Stereoselective Synthesis of Highly Functionalized Cyclohexenes via Strong-Acid-Mediated Endocyclic C-C Bond Cleavage of Monocyclopropanated Cyclopentadienes.

A stereoselective, solvent- and metal-free endocyclic C-C bond cleavage of monocyclopropanated cyclopentadienes mediated by strong acids was developed, leading to highly functionalized six-membered carbocycles with high stereocontrol. The critical step for this ring-expansion is the formation of a cyclopropyl carbocation that undergoes endocyclic ring opening via an SN2'-type attack of various nucleophiles. Subsequent synthetic transformations show the versatility of the resulting cyclohexenes for the synthesis of new compounds with nonconventional substitution patterns.