(3R,4R)-tert-Butyl 3-((6,7-Dihydro-5H-pyrrolo[2,3-d]pyrimidin-4-yl)(methyl)amino)-4-methylpiperidine-1-carboxylate is an intermediate in the synthesis of 5,6-Dihydro CP-690550 (D450240), an impurity of CP-690550 (C781350), a pyrrolo[2,3-d]pyrimidine derivative, as Janus kinase inhibitor for treatment of rheumatoid arthritis.