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

SILYLATION OF NITRO COMPOUNDS AS A PROCESS 643
It is reasonable to suggest that SENA A is initially generated, and then can
undergo a reversible second silylation. The resulting iminium cation B undergoes
stereoselective intramolecular cyclization to give the cation C, whose deprotonation
affords dihydrofurans (375) containing the nitroso acetal fragment. These
target products are generated in high yields, but the substituent R 4 in the starting
substrate must necessarily be conjugated with the carbonyl group, thus decreasing
the electrophilicity of its carbon atom. If R 2 =H, the iminium cation cannot
be trapped intramolecularly because of its fast deprotonation.
If two acyl groups are attached to the C-3 atom, the formation of the target
dihydrofuran (376) occurs apparently through another pathway (Scheme 3.214).
In the Þrst step, silylation occurs at the –CHAc2 fragment containing the most
active proton. Then the process involves double silylation of the nitro fragment,
cyclization of the resulting cation and stabilization of the intermediate accompanied
by elimination of trimethylbromosilane (292).
If the substituent R 4 is not conjugated with the carbonyl group (compounds
(377) in Scheme 3.215), silylation proceeds in a different fashion (153, 292,
470). After the formation of the initial SENAs (378), the latter undergo reversible
cyclization to give cyclic iminium cations B, which are stabilized by deprotonation
of the C-4 atom to form the cyclic intermediates 2H -5,6-dihydrooxazines C.
Ac
O2N Ac
Ph
i – Me3SiBr/ Et3N ~150 h, −30°C;
O
Me
O 2N
Me
376
O
Ph
Ph
i
Ac
Me
OSiMe3
N(OSiMe3) 2
78%
i
Me 3SiBr
Scheme 3.214
Me3SiO
N
Me3SiO Me3Si
O
Me
+
O
Me
Ph
O
+
Me
Ph
OSiMe3
Me
Br −
N(OSiMe3)2
Br−