A solution of 3-[4-(3-phenylpyrazolo[1,5-a]pyrimidin-6-yl)phenyl]propanal (2-5, 278 mg, 0.850 mmol, 1 equiv) in a 4:1 mixture of THF and t-BuOH (20 mL) was treated with 2-methyl butene (5 mL), an aqueous solution of sodium phosphate monobasic (0.14 M, 234 mg, 1.70 mmol, 2 equiv) and sodium chlorite (197 mg, 2.18 mmol, 2.57 equiv). After 1 hour at room temperature, additional solid sodium phosphate monobasic (278 mg, 1.70 mmol, 2 equiv) and sodium chlorite (197 mg, 2.18 mmol, 2.57 equiv) were added. The reaction mixture was stirred for an additional 4 hours, concentrated and the residue dissolved in EtOAc (100 mL) then washed with a 25:1 mixture of aqueous 10% sodium bisulfite solution and 10% potassium hydrogen sulfate solution (2×100 mL). The organic layer was dried over sodium sulfate and concentrated to afford 3-[4-(3-phenylpyrazolo[1,5-a]pyrimidin-6-yl)phenyl]propanoic acid (4-1) as a yellow solid. LRMS m/z: Calc'd for C21H18N3O2 (M+H) 344.4, found 344.1.
