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M. Shindo et al.
LETTER
(5) (a) In contrast, Denmark’s b-silyl alkenes stabilize oxyallyl
cations by the b-effect. See: Denmark, S. E.; Jones, T. K. J.
Am. Chem. Soc. 1982, 104, 2642. (b) In Frontier’s polarized
substrate, both of the pentadienyl and oxyallyl cations are
stabilized. See ref. 2c.
(6) For reviews on ynolates, see: (a) Shindo, M. Tetrahedron
2007, 63, 10. (b) Shindo, M. Synthesis 2003, 2275.
(c) Shindo, M. Synth. Org. Chem. Jpn. 2000, 58, 1155.
(d) Shindo, M. Chem. Soc. Rev. 1998, 27, 367.
(7) Shindo, M.; Matsumoto, K.; Shishido, K. Org. Synth. 2007,
84, 11.
(8) Shindo, M.; Kita, T.; Kumagai, T.; Matsumoto, K.;
Shishido, K. J. Am. Chem. Soc. 2006, 128, 1062.
(9) Mannekens, E.; Tourwé, D.; Lubell, W. D. Synthesis 2000,
1214.
(10) General Procedure for the Sc(OTf)3-Catalyzed Nazarov
Reaction
(11) Ohwada reported the O,O-diprotonated dication
intermediate in the TfOH-mediated Nazarov reaction:
Suzuki, T.; Ohwada, T.; Shudo, K. J. Am. Chem. Soc. 1997,
119, 6774.
(12) General Procedure for the TfOH/Tf2O-Catalyzed
Nazarov Reaction
Preparation of the TfOH/Tf2O stock solution: 2.0 mL of a
mixture, prepared from TfOH (1.0 mL, 11.3 mmol) and Tf2O
(50 mL), was dissolved in CH2Cl2 (50 mL). To the
divinylketone (0.210 mmol) was added the stock solution
(0.5 mL), containing TfOH (0.2 mmol) and Tf2O (0.01
mmol), at r.t. under argon and stirred. After 3 min, a sat. aq
NaHCO3 was added and the mixture was extracted with
CH2Cl2. The organic extracts were washed with a sat. aq
NaHCO3, and brine, dried over MgSO4, filtered and
concentrated in vacuo. The residual oil was purified through
silica gel column chromatography (hexane–EtOAc) to give
the cyclopentenone.
A solution of the (E)-b-alkoxy divinyl ketone (0.125 mmol)
in CH2Cl2 (2 mL) was added to a solution of Sc(OTf)3 (10
mol%) in CH2Cl2 (3 mL) in one portion at r.t. and the
solution was stirred. After 3 min, a sat. aq NaHCO3 was
added and the mixture was extracted with CH2Cl2. The
organic extracts were washed with a sat. aq NaHCO3, and
brine, dried over MgSO4, and concentrated in vacuo. The
residual oil was purified through silica gel column
chromatography (hexane–EtOAc, 90:10) to give the
a-ethoxy cyclopentenone.
(13) General Procedure for the Tf2O-Catalyzed Nazarov
Reaction
To a solution of the substrate (0.15 mmol) in anhyd CH2Cl2
(0.6 mL), under argon, was added TfOH (0.1 mol%) at r.t.
and stirred. After 3 min, a sat. aq NaHCO3 solution was
added and the mixture was extracted with CH2Cl2. The
organic extracts were washed with a sat. aq NaHCO3
solution and brine, dried over MgSO4, filtered and
concentrated in vacuo to afford a crude cyclopentenones.
The residual oil was purified through silica gel column
chromatography (hexane–EtOAc, 90:10) to give the desired
a-ethoxy cyclopentenone as colorless oil.
Compound 8: 1H NMR (400 MHz, CDCl3): d = 1.12 (t,
J = 7.2 Hz, 3 H), 1.27 (s, 3 H), 1.58 (s, 3 H), 2.38 (d, J = 18.4
Hz, 1 H), 2.67–2.80 (m, 3 H), 2.86 (t, J = 7.2 Hz, 2 H), 3.21–
3.33 (m, 2 H), 7.15–7.30 (m, 5 H). 13C NMR (100 MHz,
CDCl3): d = 7.98, 15.8, 23.2, 32.8, 33.1, 42.4, 59.4, 78.9,
126.4, 128.1, 128.5, 134.9, 140.2, 169.3, 208.3. IR (neat):
1705, 1643, 1088 cm–1. MS (EI): m/z = 214 [M+ – OEt].
Anal. Calcd for C17H22O2: C, 79.03; H, 8.58. Found: C,
78.81; H, 8.73.
(14) Interruption of oxyallyl cations: (a) Shoppee, C. W.; Cooke,
B. J. A. J. Chem. Soc., Perkin Trans. 1 1972, 2271. Other
examples, see: (b) Magnus, P.; Quagliato, D. J. Org. Chem.
1985, 50, 1621. (c) Asokan, C. V.; Bhattacharji, S.; Ila, H.;
Junjappa, H. Synthesis 1988, 281. (d) Hara, S.; Okamoto,
S.; Narahara, M.; Fukuhara, T.; Yoneda, N. Synlett 1999,
411. (e) Wang, Y.; Arif, A. M.; West, F. G. J. Am. Chem.
Soc. 1999, 121, 876. (f) Dhoro, F.; Tius, M. A. J. Am. Chem.
Soc. 2005, 127, 12472.
Synlett 2007, No. 7, 1096–1100 © Thieme Stuttgart · New York