To a −78° C. solution of oxalyl chloride (0.203 mL, 2.33 mmol) in 10 mL DCM was added dropwise by syringe a solution of DMSO (0.330 mL, 4.66 mmol) in 3 mL DCM. The reaction mixture was stirred 35 minutes, then a solution of (R)-tert-butyl 4-(7-hydroxy-5-methyl-6,7-dihydro-5H-cyclopenta[d]pyrimidin-4-yl)piperazine-1-carboxylate (0.556 g, 1.66 mmol) in 5 mL DCM was added slowly by syringe. The reaction mixture was stirred another 1 hour at −78° C., after which neat TEA (1.09 mL, 7.81 mmol) was added. The reaction mixture was then allowed to warm to room temperature, stirred 30 minutes, and H2O was added. The mixture was extracted with 3×75 mL DCM, and the combined extracts were dried (Na2SO4), filtered, and concentrated in vacuo. The crude was purified on silica gel (Biotage 40M): the column was flushed with about 300 mL 4:1 DCM:EtOAc, then gradient to 1:4 DCM: EtOAc to give (R)-tert-butyl 4-(5-methyl-7-oxo-6,7-dihydro-5H-cyclopenta[d]pyrimidin-4-yl)piperazine-1-carboxylate (0.440 g, 79.6% yield) as a brown foam. MS (APCI+) m/z 333 [M+H]+. 1H NMR (CDCl3, 400 MHz) δ 8.73 (s, 1H), 3.93-3.82 (m, 2H), 3.74-3.48 (m, 7H), 2.96 (dd, J=19.6, 7.3 Hz, 1H), 2.34 (dd, J=19.6, 1.5 Hz, 1H), 1.50 (s, 9H), 1.32 (d, J=6.8 Hz, 3H).
