INTRAMOLECULAR PALLADIUM-CATALYZED CYCLOADDITIONS WITH A CLEAVABLE TETHER

A facile synthesis of 2-(acyloxy)-3-methylene-4-(trimethylsilyl)butyric acids evolves from the use of tris(phenylthio)acetaldehyde as a practical glyoxylic ester equivalent. Introduction of the bifunctional donor of a [3 + 2] cycloaddition proceeds simply by esterification of an alcohol possessing TMM-PdL2 acceptor units. The synthetic flexibility of the sulfone leads to the choice of alpha, beta-unsaturated sulfones as the acceptors. Palladium-catalyzed cycloadditions proceed smoothly at temperatures as low as room temperature. Depending upon the substitution pattern and especially the conformation of the substrate, tethering the ester containing the bifunctional TMM unit at the carbon allylic to the acceptor provides substrates that predominantly to exclusively undergo cycloaddition rather than processes dominated by an alternative Pd(0)-catalyzed ionization. Since the ester linkage is easily cleaved, this sequence serves as a convenient strategy for controlled TMM-PdL2 cycloadditions. For example, the diastereofacial selectivity of this intramolecular process involves attack syn to the allylic oxygen in a six- or seven-membered-ring acceptor but anti to this oxygen in an intermolecular process. A diastereocontrolled cyclopentenone annulation is developed. Interestingly, the intramolecular cycloaddition of eight- and twelve-membered rings proceeds anti to the allylic oxygen. As an aside, the indefinite shelf life of the crystalline tris(phenylthio)acetaldehyde makes it a convenient glyoxylate synthon.