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

Abstract

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′(4H)-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.