C.H. Cheon, H. Yamamoto / Tetrahedron 66 (2010) 4257–4264
4263
column chromatography (EtOAc/MeOH (20:1)) on silica, the prod-
uct was obtained. This solid was re-dissolved in acetone, acidified
with aqueous HCl (4 N, 50 mL), re-extracted with ether, dried over
Na2SO4, and concentrated. The product was obtained as a pale
NaHCO3 solution, extracted with ether. The organic layer was
combined, dried with Na2SO4, and concentrated. Column chro-
matography on silica (hexanes/ethyl acetate (6:1)) gave the
mixture of two diastereomers 14a-
a and 14a-b in 4:1 ratio with
brown solid. 1H NMR (DMSO, 500 MHz)
(DMSO, 125 MHz)
183.2, 193.9.; 19F NMR (CDCl3, 471 MHz)
d
: no peak; 13C NMR
80% overall yield.
d
: 108.6–118.8 (m, CF2CF2CF2CF2CF2CF2CF2CF3),
d
:ꢁ71.4 (s, 2F), ꢁ80.3 (s,
Acknowledgements
3F), ꢁ114.6 (s, 2F), ꢁ120.5 (s, 2F), ꢁ121.5 (s, 2F), ꢁ121.8 (s, 2F),
ꢁ122.5 (s, 2F), ꢁ125.8 (s, 2F). MS (APCI) Exact mass calcd for
C20H2F34N2O6S2 (Mꢁ1) 1074.8. Found: 1074.7.
We would like to thank Toyota Motors Corporation for financial
support and Dr. V.H. Rawal for his kind discussion for this project.
4.2. Application of squaramides 2a–c to organic reactions
Supplementary data
4.2.1. Representative procedure for Mukaiyama aldol reaction. To
a solution of 2a (0.0020 M; 1.0 mL; 0.0020 mmol; 0.010 equiv) in
acetonitrile was added 4a (21 mg; 0.20 mmol; 1.0 equiv) and the
mixture was stirred for 10 min at room temperature. After that,
silyl enol ether 3a (42 mg; 0.22 mmol; 1.1 equiv) was added
dropwise to the reaction mixture. The reaction mixture was
allowed to stir at room temperature while the reaction was
monitored by TLC. When the aldehyde 4a was completely con-
sumed, aqueous HCl (1 N; 1 mL) was added to the reaction
mixture and stirred at the same temperature until all the silyl
ether was converted to the free alcohol. The reaction mixture
was neutralized with NaHCO3, extracted with ether, dried over
Na2SO4, and concentrated. Column chromatography (EtOAc/hex-
anes (15:85)) on silica gave the desired product 5aa in white
solid. (43 mg; 96% yield).
Supplementary data associated with this article can be found, in
References and notes
1. For a review of Brønsted acid catalysis, see: Pihko, P. M. Angew. Chem., Int. Ed.
2004, 43, 2062.
2. For a review of hydrogen bond organic catalysis, see: Doyle, A. G.; Jacobsen,
E. N. Chem. Rev. 2007, 107, 5713.
3. Huang, Y.; Unni, A. K.; Thadani, A. N.; Rawal, V. H. Nature 2003, 424, 146.
4. For reviews of chiral phosphoric acid catalysis, see: (a) Akiyama, T. Chem. Rev.
2007, 107, 5744; (b) Tereda, M. Chem. Commun. 2008, 4097.
5. For reviews of strong Brønsted acid catalysis in organic reactions, see: (a) Boxer,
M. B.; Albert, B. J.; Yamamoto, H. Aldrichimica Acta 2009, 42, 3; (b) Takasu, K.
Synlett 2009, 1905.
6. TfOH and Tf2NH are known to be highly hygroscopic.
7. The pKa1 of squaric acid is 1.5 and pKa2 is 3.5, see: West, R.; Powell, D. L. J. Am.
Chem. Soc. 1963, 85, 2577.
8. (a) Malerich, J. P.; Hagihara, K.; Rawal, V. H. J. Am. Chem. Soc. 2008, 130, 14416;
(b) Zhu, Y.; Malerich, J. P.; Rawal, V. H. Angew. Chem., Int. Ed. 2010, 49, 153.
9. (a) Ishihara, K.; Hasegawa, A.; Yamamoto, H. Angew. Chem., Int. Ed. 2001, 40,
4077; (b) Hasegawa, A.; Ishikawa, T.; Ishihara, K.; Yamamoto, H. Bull. Chem. Soc.
Jpn. 2005, 78, 1401.
10. (a) Nakashima, D.; Yamamoto, H. J. Am. Chem. Soc. 2006, 128, 9626; (b) Jiao, P.;
Nakashima, D.; Yamamoto, H. Angew. Chem., Int. Ed. 2008, 47, 2411; (c) Cheon, C.
H.; Yamamoto, H. J. Am. Chem. Soc. 2008, 130, 9246; (d) Cheon, C. H.; Yamamoto,
H. Org. Lett. accepted for publication.
11. For a review of non-covalently solid-phase bound catalysts for organic synthesis,
see: Horn, J.; Michalek, F.; Tzschucke, C. C.; Bannwarth, W. In Topics in Current
Chemistry 242: Immobilized Catalysts-Solid Phases, Immobilization and Applica-
tions; Kirschning, A., Ed.; Springer: Heidelberg, Germany, 2004; Chapter 2, p 43.
12. For a review of fluorous reverse phase silica gel in organic synthesis, see:
Curran, D. P. Synlett 2001, 1488.
13. (a) For the development of N,N-bis(triflyl)squaramide, see: Cheon, C. H.; Ya-
mamoto, H. Tetrahedron Lett. 2009, 50, 3555; (b) For the development of N,N-
bis(nonaflyl)squaramide, see: Bull. Korean Chem. Soc. 2010, 31, 539; (c) For
a review for usage of nonaflyl group in organic synthesis, see: Ho¨germeier, J.;
Reissig, H.-U. Adv. Synth. Catal. 2009, 351, 2747.
4.2.2. Representative procedure for Mukaiyama Michael reaction. To
a solution of 2a (0.0020 M; 1.0 mL; 0.0020 mmol; 0.010 equiv) in
acetonitrile was added 10a (29 mg; 0.20 mmol; 1.0 equiv) and the
mixture was stirred for 10 min at room temperature. After that, silyl
enol ether 3a0 (55 mg; 0.22 mmol; 1.1 equiv) was added dropwise
to the reaction mixture. The reaction mixture was allowed to stir at
room temperature while the reaction was monitored by TLC. When
the enone 10a was completely consumed, aqueous HCl (1 N; 1 mL)
was added to the reaction mixture and stirred at the same tem-
perature until all the silyl ether was converted to the free alcohol.
The reaction mixture was neutralized with NaHCO3, extracted with
ether, dried over Na2SO4, and concentrated. Column chromatog-
raphy (EtOAc/hexanes (15:85)) on silica gave the desired product
11aa in white solid. (49 mg; 98% yield).
14. For the original report in the patent, see: Armand, M.; Choquette, Y.; Gauthier,
M.; Michot, C. Patent Vol. EP0850920 (A2), 1998.
4.2.3. Representative procedure for Hosomi–Sakurai allylation re-
action. To a solution of 2a (2.3 mg; 0.0060 mmol; 0.030 equiv) in
acetonitrile was added 4f (30 mg; 0.20 mmol; 1.0 equiv) and the
mixture was stirred for 10 min. After that, allyltrimethylsilane
(34 mg; 0.30 mmol; 1.5 equiv) was added dropwise to the reaction
mixture. The reaction mixture was allowed to stir at room tem-
perature while the reaction was monitored by TLC. After 8 h,
aqueous HCl (1 N; 1 mL) was added to the reaction mixture and
stirred at room temperature until all the silyl ether was converted
to the free alcohol. The reaction mixture was neutralized with
NaHCO3, extracted with ether, dried over Na2SO4, and concen-
trated. Column chromatography (EtOAc/hexanes (15:85)) on silica
gave the desired product 12 in white solid. (35 mg; 92% yield).
15. 13C NMR spectra showed no decomposition of squaramide 2a after 60 days on
the benchtop under air. Moreover, even two-month old solution of 2a in CH3CN
still keeps its catalytic reactivity.
16. For a seminal reference of Mukaiyama aldol reaction, see: Mukaiyama, T.;
Narasaka, K.; Banno, K. Chem. Lett. 1973, 1011.
17. For reviews, see: (a) Ishihara, K.; Yamamoto, H. In Modern Aldol Reactions;
Mahrwald, R., Ed.; Wiley-VCH: Weinheim, Germany, 2004; Vol. 2, Chapter 2,
p 25; (b) Palomo, C.; Oiarbide, M.; Garcia, J. M. Chem.dEur. J. 2002, 8, 37.
18. An example of Lewis acid pathway in situ generated silylated Brønsted acid,
see: Boxer, M. B.; Yamamoto, H. Nat. Protoc. 2006, 1, 2434.
19. For recent examples of asymmetric Lewis acid catalysis via in situ generation
of the silylated Brønsted acids from Brønsted acids, see: (a) Zamfir, A.;
´
´
´
Tsogoeva, S. B. Org. Lett. 2010, 12, 188; (b) Gracıa-Gracıa, P.; Lay, F.; Gracıa-
Gracı´a, P.; Babalakos, C.; List, B. Angew. Chem., Int. Ed. 2009, 48, 4363; (c)
Rowland, E. B.; Rowland, G. B.; Rivera-Otero, E.; Antilla, J. C. J. Am. Chem. Soc.
2007, 129, 12084.
20. Usage of this base to differentiate reaction pathway between Lewis acid and
Brønsted acid catalysis, see: (a) Cheon, C. H.; Yamamoto, H. Tetrahedron Lett. 2009,
50, 3555; (b) Hara, K.; Akiyama, R.; Sawamura, M. Org. Lett. 2005, 7, 5621; (c)
Mathieu, B.; Ghosez, L. Tetrahedron 2002, 58, 8219; (d) Ref. 19(b); (e) Ref. 10(d).
21. Ishihara, K.; Yamamato, H. Chem. Commun. 2002, 1564.
22. For a seminal reference of Mukaiyama Michael reaction, see: Narasaka, K.; Soai,
K.; Mukaiyama, T. Chem. Lett. 1974, 1223.
23. In the crude reaction mixture, the oligomer of 2-cyclopenenone 10e was ob-
served from the reaction of the resulting enol ether with the remaining 2-cy-
clopentenone 10e.
4.2.4. Representative procedure for carbonyl-ene reaction of rac-cit-
ronellal. To a solution of 2a (68 mg; 0.10 mmol; 0.050 equiv) in
THF (20 mL) was added rac-citronellal 13 (0.31 g; 2.0 mmol;
1.0 equiv) dropwise at room temperature. The reaction was
allowed to stir at room temperature while the reaction was
monitored by TLC. After all the starting material was consumed,
the reaction mixture was quenched with saturated aqueous