Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

42 NITRILE OXIDES
O
O
111
CH(OMe)2
Isoxazolobenzodioxocines 112 were prepared in 29% to 65% yields by the
1,3-dipolar cycloaddition. of the corresponding benzenenitrile oxides to 2,5dihydro-1,6-benzodioxocine.
Similarly, monoadducts 113 were obtained from the
16-membered tetraether as the dipolarophile (278).
O
O
H
H
O
N
R
112
113
R = H, 4-OMe, 4-Me, 4-Cl, 4-NO2, 4-Br, 4-F, 3-Cl R = H, 4-Cl, 4-NO2, 3-Cl
Alkylidene-substituted heterocycles readily enter 1,3-cycloaddition reactions
with nitrile oxides to give the corresponding spiroadducts. Thus, reaction of
methylene-γ-butyrolactones with aromatic nitrile oxides proceeds at room temperature
producing spiroheterocycles, for example 114 (279). However, cycloaddition
with 5-methylene-5H -furan-2-one, carried out in reßuxing toluene gives
the unexpected product 115 (279). Some 4-substituted benzonitrile oxides undergo
1,3-dipolar cycloadditions with 3-methylenephthalide affording expected
spiroisoxazolines. These spiroadducts can be converted to the corresponding
2-(3-arylisoxazol-5-yl)benzoic acids by various methods, including thermal and
acidic treatments, as well as electrooxidation (280).
O
N
O
O
114
Me
Me
Bu
N O
O
N
O
O
115
O
N
O
O
O
N
R
Me