Phosgene → tert-butyl N-(6-chloropyridin-2-yl)carbamate

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64 mL of ethyl acetate were added to a reaction system and cooled to about −15° C. with a salt and ice bath followed by blowing in 10.6 g (107 millimoles) of phosgene. A mixed solution containing 5.5 g (42.8 millimoles) of 2-amino-6-chloropyridine, 11.1 g (85.6 millimoles) of N,N-diisopropylethylamine, 6.34 g (85.5 millimoles) of t-butanol and 43 mL of ethyl acetate were dropped into this phosgene solution over the course of 2.5 hours while holding the internal temperature at −15° C. to −5° C. Following completion of dropping, the solution was stirred for 3 hours at the same temperature range. Next, 50 mL of water and 23.2 g of 28% sodium hydroxide were sequentially added while holding the internal temperature at 0° C. or lower. The aqueous phase was extracted with ethyl acetate. The organic phase was washed with water and concentrated under reduced pressure. The resulting crude crystals were subjected to quantitative analysis by HPLC. The target (6-chloro-pyridin-2-yl)-carbamic acid tert-butyl ester was obtained at a yield of 93%. There was hardly any urea compound formed.

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