Michael addition of 1,3-cyclopentanedione, 1,3-cyclohexanedione and 1,3-cycloheptanedione to 1-(X-phenyl)-2-nitroethylenes

Michael addition of 1,3-cyclopentanedione, 1,3-cyclohexanedione and 1,3-cycloheptanedione to 1-(X-phenyl)-2-nitroethylenes was studied. 1,3-Cyclopentanedione afforded 2-(1-(X-phenyl)-2-nitroethyl)-1,3-cyclopentanediones (I), 1,3-cyclohexanedione gave 9-(X-phenyl)-8-hydroxyimino-7-oxabicyclo[4,3,0]-1-nonen-2-ones (IIa-IIe),whereas reaction with 1,3-cycloheptanedione led to 10-(X-phenyl)-8-oxabicyclo[5,3,0]-1-decen-2-ones (IIg-IIj) as well as 9,9'-hydroxyaminobis(10-(X-phenyl)-8-oxabicyclo[5,3,0]-1-decen-2-ones) (III). Products I were isolated also in the addition of 1,3-cyclohexadione and 1,3-cycloheptanedione to 1-(X-phenyl)-2-nitroethylene when X=H or an electron accepting substituent and the reaction was performed at -5 to 0°C, and were converted into II. The structure of the compounds and mechanism of the conversion of I to II is discussed on the basis of IR, 1H NMR and 13C NMR spectra.