Organic Letters
Letter
undesired two carbon ring-expanded silyl enol ethers 5 as
minor products (Table 4, entries 1, 2). It is notable that for
medium ring size (Table 4, entry 4), only (Z)-silyl enol ether
4d was obtained.
Further chemical transformations of the resulting stereo-
selective (Z)-silyl enol ethers are illustrated in Scheme 2. One-
yields through carbonyl homologation. This methodology was
successfully applied to sequential Mukaiyama aldol and [2 + 2]-
cycloaddition reactions.
ASSOCIATED CONTENT
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S
* Supporting Information
Experimental procedures and full analytical data. This material
Scheme 2. Sequential Reactions of (Z)-Silyl Enol Ethers
AUTHOR INFORMATION
Corresponding Author
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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Dedicated to Professor Sung Ho Kang on the occasion of his
honorable retirement. This paper was supported by 63
Research Fund, Sungkyunkwan University, 2012.
pot sequential reactions were successfully performed with
catalyst 1c. Benzaldehyde was added into 2a prepared in situ by
the general procedure to afford Mukaiyama aldol7 product.
After deprotection of the TMS group under acidic conditions,
β-hydroxy ketone 6 was isolated in 90% yield (Scheme 2, eq 1).
As a further application of the sequential reaction, in situ silyl
enol ether formation and [2 + 2]-cycloaddition8 were carried
out (Scheme, eq 2). This process was successfully catalyzed by
40 mol % of 1c to generate product 7 in 95% yield and a 7:1
diastereomeric ratio.
REFERENCES
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(1) For reviews, see: (a) Brownbridge, P. Synthesis 1983, 1.
(b) Brownbridge, P. Synthesis 1983, 85. (c) Kuwajima, I.; Nakamura,
E. Acc. Chem. Res. 1985, 18, 181. (d) Fleming, I.; Barbero, A.; Walter,
D. Chem. Rev. 1997, 97, 2063. (e) Kobayashi, S.; Manabe, K.; Ishitani,
H.; Matsuo, J.-I. Sci. Synth. 2002, 4, 317.
̀ ́
(2) (a) Cahiez1, G.; Figadere, B.; Clery, P. Tetrahedron Lett. 1994,
35, 6295. (b) Lessene, G.; Tripoli, R.; Cazeau, P.; Biran, C.; Bordeau,
M. Tetrahedron Lett. 1999, 40, 4037. (c) Xie, L.; Vanlandeghem, K.;
Isenberger, K. M.; Bernier, C. J. Org. Chem. 2003, 68, 641. (d) He, X.;
Hurley, E.; Noll, B. C.; Henderson, K. W. Organometallics 2008, 27,
3094. (e) Kerr, W. J.; Watson, A. J. B.; Hayes, D. Org. Biomol. Chem.
2008, 6, 1238.
A mechanistic proposal is presented in Scheme 3. Boron
Lewis acid activated ketone 8 undergoes a nucleophilic addition
(3) (a) Hudrlik, P. F.; Hudrlik, A. M.; Kulkarni, A. K. J. Am. Chem.
Soc. 1985, 107, 4260. (b) Trost, B. M.; Surivet, J.; Toste, F. D. J. Am.
Chem. Soc. 2001, 123, 2897. (c) Song, Z.; Kui, L.; Sun, X.; Li, L. Org.
Lett. 2011, 13, 1440. (d) Tsubouchi, A.; Enatsu, S.; Kanno, R.; Takeda,
T. Angew. Chem., Int. Ed. 2010, 49, 7089. (e) Dias, E. L.; Brookhart,
M.; White, P. S. J. Am. Chem. Soc. 2001, 123, 2442. (f) Aggarwal, V. K.;
Sheldon, C. G.; Macdonald, G. J.; Martin, W. P. J. Am. Chem. Soc.
2002, 124, 10300. (g) Dabrowski, J. A.; Moebius, D. C.; Wommack, A.
J.; Kornahrens, A. F.; Kingsbury, J. S. Org. Lett. 2010, 12, 3598.
(4) (a) Gao, L.; Kang, B. C.; Hwang, G.-S.; Ryu, D. H. Angew. Chem.,
Int. Ed. 2012, 51, 8322. (b) Gao, L.; Kang, B. C.; Ryu, D. H. J. Am.
Chem. Soc. 2013, 135, 14556.
Scheme 3. Proposed Mechanism for (Z)-Silyl Enol Ether
Formation Reaction
(5) (a) Hauser, C. R.; Hance, C. R. J. Am. Chem. Soc. 1952, 74, 5091.
(b) Sato, S.; Okada, H.; Matsuda, I.; Izumi, Y. Tetrahedron Lett. 1984,
25, 769. (c) Sun, C.; Li, J.; Demerzhan, S.; Lee, D. ARKIVOC 2011,
17.
(6) Lee, S. I.; Kang, B. C.; Hwang, G.-S.; Ryu, D. H. Org. Lett. 2013,
15, 1428.
(7) (a) Mukaiyama, T.; Narasaka, K.; Banno, K. Chem. Lett. 1973, 2,
1011. (b) Mukaiyama, T.; Banno, K.; Narasaka, K. J. Am. Chem. Soc.
1974, 96, 7503.
(8) Canales, E.; Corey, E. J. J. Am. Chem. Soc. 2007, 129, 12686.
(9) (a) Nakamura, K.; Osamura, Y. J. Am. Chem. Soc. 1993, 115,
9112. (b) Berson, J. A.; Suzuki, S. J. Am. Chem. Soc. 1959, 81, 4088.
(10) (a) Brook, A. G.; Limburg, W. W.; MacRae, D. M.; Fieldhouse,
S. A. J. Am. Chem. Soc. 1967, 89, 704. (b) Moser, W. H. Tetrahedron
2001, 57, 2065.
by the TMSD carbon to afford diazonium intermediate 9. This
intermediate then undergoes a 1,2-aryl shift with concomitant
nitrogen extrusion followed by a general migration tenden-
cy,3g,9 to form the α-silyl ketone intermediate 10. For 2q (R = i-
Pr), α-silyl ketone 10 was isolated under 10 mol % of catalyst.
Finally, stereoseletive (Z)-silyl enol ether 11 is generated by a
1,3-Brook rearrangement.10
In conclusion, we have developed an oxazaborolidinium ion
catalyzed reaction between alkyl aryl ketones and TMSD as a
new and effective method for the highly stereoselective
synthesis of (Z)-silyl enol ethers. While employing cyclic
ketones, ring-expanded silyl enol ethers were prepared in high
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dx.doi.org/10.1021/ol500174q | Org. Lett. 2014, 16, 2077−2079