A Lewis/Brønsted acid-mediated route to furans from nitroalkenes and enol ethers

A novel approach toward tri- and tetra-substituted furans via a Brønsted acid-mediated ring contraction of readily available 1,2-oxazine N-oxides is reported. This method provides a two-step route to obtain polysubstituted furans from simple feedstocks-nitroalkenes and enol ethers or ketals. An operationally simple procedure combining the synthesis of 1,2-oxazine N-oxides and their ring contraction in one pot was also developed. Mechanistic studies revealed that a Nef-type reaction pathway was involved in the ring contraction. The synthetic utility of the obtained furans was demonstrated by their conversion into diastereomerically pure tetrahydrofurans.