2-chloro-8-cyclopentyl-5-methyl-8H-pyrido(2,3-d)pyrimidin-7-one

It is further converted into the 6-bromo derivative or substituted directly at the 2-position for final drug formulation. 2-chloro-8-cyclopentyl-5-methyl-8H-pyrido[3-d]pyrimidin-7-one (CAS 1013916-37-4) is a crucial intermediate in the synthesis of Palbociclib (a CDK4/6 inhibitor), typically produced by the cyclization of 1-(2-chloro-4-(cyclopentylamino)pyrimidin-5-yl)ethanone with ethyl acetate.