Relative Reactivity of Benzothiophene-Fused Enediynes in the Bergman Cyclization.

To find promising analogues of naturally occurring enediyne antibiotics with a sufficient reactivity in the Bergman cyclization and moderately stable under isolation and storage, a scale of relative enediynes reactivity was created on the basis of calculated free activation energies for the Bergman cyclization within 12 known and new benozothiophene, benzene, and cinnoline annulated 9- and 10-membered enediynes. To verify the predicted reactivity/stability balance, three new carbocyclic enediynes fused to a benzothiophene core bearing 3,4,5-trimethoxybenzene, fluoroisopropyl, and isopropenyl substituents were synthesized using the Nicholas-type macrocyclization. It was confirmed that annulation of a 3,4,5-trimethoxybenzene moiety to a 10-membered enediyne macrocycle imparts high reactivity to an enediyne while also conferring instability under ambient temperature. Fluoroisopropyl-substituted 10-membered enediyne from the opposite end of the scale was found to be stable while moderately reactive in the Bergman cyclization. Along with the experimentally confirmed moderate reactivity (DSC kinetic studies), (fluoroisopropyl)enediyne showed a significant DNA damaging activity in plasmid cleavage assays comparable with the known anticancer drug Zeocin.

Electrophilic cyclization of aryldiacetylenes in the synthesis of functionalized enediynes fused to a heterocyclic core.

An efficient strategy for the synthesis of asymmetrically substituted enediynes fused to benzothiophene, benzofuran, and indole was developed. The proposed approach is based on the electrophilic cyclization of diacetylenes and Sonogashira coupling. Thus, iodocyclization of readily available ortho-functionalized (buta-1,3-diynyl)arenes was used as a direct way for the synthesis of 2-ethynyl-3-iodoheteroindenes. These substrates and their modified derivatives were easily converted by Sonogashira coupling with acetylenes to a variety of asymmetrically substituted acyclic enediynes fused to heterocycles. The tolerance of the developed methodology to a variety of functional groups is a great advantage in the synthesis of macrocyclic enediyne systems fused to a heterocyclic core. Synthesis of indole-fused 12-membered macrocyclic dienediyne was achieved using ring-closing metathesis as a key step.