Oxaenediynes through the Nicholas-Type Macrocyclization Approach

The Nicholas-type macrocyclization has been used for the first time for the synthesis of 10- and 11-membered oxaenediynes fused to a benzothiophene. The acyclic starting materials were easily synthesized by electrophilic cyclization of o-(buta-1,3-diynyl)thioanisoles followed by a Sonogashira coupling of the resulting 2-ethynyl-3-iodobenzothiophenes with functionalized alkynes. A high reactivity of the 10-membered oxacycles in the Bergman cyclization was predicted by DFT calculations and confirmed by differential scanning calorimetry. The ability of enediynes to induce single-strand PM2 DNA scissions was also found.