One-pot access to indazole fused-phenanthridinones via Rh(III)-catalyzed [4+2] annulation

An efficient synthesis of indazole fused phenanthridinones via Rh(III)-catalyzed inert C(sp2)-H activation/[4 + 2] annulation of 3-aryl-1H-indazoles with iodonium ylides has been developed, providing a strategy to access multiconjugated pi-system tetracyclic and pentacyclic aza-heterocyclics with the favorable photoluminescence properties. This "one-pot" reaction features high efficiency, excellent regioselectivity, a broad substrate compatibility and could be easily scaled up. Furthermore, the obtained products exhibited low toxicity in MDCK cells as well as selectively labeled the zebrafish larvae, which indicated that the titled products could be potentially utilized as biofluorescent probes or fluorescent dyes.