Scheme 2. Lithiation of 6 and Reaction with Electrophiles
Scheme 4. Stannylation of 6e with Ph3SnCl
(Scheme 2). Further, it is questionable if configurational
stability is still given due to extended conjugation in the
pentadienyl anion of 7.
process,8 the lithium intermediate 7e has the (S)-configura-
tion; this is formed by removal of the encircled proton (pro-R
Dienes 6 are synthesized in a straightforward manner.
After complete isomerization of allyl carbamates3 10 to vinyl
carbamates 11 via lithiation, lithium-titanium exchange,4
and protonation, the (Z)-enol carbamates 6 are obtained after
vinylic lithiation5 of 11, transmetalation to zinc, and Negishi
coupling5c,6 (Scheme 3).
Scheme 3. Synthesis of Dienes 6
Figure 1. X-ray structure of 13.9
in 6e). The structures of the further substitution products are
based on this result, combined with the known or proven
stereochemical courses of the reactions.
The lithium intermediates derived from 6 are configura-
tionally stable under the reaction conditions: after the (-)-
sparteine-mediated deprotonation of 6e (toluene, -78 °C)
and reaction with (CH3)3SiCl, the silane 15 is obtained with
the same enantiomeric purity (er ) 98:2) after a standing
time of 1 h (73%) or in situ trapping (Scheme 5). 15 is
Deprotonation of dienyl carbamate 6e with n-butyllithium/
(-)-sparteine (2) in toluene at -78 °C proceeds smoothly
within 1 h. Trapping with Ph3SnCl provides two separable
regioisomers 13 and 147 in a ratio of 68:32 and an er of
99:1 for 13 (Scheme 4). An X-ray crystal structure analysis
with anomalous diffraction revealed the (5S,1E,3Z) config-
uration of 13 (Figure 1). Because all stannylations of lithiated
2-alkenyl carbamates are known to take place in an anti-SE′
Scheme 5. Configurational Stability of 6
(3) (a) Hoppe, D.; Hanko, R.; Bro¨nneke, A.; Lichtenberg, F.; van Hu¨lsen,
E. Chem. Ber. 1985, 118, 2822. (b) Hoppe, D.; Behrens, K.; Fro¨hlich, R.;
Meyer, O. Eur. J. Org. Chem. 1998, 2397.
(4) Reviews: (a) Reetz, M. T. Organotitanium Reagents in Organic
Synthesis, 1st ed.; Springer-Verlag: Berlin, 1986. (b) Weidmann, B.;
Seebach, D. Angew. Chem. 1983, 95, 12; Angew. Chem., Int. Ed. Engl.
1983, 22, 32. (c) Reetz, M. T. Organotitanium Chemistry. In Organome-
tallics in Synthesis, 2nd ed.; Schlosser, M., Ed.; Wiley: Chichester, 2002;
p 817.
a c ) 0.76, 1.05 in CHCl3; er ) 98:2.
(5) (a) Hoppe, D.; Paulsen, H. Tetrahedron 1992, 48, 5667. (b) Hoppe,
D.; Peschke, B.; Lu¨ssmann, J.; Dyrbusch, M. Chem. Ber. 1992, 117, 1421.
(c) Sengupta, S.; Snieckus, V. J. Org. Chem. 1990, 55, 5680. (d) Kocienski,
P.; Dixon, N. J. Synlett 1989, 52.
(6) Negishi, E.; Zeng, X.; Tan, Z.; Qian, M.; Hu, Q.; Huang, Z. In Metal-
Catalyzed Cross-Coupling Reactions, 2nd ed.; de Meijere, A., Diedrich,
F., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany,
2004; Vol. 2, p 815.
assigned the (S)-configuration because all silylations of
lithiated 2-alkenyl carbamates are known to take place in
an anti-SE′ process.
(7) 14 decomposes easily not allowing complete analysis.
(8) Paulsen, H.; Graeve, C.; Hoppe, D. Synthesis 1996, 141.
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