Process development and large-scale synthesis of a PDE4 inhibitor

An efficient, scalable synthesis of the PDE4 inhibitor, 6-[1-methyl-1-(methylsulfonyl)ethyl]-8-(3-{(E)-2-(3-methyl-1,2,4-oxadiazol-5-yl)-2-[4- (methylsulfonyl)phenyl] vinyl}phenyl)-quinoline benzenesulfonate (10) is described. The synthesis is highly convergent, generating the penultimate 9 by coupling aldehyde 7 and oxadiazole 8 in a Knoevenagel reaction. The process consists of a total of nine chemical steps, five of which comprise the sequence to prepare aldehyde 7 via Skraup reaction, bromination, sulfone formation, methylation and Suzuki-Miyaura cross-coupling, and a two-step sequence for the synthesis of oxadiazole 8 that includes the methylamidoxime and oxadiazole steps. The final two steps are Knoevenagel coupling and salt formation. The process produced the drug candidate 10 in 46% overall yield from 2-bromo-4-methylaniline (1) on multikilogram scale.