Chemistry Letters Vol.33, No.7 (2004)
923
57, 6099 (2001).
tack the silicon atom to give 3. In contrast, conjugate base of
stronger Brꢀnsted acid is less nucleophilic and the attack on
the silicon atom is suppressed. Water also acted as a nucleophile
toward silicon atom, thus interrupting the formation of 4.17
Next, one-pot synthesis of quinoline derivative was exam-
ined. Although oxidizing agents such as DDQ, I2, and CAN were
not effective, Mn(OAc)3 turned out to be effective for the syn-
thesis of quinoline derivatives. Treatment of aldimine and silyl
enol ether (2.0 equiv.) with CF3SO3H (10 mol %) in CH3CN at
room temperature for 3 h, and subsequent oxidation of the mix-
ture with Mn(OAc)3 (2.0 equiv.) in AcOH at room temperature
overnight furnished numerous kinds of quinoline derivatives in
good to excellent yields (Figure 2).18
2
3
G. Babu and P. T. Perumal, Tetrahedron Lett., 39, 3225 (1998).
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3419 (2001).
4
5
6
7
8
9
OTMS
1) CF3SO3H
2) Mn(OAc)3
N
N
Ar2
R
Ar2
+
R
Ar1
Ar1
2
1
10 M. V. Spanedda, V. D. Hoang, B. Crousse, D. Bonnet-Delpon,
and J.-P. Begue, Tetrahedron Lett., 44, 217 (2003).
11 W. Zhang, X. Jia, L. Yang, and Z.-L. Liu, Tetrahedron Lett.,
43, 9433 (2002).
12 T. Akiyama, J. Takaya, and H. Kagoshima, Synlett, 1999,
1045; T. Akiyama, J. Takaya, and H. Kagoshima, Tetrahedron
Lett., 42, 4025 (2001); T. Akiyama, J. Takaya, and H.
Kagoshima, Adv. Synth. Catal., 344, 338 (2002); T. Akiyama,
J. Itoh, and K. Fuchibe, Synlett, 2002, 1269; T. Akiyama, J.
Itoh, K. Yokota, and K. Fuchibe, Angew. Chem., Int. Ed., 43,
1566 (2004).
13 For the Brꢀnsted acid-catalyzed aza Diels–Alder reaction, see;
T. Akiyama, J. Takaya, and H. Kagoshima, Tetrahedron Lett.,
40, 7831 (1999).
14 Lewis acid promoted aza Diels-Alder reaction of N-aryl
aldimine and silyl enol ether has been reported. See, Ref. 5b.
15 For recent examples of the preparation of quinolines, K. Sangu,
K. Fuchibe, and T. Akiyama, Org. Lett., 6, 353 (2004); B. R.
McNaughton and B. L. Miller, Org. Lett., 5, 4257 (2003),
and references cited therein.
Me
OMe
N
N
N
Ph
Ph
Ph
Ph
Ph
Ph
Ph
100%
93%
90%
Cl
MeO
N
N
N
Ph
Ph
Ph
Ph
Me
96%
N
90%
85%
N
Ph
Ph
16 The structure of 5 was determined by comparison of the
1H NMR spectra with the literature data.19
Cl
OMe
94%
17 pKa values of Brꢀnsted acids follow; HBF4 (À0:44),20 HClO4
88%
N
(À10).21
OMe
OMe
18 Typical experimental procedure (2,4-diphenylquinoline), To a
solution of benzylideneaniline (40.2 mg, 0.222 mmol) and ꢁ-
trimethylsiloxystyrene (88 mL, 0.444 mmol) in CH3CN
(1 mL) was added a CH3CN solution of CF3SO3H (98 mL,
0.021 mmol) (0.226 mol/L solution of CF3SO3H in CH3CN)
at room temperature. After being stirred at the temperature
N
Ph
71%
Ph
Me
O2N
.
80%
for 3 h, Mn(OAc)3 2H2O (120.1 mg, 0.448 mmol) and AcOH
(3 mL) were added. The mixture was stirred at the temperature
for 24 h. After removing the solid by filtration over Celite, the
filtrate was neutralized by addition of sat. NaHCO3 solution.
The filtrate was extracted with CH2Cl2 and the combined
extracts were washed with sat. NaHCO3 solution and brine.
Removing the solvent in vacuo gave crude material. Purifica-
tion of the crude material with p-TLC (hexane:ethyl acetate
= 7:1/v:v) furnished 2,4-diphenylquinoline (62.4 mg, 0.221
mmol) in a quantitative yield.
Figure 2. One-pot synthesis of quinolines.
In conclusion, we have found an intriguing effect of the
Brꢀnsted acid leading to quinoline derivative by [4+2] cycload-
dition reaction of silyl enol ether and aldimines. Subsequent
Mn(OAc)3 mediated oxidation in one pot conditions furnished
quinoline derivatives in good to excellent yields.
References and Notes
19 T. Nishio and Y. Omote, J. Chem. Soc., Perkin Trans. 1, 1983,
1773.
20 T. N. Sudakova and V. V. Krasnoshchekov, Zh. Neorg. Khim.,
23, 1506 (1978).
21 W. N. White, T. Vogelmann, M. Morse, and H. S. White,
J. Org. Chem., 42, 162 (1977).
1
D. L. Boger, S. M. Weinreb, ‘‘Hetero Diels-Alder Methodolo-
gy in Organic Synthesis,’’ Academic, San Diego (1987),
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to Dienes,’’ ed. by B. M. Trost and I. Fleming, Oxford (1991)
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Published on the web (Advance View) June 21, 2004; DOI 10.1246/cl.2004.922