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

SILYLATION OF NITRO COMPOUNDS AS A PROCESS 717
steps. For this purpose, cyclic nitronates (568) are synthesized initially, and their
silylation through cations (569) and oxazines A afford silyl derivatives (570)
whose desilylation produces the target oximes. However, the desired reaction
is complicated by a side reaction associated with the ring-chain tautomerism of
cations (569) (for more details, see Section 3.5.2.5).
An increase in the size of the carbocycle and steric hindrance of the base
leads to a decrease in the contribution of the target enoxime in the reaction
products. Hence, in each particular case it is necessary to perform special experiments
to elucidate whether the scheme is applicable for the synthesis of conjugated
enoximes containing a remote functional group and to Þnd optimal
conditions.
3.5.6.3. Approaches to the Synthesis of Unfunctionalized Conjugated Enoximes
The role of conjugated enoximes as biologically active compounds and as valuable
reagents and intermediates in organic synthesis was considered in the beginning
of Section 3.5.6.
Therefore, it is important to develop special procedures for the synthesis of
enoximes (573) from available AN through relatively stable intermediate BENA
(572) and unstable α-nitrosoalkenes A (Scheme 3.278).
Such procedures are presently lacking, although uncontrolled transformations
of selected BENA into silyl derivatives of enoximes were documented (469).
Another possible route to the synthesis of conjugated enoximes (573) is based
on the sequence presented in Scheme 3.279.
Here, six-membered cyclic nitronate (575) is initially assembled from the
simple reagents according to known procedures, and this nitronate is silylated to
give the intermediate A, whose [4 + 2]-cycloreversion affords silyl derivative of
enoxime (574). Finally, desilylation of the latter compound gives rise to enoxime
(573). This chain of transformations was repeatedly documented in the literature
(see, e.g., Refs (153, 264)). However, this sequence was not optimized as a
method for the synthesis of conjugated enoximes. It can be concluded that the
absence of an alkyl substituent at the C-3 atom (R 1 =H) and the use of sterically
unhindered bases (e.g., of pyridine) is favorable for realizing the desired
pathway.
R2 R3 R1 NO2 R2 R3 R1 N
R2 R3 R
N
1
R
O
2
R3 SiO
OSi
AN 572
H
A 573
N
Si – trialkylsilyl (SiMe 3 most preferable)
Scheme 3.278
R 1
OH