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

50 NITRILE OXIDES
R
N
Ph
N O O
Ph
R
Ph
N
Ph
N O O
148 149
R
R = H, Me, OMe, Cl
2-Methyl-4,5-dihydrooxazole (319), 2-phenyl-4,5-dihydrooxazole (320), and
2,4,4-trimethyl-4,5-dihydrooxazole (319), which are examples of cyclic imidate
esters, undergo 1,3-dipolar cycloaddition reactions with benzonitrile N-oxide to
give the 7a-substituted 3-phenyl-5,6-dihydro-7aH -oxazolo[3,2-d]-1,2,4oxadiazoles
150. 2-Methyl-4,5-dihydrothiazole gives the thia analog of 150 (319).
Alkanoyl- and aroylformonitrile oxides (RCOCNO), 2-methyl- 4,5-dihydrooxazole,
2-methyl-4,5-dihydrothiazole (319) as well as 2-phenyl-4,5-dihydrooxazole
(320) give the open-chain compounds 151 (X = O and S, respectively)
(Scheme 1.30).
Among six-membered unsaturated nitrogen heterocycles, cycloaddition reactions
of nitrile oxides at the C=N bond have been described for individual
3,4-dihydroisoquinolines, such as the reactions of 6,7-dimethoxy-3,4-dihydroisoquinoline
and its 1-methyl- and 1-cyanomethyl-substituted derivatives with
acylcarbonitrile oxides (321). They have also been described for 1,3,4-oxadiazin-
6-ones (322), and for fused dihydro-1,3-oxazine derivatives (323, 324).
Reactions of 2,5-diaryl-1,3,4-oxadiazin-6-ones 152 (R = H, Me, MeO, Cl,
NO2) with stable nitrile oxides R 1 CNO (R 1 = 2,4,6-Me3C6H2, 2,6-Cl2C4H3)
X
N
X = O, S
Me(Ph)
R′ = Me or Ph
PhCNO
RCOCNO
Scheme 1.30
N
X O
Me(Ph)
N
150
Ph
N
O
Ph
RCOXCH 2CH 2N(CN)COR′
151
R