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

490 NITRONATES
R
R′
R
R′
O
N
H
O
73
O
N
O H
74
O
N
OCMe 3
74a
R
R′
R
R′
N + O (1) N
OH
X
O
N
OH
+
O
(2)
N + (3)
OH
Scheme 3.72
H
N XH
+ (1′)
corresponding oxime (MeO2C)2C=N–OH in 80% to 90% yield (235). Decomposition
of (73a) was studied in protic and aprotic solvents of different polarity
(CH2Cl2, methanol, ethanol, dioxane, acetonitrile, monoglyme, ethyl acetate, and
DMF). It was found that the reaction rate is virtually independent of the nature
of the solvent (k ≈ 5·10 −4 –10·10 −4 min −1 ). Therefore, the results of the study
(235) fully conÞrm that thermal decomposition of nitronate (73a) occurs as a
monomolecular electrocyclic reaction. More than 30 years later, thermal decomposition
of nitronate (73a) was reinvestigated by Japanese researchers (49), who
obtained very close results. These authors also stated that thermal decomposition
of nitronate (73a) is completely inhibited by the addition of 10% mol. boron trißuoride
etherate, which however causes spontaneous decomposition of both isomers
of nitronate (MeO2C)CH=N(O)OMe giving rise to bis-carbomethoxyfuroxan
rather than the corresponding oxime (MeO2C)CH=N–OH (see Eq. 1 in Scheme
3.73).
Data on the chemistry of acyclic nitronates conÞrms that the process described
by Equation 1 in Scheme 3.72 has a general character (see, e.g., Ref. 56) or the
results of thermolysis of nitronate (73b) (236) described by Equation 2 in Scheme
3.73).
It should be noted that the mechanism of thermal decomposition of acyclic
nitronates was not studied in detail. It is known that cis isomers of alkyl nitronates
derived from primary AN are much less stable than the analogous trans isomers
(49, 237a).