Transition metal templates as guides for cycloadditions

Abstract

Cycloaddition reactions, represented by the Diels-Alder reaction, set a standard of chemo-, regio-, diastereo-, and enantioselectivity for the development of new synthetic reactions. Cycloadditions to carbocyclic rings other than six members becomes an important objective. Palladium templates convert the novel bifunctional conjunctive reagents, 2-trialkyl (or trialkoxy) methylallyl carboxylates, into reactive intermediates that transfer a trimethylenemethane or substituted trimethylenemethane unit to suitable acceptors. In direct analogy to the Diels-Alder reaction, typical Diels-Alder dienophiles undergo smooth cycloadditions in chemo-, regio-, and diastereoselective fashion. A novel cyclopentenone annulation has evolved. Using co-catalysts, carbonyl groups also become acceptors. Transfer of substituted trimethylenemethane fragments to both types of acceptors can be controlled to provide regiocomplementary products derived from kinetic or thermodynamic reactive intermediates. Such a reaction can be generalized into [2n + 3] cycloadditions in which seven and nine, as well as five membered rings may be formed. Application of this methodology in natural products synthesis is probed. © 1988 IUPAC