Synthesis of heterocycles by 1,3-dipolar cycloaddition reactions with the participation of nitroalkenes (review)

An experimental and theoretical study of the hetero Diels–Alder reactions between (E)-2-aryl-1-cyano-1-nitroethenes and ethyl vinyl ether: one-step or zwitterionic, two-step mechanism?

“…Nitronates are very useful precursors in organic synthesis [1][2][3][4][5]. The general, most universal method of the preparation of six-membered cyclic nitronates is a hetero Diels-Alder (HDA) reaction between conjugated nitroalkenes (CNA) as heterodienes and ethylenic dienophiles [3][4][5][6].…”

Unusual transformations of 2-isoxazolines

“…68 The instability of 4-and 5-nitro-2-isoxazolines was repeatedly observed in their synthesis from nitrile oxides and nitroalkenes, when the primary product of the cycloaddition, the corresponding 2-isoxazoline, converted into the isoxazole with elimination of nitrous acid. 46 Some nitroisoxazolines formed in this reaction are rather stable under reaction conditions; however, they undergo rapid aromatisation on heating. Thus, the interaction of aromatic nitrile oxides with nitroethylene resulted in the 5-nitro-2-isoxazolines 121, which on boiling in toluene gave high yields of the isoxazoles 122.…”

“…Thus, 4-nitro-5-methylisoxazolines 123 aromatise upon prolonged heating, whereas the derivatives of 5-nitro-2-isoxazoline 124 aromatise spontaneously under reaction conditions. 45,46 The 1,3-dipolar cycloaddition of benzonitrile oxide to a-substituted nitroethylenes of the type 125 at 0 8C gives the isoxazolines 126, which spontaneously convert at 20 8C into the isoxazoles 127 with the elimination of the nitro group: 69 The nitro group can be eliminated more readily from isoxazolines than the cyano and carboxy groups. 70,71 Thus, the nitro group eliminates preferentially from C(5) of the 5-nitro-5-alkyl(aryl)isoxazolines 128 and 129 to give the corresponding 3,5diaryl-(130) 72 or 3,5-dialkylisoxazoles (131).…”

“…74 The adducts of benzonitrile oxide with halonitroethylenes aromatise to give 4-and 5-nitroisoxazoles which can be explained by the ability of intermediate isoxazolines to eliminate the hydrogen halide molecule more readily than HNO 2 . 46 It should be noted that spontaneous elimination of hydrogen halide under conditions of nitrile oxide synthesis was often observed when halogenoalkenes were used as dipolarophiles. 57,77 Hence, in some cases unstable 4-and 5-nitroisoxazoles can be obtained by cycloaddition to nitroalkenes containing a substituent group, which eliminates more readily than the nitro group.…”

“…57,77 Hence, in some cases unstable 4-and 5-nitroisoxazoles can be obtained by cycloaddition to nitroalkenes containing a substituent group, which eliminates more readily than the nitro group. 46 The nitro group can be split off, not as nitrous acid, but as a stable compound with another substituent in the isoxazoline ring. Such an ability for elimination is useful in the synthesis of some isoxazoles.…”