Unexpected Base-Promoted Synthesis of Tetrasubstituted Furans via Palla­dium-Catalyzed Oxidation and Cyclization of Carbon–Carbon Triple Bond with Molecular Oxygen

Abstract

A new approach to the synthesis of tetrasubstituted furans using aromatic alkynes catalyzed by PdCl₂ in DMA (N,N-dimethylacetamide)–H₂O with dioxygen as the sole oxidant is described, in which the oxygen atom of the furan was from water. The preliminary mechanistic understanding of this transformation was investigated.

Key words

palladium - oxidation - furans - alkynes - molecular oxygen

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• 8 Typical Procedure for Substrate 1a: The reaction was carried out in a 150-mL Teflon-lined 316-L stainless steel autoclave and a magnetic stirrer. A mixture of PdCl₂ (5 mol%, 0.025 mmol, 4.4 mg), NaOAc (5 mol%, 0.025 mmol, 2.1 mg), 1a (0.5 mmol, 89 mg), H₂O (0.1 mL) and DMA (2 mL) was placed in the 150-mL Teflon-lined 316-L stainless steel autoclave. Then, 0.1 MPa of O₂ was introduced. The mixture was stirred for 6 h at 80 °C. After the reaction, the reactor was quickly cooled to r.t. The excess of O₂ was depressurized slowly. The products were extracted using a Et₂O–H₂O mixture (1:1; 3 × 20 mL). The Et₂O layers containing the products were dried over Na₂SO₄, filtrated and concentrated under reduced pressure. The resultant crude mixture was purified by column chromatography (silica gel) with a mixture of EtOAc–hexane (1:20) as eluent to afford 2a (70 mg, yield 76%). The product was characterized by NMR and confirmed by comparison with the reported authentic NMR data
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