A. Claraz, G. Landelle, S. Oudeyer, V. Levacher
SHORT COMMUNICATION
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an ammonium aryloxide bearing an OTMS group on the
quinoline moiety (intermediate D).[18] Nevertheless, to date,
we cannot rule out either of these two mechanisms and the
modest level of enantioselectivity observed could be ex-
plained by competition between the two pathways.
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Conclusions
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We have reported an enantioselective organocatalytic
protonation reaction of silyl enolates 1a–m by using a new
and simple betaine-type catalysts derived from readily ac-
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simple conditions (DME, –20 °C), moderate enantio-
selectivities have been attained for the tetralone series of
substrates. Note that the benzosuberone series, which gen-
erally provides low ees, furnished modest but unprecedented
enantioselectivity of about 40%. These betaine catalysts are
the subject of further study.
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Experimental Section
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A mixture of betaine QN+5 (12.2 mg, 0.025 mmol) and 4-meth-
oxyphenol (34.1 mg, 0.275 mmol) in dry DME (0.25 mL) was
stirred for a few minutes at room temperature until complete solu-
bilization of the reagents. The reaction flask was placed at –20 °C
and silyl enol ethers 1a–m (0.25 mmol) were added as a solution in
DME (0.25 mL). The reaction mixture was stirred at –20 °C until
complete disappearance of the silyl enol ethers (monitored by GC-
FID). The solvent was removed under vacuum and the residue
purified by flash chromatography (petroleum ether/Et2O = 95:5) to
afford the pure ketones 2a–m, which were analyzed by HPLC using
chiral column to determine the enantiomeric excess.
[11] A. Claraz, S. Oudeyer, V. Levacher, Adv. Synth. Catal. 2013,
355, 841–846.
[12] For a recent review on cooperative ion-paring in asymmetric
organocatalysis, see: J.-F. Briere, S. Oudeyer, V. Dalla, V. Lev-
acher, Chem. Soc. Rev. 2012, 41, 1696–1707.
[13] For recent examples of the enantioselective protonation of chi-
ral quaternary ammonium enolate intermediates derived from
cycloalkanone and cycloalkenone enol chloroacetates, see: a)
E. Yamamoto, A. Nagai, A. Hamasaki, M. Tokunaga, Chem.
Eur. J. 2011, 17, 7178–7182; b) E. Yamamoto, D. Gokuden, A.
Nagai, T. Kamachi, K. Yoshizawa, A. Hamasaki, T. Ishida, M.
Tokunaga, Org. Lett. 2012, 14, 6178–6181.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, 1H and 13C NMR spectra of QN+5–
QN+8, 1c, 1k, 2k, HPLC traces for 2a–m.
Acknowledgments
[14] W.-Q. Zhang, L.-F. Cheng, J. Yu, L.-Z. Gong, Angew. Chem.
2012, 124, 4161; Angew. Chem. Int. Ed. 2012, 51, 4085–4088.
[15] H. Li, Y. Wang, L. Tang, L. Deng, J. Am. Chem. Soc. 2004,
126, 9906–9907.
This work was supported by the Centre National de la Recherche
Scientifique (CNRS), the University of Rouen and INSA of Rouen,
the région Haute-Normandie, the FEDER “BIOFLUORG” (con-
vention no. 33236) and Labex SynOrg (ANR-11-LABX-0029).
G. L. and A. C., respectively, thank the région Haute-Normandie
and the CRUNCh for a grant.
[16] See the Supporting Information for further details.
[17] T. Poisson, S. Oudeyer, V. Dalla, F. Marsais, V. Levacher, Syn-
lett 2008, 2447–2450.
[18] All our attempts to synthesize and isolate intermediate D
failed.
[1] For general reviews on enantioselective protonation reactions,
Received: September 4, 2013
Published Online: October 24, 2013
see: a) A. Claraz, S. Oudeyer, V. Levacher, Curr. Org. Chem.
7696
www.eurjoc.org
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 7693–7696