Mild, Modular, and Convergent Synthesis of Helical Naphtho[2,1-c]chromenes via a Multistep Cyclization/Aromatization Cascade Sequence

Tetracyclic 6 H -naphtho[2,1- c ]chromenes are expeditiously synthesized through a BF 3 ·OEt 2 -mediated, three-step cascade reaction, creating new central pyran and aromatic rings. The cascade involves the addition of phenol-derived alkynyl substrates to BF 3 -activated aldehydes followed by alkyne-Prins cyclization, Friedel–Crafts reaction, and final elimination. Aliphatic and electron-deficient aromatic aldehydes afford the products in 50–74% isolated yields, but benzaldehyde and tolualdehyde resulted in lower yields. X-ray analysis of a p -bromophenyl derivative ( 5aA ) shows the two aromatic moieties are twisted by 28° to create a helical backbone.