Evidence for the formation of the oxacyclopentenyl cation from a cyclopropyl ketone

Abstract

On treatment with stannic chloride in nitromethane or concentrated sulphuric acid trans-2-phenyl-cyclopropyl phenyl ketone (1) gives the same intermediate as shown by an in situ¹H and ¹³C n.m.r. investigation. In concentrated sulphuric acid this intermediate had been assigned an oxacyclopentenyl cation structure (2); this is confirmed here. The assignment is supported chemically by synthesisting (2) in an inert solvent and treating it with methylmagnesium iodide. The expected trisubstituted tetrahydrofuran (7) is isolated in 53% yield.