Heterocyclic core modifications in trypanosomacidal 2-[(phenylheteroaryl)ethyl]ureas

Abstract

The protozoan parasites Trypanosoma brucei and Trypanosoma cruzi, which cause human African trypanosomiasis (HAT) and Chagas disease, respectively, are responsible for considerable human suffering. Reduced case numbers and improved treatment options for HAT provide hope, but the outlook for Chagas disease is less promising, and safer, more efficacious chemotherapy is sorely needed. We previously reported the discovery and optimisation of a novel class of potent and selective trypanosomacidal 2-[(2-phenylthiazolyl)ethyl]ureas active against both T. brucei brucei and T. cruzi. In the current work, replacement of the core thiazole with alternative heterocycles has revealed that a contiguous arrangement of phenyl substituent, hydrogen-bond-accepting nitrogen, and alkyl linker are required to maintain activity. Compared to the parent thiazole, increased polarity of the core heterocycle in triazoles, tetrazoles and pyrimidines, leads to a drop in potency against T. b. brucei. A 2,6-disubsituted pyridine is tolerated but in general, 5-membered heterocycles are preferred. Analogues with oxazole, pyrazole and isomeric ('reverse') pyrazole cores displayed comparable T. b. brucei potency and selectivity to the parent thiazole, and in some cases improved lipophilic ligand efficiencies and metabolic stability. These compounds possessing more polar core heterocycles were generally 2-4 times less potent against T. cruzi (compared to T. b. brucei). This study demonstrates robust structure-activity relationships across a variety of heterocyclic scaffolds, providing many options for further optimisation of this class of compounds.

Fig. 1. Structures of drugs in clinical trial or currently used for the treatment of African (blue) and American (black) trypanosomiasis.

Fig. 2. HTS hits relevant to the current work. Selectivity index (SI) = IC₅₀ [human embryonic kidney (HEK293) cells]/IC₅₀ [trypanosome].

Fig. 3. Lead compound 3 and key characterisation data. A ‘traffic lights’ system is used to indicate excellent/good (green), adequate (orange) and poor (red) properties. Mw = molecular weight, PSA = polar surface area, PPB = protein plasma binding, HLM = human liver microsomes.

Fig. 4. Early pre-leads (4, 5) and generic structures (6, 7) of compounds targeted in this study.

Scheme 1. Synthesis of 4-phenyltriazoles. Reagents, conditions and yields: a) NEt₃, DCM, 4 : 1 ratio of 9 and 10, 75%; b) NaN₃, DMF, 85 °C, 73%; c) CuSO₄·5H₂O, sodium ascorbate, 2 : 1 t-BuOH/H₂O, 87%; d) TFA, DCM, 0 °C → RT, quant., (62% after recryst. from i-PrOH); e) DCM, NEt₃, BzCl, 0 °C → RT, mixture of 15 and 16, yield not determined; f) conc. HCl, MeOH, 0 °C → RT, quant., (98% after recryst. from EtOH); g) crude hydrochloride 17, DCM, NEt₃, BzCl, 0 °C → RT, 15 only, 67% (after recryst. from EtOAc).

Scheme 2. Synthesis of 1-phenyltriazoles. Reagents, conditions and yields: a) Cs₂CO₃, MeCN, 0%; b) Boc₂O, DCM, 0 °C → RT, 74%; c) PhN₃, CuSO₄·5H₂O, sodium ascorbate, 2 : 1 t-BuOH/H₂O, 60%; d) 1. conc. HCl, MeOH, 0 °C → RT, evap. 2. BzCl, NEt₃, DCM, 0 °C → RT, 41% (22b) or AcCl, EtOH, 0 °C → RT, evap. 2. cyclopentanecarbonyl chloride, NEt₃, DCM, 0 °C → RT, 94% (22c).

Scheme 3. Synthesis of 1,2,4-triazoles. Reagents, conditions and yields: a) NaH, DMF, 100 °C, 47%; b) 1. AcCl, EtOH, 0 °C → RT, evap. 2. BzCl or cyclopentanecarbonyl chloride, NEt₃, DCM, 0 °C → RT, 65% (25b), 66% (25c). Representation of the crystal structure of 25a. Ellipsoids are shown at 50% probability amplitudes with hydrogen atoms assigned arbitrary radii.

Scheme 4. Synthesis of tetrazoles. Reagents, conditions and yields: a) K₂CO₃, DMF, 80 °C, 71% (27a); b) 1. conc. HCl, MeOH, 0 °C → RT, evap. 2. BzCl, NEt₃, DCM, 0 °C → RT, 66% (27b) or AcCl, EtOH, 0 °C → RT, evap. 2. cyclopentanecarbonyl chloride, NEt₃, DCM, 0 °C → RT, 59% (27c). Representation of the crystal structure of 27a. Ellipsoids are shown at 50% probability amplitudes with hydrogen atoms assigned arbitrary radii.

Scheme 5. Synthesis of 3-phenylpyrazoles. Reagents, conditions and yields: a) K₂CO₃, DMF, 70 °C, 55%; b) conc. HCl, 1,4-dioxane, quant.; c) K₂CO₃, DMF, 73% d) conc. NH₃, 60 °C, 60%; e) DAIB, MeOH, 51% f) 4 M HCl, reflux, quant. g) BzCl, NEt₃, DCM, 0 °C → RT, 57% (30b); cyclopentanecarbonyl chloride, NEt₃, DCM, 0 °C → RT, 98% (30c); h) CDI, DMF, MeCN, 46%; i) NEt₃, piperidine, DCM, 94%.

Scheme 6. Synthesis of 1-phenylpyrazoles. Reagents, conditions and yields: a) Cs₂CO₃, CuBr₂, DMF, 190 °C, μW, 80%; b) NaNO₂, THF 4 M HCl, KI; c) NEt₃, cat. PdCl₂(dppf), TBAB, DMF/H₂O, 50 °C, 62% (two steps); d) H₂, 5% Pd/C, EtOH, 50 °C, 85%; e) DPPA, i-Pr₂NEt, t-BuOH, 50% f) TsOH, MeCN, 60% g) 1. p-nitrophenyl chloroformate, NEt₃, DCM/THF; 2. piperidine, 72%. TBAB = tetrabutylammonium bromide; DPPA = diphenyl phosphoryl azide.

Scheme 7. Synthesis of oxazoles. Reagents, conditions and yields: a) KCN, DMF, 60 °C, 86%; b) 1. Boc₂O, NiCl₂·6H₂O, NaBH₄, 0 °C, 2. diethylaminetriamine, 76%; c) conc. HCl, dioxane, 0 °C → RT, 96%; d) NEt₃, DCM, 55%.

Scheme 8. Synthesis of 2,6-disubstituted pyridines. Reagents, conditions and yields: a) PhB(OH)₂, K₂CO₃, TBAB, 1,4-dioxane/H₂O, cat. PdCl₂(dppf), 40 °C, 93%; b) methyl or benzyl acrylate, cat. P(o-Tol)₃, cat. Pd(OAc)₂, NEt₃, μW 90 °C, 50% (56); c) H₂, 5% Pd/C, EtOH, 60 °C; d) 1. LiOH, THF/H₂O, 2. H₃O⁺ pH 7, 55% (from 55); e) H₂, 5% Pd/C, EtOH, 60 °C, 70%; f) DPPA, NEt₃, t-BuOH, reflux, 26%; g) TsOH, MeCN, 73%; h) cyclopentanecarboxylic acid, NEt₃, EDCI, DMAP, DMF, 40 °C, 94%; i) 1. p-nitrophenyl chloroformate, NEt₃, DCM, 2. piperidine, 40%.

Scheme 9. Synthesis of 2,5-disubstituted pyridines. Reagents, conditions and yields: a) Ph(BOH)₂, Na₂CO₃, THF/H₂O, cat. Pd(PPh₃)₄, 75 °C, 48%; b) cat. P(o-Tol)₃, cat. Pd(OAc)₂, NEt₃, MeCN, μW 90 °C, 56%; c) H₂, 5% Pd/C, EtOH, 50 °C, 64%; d) DPPA, NEt₃, t-BuOH, reflux, 34%; e) TFA, DCM, quant.; f) cyclopentanecarboxylic acid, NEt₃, EDCI, DMAP, DMF, 40 °C, 58%; g) 1. p-nitrophenyl chloroformate, NEt₃, DCM, 2. piperidine, 26%.

Scheme 10. Synthesis of pyrimidines. Reagents, conditions and yields: a) Ph(BOH)₂, Na₂CO₃, MeCN, cat. Pd(PPh₃)₄, 90 °C, 47%; b) HI; c) t-butyl acrylate, cat. Pd(OAc)₂, cat. P(o-Tol)₃, cat. PBu₃.HBF₄, MeCN, μW 70 °C 32% (two steps from 73); d) tributyl(vinyl)tin, cat. Pd(OAc)₂, PPh₃, THF, reflux, 83%; e) BnNH₂, EtOH, cat. AcOH, 30%; f) 1 : 1 conc. NH₃/EtOH, 0.08 M, sealed tube, 70 °C, 76%; g) cyclopentanecarboxylic acid, EDCI, cat. DMAP, DMF, 27%.

Scheme 11. Synthesis of 4-phenyl (‘reverse’) thiazoles. Reagents, conditions and yields: a) EtOH, 80 °C, 70%; b) 1. Boc₂O, NiCl₂·6H₂O, NaBH₄, 0 °C, 2. diethylaminetriamine, 95%; c) 4 M HCl in dioxane, 0 °C → RT, used directly in next step; d) NEt₃, DCM, 45% (two steps).

Fig. 5. Active and inactive t-butyl carbamates.