Synthesis of fluorescent cyclopentene-fused-phenanthrenes via boron trifluoride-promoted cascade cationic cyclization

Abstract

A metal-free, BF₃-promoted cascade cyclization of biaryl-embedded enynones with nitriles is disclosed, enabling rapid access to amide-functionalized cyclopentene-fused phenanthrenes under mild conditions. The transformation is initiated by electrophilic activation of the enone, which triggers two consecutive intramolecular cyclizations followed by a Ritter-type reaction. This process builds two new C-C bonds, one C-N bond, and one C=O bond, affording structurally intricate tetracyclic frameworks with high efficiency. In contrast to previously reported related cyclizations, this strategy uniquely exploits both electrophilic sites of the enone and incorporates external nucleophiles beyond water, thereby substantially expanding the synthetic scope of enynone-based cationic cascade processes. The protocol exhibits broad applicability to enynone substrates and accommodates a diverse array of nitriles, underscoring its synthetic versatility. Mechanistic studies, supported by DFT calculations and experimental evidence, elucidate the origins of reactivity and selectivity. Notably, the resulting polycyclic products display remarkable photophysical properties, with solid-state fluorescence quantum yields of up to 0.35, highlighting their potential as fluorescent materials and molecular probes.