Double Coupling Reactions of 3,4-Bis(stannyl)furanone: Facile Preparation of Diaryl- and Dibenzylfuranones

Abstract

The palladium-catalyzed cross-coupling reaction of 3,4-bis(tributylstannyl)furan-2(5H)-one using chelating ligand or polar solvent gives mixtures of single and double coupled products, even when one equivalent of halide coupling partner is used. After optimization, the double coupling reaction was shown to be general, with the use of two equivalents of aryl iodides giving 3,4-disub­stituted furanones, The reaction using benzyl bromides proceeds at lower temperatures than the corresponding coupling using aryl ­iodides, giving dibenzylfuranones. The methodology has been ­exemplified in a synthesis of (±)-hinokinin.

Key words

furanon - butenolide - Stille coupling - diarylfuranone - lignan - hinokinin

References and Notes

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4

It is not possible to directly discern the yields of the individual coupling reactions: assuming a maximum yield of 100% for the second coupling (normally a more efficient process, see ref. 2), the maximum possible yield for the first step is 51%. Our original observations in monocoupling of 1 with PhI gave 2 (Ar = Ph) in a yield of 51%.

5

Representative Experimental Procedure.
To a flame-dried flask (under argon atmosphere) charged with PdCl₂(PPh₃)₂ (2 mol%), CuI (8 mol%), AsPh₃ (8 mol%) was added 3,4-bis(tributylstannyl)furan-2(5H)-one (500 mg, 0.76 mmol) as a solution in dry, deoxygenated DMF, followed by iodobenzene (310 mg, 1.52 mmol), also added dropwise as a solution in DMF (2.0 mL). After reaction was complete (24 h) the mixture was diluted with aq KF (1 M, 10.0 mL) and extracted with Et₂O (3 × 30.0 mL), washed with H₂O (3 × 15.0 mL) and brine (3 × 15.0 mL). Solvent was removed under reduced pressure and the crude product was purified via flash chromatography (silica gel, 3:2 PE-Et₂O; R _f = 0.24). Recrystallization (CH₂Cl₂-PE) gave 3,4-diphenylfuran-2(5H)-one as pale yellow crystals (56%, 101 mg); mp 104-105 °C (CH₂Cl₂-PE). IR (CHCl₃): 1751, 1646, 1489 cm^-1. ¹H NMR (250 MHz, CDCl₃): δ = 5.10 (2 H, s), 7.23-7.33 (10 H, m). ¹³C NMR (60 MHz, CDCl₃): δ = 71.0 (OCH₂), 126.6, 127.9, 129.2, 129.3, 129.4, 129.7, 130.6, 131.0, 131.2, 156.6, 173.9. MS (CI, NH₃): m/z calcd for C₁₆H₁₃O₂: 237.0916. Found [MH]⁺: 237.0915; m/z (%) = 179.0 (15).

7

Representative Experimental Procedure.
To a flame-dried flask (under an argon atmosphere) charged with benzyl bromide (226 mg, 1.33 mmol), PdCl₂(PPh₃)₂ (5 mol%), AsPh₃ (8 mol%) and CuI (8 mol%) was added THF (5.0 mL) and the mixture warmed to 50 °C. 3,4-Bis(tributyl-stannyl)furan-2(5H)-one (450 mg, 0.66 mmol) in THF (5.0 mL) was added dropwise via syringe. After reaction was complete the mixture was concentrated under reduced pressure. Purification via flash chromatography (silica gel, PE-Et₂O 1:1; R _f = 0.50) gave 3,4-dibenzylfuranone as a clear colorless oil (78 mg, 45%). IR (CHCl₃): 3063, 1754, 1668 cm^-1. ¹H NMR (250 MHz, CDCl₃): δ = 3.71 (2 H, s), 3.74 (2 H, s), 4.51 (2 H, s), 6.99-7.02 (2 H, dd, J = 7.0, 2.0 Hz), 7.22-7.29 (8 H, m). ¹³C NMR (60 MHz, CDCl₃): δ = 30.0, 34.0, 71.7, 127.0, 127.2, 127.7, 129.0, 129.1, 129.2, 129.5, 136.3, 138.5, 160.3, 175.2. MS (CI, NH₃): m/z calcd for C₁₈H₁₆O₂: 265.1231. Found [MH]⁺: 265.1221; m/z (%) = 219 (20), 91 (25).

9

Data for (±)-Hinokinin.
IR (CHCl₃): 1769, 1495, 1245, 1033 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 2.45-2.60 (3 H, m, ArCH₂CH, ArCH₂CH and ArCH2CHCHC=O), 2.86 (1 H, dd, J = 14.5, 7.0 Hz, ArCH₂CHC=O), 2.99 (1 H, dd, J = 14.0, 5.0 Hz, ArCH₂CHC=O), 3.87 (1 H, dd, J = 9.00, 7.00 Hz, OCH₂), 4.14 (1 H, dd, J = 10.0, 7.00 Hz, OCH₂), 5.95 (4 H, m, OCH₂O), 6.46-4.48 (2 H, m, 2 × Ar,), 6.62 (2 H, m, 2 × Ar), 6.64 (2 H, m, 2 × Ar). ¹³C NMR (100 MHz, CDCl₃): δ = 35.2, 38.8 (ArCH₂), 41.7 (ArCH₂CHCHC=O), 46.9 (ArCH₂CHCHC=O), 71.6 (OCH₂), 101.4 (OCH₂O) 108.6, 108.9, 109.2, 109.8, 121.9, 122.6, 131.7, 132.0, 132.1, 146.8, 146.9, 148.3 (2 × Ar), 178.8 (C=O). MS (CI, NH₃): m/z calcd for C₂₀H₁₉O₆: 355.1181. Found [MH]⁺: 355.1164; m/z (%) = 135 (45).