Process Development and Synthesis of Birinapant: Large Scale Preparation and Acid-Mediated Dimerization of the Key Indole Intermediate

Birinapant/TL32711 (1) is a novel bivalent antagonist of the inhibitor of apoptosis (IAP) family of proteins which is currently in clinical development for the treatment of cancer and hepatitis B virus (HBV) infection. In this report, we present a detailed description of the 1 drug substance synthesis used to support our ongoing clinical studies. Key transformations in this process included the development of a scalable, high-yielding route to acyl indole 14 as well as a two-step dimerization/oxidation of indole 19 that afforded biindole 21 in excellent yield and purity (70% yield, 2 steps; >95 area% purity by HPLC analysis). In addition, partial defluorination of 21 was observed following hydrogen-mediated benzyloxycarbonyl (Cbz) protective group removal which was obviated by the use of HBr/HOAc for this transformation. The use of commercially available amino acid derivatives afforded related impurities which proved difficult to purge in subsequent steps. Thus, defining the impurity specification for these reagents was critical to providing 1 drug substance of >99 area% chemical purity. Using this process, we have successfully prepared 1 drug substance multiple times on >500-g-scale in support of our clinical development program.