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

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Reaction Conditions

Reaction Steps

A mixture of 3 (3.6 g, 11 mmol) and N-(tert-butoxycarbonyl)-(2S,4S)-4-hydroxyproline methyl ester (0.75 g, 3.0 mmol) was dissolved in 30 mL dry CH2Cl2 under Ar(g). The solution was cooled to −78° C., and diethylaminosulfur trifluoride (DAST; 5.6 mL, 42 mmol) was than added by syringe. The mixture was stirred for an additional 45 min at −78° C. and then allowed to warm to room temperature. After 20 h, the mixture was transferred to a separatory funnel and carefully washed with 80 mL of saturated NaHCO3(aq). A considerable amount of gas was released during wash. The organic layer was washed with an additional 2×30 mL of saturated NaHCO3(aq) and 30 mL of water, dried over MgSO4(s), and concentrated to a dark oil by rotary evaporation at reduced pressure. Flash chromatography (hexanes/EtOAc 4:1) afforded 4 (1.98 g, 6.12 mmol, 55%) as an oily yellow solid.

Provenance & Evidence