A RhII-Catalyzed [4 + 3]-Cycloaddition via the Stereoselective Cyclopropanation of Vinyl Allenes En Route to Oxepino[b]indoles and Subsequent Elaboration to Spirooxindole Frameworks

Oxepino[b]indoles were obtained in good to excellent yields employing a [4 + 3]-cycloaddition initiated by a stereo- and regioselective, RhII-catalyzed cyclopropanation between a vinyl allene and diazooxindole to afford an intermediate cyclopropyl allene that engaged the oxindole carbonyl in a spontaneous hetero-[3,3]-rearrangement. A survey of functional group tolerance revealed a diverse array of substrates amenable to oxepino[b]indole formation. In addition to the intriguing architecture of the cycloadducts, exposure to either Brønsted acid or base enables the assembly of functionalized spirroxindoles via the unusual conversion of a 5-7 fused ring system to a 5-5 spirocycle.