Palladium-Catalyzed Reaction of Arenediazonium Tetrafluoroborates with Methyl 4-Hydroxy-2-butenoate: An Approach to 4-Aryl Butenolides and an Expeditious Synthesis of Rubrolide E

Abstract

The palladium-catalyzed reaction of arenediazonium ­tetrafluoroborates with methyl 4-hydroxy-2-butenoate in MeOH under mild conditions gives 4-arylbutenolides usually in good to high yields through a domino vinylic substitution/cyclization process. The reaction tolerates a variety of useful substituents including the whole range of halogen substituents, nitro, ether, cyano, keto, and ester groups and can be performed as a one-pot process generating the arenediazonium salt in situ. By using this method, the marine antibiotic rubrolide E has been synthesized via an expeditious and efficient sequential protocol that omits the isolation of the butenolide intermediate (two operative steps, 52% overall yield).

Key words

diazonium salts - Heck reaction - cyclization - buten­olides - rubrolides

References and Notes

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9

Typical Procedure for the Preparation of Butenolides (3) To a stirred solution of 2b (58.0 mg, 0.50 mmol) and Pd(OAc)₂ (5.6 mg, 0.025 mmol) in anhyd MeOH (5.0 mL), 1b (250.0 mg, 1.0 mmol) was added at r.t. under argon. The reaction mixture was warmed at 40 ˚C and stirred at that temperature for 4 h (the reactor was protected from light with aluminum film). After cooling, the reaction mixture was diluted with EtOAc, washed with brine, dried over Na₂SO₄, and concentrated under reduced pressure. The residue was purified by chromatography on SiO₂ [n-hexane-EtOAc, 60:40] to afford 83.9 mg of 3b (77% yield); mp: 192-194 ˚C. IR (KBr): 1745, 1710, 1280 cm^-¹. ^¹H NMR (400 MHz, CDCl₃): δ = 8.15 (d, J = 8.4 Hz, 2 H), 7.60 (d, J = 8.4 Hz, 2 H), 6.21 (t, J = 1.6 Hz, 2 H), 5.16 (d, J = 1.6 Hz, 2 H), 3.97 (s, 3 H). ^¹³C NMR (100.6 MHz, CDCl₃): δ = 173.2, 166.0, 162.5, 133.6, 132.9, 130.5, 126.5, 115.3, 70.9, 52.6. MS: m/z (%) = 216 (8) [M⁺], 75 (42), 59 (100).

14

One-Pot Procedure for the Preparation of Butenolides (3) from Anilines
A solution of BF₃˙OEt₂ (140 µL, 1.1 mmol) in anhyd THF (1 mL) was cooled at -15 ˚C, and 4-carbomethoxylaniline (151.2 mg, 1 mmol) was added. Then, tert-butyl nitrite (160 µL, 1.3 mmol) in 1 mL of the same solvent was added dropwise to the rapidly stirred solution over a 10 min period. After that, the reaction temperature was maintained at -15 ˚C for 10 min, allowed to warm to 5 ˚C (ice-water bath) over a 20 min period, warmed to r.t., and stirred at the same temperature till the disappearance of the starting aniline. The reaction mixture was subsequently concentrated under reduced pressure and diluted with anhyd MeOH (5 mL). Then, 2b (58.0 mg, 0.50 mmol) and Pd(OAc)₂ (5.6 mg, 0.025 mmol) were added, the reaction mixture was warmed at 40 ˚C, and stirred at that temperature for 4 h (the reactor was protected from light with aluminum film). After cooling, the reaction mixture was diluted with EtOAc, washed with brine, dried over Na₂SO₄, and concentrated under reduced pressure. The residue was purified by chromatography on SiO₂ [n-hexane-EtOAc, 60:40] to afford 80.0 mg of 3b (73% yield).