10.1002/anie.201813880
Angewandte Chemie International Edition
(±)-5s could also be isolated, albeit in low yield due to its volatility.
Compound (±)-5t bearing an alkyl group required a prolonged
reaction time of 72 h but could be isolated in 63% yield. By
converting (±)-5p into its dinitro benzoate the cis-configuration in
the seven-membered ring could be unambiguously assigned by
X-ray crystallographic analysis (Figure 1).
It has been shown that a Z-configuration within the
substrates results in a significant increase of the activation barrier
for the 3,3-sigmatropic rearrangement of divinylcyclo-
propanes.[9b-9d, 13] Such a rearrangement usually requires higher
reaction temperatures not compatible with the temperature
sensitive organocatalyst (–)-2a.[14] Therefore, the more stable
diarylprolinol silyl ether (–)-2b was used for the conversion of 4-
(2-(Z)-vinylcyclopropyl)but-2-enals (Table 6). Using this strategy,
we failed to generate the alkylated species (±)-5u and (±)-5v.
Acknowledgements
This work was supported by the Fonds der Chemischen Industrie
(Ph.D. scholarship to C.A.). Lilian Maria Maas (FU Berlin) is kindly
acknowledged for providing substrate (±)-1x. We thank Dr. Volker
Martin Schmiedel (FU Berlin) for fruitful discussion and Christiane
Groneberg (FU Berlin) for HPLC support.
Keywords: organocatalysis
• pericyclic reactions • Cope
rearrangement • cycloheptadienes • cyclopropanes
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good to excellent yield along with good diastereoselectivity.
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In conclusion, we have developed an organocatalytic Cope
rearrangement that is initiated by dienamine formation. The
reaction is driven by release of ring strain and proceeds under
mild conditions and with high functional group tolerance. The
substrates were successfully transformed into the corresponding
cycloheptadienes via in situ generation of a divinylcyclopropyl
motif by means of secondary amine catalysis. The optimized
reaction conditions afforded good yields and good to excellent
diastereoselectivities. The E-4-(2-vinylcyclopropyl)-but-2-enals
reacted significantly slower than the Z-substrates and required
higher reaction temperatures. Overall, the dienamine induced
DVCPR opens up a diastereoselective route to substituted
cycloheptadienes and greatly extends the utility of
organocatalysis.
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