Solvent-free microwave synthesis of 2-, 5- and 9-substituted benzo[a]phenoxazinium chlorides

Abstract

Benzo[a]phenoxazinium salts are usually synthesised by condensation reaction of nitrosoanilines or nitrosonaphthylamines with 1-naphthylamines, 3-aminophenols or 2-napthol. In the last case, monoaminosubstituted benzo[a]phenoxazinium salts should be transformed into the diaminosubstituted derivatives by reaction with amines in the presence of an oxidation agent. The use of a strong mineral acid, such as perchloric or hydrochloric acids, in N,N-dimethylformamide, methanol or ethanol as a solvent, under reflux conditions, is frequently reported for the cyclisation [1-6]. As a continuation of our previous research [3-6], the present work describes for the first time the efficient synthesis of benzo[a]phenoxazinium chloride 1 by condensation reactions of nitrosofenol 2 and 5-aminonaphthalen-2-ol or its derivatives 3 under microwave irradiation in solvent-free conditions or using N,N-dimethylformide, in comparison to conventional heating conditions. These new compounds possess a combination of substituents at the tetracyclic ring that includes the hydroxyl, aminopropoxyl, as well as amine groups, and monoalkylated amines.