Issue ICHC-20 ARKIVOC 2006 (vii) 370-389
the negatively charged carbon, by taking advantage of the fact that carbon-silicon bonds are easily cleaved by nucleophiles such as halide ion. Thus, desilylation of a-silyliminium salts,7 treatment of a-silylimine derivatives with an acyl halide,8 trimethylsilyl triflate9 or trifluoroacetic acid,10 and the water-induced desilylation of a-silylimine derivatives11 have been developed.
Me
MeO2C CO2Me
X
X
MeO2C CO2Me R2
1) MeOSO2CF3
TMS
N
N R2 R2
2) CsF
N
R1
R1
room temp.
R1 X = O, S
On the other hand, N-unsubstituted azomethine ylides exist as tautomers of a-iminoester derivatives under thermal conditions. Namely, the highly acidic hydrogen adjacent to the imine migrates to the nitrogen. Thus generated azomethine ylides undergo 1,3-dipolar cycloaddition to give pyrrolidine derivatives.12
AB
R1
R1
A
B
R1 R2
MeO2C
N R2 MeO2C
N R2 ~100 °C
MeO2C N
H
H
Azomethine imines are much less studied than azomethine ylides. One of the frequently employed methods for the generation of azomethine imines is a condensation of N,N’disubstituted hydrazines and aldehydes.13
N H NH O Ph xylene, 140 °C – H2O N N O Ph Ph N N O Ph Ph PhCHO Hydrazones have been also used as azomethine imine precursors to achieve cycloadditions.14 The azomethine imines, generated by a formal 1,2-prototropic shift, undergo intermolecular cycloaddition with dipolarophiles to give pyrazolidine derivatives in low to moderate yields.
ISSN 1424-6376 Page 372 ©ARKAT
