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

REACTIONS OF DIPOLAR 1,3-CYCLOADDITION ([3 + 2] CYCLOADDITION) 317
(a)
(b)
Ph N
R Ph
C
+ +
−
O
R
R = Ph
R = H
Ph
+ −
Ph N
O
CH2
CH(CH3)7−Me
+
R 2
R1 C
NO 2
H
R H
Ph H
H
N
Ph O (CH2) 7Me
R = Ph
R = H
R H
Ph H
(CH2)7Me
N
Ph O H
R = H
R
Ph
N
Ph O
R
Ph
N
Ph O
NO2 R2 R 1
R 1
R 2
NO2
R = H, Ph; R 1 ,R 2 = H, Alk, Ph, p-XC 6H 4, thienyl, etc.
A
B
Scheme 2.236
However, α,N-diphenyl nitrone with the same alkene forms a mixture of stereoisomeric
2,3-diphenyl-5-octylisoxazolidines (Scheme 2.236a, routes A and B) (734).
Owing to the strong electronic inßuence of the nitro group, the nitro-oleÞnes
are of special interest. Analysis of frontier molecular orbital interaction in 1,3cycloaddition
reactions of α,α,N -triphenyl and α,N-diphenyl nitrones with α- and
β-substituted nitroethylenes has been carried out. In all of the examples studied,
orbital effects lead to the formation of the corresponding 4-nitroisoxazolidines
(Scheme 2.236b, route A) (736). The 1,3-cycloaddition reactions of α,N-diphenyl
nitrone with trans-1-nitro-1-propene and trans-3,3,3-trichloro-1-nitro-1-propene
occur via a concerted mechanism despite the high π-deÞcient character of the
dipolarophiles. This mechanism is indicated by the cis-stereospeciÞcity of the
cycloaddition, the obtained values of activation parameters and the weak solvent
effect on the reaction kinetics (737).
To study asymmetric induction from the nitrone part in 1,3-dipolar cycloaddition
to styrene, D-erythrose derived nitrones (479 a–c) have been used. Cycloaddition
of nitrones (479 a–c) to styrene, in boiling toluene for 10 h, affords
a mixture of four diastereomeric 3,5-disubstituted isoxazolidines (481 a–c–
484 a–c) in high yields (82%–94%) (Scheme 2.237) (208).
Under similar reaction conditions the D-threose derived nitrone (480) is converted
in high regioselectivity to diastereomeric cycloadducts (485–488) inan
overall yield of 84% (Scheme 2.238).
A
B