Abstract
The cyclization of 2-(1-alkynyl)-2-alken-1-ones proceeded smoothly in the presence of various nucleophiles with a catalytic amount of Bu4 N[AuCl4 ] in [bmim]BF4 , leading to the formation of highly substituted furans. This system is air-stable and recyclable. The catalyst is inexpensive and readily available.
Key words
gold-catalyzed - cyclization reaction - ionic liquids - furans - recyclable
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Representative Procedure.
To the appropriate 2-(1-alkynyl)-2-alken-1-one (0.2 mmol) and nucleophile (1.5 equiv) in [bmim]BF4 (1 mL) was added Bu4 N[AuCl4 ] (1 mol%). The mixture was stirred at r.t. for 3 h unless otherwise specified. The mixture was extracted with Et2 O (2 ¥ 5 mL). The isolated ether layer was concentrated in vacuo and the residue was purified by flash chromatography on silica gel.
12
Recycling Procedure.
To the 2-phenylethynyl-2-cyclohexen-1-one (1.0 mmol) and MeOH (1.5 equiv) in [bmim]BF4 (3 mL) was added Bu4 N[AuCl4 ] (1 mol%). The mixture was stirred at r.t. for 3 h. The product was extracted from the reaction mixture by addition of Et2 O (2 ¥ 20 mL). The ether layer was separated. The ionic liquid layer containing Au catalyst was pumped for several minutes and reused for the next reaction.
13
Spectral Data of the Representative Product 2a.
1 H NMR (300 MHz, CDCl3 ): δ = 1.79-1.93 (m, 2 H), 1.94-2.05 (m, 2 H), 2.59-2.75 (m, 2 H), 3.43 (s, 3 H), 4.28 (t, J = 3.9 Hz, 1 H), 6.64 (s, 1 H), 7.18-7.23 (m, 1 H), 7.31-7.36 (m, 2 H), 7.60-7.63 (m, 2 H). 13 C NMR (75 MHz, CDCl3 ): δ = 18.8, 23.2, 28.3, 56.1, 72.3, 105.3, 119.9, 123.3, 126.8, 128.5, 131.1, 152.2, 152.8.