Structure-Activity Relationship of Phenylpyrazolones against Trypanosoma cruzi

Abstract

Chagas disease is a neglected parasitic disease caused by the parasitic protozoan Trypanosoma cruzi and currently affects around 8 million people. Previously, 2-isopropyl-5-(4-methoxy-3-(pyridin-3-yl)phenyl)-4,4-dimethyl-2,4-dihydro-3H-pyrazol-3-one (NPD-0227) was discovered to be a sub-micromolar inhibitor (pIC₅₀ =6.4) of T. cruzi. So far, SAR investigations of this scaffold have focused on the alkoxy substituent, the pyrazolone nitrogen substituent and the aromatic substituent of the core phenylpyrazolone. In this study, modifications of the phenyldihydropyrazolone scaffold are described. Variations were introduced by installing different substituents on the phenyl core, modifying the geminal dimethyl and installing various bio-isosteres of the dihydropyrazolone group. The anti T. cruzi activity of NPD-0227 could not be surpassed as the most potent compounds show pIC₅₀ values of around 6.3. However, valuable additional SAR data for this interesting scaffold was obtained, and the data suggest that a scaffold hop is feasible as the pyrazolone moiety can be replaced by a oxazole or oxadiazole with minimal loss of activity.

Keywords: Benznidazole; Trypanosoma cruzi; neglected parasitic diseases; phenylpyrazolones; structure-activity relationships.

Figure 1

Current drugs for Chagas disease: benznidazole (1) and nifurtimox (2). NPD‐0227 (3) was obtained from earlier hit optimization against T. cruzi.10

Scheme 1

Preparation of pyrazolones 57–68 (Table 1) with modifications on the central phenyl ring. a) KMnO₄, KPO₄H₂, tBuOH, RT, 16 h, 79 %; b) Br₂, dioxane, RT, 16 h‐9d, 68–95 %; c) Br₂, AcOH, 60 °C, 4 h, 75 %; d) Br₂, Fe, CHCl₃, RT, 8 h, 35 %; e) i: (COCl)₂, DMF, CH₂Cl₂, RT, 4 h, ii: LDA, methylisobutyrate, THF, −78 °C to RT, 2 h; f) i: 3‐ethoxy‐2,2‐dimethyl‐3‐oxopropanoic acid, (COCl)₂, DMF, CH₂Cl₂, ii: AlCl₃, CH₂Cl₂; g) N₂H₄, EtOH, RT, 16 h, 6–91 %; h) 2‐bromopropane, NaH, DMF, RT, 16 h, 29–85 %; i) 3‐pyridinyl‐B(OH)₂, Pd(dppf)Cl₂ ⋅ CH₂Cl₂, Na₂CO₃, DME/H₂O, 120 °C, 1 h, 6–88 %.

Scheme 2

Preparation of pyrazolone 74 (Table 1) with modification on the central phenyl ring. a) i: (COCl)₂, DMF, CH₂Cl₂, RT, 4 h; ii: LDA, methylisobutyrate, THF, −78 °C to RT, 2 h; b) N₂H₄, EtOH, RT, 16 h, 24 % over two steps; c) 2‐bromopropane, NaH, DMF, RT, 16 h, 75 %; d) CuCN, DMF, 150 °C, 18 h, 59 %; e) 3‐pyridinyl‐B(OH)₂, Pd(dppf)Cl₂ ⋅ CH₂Cl₂, Na₂CO₃, DME/H₂O, 120 °C, 1 h, 61 %.

Scheme 3

Introduction of variations on the gem‐dimethyl position of NPD‐0227 (3), resulting in 89–92 (Table 2). a) i: (COCl)₂, DMF, CH₂Cl₂, RT, 4 h; ii: LDA, methylisobutyrate, THF, −78 °C to RT, 2 h; b) ethyl potassium malonate, TEA, CDI, MgCl₂, ACN, THF, 44 %; c) K₂CO₃, 1,4‐dibromobutane, DMSO, RT, 51 %; d) N₂H₄, EtOH, RT, 16 h, 20–65 % over two steps; e) 2‐bromopropane, NaH, DMF, RT, 16 h, 74–96 %; f) 3‐pyridinyl‐B(OH)₂, Pd(dppf)Cl₂ ⋅ CH₂Cl₂, Na₂CO₃, DME/H₂O, 120 °C, 1 h, 22–60 %.

Scheme 4

Preparation of dihydropyrazolo‐oxazole 95 (Table 3). a) N₂H₄, EtOAc, RT, o/n, 67 %; b) K₂CO₃, 1,2‐dibromoethane, DMF, 80 °C, 2 h, 37 %; c) 3‐pyridinyl‐B(OH)₂, Pd(dppf)Cl₂ ⋅ CH₂Cl₂, Na₂CO₃, DME/H₂O, 120 °C, 1 h.

Scheme 5

Preparation of thiadiazole 98 (Table 3). a) 3‐pyridinyl‐B(OH)₂, Pd(dppf)Cl₂ ⋅ CH₂Cl₂, Na₂CO₃, DME/H₂O, 120 °C, 1 h, 91 %; b) hydrazinecarbothiamide, TFA, 60 °C, 6 h, 71 %.

Scheme 6

Preparation of the thione analogue of previous optimized hit NPD‐0227, 99 (Table 3). a) Lawessons reagent, THF, reflux, 32 h, 76 %.

Scheme 7

Preparation of the dihydropyridazinone analogue 104 (Table 3). a) AlCl₃, succinic anhydride, PhNO₂, 60 °C, 4 h, 42 %; b) N₂H₄ ⋅ H₂O, EtOH, reflux, 1 h, 89 %; c) 2‐bromopropane, NaH, DMF, 50 °C, 3 h, 87 %; d) 3‐pyridinyl‐B(OH)₂, Pd(dppf)Cl₂ ⋅ CH₂Cl₂, Na₂CO₃, DME/H₂O, 120 °C, 1 h, 63 %.

Scheme 8

Preparation of cyclopropylpyrolotriazole 109 (Table 3). a) Lawessons reagent, toluene, reflux, o/n, 69 %; b) N₂H₄ ⋅ H₂O, THF, RT→70 °C, 5 h, 82 %; c) cyclopropanecarbonylchloride, pyridine, 70 °C, 2 h, then DMF, 150 °C, 2 h, 47 %; d) 3‐pyridinyl‐B(OH)₂, Pd(dppf)Cl₂ ⋅ CH₂Cl₂, Na₂CO₃, DME/H₂O, 120 °C, 1 h, 30 %.

Scheme 9

Preparation of oxazoles 120–124 (Table 4). a) BrCH₂COR, NaH, DMF, RT, 30 min, 83–94 %; b) NH₄OAc, AcOH, 170 °C, 6 h, 11–36 %; c) 3‐pyridinyl‐B(OH)₂, Pd(dppf)Cl₂ ⋅ CH₂Cl₂, Na₂CO₃, DME/H₂O, 120 °C, 1 h, 36–74 %.

Scheme 10

Preparations of oxadiazoles 132–136 (Table 4). a) H₂SO₄, EtOH, reflux, 6 h, 97 %; b) N₂H₄ ⋅ H₂O, EtOH, reflux, 16 h, 53 %; c) R‐COOH, POCl₃, reflux, 1 h, 42–80 %; d) 3‐pyridinyl‐B(OH)₂, Pd(dppf)Cl₂ ⋅ CH₂Cl₂, Na₂CO₃, DME/H₂O, 120 °C, 1 h, 32–65 %.

Scheme 11

Synthesis of thiazoles 141 and 142 (Table 4). a) (COCl)₂, DMF, CH₂Cl₂, RT, 18 h; b) 30 % NH₄OH in H₂O, CH₂Cl₂, RT, 5 min, 75 % over two steps; c) Lawessons reagent, toluene, reflux, 18 h, 13 %; d) BrCH₂COR, propan‐2‐ol, RT, 2 h; e) 3‐pyridinyl‐B(OH)₂, Pd(dppf)Cl₂ ⋅ CH₂Cl₂, Na₂CO₃, DME/H₂O, 120 °C, 1 h, 35–44 % over two steps.