Construction of the Fused Pentacycle of Talatisamine via a Combination of Radical and Cationic Cyclizations

The fused 6/7/5/6/6-membered (ABCDE) ring system of talatisamine was synthesized in 22 steps. After preparation of the AE-ring structure from 2-(ethoxycarbonyl)­cyclohexanone, elaboration of the carboskeleton was realized by sequential additions of allyl magnesium bromide and the lithiated C-ring. The C11-bridgehead radical derived from the ACE-ring underwent the 7-endo cyclization with the enone moiety to form the B-ring in C10-stereoselective and C11-stereospecific manners. The 6-endo cyclization of the remaining D-ring was in turn attained by using the silyl enol ether as the nucleophile and the PhSeCl-activated olefin as the electrophile. These radical and cationic cyclizations were demonstrated to be highly chemoselective, and they significantly contributed to streamlining the route to the intricately fused pentacycle of talatisamine.