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

N
Ar
COAr′
H
MeOH
N
O
Ar
HN
R NH
H
Ar′
OMe
COAr′
H
OH
OMe
Ar
REACTIONS OF NITRILE OXIDES 49
R = Ph, 4-MeC 6H 4, 4-ClC 6H 4; Ar = Ph, 4-MeC 6H 4; Ar′ = Ph, 4-MeC 6H 4
H 2O
Scheme 1.29
Ar
−MeOH, −H2O
OH
NH-CH=C-Ar′
OMe OH
R NHCOAr
equilibrating fused and spiro adducts) and on the C(2)=C(3) double bond of 142.
Site selectivity is highly enhanced by carrying out the reaction in polar solvents,
only the attack at the C=N moiety has been observed in methanol (312). The
relative stabilities of rapidly equilibrating mixtures of fused (144) andspiro(143)
isomers have been well reproduced by B3LYP/6–31G* calculations (313).
1.3.4.2.2. Nonaromatic Unsaturated Heterocycles Reactions of aromatic nitrile
oxides with 1-azirines are followed by the ring opening of the latter to give
4-benzamidoisoxazoles 145 (314). The structure of 145 (R = 4-ClC6H4, Ar=
Ar ′ = Ph) was established by single-crystal X-ray analysis. A mechanism for the
formation of 145 has been proposed, (see Scheme 1.29).
1,3-Dipolar cycloadditions of 2-ethoxy- and 2-ethylthio-1-azetines 146 (Z =
O, S) with nitrile oxides give 4,5,6,6a-tetrahydroazeto[1,2-d]oxadiazoles, for
example 147 (315, 316).
N
O
145
Me
Me
ZEt
Me
Me ZEt
O
N
Me N
Me N
Me
Me
Ph
146 147
Cyclic imidate esters, 2-ethoxypyrrolin-5-one and 2-ethoxy-1H -indol-3-one,
undergo 1,3-dipolar cycloaddition reactions with nitrile oxides, the reaction site
being at the pyrroline C=N bond (317). Rigid and sterically congested pyrroline
spiro compounds 148 demonstrate complete diastereofacial selection in site and
regiospeciÞc cycloaddition reactions with nitrile oxides to give products 149
(318).
Ar′
RCNO