ORGANIC
LETTERS
2009
Vol. 11, No. 16
3778-3781
B-Alkylcatecholborane-Mediated Tandem
Radical Conjugated Addition-Aldol
Cyclization
Alice Beauseigneur,† Cecilia Ericsson,†,‡ Philippe Renaud,*,† and Kurt Schenk§
Department of Chemistry and Biochemistry, UniVersity of Bern, Freiestrasse 3,
CH-3012 Bern, Switzerland, Department of Biochemistry and Organic Chemistry,
Uppsala UniVersity, Box 576, S-751 23 Uppsala, Sweden, and Laboratoire de
´
Crystallographie, Ecole Polytechnique Fe´de´rale de Lausanne, Le Cubotron - Dorigny,
CH-1015 Lausanne, Switzerland
Received July 1, 2009
ABSTRACT
A one-pot procedure involving radical conjugate addition of B-alkylcatecholboranes to enones followed by intramolecular aldol reaction is
reported. Application to the stereoselective synthesis of monocyclic and bicyclic products with up to four contiguous stereogenic centers is
presented.
Formation of C-C bonds is a key process in synthetic
organic chemistry. Running tandem processes involving more
than one C-C bond formation is highly attractive since they
allow a rapid increase in molecular complexity.1 In recent
years, we have explored the use of radical reactions in
synthesis and have developed a wide range of C-C bond-
forming radical reactions such as conjugate addition, ally-
lation, alkynylation, and vinylation using organoboranes as
radical precursors.2,3 Interestingly, the radical conjugated
addition reaction of organoboranes to various R,ꢀ-unsaturated
ketones occurs via formation of boron enolates.3,4 Since
boron enolates are known to participate efficiently in aldol
reactions, a tandem process involving a radical conjugate
addition followed by an aldol reaction could be envis-
aged.5,6 Tandem conjugate addition-aldol reactions using
organometallic species are well documented in the literature.7
Examples of radical mediated conjugated addition followed
by an aldol processes involving boron8 and zinc9 enolate
intermediates have been reported. In this communication, we
describe a process involving efficient radical mediated
conjugate additions of B-alkylcatecholboranes to enones
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1991, 64, 403–409. Ueda, M.; Miyabe, H.; Sugino, H.; Miyata, O.; Naito,
T. Angew. Chem., Int. Ed. 2005, 44, 6190–6193. Miyabe, H.; Ueda, M.;
Fujii, K.; Nishimura, A.; Naito, T. J. Org. Chem. 2003, 68, 5618–5626.
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† University of Bern.
‡ Uppsala University.
§
´
Ecole Polytechnique Fe´de´rale de Lausanne.
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10.1021/ol9014943 CCC: $40.75
Published on Web 07/24/2009
2009 American Chemical Society