The formation of the d-homocyclopamine ring system in IPI-926 is the key step in its semisynthesis and proceeds via a chemoselective cyclopropanation followed by a stereoselective acid-catalyzed carbocation rearrangement. In order to perform large-scale cyclopropanation reactions, we developed new iodomethylzinc bis­(aryl)­phosphate reagents that were found to be both effective and safe. These soluble reagents can be prepared under mild conditions and are stable during the course of the reaction. Importantly, they have favorable energetics relative to other cyclopropanating agents such as EtZnCH2I. Herein, we describe the process optimization studies that led to successful large-scale production of the d-homocyclopamine core necessary for IPI-926.

A design of experiments (DoE) approach was taken to optimize purity and reaction yield of the final debenzylation and hydrochloride salt formation of IPI-926. The study involved a careful dissection of the different process steps to enable an independent investigation of these steps while ensuring that process streams were representative. The results enabled a streamlined process from the final chemical transformation to the salting and isolation and led to the elimination of variability in the process as well as a robust control of impurities. The optimized process was applied to production and demonstrated on the kilogram scale.