(Diethylamino)sulfur trifluoride → 4-Fluoro-4-[4-[[(phenylmethoxy)carbonyl]amino]-2-fluorophenyl]-1-piperidinecarboxylic acid phenylmethyl ester

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To a solution of diethylaminosulfur trifluoride (DAST, 0.65 mL) in dry methylene chloride (49 mL) at −78° C. under N2 is added a solution of 4-hydroxy-4-[4-[[(phenylmethoxy)carbonyl]amino]-2-fluorophenyl]-1piperidinecarboxylic acid phenylmethyl ester (EXAMPLE 74, Step 1, 2.25 g) in dry methylene chloride (47 mL) over 2 mins. The resulting mixture is stirred at −78° C. for 1 hr and at ambient temperature for 30 mins and is then adjusted to ph 8 with saturated aqueous sodium bicarbonate (50 mL), diluted with water (50 mL), and the layers are separated. The organic phase is washed with water (25 mL) and saline (25 mL), dried over anhydrous sodium sulfate and concentrated under reduced pressure, and the residue is chromatographed on silica gel (230-400 mesh, 150 g) eluting with methanol/methylene chloride (0.5/99.5). Those fractions with an R7=0.27 by TLC (ethyl acetate/hexane, 25/75) are pooled and concentrated to give the title compound (contaminated with approx. 15% of the elimination side product). An analytical sample is prepared by radial chromatography (100 μ silica gel rotor, ethyl acetate/hexane (20/80) eluent), mp 116-118° C.

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