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

722 NITRONATES
bubbling a solution of nitrile (586) through a layer of silica gel. Catalysis by the
ßuoride ion leads to complete removal of the silyl protecting group to form the
Z isomer of β-acetylacrylonitrile (591a).
Probably, silylation will be a convenient procedure for the trans→cis isomerization
of β-functionalized ene nitriles. Special experiments demonstrated that
silylation of E isomer (591b) with an excess of Et3N/Me3SiOTf in CH2Cl2, followed
by the removal of the silyl protecting group, gives the thermodynamically
unfavorable Z isomer (591a) in a total yield of 75%.
Several examples of exhaustive silylation of primary AN, conjugated
enoximes, and ene nitriles (470, 554) are presented in Scheme 3.283. As can
be seen from these data, silylation of the C,C double bond is accompanied by
the introduction of trimethylsilyl groups at the allylic position with respect to
this bond. In the presence of a substituent in the β-position, the reaction is
stereoselective at the C,C double bond.
It should be noted that total silylation of AN remains to be optimized. However,
a convenient and simple procedure is expected to be developed for the
synthesis of anionic intermediates A from the corresponding primary AN (see
the lower line in Scheme 3.283).
The latter can be involved in coupling reactions with various electrophiles to
give interesting products (550). The development of this strategy is hindered by
the lack of convenient procedures for the synthesis of α-silylated conjugated ene
nitriles (see, e.g., Ref. 551).
Ph
NO 2
NO2
Me3SiOTf(10 equiv.)/Et3N(50 equiv.)
20°C, 7 days
Me3SiOTf(10 equiv.)/Et3N(15 equiv.)
0°C, 7 days
Me3SiOTf(10 equiv.)/Et3N(50 equiv.)
Me3Si SiMe3 30%
593
SiMe3 CN
20°C, 7 days SiMe3
N
OH Me3SiOTf(11eq.)/Et3N(12
Ph
equiv.)
CN
12 days,CH2Cl2 595
596 90%
SiMe3
CN
RCH2NO 2 SiMe 3
F
−SiMe3F
Scheme 3.283
SiMe3 CN
Me3Si
592
SiMe3 CN
42%
CN
594
A
CN
22%
SiMe3
CN ~100%