Transition Metal-Catalyzed Intramolecular [4 + 2] Cycloadditions: Initial Studies on Stereochemistry and on Steroid and Vitamin D Analog Syntheses

摘要

The nickei(0)-catalyzed intramolecular [4 + 2] cycloaddition between dienes and unactivated π-systems has beea shown to be an efficient complement to the uncata-lyzed (Diels-Alder) reaction. Several representative examples have now been reported forwhich the latter reaction fails, or proceeds only slowly, even at elevated temperatures, while the former occurs efficiently and rapidly, often at room temperature. Notwithstanding the synthetic and operational advantages of the catalyzed cycloaddition,its lull utilization in Synthesis has been limited by the unknown relationship between the geometry of the starting diene and the stereochemistry of the cycloadduct. We describe herein the first study of thus stereochemical feature of the nickel(0)-catalyzed intramolecular cycloaddition. Developed in the context of a strategy for the synthesis of steroids and vitaroin D analogs, this study also furnishes the first solution to a probiem previously encountered in attempts to apply this metal-catalyzed cycloaddition to the synthesis of bicyclic syst-ans bearing an angular methyl group, a prominent feature of numerous natural and non-natural polycycles.