Abstract
This review summarizes the novel type of palladium-catalyzed cascade ring expansion reactions of cyclobutanols with various unsaturated bonds. The intramolecular cascade ring expansion-cyclizations of isopropenylcyclobutanol proceeds smoothly in the presence of the palladium (II) complex. The diastereoselectivity of the obtained naphthohydrindans can be controlled by the choice of the reaction solvents. As an application utilizing this reaction, total synthesis of (+)-equilenin has been achieved. The reaction of allenylcyclobutanols with a iodoalkenyl side chain with a palladium (0) catalyst produces the cyclized products that have seven- and eight-membered rings. The reaction can be successfully applied to the stereospecific synthesis of α-substituted cyclopentanones with a quaternary carbon stereocenter. The cyclobutanols containing a propargylic component react with phenols in the presence of the palladium (0) catalyst to afford the phenoxy-substituted cyclopentanones via a nucleophilic addition-ring expansion process. The reaction also proceeds stereospecifically to afford the corresponding products with high efficiency.
