Regioselective Reactions of 1-Alkylidene-2-oxyallyl Cations with Furan: Control of [4 + 3] Cycloaddition, [3 + 2] Cycloaddition, and Electrophilic Substitution

The ring opening of alkylidenecyclopropanone acetal under acidic conditions produces the 1-alkylidene-2-oxyallyl cation as an intermediate, which reacts with furan to give the [3 + 2] and [4 + 3] cycloadducts as well as an electrophilic substitution product. The product distribution is controlled by the oxy substituents of the cation and by the solvent employed.