Synthesis of Carbon 14 and Deuterium‐Labelled Nerandomilast (BI 1015550)

(R)-2-(4-(5-Chloropyrimidin-2-yl)piperidin-1-yl)-4-((1-(hydroxymethyl)cyclobutyl)amino)-6,7-dihydrothieno[3,2-d]pyrimidine 5-oxide (BI 1015550, 1) is a potent and selective inhibitor of phosphodiesterase type 4 (PDE4) being developed for the treatment of idiopathic pulmonary fibrosis (IPF) and progressive pulmonary fibrosis (PPF). We report the synthesis of this drug candidate labelled with carbon 14 and deuterium. The carbon 14 synthesis was completed in three radioactive steps in 27% overall yield, with a specific activity of 52 mCi/mmol (1.92 GBq/mmol), radiochemical purity, and enantiomeric excess higher than 99%. The deuterium labelled compound was prepared in seven steps in 67% overall yield and with isotopic enrichment, chemical purity, and enantiomeric excess higher than 99%.