ISOXAZOL-5(4H)-ONES - X-RAY CRYSTAL-STRUCTURES OF A 4-HYDROXYISOXAZOL-5(4H)-ONE AND A [4,4'-BIISOXAZOLE]-5(4H),5'(4'H)-DIONE FORMED BY OXYGEN OXIDATION OF 3-TERT-BUTYL-4-METHYLISOXAZOL-5(4H)-ONE

Treatment of ethyl (RS)-2,4,4-trimethyl-3-oxopentanoate 4 with hydroxylamine under basic conditions gave 5-tert-butyl-4-methylisoxazol-3-ol 5 and a tautomeric mixture of 3-tert-butyl-4-methylisoxazol-5 (2H)-one 6 and (RS)-3-tert-butyl-4-methylisoxazol-5(4H)-one 7. During the attempts to crystallize the inseparable mixture of tautomers 6 and 7 two other compounds, (RS)-3-tert-butyl-4-hydroxy-4-methylisoxazol-5 (4H)-one 8 and (RS,RS)-3,3'-di-tert-butyl-4,4'-dimethyl-[4,4'-biisoxazole]-5(4H), 5'(4'H)-dione 9, were progressively formed. The structures of compounds 8 and 9 were established by X-ray analyses. Compounds 8 and 9 are proposed to be formed by oxidation of compound 7 with oxygen, but the mechanisms underlying these two oxidative processes are unknown.