Morpholinosulfur trifluoride → N-tert-Butoxycarbonyl-(2R,4S)-4-fluoroproline benzyl ester

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Reaction Steps

Morpholinosulfur trifluoride (morph-DAST; 3.5 mL, 29 mmol) was added by syringe to a solution of 1 (6.5 g, 20 mmol) in dry CH2Cl2 (200 mL) under Ar(g) at −78° C. The solution w allowed to warm to room temperature and stirred overnight. Following a quench with methanol, the reaction mixture was washed with 2×100 mL saturated NaHCO3(aq). An emulsion resulted, so the mixture was diluted with H2O and CH2Cl2 and filtered through Celite. The biphasic filtrate was separated, and the organic layer was washed with water, dried over MgSO4(s), and concentrated by rotary evaporation under reduced pressure. The resulting oil was purified by flash chromatography (1.5% v/v MeOH in CH2Cl2), which afforded 11 (3.1 g, 9.6 mmol, 48%) as a yellow oil. An additional 2.32 g of 11 that were contaminated with a minor impurity, as indicated by TLC, were also obtained from the column.

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