8
762
F. Acquadro et al. / Tetrahedron Letters 43 (2002) 8759–8763
plane) and that of the methoxy, leading to nine-mem-
bered pre-transition cyclic complexes. In the absence of
specific mechanistic studies or calculations, attempts to
rationalize the isomer ratios can appear very unrealistic.
It is however known that N-carbamate piperidines tend
to place their a-substituent into an axial orientation
the opening of (strained) carbocycles or to unsaturated
heterocycles (such as that described by Ishimaru and
3c
Kojima) would also set the stage for a new access to
1,3-disubstituted dienes of which reactivity has already
15
been studied. These developments are on the way.
14
(
the Paulsen effect). When one takes into account this
preference, the Z-configuration of the double bond in
the major isomers of 3 and 5, and the necessity for the
lithium to lie in the CNCO plane, the number of
degrees of freedom decreases sharply and only two
chelated transition states can be built from a molecular
model (Scheme 4). In the first one (A), the nine-mem-
bered ring is roughly chair-like (the tether standing
Acknowledgements
The Consiglio Nazionale delle Ricerche and the Centre
National de la Recherche Scientifique are thanked for
an exchange grant (CNR-CNRS c8724, 2000–2003).
‘
remote’ of the carbamate moiety) while it is the oppo-
References
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(
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6b
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