Pirfenidone structural isosteres: design, synthesis and spectral study

Introduction

Liver fibrosis represents the results of a sustained wound healing response to chronic hepatic cell injury that originates due to various causes, including viral infections, e.g., hepatitis B and C viruses; and metabolic disorders such as that of alcohol ingestion (Friedman, 2003; Guzman, 2008). The end-stage progression of hepatic fibrosis is known as cirrhosis and it is characterized by regenerative nodules surrounded by dense functionless fibrotic tissue (Guzman, 2008).

Several studies on experimental animal models of liver fibrosis showed possible spontaneous resolution of fibrous tissues after the removal of the fibrogenic stimulus (Issa et al., 2001). Similar results were also observed in human patients with liver fibrosis due to autoimmune hepatitis (Dufour et al., 1997) and biliary etiology (Hammel et al., 2001). However, the need for other means of treatment is vital, especially when the removal of the causative factor is unlikely. Recently, the different antifibrotic agents that inhibit the accumulation or resolve the already accumulated excess fibrous tissues in extracellular matrix (ECM) have been summarized (El Bialy et al., 2011a). These agents are thought to target one or more of the three stages involved in the process of fibrosis formation, i.e., the triggering stage, fibrogenesis and ECM accumulation (El Bialy et al., 2011a). One of these agents is pirfenidone (1a in Scheme 1) that was recently approved for the treatment of idiopathic pulmonary fibrosis (IPF) (Liu et al., 2009). Pirfenidone showed promising results in a liver fibrosis model by the inhibition of collagen synthesis, reduction of the production of number of cytokines and fibroblast proliferation (Di Sario et al., 2004). In the present investigation, the design and synthesis of new compounds with possible antifibrotic activity based on the structure of pirfenidone are reported. Structural modifications were designed to test the effect of altering the electronic character of phenyl group (1a–c), replacing the methyl group with the approximately same size bromine substituent but with the bromine being slightly more lipophilic (2a–c), and by replacing the pyridone with the more hydrophilic pyrimidone ring (3a–c). Although some of these compounds appeared previously in the literature (Stajer et al., 1987; Li and Dixon, 2004; Blatt et al., 2006; Kossen et al., 2009), we did not find associated physical or spectral data.

Scheme 1.

Synthesis of compounds 1a–c, 2a–c and 3a–c. Reagents and conditions: (i) Cu(OAc)₂ ‧H₂O, pyridine, molecular sieves 4 Å, CH₂Cl₂.

Results and discussion

Synthesis of compounds 1a–c, 2a–c and 3a–c was achie­ved employing a one-step coupling reaction performed under mild reaction conditions known as the Chan-Lam-Evans reaction (Chan and Lam, 2005; Rauws and Maes, 2012). The reaction involved the use of different arylboronic acids and CuII-containing catalyst in CH₂Cl₂ at room temperature (Chan and Lam, 2005; Sanjeeva Rao and Wu, 2012) (Scheme 1).

Boronic acids are synthesized from other boron-containing compounds (Hall, 2005) and used in many reactions forming C–C bonds in the presence of a palladium catalyst as in Suzuki-Miyaura coupling reaction (Suzuki, 2005), or C-heteroatom (N or O) bonds using copper catalyst as in Chan-Lam coupling reaction (Chan and Lam, 2005; Rauws and Maes, 2012; Sanjeeva Rao and Wu, 2012).

Herein, the appropriate arylboronic acid was allowed to react with the N-containing system (pyridone/pyrimidone), in the presence of Cu(OAc)₂ as a catalyst. The reaction is thought to start with a transmetalation step where the aryl moiety is transferred from the boronic acid to the Cu(OAc)₂ forming ArCu^IIOAc. Another portion of Cu(OAc)₂ oxidizes the produced ArCu^II(OAc) into ArCu^III(OAc)₂. The latter Cu^III species easily forms the C–N bond in the final product with reductive elimination of Cu^IOAc. The final step represents the oxidation of the produced Cu^IOAc into Cu^II(OAc)₂ completing the catalytic cycle (King et al., 2009).

The structures of all compounds 1a–c, 2a–c and 3a–c were verified using microanalytical and spectral analysis. The ¹H NMR spectra show peaks at δ 7.55–7.37 integrated as five protons to confirm the presence of an unsubstituted phenyl group incorporated into the pyridone and pyrimidone ring systems in compounds 1a, 2a and 3a. Compounds with para-methoxyphenyl group (1b, 2b and 3b) exhibit in their ¹H NMR spectra the characteristic singlet of methoxy group at δ 3.8. The shielding effect of this group is noticeable especially on the adjacent protons in its ortho positions, where, for example, a doublet peak equivalent to two protons (H-3′-5′) appears at δ 7.03 in compound 2b compared to an apparent triplet at δ 7.51 in the unsubstituted compound 2a.

Similarly, the strong deshielding effect of the -CN group in compounds 1c, 2c and 3c on the neighboring protons is obvious. For example, a doublet peak equivalent to two protons appears at δ 8.02 (H-3′-5′) in the ¹H NMR spectrum of compound 3c compared to an apparent triplet at δ 7.55 for the unsubstituted compound 3a.

Structures of all compounds were also confirmed by studying their ¹³C NMR spectra. Peaks that represent classical aromatic carbons appear at δ 126.7–129.2 in compounds 1a, 2a and 3a corresponding to five unsubstituted phenyl carbons (C2′/6′), (C3′/5′) and C4′. The sixth carbon C1′ signal appears deshielded at δ 137.1–141.0 due to its direct attachment to the heterocyclic N atom. Compounds 1b, 2b and 3b were characterized by having a peak at δ 55.4 corresponding to the -OCH₃ group, whereas compounds 1c, 2c and 3c have a peak at δ 116.4–118.3 corresponding to the -CN group.

The effect of the presence of the methoxy group in compounds 1b, 2b and 3b on the directly attached carbon (C-4′) and on carbons in its ortho positions (C3′/C5′) is evident; where C-4′ appears as more deshielded and C3′/C5′ as a more shielded peak than their corresponding carbons in the unsubstituted phenyl-containing compounds 1a, 2a and 3a. Conversely, the carbon directly attached to -CN in compounds 1c, 2c and 3c is more shielded, whereas carbons at ortho positions (C3′/C5′) appear to be more deshielded than their corresponding carbons in compounds 1a, 2a and 3a (Tables 1–3).

In anticipation of performing further modifications of the pirfenidone structure, the synthesis of a useful synthon 4 was achieved using the method adopted previously (El Bialy et al., 2011b) as outlined in Scheme 2. The previously prepared compound 2b was boronated by palladium catalyzed cross-coupling reaction via its interaction with bis(pinacolato)diboron in the presence of bis(dibenzylideneacetone)palladium, [Pd(dba)₂] as catalyst, tricyclohexylphosphine (PCy₃) as ligand, KOAc as base and anhydrous dioxane as solvent, at 80–90°C (El Bialy et al., 2011b).

Scheme 2.

Synthesis of compound 4. Reagents and conditions: (i) bis(dibenzylideneacetone) palladium, tricyclohexylphosphine, KOAc, dioxane, 80–90°C.

The structure of compound 4 was verified by microanalytical and spectral analysis. Its ¹H NMR spectrum exhibits the presence of the three peaks at δ 7.67, 7.56 and 6.45, of one proton each, characteristic for the three pyridone protons. It also shows the two doublet peaks, of two protons each, showing an AB-system pattern at δ 7.33 and 7.03, characteristic for the four protons of the substituted phenyl group, which is also consistent with the results of compound 2b. In addition, it shows one singlet at δ 1.25, equivalent for 12 protons assigned for the four methyl groups of the boronate ester.

¹³C NMR spectrum of compound 4 shows eight peaks in the aromatic region equivalent to ten carbons. In the aliphatic region, it shows three different peaks which match the proposed structure, bearing in mind that the carbon atom directly attached to boron atom does not appear in the spectrum, which is consistent with the previously reported results for similar compounds (El Bialy et al., 2011b).

Conclusion

The synthesis of 1a–c, 2a–c and 3a–c obtained under mild conditions by arylation of the appropriate heterocyclic amines with aryl boronic acids under Chan-Lam-Evans conditions is reported and analysis of their ¹H and ¹³C NMR spectral data is presented. The biological evaluation of the possible antifibrotic activity of these compounds will be completed and published elsewhere.

Experimental

Melting points were recorded using a Thomas-Hoover (Uni-Melt) capillary melting point apparatus and are uncorrected. Thin layer chromatography (TLC) analysis was carried out on silica gel 60 F 254 precoated aluminum sheets and detected under UV light. ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) spectra were recorded in DMSO-d₆ employing a Bruker Avance 400 MHz spectrometer. Elemental analyses were obtained from Atlantic Microlab Inc. (Norcross, GA, USA). All chemicals and solvents were purchased from Aldrich Chemical Co. or VWR.