Chemistry of the α,β-Double Umpolung of 2-Propiolate and 2-Butynoate to Show the Domino Synthesis of 2,5-Dihydrofurans

The umpolung, also known as the polarity reversal, allows reactions that are difficult or not possible classically. Herein, we report the α,β-double umpolung of 2-propiolate and 2-butynoate for the synthesis of 2,5-dihydroxyfurans in one pot. The reactions were optimized to suppress the formation of classical Michael adducts. A total of 24 unprecedented cyclic allyl ethers were synthesized with yields of up to 95% in eco-friendly toluene. It was observed that the methyl group in 2-butynoate led to steric hindrance compared to 2-propiolate, and a competition between the hydroxy group of the ketone and PPh₃ was observed. The significance of the protocol was showcased by the postsynthetic transformation of methyl 4-phenyl-2,5-dihydrofuran-2-carboxylate to several synthetically important derivatives, including methyl 4-phenylfuran-2-carboxylate, tetrahydrofuran, and a β,γ-diol-ester.