Trimethylsulfoxonium chloride (79 mg, 0.62 mmol) in DMSO (1.5 mL) was treated with sodium hydride (21 mg, 60% oil dispersion, 0.53 mmol) and stirred at ambient temperature for 10 min. 5-(3,3-Dimethyl-but-1-ynyl)-3-[((1R)-4-methyl-cyclohex-3-enecarbonyl)-(4-oxo-cyclohexyl)-amino]-thiophene-2-carboxylic acid methyl ester 507 in THF (1 mL+0.5 mL) was added dropwise and the reaction mixture was stirred for 45 min. The orange solution was treated with 5% citric acid until pH 3 and partitioned between water and ethyl acetate. The organic layer was separated and the aqueous was extracted again with ethyl acetate. The combined organics were washed with 5% LiCl, water and brine, and dried over sodium sulfate. After filtration and concentration, the residue was purified by flash chromatography (20-75% EtOAc:hexanes) to afford 0.134 g (70% yield) of 5-(3,3-dimethyl-but-1-ynyl)-3-[((1R)-4-methyl-cyclohex-3-enecarbonyl)-(1-oxa-spiro[2.5]oct-6-yl)-amino]-thiophene-2-carboxylic acid methyl ester 508 as a white powder.
