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

456 NITRONATES
NO2
CO2DBHA
23b
R 1
Z
R 2 NO 2
23c–f
X
NO2
LA or Δ
Hal −
O
O N
24b
Z
ODBHA
O
O N
R1 (2)
O
R 2
24c–f 60%–100%
Scheme 3.28
R 1
R 2
Z
24
DBHA−
Me
Me
CO 2Et
c
Ph
H
CO 2Et
d
Ph
H
CN
e
OMe
Ph
Me
CO 2Ph
f
M NO2 O N
X
X
X
NO2 O
23 24
Scheme 3.29
cyclopropanes (23c–f) presented in Scheme 3.28 undergo smooth thermal or
Lewis acid catalyzed isomerization to give the corresponding nitronates (24c–f)
(Eq. 2).
In particular, keeping cyclopropane (23b) in boron trißuoride etherate at 20 ◦ C
leads to a quantitative isomerization to give the corresponding nitronate (24b).
In cyclopropane, the C,C bond between the atom bearing the nitro group and
the most substituted atom of the ring is cleaved.
Therefore, all prerequisites are present for the development of a general strategy
for the synthesis of nitronates (24) from α-functionalized primary AN through
stabilized carbenium intermediates and nitrocyclopropanes (23). This approach
allows the stereoselective synthesis of nitronates (24) from simple molecules
(Scheme 3.29).
Cycloaddition of Carbenes to conjugated Nitro OleÞns (28). From the above it
is evident that there is another synthetic route to nitronates (24 g) with the use
of carbenium intermediates based on [1 + 2]-cycloaddition of carbenes RR 1 C: to
conjugated nitro oleÞns (28) followed by isomerization of intermediate nitrocyclopropanes
(23 g). However, this strategy was used only in one study (see
Scheme 3.30).
(1)
(2)