Synthesis of an A/B-cis-Fused Cyclopenta[b]fluorene (6/5/6/5) Ring System for Embellicine A via the Eight-Membered...

The first synthesis of an A/B-cis-fused cyclopenta[b]fluorene (6/5/6/5) ring system was successfully achieved via an intramolecular Diels-Alder (IMDA) reaction of an eight-membered silylene-tethered precursor. In this key step, a significant conformational change in the diene moiety was achieved by introducing an eight-membered silylene tether, and the desired A/B-cis-fused tetracyclic compound was obtained with complete stereoselectivity. After several functional group manipulations on the D-ring, a fully elaborate tetracyclic fragment of embellicines was successfully synthesized.