A mixture of 5-bromopyrazin-2-ol B4a (525 mg, 3 mmol), tert-butyl 4-(methylsulfonyloxy)piperidine-1-carboxylate (1.117 g, 4 mmol), 18-crown-6 (79 mg, 0.3 mmol), K2CO3 (829 mg, 6 mmol) in butan-2-one (19 mL) is subjected to microwave irradiation at 130° C. for 15 min. The solids are filtered off, washed with ethyl acetate and purified by flash chromatography (hexanes/ethyl acetate gradient) to afford tert-butyl 4-(5-bromopyrazin-2-yloxy)piperidine-1-carboxylate B4b as a white solid: MS calcd. For C14H21BrN3O3 ([M+H]+): 358.1, found: 358.1.
