To a stirred solution of 0.9M KHMDS (40.9 mL, 36.8 mmol) in THF (100 mL) at −78° C. was added a THF (50 mL) solution of ethyl 7-chloro-6-(2-ethoxy-2-oxoethyl)-5-methylpyrazolo[1,5-a]pyrimidine-2-carboxylate (10 g, 30.7 mmol, for synthesis see WO2013025584) was added over the course of 20 min. After 30 min, a THF (15 mL) solution of 3-phenyl-2-(phenylsulfonyl)-1,2-oxaziridine (10.43 g, 39.9 mmol) was added to the red reaction mixture and stirring was continued for an additional 30 min at −78° C. Then, the resulting orange reaction mixture was quenched with sat. aq. NH4Cl (50 mL), diluted with EtOAc (200 mL), washed with water (100 mL), brine (100 mL), dried (Na2SO4), filtered and concentrated in vacuo to give a solid. This solid was triturated with small amount of ethyl acetate and solids were filtered, washed with hexanes and dried under high vacuum to afford ethyl 7-chloro-6-(2-ethoxy-1-hydroxy-2-oxoethyl)-5-methylpyrazolo[1,5-a]pyrimidine-2-carboxylate (5 g, 16.85 mmol, 43% yield, 90% pure) as light yellow solid. 1H NMR (400 MHz, CDCl3) δ 7.21 (s, 1H), 5.76 (s, 1H), 4.58-4.49 (m, 2H), 4.33 (dtt, J=10.7, 7.1, 3.7 Hz, 2H), 2.71-2.64 (s, 3H), 1.48 (t, J=7.2 Hz, 3H), 1.29-1.24 (m, 3H). LCMS (M+1)=342.16.
