Design and synthesis of N-substituted indazole-3-carboxamides as poly(ADP-ribose)polymerase-1 (PARP-1) inhibitors(†)

Regioselective N-alkylation of the 1H-indazole scaffold; ring substituent and N-alkylating reagent effects on regioisomeric distribution

The indazole scaffold represents a promising pharmacophore, commonly incorporated in a variety of therapeutic drugs. Although indazole-containing drugs are frequently marketed as the corresponding N-alkyl 1H- or 2H-indazole derivative, the efficient synthesis and isolation of the desired N-1 or N-2 alkylindazole regioisomer can often be challenging and adversely affect product yield. Thus, as part of a broader study focusing on the synthesis of bioactive indazole derivatives, we aimed to develop a regioselective protocol for the synthesis of N-1 alkylindazoles. Initial screening of various conditions revealed that the combination of sodium hydride (NaH) in tetrahydrofuran (THF) (in the presence of an alkyl bromide), represented a promising system for N-1 selective indazole alkylation. For example, among fourteen C-3 substituted indazoles examined, we observed > 99% N-1 regioselectivity for 3-carboxymethyl, 3-tert-butyl, 3-COMe, and 3-carboxamide indazoles. Further extension of this optimized (NaH in THF) protocol to various C-3, -4, -5, -6, and -7 substituted indazoles has highlighted the impact of steric and electronic effects on N-1/N-2 regioisomeric distribution. For example, employing C-7 NO2 or CO2Me substituted indazoles conferred excellent N-2 regioselectivity (≥ 96%). Importantly, we show that this optimized N-alkylation procedure tolerates a wide structural variety of alkylating reagents, including primary alkyl halide and secondary alkyl tosylate electrophiles, while maintaining a high degree of N-1 regioselectivity.

Discovery and structure-activity relationship of novel 2,3-dihydrobenzofuran-7-carboxamide and 2,3-dihydrobenzofuran-3(2H)-one-7-carboxamide derivatives as poly(ADP-ribose)polymerase-1 inhibitors

Novel substituted 2,3-dihydrobenzofuran-7-carboxamide (DHBF-7-carboxamide) and 2,3-dihydrobenzofuran-3(2H)-one-7-carboxamide (DHBF-3-one-7-carboxamide) derivatives were synthesized and evaluated as inhibitors of poly(ADP-ribose)polymerase-1 (PARP-1). A structure-based design strategy resulted in lead compound 3 (DHBF-7-carboxamide; IC₅₀ = 9.45 μM). To facilitate synthetically feasible derivatives, an alternative core was designed, DHBF-3-one-7-carboxamide (36, IC₅₀ = 16.2 μM). The electrophilic 2-position of this scaffold was accessible for extended modifications. Substituted benzylidene derivatives at the 2-position were found to be the most potent, with 3′,4′-dihydroxybenzylidene 58 (IC₅₀ = 0.531 μM) showing a 30-fold improvement in potency. Various heterocycles attached at the 4′-hydroxyl/4′-amino of the benzylidene moiety resulted in significant improvement in inhibition of PARP-1 activity (e.g., compounds 66–68, 70, 72, and 73; IC₅₀ values from 0.718 to 0.079 μM). Compound 66 showed selective cytotoxicity in BRCA2-deficient DT40 cells. Crystal structures of three inhibitors (compounds (−)-13c, 59, and 65) bound to a multidomain PARP-1 structure were obtained, providing insights into further development of these inhibitors.