Angewandte
Chemie
Keywords: allylation · allylic alkylation · asymmetric catalysis ·
.
boron · copper
[
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[
[
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3
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[
[
Scheme 2. Formation of 9 and its addition to a model aldehyde and
ketone.
1
1
Unfortunately, there is little opportunity for using these
species in enantioselective additions because the fluoride
substituents cannot be readily modified to incorporate chiral
b)H. Malda, A. W. van Zijl, L.A. Arnold, B. Feringa, Org. Lett.
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[
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[
18]
[10] For the simple two-step synthesis of 6 from 3-chloropropyne,
see: M. Gravel, B. B. Tourꢀ, D. G. Hall, Org. Prep. Proced. Int.
directing groups. Herein, we report promising preliminary
results for the first chiral a-substituted allylic trifluoroborate
salts. We were delighted to find that the preparation of salt 9
2004, 36, 573 – 579.
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[
19]
from the corresponding pinacolate 1a occurs with ease and
without racemization as shown by its addition to a model
aldehyde and ketone (Scheme 2). Ongoing efforts to increase
the E/Z selectivity may lead to an efficient class of highly
reactive ketone allylation agents.
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1
[
8
In summary, we have developed a simple and efficient
catalytic enantioselective preparation of a chiral a-substituted
allylic boronate reagent. It was further demonstrated that this
reagent can add with very high diastereoselectivity to
aldehydes in a convenient one-pot fashion using a low-
temperature procedure promoted by a Lewis acid. This
preparative method also provides an efficient route to
reactive allylic trifluoroborate salts. Further studies will aim
to extend this approach to carbonyl crotylation reactions and
additions to imines.
[
[
14] The uncatalyzed reaction gives a 3:1 E/Z selectivity.
15] V. Rauniyar, D. G. Hall, J. Am. Chem. Soc. 2004, 126, 4518 –
4519.
[16] The use of stoichiometric BF
was necessary for a high E/Z ratio.
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3
[
1
Synthesis 2000, 990 – 998.
[
18] For a phase-transfer-catalysis approach using chiral ammonium
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[19] D. S. Matteson, G. Y. Kim, Org. Lett. 2002, 4, 2153 – 2155.
Received: March 6, 2007
Published online: June 20, 2007
Angew. Chem. Int. Ed. 2007, 46, 5913 –5915
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5915