Crystal structures of salts of bedaquiline

Bedaquiline [systematic name: 1-(6-bromo-2-methoxyquinolin-3-yl)-4-(dimethylamino)-2-(naphthalen-1-yl)-1-phenylbutan-2-ol, C₃₂H₃₁BrN₂O₂] is one of two important new drugs for the treatment of drug-resistant tuberculosis (TB). It is marketed in the US as its fumarate salt {systematic name: [4-(6-bromo-2-methoxyquinolin-3-yl)-3-hydroxy-3-(naphthalen-1-yl)-4-phenylbutyl]dimethylazanium 3-carboxyprop-2-enoate, C₃₂H₃₂BrN₂O₂⁺·C₄H₃O₄^-}, and about a dozen other salts of bedaquiline have been described in patent literature, but none have so far been structurally described. In a first communication, we present the crystal structure of bedaquilinium fumarate and of two new benzoate salts, as well as that of a degradation product of the reaction of bedaquilinium fumarate with sodium ethoxide, 3-benzyl-6-bromo-2-methoxyquinoline, C₁₇H₁₄BrNO. The fumarate and benzoate salts both feature cations monoprotonated at the dimethylamino group. The much less basic quinoline N atom remains unprotonated. Both salts feature a 1:1 cation-to-anion ratio, with the fumarate being present as monoanionic hydrofumarate. The conformations of the cations are compared to that of free base bedaquiline and with each other. The flexible backbone of the bedaquiline structure leads to a landscape of conformations with little commonalities between the bedaquiline entities in the various structures. The conformations are distinctively different for the two independent molecules of the free base, the two independent molecules of the hydrofumarate salt, and the one unique cation of the benzoate salt. Packing of the salts is dominated by hydrogen bonding. Hydrogen-bonding motifs, as well as the larger hydrogen-bonded entities within the salts, are quite similar for the salts, despite the vastly differing conformations of the cations, and both the hydrofumarate and the benzoate structure feature chains of hydrogen-bonded anions that are surrounded by and hydrogen bonded to the larger bedaquilinium cations, leading to infinite broad ribbons of anions, cations, and (for the benzoate salt) water molecules. The benzoate salt was isolated in two forms: as a 1.17-hydrate (C₃₂H₃₂BrN₂O₂⁺·C₇H₅O₂^-·1.166H₂O), obtained from acetone or propanol solution, with one fully occupied water molecule tightly integrated into the hydrogen-bonding network of anions and cations, and one partially occupied water molecule [refined occupancy 16.6 (7)%], only loosely hydrogen bonded to the quinoline N atom. The second form is an acetonitrile solvate (C₃₂H₃₂BrN₂O₂⁺·C₇H₅O₂^-·0.742CH₃CN·H₂O), in which the partially occupied water molecule is replaced by a 74.2 (7)%-occupied acetonitrile molecule. The partial occupancy induces disorder for the benzoate phenyl ring. The acetonitrile solvate is unstable in atmosphere and converts into a form not distinguishable by powder XRD from the 1.17-hydrate.