N-tert-Butyl-5-methylisoxazolium Perchlorate

Experimental procedure¹⁾:
Under nitrogen, N-tert-butyl-5-methylisoxazolium perchlorate (240 mg, 1 mmol) is added to a mixture of 3-quinoline carboxylic acid (173 mg, 1 mmol) and triethylamine (140 µL, 1 mmol) in DMF (2 mL). The mixture is stirred at room temperature for 4 h and then poured on iced water (50 mL). The precipitate is collected, washed with water and dried. The intermediate ester 1 (131 mg, 0.44 mmol) is isolated in 44% yield as a yellow powder. Then the product (110 mg, 0.36 mmol) in DMF (3 mL) is added to a freshly prepared guanidine 2 (123 mg, 0.39 mmol) and the mixture is stirred at 140 °C for 2 h. The solvent is evaporated under reduced pressure. Water (10 mL) is added to the residue, and after 12 h at 4 °C, the precipitate formed is filtered, washed with water, and dried to obtain compound 3 in 85% yield.