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.
Ringopening reactions of 2-cyclohexylidene-3,3-dimethylcyclopropanone acetal (1) are readily induced by treatment of hydrogen chloride in various solvents. Bond cleavage takes place at the C1–C2 or C2–C3 bond, and the ratio of C1–C2/C2–C3 cleavages changes from >99/1 to <1/99 depending on the solvent. The two modes of bond cleavage must be initiated by protonations at the carbon–carbon double bond