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

474 NITRONATES
(DMAP) provides the only approach to silylation of nitroalkanes containing perßuoroalkyl
substituents at the α-C atom (133).
The use of more polar solvents instead of benzene makes it difÞcult to separate
the triethylamine salt formed as a by-product, but can substantially accelerate
silylation. For example, the use of CH2Cl2 increases the yields of silyl esters of
primary AN (188).
An increase in electrophilicity of SiX (e.g., the use of silyl trißates) is possible
only in the absence of protons bonded to the β-C atom in silylated AN (see entry
10 in Table 3.1).
In other cases, the use of silyl trißates leads to double silylation of the starting
AN. (This process will be considered separately in Section 3.5.1. Silylation of
AN containing functional groups at the β-carbon atom will also be considered
separately in Section 3.5.5.)
3.2.3.3. Silylation of Products of Conjugated Addition of Nucleophiles to α-
NitrooleÞns Nitroalkane anions can be generated not only by deprotonation of
nitroalkanes (various modiÞcations of these process were considered above) but
also by the conjugated addition of nucleophiles 56 to α-nitroalkenes (42) (Scheme
3.56, Table 3.2).
In this approach, the SENA skeleton is assembled from nitroalkene (42) and
nucleophile 56.With the exception of two examples (entries 1 and 2 in Table 3.2),
the reaction does not stop at SENA 51, which either undergoes intramolecular
cyclization through [3 + 2]-cycloaddition to give fused heterocycles (as a rule after
elimination of trimethylsilanol) (198–200) or is involved in [3 + 2]-cycloaddition
with specially added methyl vinyl ketone or methyl acrylate to form (after elimination
of silanol) substituted isoxazolines in rather high yields (201).
Generally, anionic intermediates A smoothly react with trialkylchlorosilanes
as the temperature is raised to room temperature. To improve the solubility of
intermediate magnesium nitronates (A in scheme 3.56), it is advantageous to
add HMPA to the reaction mixture. The addition of Et3N stabilizes SENAs as
intermediate.
Cunico and Motta reported (202) on the very interesting one-pot synthesis
of SENAs, such as β-nitrocarboxylic acid N,N -dimethylamides derivatives, from
silylated N,N -dimethylformamides 57a,b and conjugated nitroalkenes (Scheme
3.57).
R
R 1
O
N
O
+ R
42
2-E 56
Si - trialkylsilyl;
O
R N O −
E +
SiCl
−78°C 20°C
R
A
51
2 R1 R2 R1 Scheme 3.56
O
R N OSi + ECl