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

484 NITRONATES
O
N OBR3 2
R 1 R 2
62a-c
+ R 4 Me 2SiX
R 1 = R 2 = R 3 = Me, R 4 = Et, X = NMe2 (50%);
R
Si
4Me2 X
BR3 2
O
N O
R 1 R 2
R 1 = H, R 2 = CMe2CO2Me, R 3 = Me, R 4 = Et, X = NMe2 (55%);
R 1 = R 2 = Me, R 3 = Pr i , R 4 = Et, X = NMe 2 (55%)
Scheme 3.62
A
N
R2 OSiMe2R4 R1 O
51o-r
+
(R 3 2BX) 3
involvement of silyl nitronates and the corresponding boron halides takes place
(for more details, see below in Section 3.2.4.2).
3.2.4. Synthesis of other Types of Nitronates
3.2.4.1. Synthesis of Acyl Nitronates The most general approach to the synthesis
of acyl nitronates is based on the reactions of anions of the corresponding
AN with acyl halides or carboxylic acid anhydrides in the presence of bases.
Here, we will not consider a series of studies, where the formation of intermediate
O-acyl nitronates was postulated without conclusive proof or by detection
of their decomposition products.
Acyl nitronates (63) derived from primary AN are characterized by two types
of such transformations: the rearrangement into α-acetoxy aldoximes (219) and
elimination of the corresponding carboxylic acids to form nitrile oxides (Scheme
3.63).
As a rule, acyl nitronates derived from secondary AN are more stable. For
example, the reaction of acetic anhydride with the Na salt of 2-nitropropane in
the presence of K2CO3 afforded the corresponding acyl nitronate in a yield lower
RCH2NO2
Ac–acyl.
AcHal or Ac2O
base
RCH
63
O
N
OAc
Scheme 3.63
H
RC N
OAc
−AcOH R C N O
O R C NOH
OAc