Phase transfer catalyzed synthesis of 3-methyl-4h-pyrazolo[3,4-d]isoxazole from 3,5-dimethyl-4-isoxazolyldiazonium tetrafluoroborate

Reaction of 3,5-dimethyl-4-isoxazolyl-diazonium tetrafluoroborate (2) with two equivalents of potassium acetate and five mole percent of 18-crown-6 in ethanol-free chloroform produce 3-methyl-4H-pyrazolo[3,4-d]isoxazole (3) in good to excellent yield. Pyrazole (3) was subjected to acylation/aroylation to afford the corresponding 4-N-acetyl/aroyl derivatives by reaction with CH3COCl/ArCOCl in Et3N.