Trifluoroacetic acid → (S)-1-((S)-2-tert-Butoxycarbonylamino-3-difluoromethoxy-propionyl)-hexahydro-pyridazine-3-carboxylic acid 2,2,2-trichloro-ethyl ester

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A cooled (0° C.) solution of (S)-tetrahydro-pyridazine-1,2,3-tricarboxylic acid 1,2-di-tert-butyl ester 3-(2,2,2-trichloro-ethyl)ester (923 mg, 2.0 mmol) in dichloromethane (3 mL) was treated with trifluoroacetic acid (3 mL). After stirring at room temperature for 1.5 h, the volatiles were removed in vacuo and residual trifluoroacetic acid was azeotroped off with toluene. The residue was combined with (S)-2-tert-butoxycarbonylamino-3-difluoromethoxy-propionic acid (260 mg, 1.02 mmol) and dissolved in anhydrous acetonitrile (10 mL). The cooled (0° C.) mixture was subsequently treated with 1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxide hexafluorophosphate (543 mg, 1.43 mmol) and N,N-diisopropylethylamine (0.71 mL, 4.08 mmol). After stirring at room temperature for 3 days, the volatiles were removed in vacuo and the residue was partitioned between ethyl acetate and water. The organics were separated, dried over anhydrous sodium sulfate, filtered and the volatiles were removed in vacuo. The residue was purified by silica gel chromatography eluting with a stepwise gradient of iso-hexanes/ethyl acetate 1:0 to 7:3 to provide the title compound (143 mg, 28%) as a pale Greene gum and as a complex mixture of rotamers. LCMS (m/z) 498.0, 500.0 [M+H], Tr=2.91 min.

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