, oxoacetonitriles, and primary amines [13]. The mild reaction conditions (AcOH as a catalyst, EtOH, 70 • C, 3 h), applicability on a large scale, and high atom efficiency (water is the only molecule lost during the reaction) warranted the application of this synthesis to important pyrrole-based lead drug candidates [13]. In this work, we report the synthesis of a key pyrrole framework and develop it further for the synthesis of several pyrrole lead drug candidates, including COX-2 selective inhibitor, antituberculosis lead candidates BM212 2a, BM521 2b, and BM533 2c, and several analogues.

Scheme 3. Synthesis of pyrroles 15-17
To demonstrate the feasibility of converting compounds 15, 16, and 17 to BM212 2a, BM521 2b, and BM533 2c, respectively, and hence the power of the three-component reaction, we demonstrated the synthesis of BM212 2a from pyrrole 15. Hence, the cyano group of pyrrole 15 was reduced using diisobutylaluminium hydride (DIBAL-H) to give carbaldehyde 18 in 93% yield (Scheme 4) [14]. When carbaldehyde 18 was treated with 1methylpiperazine in the presence of AcOH in DCM for 2 h, followed by the addition of NaBH(AcO)3 and stirring overnight, BM212 2a was obtained in 95% yield after silica gel column chromatographic purification [12]. BM521 2b and BM533 2ccan be obtained in a similar fashion from pyrroles 16 and 17, respectively. α-Hydroxyketones 3 and 11-13 were synthesized from the corresponding substituted phenacyl bromide using sodium formate (General Procedure 1, experimental section and [13]). Scheme 4. Synthesis of BM212 2a from pyrrole product 15, and X-ray single crystal structure of BM212 2a (CCDC 2191163) (displacement ellipsoids are drawn at the 50% probability level).

Materials and Methods
Scheme 4. Synthesis of BM212 2a from pyrrole product 15, and X-ray single crystal structure of BM212 2a (CCDC 2191163) (displacement ellipsoids are drawn at the 50% probability level).

Materials and Methods
All chemicals and AR grade solvents were obtained from Sigma-Aldrich (Saint Louis, MO, USA), Merck (Lebanon, NJ, USA), or Alfa Aesar (Tewksbury, MA, USA) and were used as received without further purification. IR spectra were recorded using a Bruker MPA FT-IR machine (Karlsruhe, Germany). 1 H NMR spectra were recorded at 400 MHz Bruker Avance III 400 (BBFO 400). 13 C NMR spectra were recorded at 101 MHz Bruker Avance III 400 (BBFO 400). HRMS were measured using a hybrid Quadrupole Time-of-Flight (Q-TOF) on a Qstar XL MS/MS system (Milford, CT, USA). Single-crystal X-ray crystallographic analysis was done using Bruker D8 Quest (Karlsruhe, Germany). Analytical TLC was performed using Merck 60 F 254 precoated silica gel plates (0.2 mm thickness) (Oakville, ON, Canada). The plates were visualized under UV (254 nm) or stained in ceric ammonium sulfate solution with heating to detect the reaction spots. Flash chromatography was performed using Merck silica gel 60 (230-400 mesh) (Oakville, ON, Canada). Copies of the 1 H NMR, 13 C NMR, and single-crystal X-ray data of pyrrole 20 can be found in the Supplementary Materials.

General Procedure 1: Preparation of Substituted Phenacyl Alcohols 3 and 11-13
A solution of the phenacyl bromide (20 mmol) and sodium formate (16 mmol) in an ethanol/water mixture (30 mL, EtOH: H 2 O = 9:1) was stirred at 90 • C for 12 h (Scheme 5). Once the reaction was completed (TLC), the mixture was allowed to cool to room temperature and ethanol was removed under vacuum. Water (30 mL) was added to the residue, and the resulting mixture was extracted with ethyl acetate (3 × 30 mL). The combined organic layers were dried over Mg 2 SO 4 and the solvent was evaporated using under vacuum. The residue was then purified using column chromatography using EtOAc/Hexane as the eluent (2:3 for 3; 1:4 for 11-13).

General Procedure 1: Preparation of Substituted Phenacyl Alcohols 3 and 11-13
A solution of the phenacyl bromide (20 mmol) and sodium formate (16 mmol) in an ethanol/water mixture (30 mL, EtOH: H2O = 9:1) was stirred at 90 °C for 12 h (Scheme 5). Once the reaction was completed (TLC), the mixture was allowed to cool to room temperature and ethanol was removed under vacuum. Water (30 mL) was added to the residue, and the resulting mixture was extracted with ethyl acetate (3 × 30 mL). The combined organic layers were dried over Mg2SO4 and the solvent was evaporated using under vacuum. The residue was then purified using column chromatography using EtOAc/Hexane as the eluent (2: AcOH (1.0 eq.) was added dropwise to a stirred solution of substituted phenacyl alcohols 3 and 11-13 (1.0 eq.), oxoacetonitriles 4 and 7 (1.0 eq.), and primary amine 5, 6, 8, and 14 (1.1 eq.) in EtOH (3 mL) at room temperature. The resulting mixture was heated at 70 °C for 3 h (TLC). The reaction mixture was then evaporated to dryness under vacuum to give the crude product as a foam. The foam was purified using silica gel column chromatography with 5-35% EtOAc/Hexane as eluent t yield the pure products. All reactions were conducted using 1.0 mmol of the substrates 3 and 11-13. This general procedure was used to prepare N-substituted 2,3,5-functionalized pyrroles 1a, 9-10 and 15-17.