Process Development toward a Pro-Drug of Buprenorphine

This paper describes the process development conducted toward the multi-kilogram synthesis of a hemiadipic acid pro-drug of buprenorphine. The major challenges were to control the levels of the dibuprenorphine monoadipate impurity and to improve the robustness of the final recrystallization. Modification of the reaction conditions limited the levels of the impurity formed during the reaction, and an additional point of control was introduced, a reslurry with dichloromethane. The solid-state properties of the pro-drug were fully investigated, and extensive polymorph screening was undertaken. An unusual low-temperature endothermic event was observed in the differential scanning calorimetry thermogram. This low-temperature event was further investigated using variable-temperature X-ray powder diffraction and variable-temperature solid-state nuclear magnetic resonance spectroscopy. It was concluded that this event was likely to be associated with a reversible, minor conformational change of the adipate side chain.