M. Lachia et al. / Tetrahedron Letters 53 (2012) 4514–4517
4517
Scheme 6. Asymmetric synthesis of (+)-GR-24 (+)-20. Reagents and conditions: (i) EDCI (1.4 equiv), HOAt (1.4 equiv), Et3N (3 equiv), DMF, 16 h, 93%; (ii) Tf2O (1.1 equiv),
collidine (1.2 equiv), CH2Cl2, rt, 6 h; (iii) CCl4, H2O, reflux 4 h, 68%; (iv) H2O2 (3 equiv), AcOH, 0 °C, 92%; (v) tBuOK (1.2 equiv), HCO2Et (3 equiv), THF, 0 °C; then 19, 20% of (+)
20 and 20% of (+) 21.
Mesmaeker, A.; Ernst, B. Terahedron Lett. 1988, 29, 2303–2306; (f) Ernst, B.;
De Mesmaeker, A.; Greuter, H.; Veenstra, S. J. Strain and its implications in
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formation of the lactone (+)-18 in only 44% ee.
15. A typical procedure for the ketene-iminium cycloaddition: To a solution of amide
17 (0.100 g, 0.386 mmol) in dichoromethane (10 mL) was added collidine
(0.061 mL, 0.463 mmol) followed by triflic anhydride (0.072 mL, 0.424 mmol).
The solution was stirred at room temperature for 24 h. The solvents were
removed in vacuo and the residue was taken up in carbon tetrachloride (4 mL)
and water (4 mL) and the biphasic mixture was stirred at 70 °C for 6 h. The
aqueous layer was extracted with dichloromethane and the combined organic
layers were dried and concentrated. The residue was purified by flash
chromatography eluting with cyclohexane and ethyl acetate (20/1) to give
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48 mg of colorless oil. ES: 159 (20%), 143 (100%); IR (liq.) cmÀ1: 2921, 1777; 1
H
NMR (400 MHz, CDCl3) d 7.29–7.36 (2H, m), 7.23–7.29 (2H, m), 4.65–4.80 (1H,
m), 3.46 (1H, dd, J = 16.9, 8.1 Hz), 3.36 (1H, ddd, J = 18.0, 8.1, 4.8 Hz), 3.11–3.19
(1H, m) 3.08 (1H, d, J = 16.9 Hz), 2.88 (1H, ddd, J = 18.0, 5.9, 3.3 Hz).