A solution of DMSO (5.45 mL, 76.8 mmol) in DCM (50 mL) was added dropwise by addition funnel to a −78° C. solution of oxalyl chloride (3.35 mL, 38.4 mmol) in DCM (150 mL). The reaction mixture was stirred for 35 minutes, and then a solution of (5R)-tert-butyl 4-(7-hydroxy-5-methyl-6,7-dihydro-5H-cyclopenta[d]pyrimidin-4-yl)piperazine-1-carboxylate (9.17 g, 27.4 mmol) in DCM (80 mL) was added slowly by addition funnel. The reaction mixture was stirred another 1 hour at −78° C., after which neat triethylamine (18.0 mL, 129 mmol) was added to the mixture. The reaction mixture was then allowed to warm to room temperature, and then it was stirred for 30 minutes. H2O was added. The mixture was extracted with DCM (3×200 mL), and the combined extracts were dried (Na2SO4), filtered, and concentrated in vacuo. The crude was purified on silica gel (Biotage 65M): the column was flushed with ca. 800 mL 4:1 DCM:EtOAc, then gradient to 1:1 DCM:ethyl acetate until product eluting, then 1:4 DCM:EtOAc eluted product to give (R)-tert-butyl 4-(5-methyl-7-oxo-6,7-dihydro-5H-cyclopenta[d]pyrimidin-4-yl)piperazine-1-carboxylate (7.5 g, 82.3% yield) as a foam. The foam was concentrated from DCM/hexanes (3×), which also gave a foam. HPLC>95% area. LC/MS (APCI+) m/z 333 [M+H]+.
