Regiocontrolled Halohydroxylations of Bicyclic Vinylidenecyclopropanes: A Versatile Strategy for the Construction of Diverse Highly Functionalized Carbocyclic Scaffolds

Highly regioselective halohydroxylations of bicyclic vinylidenecyclopropanes that lead to four types of products 2, 3, 4, and 6 were developed. The halohydroxylation reaction occurs at room temperature to give rise to ring-keeping products vinylbicyclo[(n+2).1.0]alkanols 2 in 55-90% yields with excellent regio- and diastereoselectivity; the reaction of bicyclic vinylidenecyclopropanes with 2.0 equiv of N-bromosuccinimide (NBS) at 100 °C affords alkylidenebicyclo[(n+2).2.0]alkanones 3 in 48-75% yields by means of further proximal cleavage of the cyclopropane ring. The structures of both types of compounds 2 and 3 have been elucidated by X-ray crystal diffraction. An interesting sequential reaction that consists of a couple electrophilic addition and elimination reactions was developed when the reaction of bicyclic vinylidenecyclopropanes with N-halosuccinimide (NXS; 3.0 equiv) was performed under the specified conditions to furnish a variety of divinyl ketones 4 by means of proximal cleavage of the cyclopropane ring. In addition, vinylidenecyclopropanes that bore one aryl group at the cyclopropyl ring reacted with NBS or I(2) at room temperature, thereby producing the corresponding divinyl ketones 4 in moderate to good yields with excellent E selectivity. Unexpectedly, 2-vinylic cyclohex-2-enols 6 were generated through a very different distal C-C bond cleavage of the cyclopropane due to the significant ring-size effect. Possible mechanisms are proposed on the basis of the obtained intermediates.