Solid 77% max. m-chloroperbenzoic acid (“m-CPBA”; 23.9 g, 107 mmol) was added portionwise to a 0° C. solution of (R)-tert-butyl 4-(5-methyl-6,7-dihydro-5H-cyclopenta[d]pyrimidin-4-yl)piperazine-1-carboxylate (20.0 g, 62.8 mmol) in CHCl3 (310 mL) The reaction mixture was stirred 5 for minutes, then warmed to room temperature and stirred for an additional 90 minutes. HPLC looked similar after 7.5 hours. The reaction mixture was cooled to 0° C., and then NaHCO3 (13.2 g, 157 mmol) and another 0.5 equivalents of m-CPBA were added. The reaction mixture was stirred overnight (14 hours). The reaction mixture was cooled to 0° C., and a solution of Na2S2O3 (29.8 g, 188 mmol) in H2O (50 mL) was added dropwise by addition funnel. This was followed by a solution of Na2CO3 (24.6 g, 232 mmol) in H2O (70 mL) by addition funnel (mixture turns homogeneous). The reaction mixture was stirred for 30 minutes, and then the mixture was extracted with CHCl3 (3×150 mL). The combined extracts were dried (Na2SO4), filtered, and concentrated to give (R)-4-(4-(tert-butoxycarbonyl)piperazin-1-yl)-5-methyl-6,7-dihydro-5H-cyclopenta[d]pyrimidine 1-oxide (21.0 g, 100%). LC/MS (APCI+) m/z 335.1 [M+H]+.
