Gold(I)-Catalyzed Intramolecular [4+3]-Cycloaddition Reactions with Furan Propargyl Esters as the Substrates: Carbenoid vs. Stabilized Allyl Cation

Abstract

The tricyclic ring system with an oxabicyclo[3.2.1]octadiene and a fused six-membered ring was produced efficiently using the readily available propargyl ester furan substrate in the presence of a Au(I) complex. The reaction involves a tandem 3,3-rearrangement of the propargyl ester followed by an intramolecular [4+3]-cycloaddition reaction. Both the primary ligand of the gold complex (N-heterocyclic carbene; NHC) and a neutral dynamic ligand (PhCN) are important for the success of the reaction.

Key words

gold-catalyzed - cycloadditions - furan - propargyl ester - carbenoid

• References and Notes

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• 27 General Procedure for the Au(I)-Catalyzed Intramolecular [4+3]-Cycloadditions: To a round-bottom flask equipped with a stirring bar under an atmosphere of nitrogen were added AgSbF₆ (2 mg, 0.007 mmol) and IPrAuCl (0.007 mmol) in freshly distilled CH₂Cl₂ (0.5 mL) and stirred for 5 min. The propargyl ester (0.07 mmol) dissolved in freshly distilled CH₂Cl₂ (0.5 mL) was added dropwise to the reaction and stirred at r.t. and monitored with ¹H NMR. The reaction mixture was diluted with Et₂O, filtered through Celite, the solvent removed under reduced pressure, and the residue purified over silica gel column. [4+3]-Cycloaddition Product 26: ¹H NMR (300 MHz, CDCl₃): δ = 0.97 (t, J = 7.35 Hz, 3 H), 1.04–1.20 (m, 1 H), 1.22–1.48 (m, 3 H), 1.68–1.89 (m, 4 H), 1.97 (d, J = 13.5 Hz, 1 H), 2.17 (s, 3 H), 2.43 (d, J = 17 Hz, 1 H), 2.89–2.92 (m, 1 H), 5.02 (d, J = 4.5 Hz, 1 H), 5.93 (d, J = 6.0 Hz, 1 H), 6.67 (d, J = 6.0 Hz, 1 H). ¹³C NMR (125 MHz, CDCl₃): δ = 11.90, 18.83, 20.45, 23.27, 24.29, 29.70, 32.21, 42.05, 80.00, 83.42, 126.75, 129.47, 139.44, 141.89, 168.66. LCMS: m/z [M + Na] calcd for C₁₅H₂₀O₃: 271.1; found: 271.1. [4+3]-Cycloaddition Product 27: ¹H NMR (500 MHz, CDCl₃): δ = 1.24–1.46 (m, 4 H), 1.91–1.74 (m, 4 H), 1.99 (d, J = 13.0 Hz, 1 H), 2.11 (s, 3 H), 2.20–2.11 (m, 2 H), 2.45 (d, J = 14.5 Hz, 1 H), 3.05 (m, 1 H), 5.04 (m, 3 H), 5.81 (m, 1 H), 5.96 (d, J = 6.0 Hz, 1 H), 6.71 (d, J = 6.0 Hz, 1 H). ¹³C NMR (125 MHz, CDCl₃): δ = 20.5, 23.2, 24.0, 24.4, 25.0, 31.4, 32.2, 39.9, 79.9, 83.4, 115.2, 126.6, 129.7, 137.8, 139.2, 142.0, 168.6. LCMS: m/z [M + Na] calcd for C₁₇H₂₂O₃: 297.1; found: 297.1. Enyne Cyclization Product 28: ¹H NMR (500 MHz, CDCl₃): δ = 0.75 (dd, J = 7.8, 5.4 Hz, 1 H), 1.04 (t, J = 4.8 Hz, 1 H), 1.10–1.38 (m, 2 H), 1.65–1.94 (m, 4 H), 2.04–2.06 (m, 2 H), 2.07 (s, 3 H), 2.14–2.25 (m, 1 H), 2.62 (t, J = 7.5 Hz, 2 H), 5.35 (d, J = 15.5 Hz, 1 H), 5.37–5.49 (m, 2 H), 5.98 (d, J = 3.0 Hz, 1 H), 6.29 (dd, J = 3.0, 1.8 Hz, 1 H), 7.31 (d, J = 1.5 Hz, 1 H). ¹³C NMR (125 MHz, CDCl₃): δ = 13.00, 21.25, 24.36, 26.02, 26.77, 27.34, 27.78, 31.99, 32.06, 78.24, 104.73, 110.03, 127.36, 131.80, 140.69, 156.21, 171.42. LCMS: m/z [M + Na] calcd for C₁₇H₂₂O₃: 297.1; found: 297.1.