To [8-benzyl-10-(1,2-dihydroxyethyl)-11-oxa-3,8-diazaspiro[5.5]undecan-3-yl]-(4-isopropoxy-3-methyl-phenyl)methanone (1.53 g, 3.17 mmol) in THF (28 mL) was added NaIO4 (1.36 g, 6.34 mmol) followed by the addition of water (10 mL). The reaction mixture was stirred at room temperature for 2 hours, then filtered through celite, and concentrated in vacuo. The residue was partitioned between sat. aq. sodium bicarbonate (50 mL) and ethyl acetate (50 mL) and the aqueous layer was extracted further with ethyl acetate (3×50 mL). The organics were combined, washed with sat. aq. sodium bicarbonate (50 mL), dried over MgSO4, and concentrated in vacuo. To the intermediate aldehyde (˜1.4 g) was added 2-methylpropan-2-ol (17 mL) and 2-methylbut-2-ene (9 mL, 85.08 mmol) and the reaction mixture was cooled to 0° C. A solution of NaClO2 (938 mg, 8.29 mmol) and NaH2PO4 (1.15 g, 8.31 mmol) in water (17 mL) was added dropwise over 5 minutes, and the reaction mixture was stirred for 30 minutes. The reaction mixture was warmed to room temperature, then extracted with ethyl acetate (4×50 mL) and the combined organic layer was washed with brine (15 mL), dried over MgSO4, filtered and concentrated in vacuo to give 8-benzyl-3-(4-isopropoxy-3-methyl-benzoyl)-11-oxa-3,8-diazaspiro[5.5]undecane-10-carboxylic acid as a white foam (1.48 g, 100%). ESI-MS m/z calc. 466.6. Found 467.5 (M+1)+; Retention time: 1.27 minutes (3 min run).
