Manufacture of the PI3K β‑Sparing Inhibitor Taselisib. Part 1: Early-Stage Development Routes to the Bromobenzoxazepine Core

Two convergent regioselective routes for the synthesis of the tetracyclic imidazobenzoxazepine triazole 1, a key intermediate toward the synthesis of taselisib, are described. In the first-generation route, a chemoselective Negishi cross-coupling reaction was developed between iodoimidazole 3 and triazole 7, which enabled the delivery of initial kilogram quantities of 1. Because of the inefficiencies in the preparation of the imidazole 3, a second-generation route via a highly regioselective imidazole ring formation between α-chloroketone 11 and aryl amidine 12 was developed. The resulting imidazole 14 provided the handle to efficiently install the seven-membered benzoxazepine ring system in one pot with two-step N-alkylation and SNAr tandem reactions.