Building Three-Dimensional Complexity by Intramolecular 2-Aminoallyl Cation-Diene (4+3) Cycloaddition.

Reliable methods for rapidly constructing C(sp)-rich three-dimensional polycycles are in high demand for organic synthesis and medicinal chemistry. Although there are various mature systems for synthesizing five- or six-membered polycycles, a catalytic platform for accessing diverse cycloheptanoid-containing polycyclic scaffolds is lacking. Herein, we describe a method for copper-catalyzed intramolecular 2-aminoallyl cation-diene (4+3) cycloaddition reactions. By using 1,3-diene-tethered ethynyl methylene cyclic carbamates as substrates, we were able to construct various cycloheptanoid-containing polycyclic scaffolds, which are present in many bioactive molecules. The cycloaddition products were rich in functionality that could undergo various chemical transformations. The synthetic utility of the method was illustrated by total synthesis of the natural products (±)-mint ketone and (±)-aphanamol I. Mechanistic studies indicated that the cycloadditions proceed by a concerted [4π+2π] mechanism and that an endo-selective pathway is favored.