Synthesis of [4.6.4.6]fenestradienes and [4.6.4.6]fenestrenes based on an 8pi-6pi-cyclization-oxidation cascade

Fenestranes are regarded as a particularly challenging synthetic targets, and only few syntheses have been reported in the recent past. These rare compounds of synthetic and theoretical interest are a class of tetracyclic skeletons, defined as doubly a,a'-bridged spiroalkanes. The reported results are focused on the synthesis of new and original [4.6.4.6]fenestradienes 3a-f and [4.6.4.6]fenestrenes 4a-e. Our approach implies the formation of this tetracyclic structure by a reaction cascade, based on consecutive transformations starting from the trienyne 1a-f: an initial soft hydrogenation using a P-2 Nickel catalyst at room temperature, followed by a conrotatory 8p electrocyclization and a disrotatory 6p electrocyclization and a final oxidation. Several examples of this type of new compounds are described in this Communication.