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

640 NITRONATES
MeO
Me
MeO
Me
MeO
Me
An
O N
368
−TMSOTf
An
An
O
O N
+
A
O N
B
+
TMSOTf
369a–d
R 1
TfO
OTMS
_
R 1
R 2
OTMS
OTMS
R 2
369a–d
O
TMSOTf
−MeOTMS
−TMSOTf
TMSOTf
−30 ο , 0.5–100 h
(368):369:TfOTMS = 1:2.5:3.5
Me
An
N
OH
An O
370d
R 1
N O
O Me O
R 2
R 2
R 1
An Ac
O N
An
371a (54%)
371b (62%)
371c (65%)
R 1
N O
O Me OH
C D
An - 4-MeO-C 6H 4; TMS - SiMe 3; 369a - R 1 = H, R 2 = Me; 369 b - R 1 = H, R 2 = An;
369c - R 1 , R 2 = (CH2)4; 369d - R 1 , R 2 = (CH2)3.
Scheme 3.211
The formation of a new C,C bond (intermediate B) is the key step in these
transformations. In the presence of Lewis acids, nitroso acetal B is rearranged to
give diketo oxime D.
Depending on the steric effects of the substituents R 1 and R 2 , the latter intermediate
undergoes cyclization to form either isoxazoles (371a–c) or N-hydroxypyrrole
(370 d) after elimination of water.
3.5.2.4. Intramolecular Trapping of Bis-N,N-Oxyiminium Cations Special γfunctionalized
nitro compounds (372) were constructed with the aim of performing
intramolecular trapping of bis-N,N-siloxyiminium cations prepared in process
of double silylation of (372). Monosilylation of the latter compounds can afford
different silyl derivatives (373a–c) (Scheme 3.212) (486, 487).
The nature of products (373) depends on a number of factors (486, 487). As
can be seen from the lower part of the scheme, the reactions of compounds, in
which R 3 is H or alkyl, using weak silyl Lewis acids (X is a poor leaving group)
O2
−H2O
−H2O
R 2