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

Me 2N
CH 2
N OSiMe 3
R 1
R 2
+
O
N
OSi
Me
Me
NO2 95%
Me2N Me
NO2 97%
331a 331b
SILYLATION OF NITRO COMPOUNDS AS A PROCESS 615
R 1
O
R N
O
2
N CH2 O Si
Me 2H+N
Si
O
96%
331e
Me 2N
CO2Me
N
O
Scheme 3.194
− (Si) 2O
NO 2
NO2 NO2 85%
331c
Me 2N
N R 1
CH 2
Me
65%
331d
However, Michael addition has been successfully performed in recent years
with the use of nucleophilics catalysis (ßuoride anion or amines) (132) (Scheme
3.195).
Only few SENAs are involved in reactions with standard Michael substrates,
and the yields of the target products are low (see Scheme 3.195). However,
a similar process with unstable intermediates, conjugated nitrosoalkenes, was
studied in sufÞcient detail (see Section 3.5.4.2.2.).
Recently, Japanese researchers have performed several asymmetric syntheses
with SENAs (460–462) catalyzed by chiral ammonium cations A or A’ (see
Scheme 3.196).
Few of these studies (460, 462) dealt with the Michael reaction; one study
(461) with the Henry reaction. The efÞciency, stereoselectivity, and enantioselectivity
of this process are rather high. The mechanism of the transformations
is poorly known. Presumably, the chiral cation should shield the Si surface
of nitronate, thus providing the Re approach of the substrate. In addition, the
approach of the reagents, resulting in generation of syn isomers, is considered
less favorable than the approach yielding anti isomers.
3.5.1.2. Double Silylation of Aliphatic Nitro Compounds–Optimization and
Procedures Double silylation of AN with standard silylating agents SiX/Base
is mechanistically shown in Chart 3.20 as a four-step process.
−
331
NO 2
NO 2
R 2
NO 2