Pyridine → Methyl (S)-4,4-difluoro-2-isocyanatopentanoate

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The compound from example 6 (6.76 g, 28.8 mmol) was initially charged and dissolved in 240 ml of dichloromethane and 9.46 ml (117 mmol, 4.05 eq) of pyridine, cooled down to 0° C. in an ice bath for 15 min and admixed with 19.86 ml of a 20% phosgene solution in toluene (37.54 mmol, 1.3 eq) within 20-30 seconds. The mixture was stirred at 0° C. for a further 2 h, then the reaction mixture was extracted with cold half-molar HCl and ice. The water phases were reextracted with dichloromethane, and the combined organic phases were washed with ice and saturated NaCl solution, then dried over MgSO4 and filtered off from the desiccant. Concentration by evaporation under reduced pressure left 5.21 g of brown oil (corresponded to a yield of 94%), which was reacted further directly.

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