Synthetic Exploitation of the Ring-Opening of 3,4-Dinitrothiophene. Part 4. Synthesis of 1,4-Disubstituted 3-Hydroxyimino-2-nitro-1-butenes and their Cyclization to 4-Nitroisoxazoles

The reduction of a single nitrovinyl moiety in 1,4-dialkyl- and 1,4-diaryl-2,3-dinitro-1,3-butadienes 1 with stannous chloride dihydrate in ethyl acetate furnishes the corresponding 3-hydroximino-2-nitro-1-butenes 4 in satisfactory yields. The oxidative cyclization of the latter compounds gives variable yields of 3,5-disubstituted 4-nitroisoxazoles 5, most likely arising from aromatization of the corresponding 4,5-dihydroisoxazoles, formed through an intramolecular Michael addition of the hydroximino group in 4 to the nitroalkene functionality. In the case of the 1-naphthyl derivative 4d such oxidative cyclization mainly leads to the competitive formation of 3-[(1-naphthyl)methyl]-2-nitrobenzo[f]quinoline N-oxide 7.