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

598 NITRONATES
O
O
OR
R - (+)menthyl
EtO 2C
N
O
O
281a
OR
O 2N
EtO 2C
EtONa
OEt
O
CH(NO 2)CO 2Et
EtO 2C
O
O
O
OR
280
92%
K2CO3 O
O
281
OR
H2C CHCO2Et
30°–40°/ 24 h
R′X
EtO
O
2C CO
N
2Et
O
O
OR
282a
Scheme 3.176
CO2Et
N
OR′
R′ – prim. alkyl 70%–96%
R′ – sec. alkyl 63%–66%
X – Cl, Br, I
the enantioselectivity of the [3 + 2]-cycloaddition is associated with discrimination
of one of the facial attacks of oleÞn on 1,3-dipole (281a) due to shielding
by the menthyl radical.
Silyl nitronates containing chiral inductors have not been as yet used in
intermolecular [3 + 2]-cycloaddition reactions. In this case, the facial discrimination
was generally created by introducing chiral nonracemic fragments into
dipolarophiles (see review 433).
Several SENAs derived from primary AN were involved in the reaction with
ceptem (282) (Scheme 3.177, Eq. 1) (434) to prepare the diastereomeric pure
cycloadducts, which were then transformed into isoxazolines (283). However, the
conÞgurations of the new stereocenters in products (283) were not determined.
The reactions of some SENAs with chiral dipolarophiles (284a,b) were also
described (411) (Scheme 3.177, Eq. 2). It should be noted that the yields of the
target cycloadducts were not always high due to steric hindrance in vicinally
substituted dipolarophiles. Also the facial selectivity is rather moderate.
Another approach is based on the use of the so-called Oppolzer’s sultams
products (286a,b), (Scheme 3.177, Eq. 3) (435–437), or on polycyclic lactam
(287) (Scheme 3.177, Eq. 4) (438) as dipolarophiles.
In the former case, both “antipodes” of sultams were used and the initially
formed cycloadducts (288a–e) were transformed into more stable isoxazolines
(289a–e) and (289 ′ a–e) with retention of conÞguration of the new stereocenter
at C-5. For Oppolzer’s sultam, (435–437) the yields of the target products were
high and the ratio of diastereomers was close to 1:10.
Interestingly, nonpolar solvents facilitate an increase in the enantioselectivity
of the process, and the introduction of a substituent at C-4, but, leads to a decrease