Selected Rosindone Derivatives: Synthesis, Structural Characterization, and Photophysical Studies

The reaction of 7-phenylbenzo[a]phenazine-5(7H)-one (rosindone) with one equivalent of a nitrating mixture (conc. H₂SO₄/HNO₃) produced 6-nitro-7-phenylbenzo[a]phenazine-5(7H)-one (NROS), as confirmed by ¹H NMR spectroscopy. The selectivity of the reaction is assigned to an electronic effect. The attempted sulphonation of rosindone with chlorosulfonic acid produced the chlorinated derivative 6-chloro-7-phenylbenzo[a]phenazine-5(7H)-one (CLROS), instead but in low yield. The nitro group of NROS was reduced with SnCl₂ to produce a blue solid identified as 6-amino-7-phenylbenzo[a]phenazine-5(7H)-one (AMROS). The X-ray crystal structure confirmed the positioning of the amino group α to the carbonyl group. The reaction of rosindone with one equivalent of lithium trimethylsilylacetylide underwent a Michael addition to introduce the nucleophile β to the carbonyl group. The compound 6a-ethynyl-6a,7-dihydrobenzo[a]phenazine-5(6H)-one (rac-ACROS) was identified by an X-ray crystal structure determination. Coupling of rac-ACROS under standard Glazer conditions produced the dyad of the compound linked via the two ethynyl subunits. A different N-phenyl group was readily introduced into the rosindone basic structure by the reaction of methyl 2-((2-aminophenyl)amino)benzoate with 2-hydroxy-1,4-naphthoquinone to produce methyl 2-(5-oxobenzo[a]phenazine-7(5H)-yl)benzoate (MROS).