Approach to a new dihydrofuran-fused cyclic system by a remarkable switching of endo/exo selectivity of a [4+2] cycloaddition reaction

An efficient synthesis of a new class of dihydrofuran-fused tetracyclic compounds, possessing a 2-oxa-furano-steroidal framework, has been achieved by successive electrocyclic reactions of benzocyclobutene derivatives. It has been revealed that the stereoselectivity of a key intramolecular [4+2] cycloaddition reaction can be controlled by installation of a bulky silyl substituent onto the furan ring resulting in the formation of the exo adducts, the opposite selectivity to that observed in our past related studies. Preliminary examinations of the bioactivities of the synthesized compounds have been performed and have revealed that they have potential as anti-influenza agents.