Isoxazole-4-sulfonic acids, preparation

An aq. soln. of NaHCOg added to a stirred soln. of / -ionone oxime (prepared from / -ionone, hydroxylamine hydrodiloride, and K-carbonate in water-ethanol) in tetrahydrofuran, protected from light, KI and iodine in water added, and refluxed 4 hrs. -> isoxazole deriv. (Y 91%) mixed with liq. NHg, tetrahydrofuran, and rerf-butanol, treated with Na until the soln. remains dark blue, stirred 15 min., and the crude / -aminoketone refluxed 24 hrs. with a trace of p-toluene-sulfonic acid in toluene / -damascone (Y 84%). F. e. s. G. Budii and J. C. Vederas, Am. Soc. 94, 9128 (1972) s. a. K. H. Sdiulte-Elte, B. L. Muller, and G. Ohloff, Helv. 56, 310 (1973). 

Electrophilic sulfonation of isoxazole is of no preparative value the substitution of only the phenyl-substituent of 5-phenylisoxazole with chlorosulfonic acid makes the same point. Both isothiazole and pyrazole can be satisfactorily sulfonated at C-4. 

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