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

472 NITRONATES
R
NO2 + Li2S LiHS + N
OLi
R = H (98%); F (77%)
R
Scheme 3.53
O
Me 3SiCl
− LiCl
R
N
O
OSiMe 3
Sodium hydride in THF can be used with advantage to deprotonate nitro
derivatives of carbohydrates in the presence of Bu t Me2SiCl (182, 183). In these
cases, the yields of respective SENAs are 60% to 80%.
However, a strong nitrogen base–DBU– is the silylating agent of choice for
silylation of AN (Scheme 3.54) (184). This procedure is versatile and allows one
to rapidly synthesize most of the target anions with a high degree of conversion
of the starting AN. The resulting anions of nitro compounds (54) rapidly react
with trialkylhalosilane to give SENA(51) in high yield.
Nevertheless, silylation of AN with a Et3N/SiX mixture is nowadays most
widely used (Scheme 3.55, Table 3.1).
Apparently, the 52⇆54 step is the rate-determining step of this reaction, the
equilibrium being shifted toward undissociated AN 52 due to low acidity of
unfunctionalized AN. In most cases, silylation products can be prepared with the
use of appropriate solvents and oleÞns as trapping agents. However, the conversion
of many AN into the corresponding SENAs in the absence of trapping agents
in benzene is low (see Table 3.1). It is particularly difÞcult to prepare SENAs
from secondary AN (see, e.g., entry 7 in Table 3.1). The reaction with the use of
triethylamine in the presence of a catalytic additive of 4-dimethylaminopyridine
DBU
[RR′C(NO2)] − 52
DBU
54
H
SiCl
−DBU HCl
O
RR′C N
51 OSi
85%-95%
+
RR′CHNO 2
R
CHNO2
R′
52
N
Et 3N
N
O
DBU; Si - trialkylsilyl
R R′
54
Scheme 3.54
N O−
Et3NH + SiX
−Et3NHX O
RR′C N
OSi
51
Si − trialkylsilyl group; X − Cl, OTf
Scheme 3.55