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

Abstract The reaction of 7‐phenylbenzo[ a ]phenazine‐5(7H)‐one (rosindone) with one equivalent of a nitrating mixture (conc. H 2 SO 4 /HNO 3 ) produced 6‐nitro‐7‐phenylbenzo[ a ]phenazine‐5(7H)‐one (NROS), as confirmed by 1 H 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(7 H )‐one (CLROS), instead but in low yield. The nitro group of NROS was reduced with SnCl 2 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(6 H )‐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(5 H )‐yl)benzoate (MROS).