tert-Butanol → tert-Butyl (1-(4-bromo-3-methylisoxazol-5-yl)cyclopropyl)carbamate

To a suspension of 1-(4-bromo-3-methylisoxazol-5-yl)cyclopropanecarboxylic acid (1.60 g, 6.50 mmol) and powdered 4 Å molecular sieves (pre-dried-0.767 g, 48% wt) in toluene (15 mL) was sequentially added N,N-diisopropylethylamine (1.50 mL, 8.59 mmol), diphenyl phosphorazidate (1.576 mL, 7.05 mmol), and tert-butanol (16.80 mL, 176 mmol). The reaction vessel was fitted with a reflux condenser and the mixture was heated to 100° C. for 1 h. After 1 h, the reaction mixture was cooled to room temperature and filtered over a plug of Celite. The filter cake was washed with EtOAc (3×), and the filtrate was concentrated to give a brown oil. The oil was purified on Biotage system (5% EtOAc:95% Hexanes to 20% EtOAc:80% Hexanes). The product tert-butyl 1-(4-bromo-3-methylisoxazol-5-yl)cyclopropylcarbamate (1.55 g, 4.89 mmol, 75% yield) was isolated as white crystals after concentration. LC/MS m/z 317 [M+H]+.