To a 1000 ml round bottom flask under N2 was added (2R)-2-(cyclopentylmethyl)-4-[(1,1-dimethylethyl)oxy]-4-oxobutanoic acid (Tetrahedron, 2001, 57(36), 7675-7683) (38.1 g, 149 mmol) in acetonitrile (650 mL) at 0° C., followed by N-methylmorpholine (82 mL, 744 mmol) and 4-[4,6-bis(methyloxy)-1,3,5-triazin-2-yl]-4-methylmorpholin-4-ium (53.7 g, 164 mmol). The solution was stirred at the same temperature for 2 hours and then (5S)-4,5-dihydro-1H-pyrazole-5-carboxylic acid, hydrochloride (22.4 g, 149 mmol) was added. The solution was allowed to warm up to room temperature and stirring continued overnight. LCMS showed that the reaction was complete. The acetonitrile was removed under reduced pressure, and the residue was taken up in DCM (500 mL) and neutralized by the addition of 2 N HCl. The organic phase was washed with brine, dried over Na2SO4, filtered and concentrated. The residue was purified by Combiflash automated silica gel chromatography (0-5% methanol containing 1% formic acid/DCM) to provide (5S)-1-{(2R)-2-(cyclopentylmethyl)-4-[(1,1-dimethylethyl)oxy]-4-oxobutanoyl}-4,5-dihydro-1H-pyrazole-5-carboxylic acid (27.7 g, 79 mmol, 53% yield) as a clear oil. LCMS: [M+H]+: 353.4.
