Diastereoselection in the Formation of Contiguous Quaternary Carbon Stereocenters by the Intramolecular Heck Reaction

The second in a series of two papers, this study examines the origins of diastereoselection in the second ring closure of the highly diastereoselective double Heck cyclization of cyclohexenes 1 and 3 that form contiguous quaternary stereocenters. Seven model substrates were synthesized and cyclized to examine the structural features responsible for imparting diastereoselection in the second intramolecular Heck reaction. These studies demonstrate that stereoselection in the formation of the second spirooxindole ring results from the avoidance of steric interactions in the insertion step with the spirooxindole formed in the first Heck cyclization. An axial substituent (carbonyl or arene) is required at the allylic position for high levels of diastereoselection to be realized.