Organocatalytic Enantioselective [3+3] Annulation toward Chromene-Fused Helicenes Bearing Central and Helical Chirality With Chiroptical Properties

Abstract

The enantioselective catalytic construction of structurally sophisticated helicenes integrating both helical and central chirality remains a formidable challenge, and organocatalytic strategies for helicene synthesis are still severely underexplored compared with transition-metal catalysis. Benefiting from the inherent stereocontrolling ability of chiral phosphoric acid (CPA), we herein report a facile, metal-free asymmetric [3+3] annulation strategy for efficient synthesis of chromene-based helicenes with dual central and helical chirality. The reaction employs readily available benzo[c]carbazol-2-ols (easily prepared via one-step Bucherer-type synthesis) and propargylic alcohols as substrates. A dual hydrogen-bonding activation mode between the substrate and CPA catalyst guarantees good diastereo- and enantioselectivities, delivering complex chiral azachromenohelicenes in high yields. The obtained enantioenriched helicenes possess high configurational stability and strong circular dichroism responses. This work provides a scalable organocatalytic platform for constructing multifunctional chiral helicene scaffolds, offering promising candidates for advanced chiroptical functional materials.