The synthesis of OSU 6162: Efficient, large-scale implementation of a Suzuki coupling

The synthesis of the chiral, nonracemic 3-aryl piperidine, OSU 6162 ((1) under bar), a potential CNS agent from Pharmacia Corporation, is presented. The key construction in the described synthesis is a palladium-catalyzed aryl cross-coupling reaction between bromosulfone ((4) under bar) and pyridyl borane ((14) under bar). Initially developed conditions for this Suzuki reaction, conducted in tetrahydrofuran/aqueous hydroxide, delivered free base ((6) under bar) or hydrochloride salt ((15a) under bar) in reproducible 80% yield. However, by changing the solvent to toluene and the base to carbonate, significant decreases in catalyst requirement were realized, and the methane sulfonate salt ((15b) under bar) of the coupled product could be obtained in reproducible 92-94% yield on 200-kg input. The success of the Suzuki reaction was critically dependent on a bulk source of the pyridyl borane coupling partner. Cryogenic conditions were developed for its generation via lithium-halogen exchange to generate thermally labile 3-lithiopyridine followed by transmetalation with diethylmethoxy borane. This highly exothermic series of transformations yielded crystalline diethyl-3-pyridyl borane in reproducible 75-80% yield on scales ranging up to 200-kg input. Selective reduction of the biaryl, classical resolution and introduction of the propyl group via the Gribble reductive amination procedure completed the synthesis of OSU 6162 free base. This route was employed to deliver over 35 kg of clinical-quality bulk drug in short order.