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

SYNTHESIS OF NITRONES 171
(where R 1 ,R 2 ,R 3 ,R 4 = H, Me, Ph; X = H,Cl, NO2) (300, 301). Cyclic nitrone
(133) results from the intramolecular addition of the oxime group to the oleÞnic
bond (302)(Scheme 2.48).
Selective formation of nitrones can be achieved using Pd (II) [Pd(cod)Cl2] as
a catalyst, in the reaction of oximes with allylic acetate (Scheme 2.49) (303).
The reaction of Z -2-furaldoximes (134) with oxirane affords in good yield
N-(2- hydroxyethyl)-α-2-furylnitrone (135) the product of N -alkylation. Ebenzaldoxime,
predominantly gives the O-alkylation product (136), whereas its
Z -isomer leads to a mixture with an insigniÞcant amount of N -alkylation product
(138) (Scheme 2.50) (304).
A mixture of nitrones (142) and dioxazines (143) forms from a mixture of Z -
and E-isomers of oximes (141), obtained from acetonide of erythrulose (140).
The reaction is carried out in a mixture of acetone and 2,2-dimethoxypropane,
in the presence of catalytic quantities of TsOH (Scheme 2.51) (305).
Alkylation of Z-aldoximes (144) and (145) with bromo esters (146) and (147)
provides good yields of high purity DEEPN and EPPyON nitrones (Scheme
2.52) (306).
Ph
Ph
NOH
R
OAc
Pd(cod) Cl2 90° C
O +
N
R = 4-F3C-C6H4, 1-Naph, 4-Cl-C6H4, 4-Me-C6H4, cyclohexyl
NOH
Ph
+
+
R
OAc
Pd(cod) Cl2 90°C, 15 h
Scheme 2.49
−
−
Ph
O +
N
HO N
Ar H
O
NaOMe
O
134
135 136
137
138 139
N
Ar H
O
HO
N
Ar H
+
−
+
OH
Ar = 2-Furyl (134, 135, 136)
Ph (137, 138, 139)
Scheme 2.50
Ph
Ph
R
Ph