Synthesis of partially hydrogenated oxa[5] and oxa[6]helicenes from β-chlorovinylaldehydes

Synthesis of partially hydrogenated oxa[5]helicenes is described starting from easily available β-chlorovinylaldehydes. The short sequence involved a Suzuki–Miyaura type coupling between β-chlorovinylaldehydes and arylboronic acids bearing ortho-methoxy groups. The presence of both the formyl and the methoxy groups allowed after reduction and demethylation respectively, the construction of the central dehydropyran ring. The molecular structure of the extended benzopyrene-based oxa[5]helicenes has been fully determined in solution and in the solid state. The strategy could be extended to oxa[6]helicene. Atroposelective Suzuki–Miyaura couplings were the key steps of the nonracemic preparation of oxa[5]helicenes. Ee observed are in good agreement with the theoretically calculated racemization barriers.