Cyclohepta[b][1,4]benzothiazines and Their Diazine Analogues. 2. Formation and Properties of Cyclohepta[b]quinoxalines

Abstract 2-Chloro- or 2-methoxytropone reacts with o-phenylenediamine to give 2-(o-aminoanilino)tropone, which upon heating provides 6H-cyclohepta[b]quinoxaline (10a). Upon acidification, 10a reversibly gives a green cation, 9a. 10a is easily converted, especially under basic conditions, into the oxidative dimer 16a, which reproduces 9a by reduction with Zn in acetic acid. The isopropyl derivatives of these compounds are prepared by the same method from 5-isopropyl-2-methoxytropone. The 5-methyl (21a) and 5,11-dimethyl derivatives 23 are made by the reaction of 2-chloro- or 2-methoxytropone with N-methyl- and N,N′-dimethyl-o-phenylenediamines. However, the methylation of 21a with methyl fluorosulfate affords 5,11-dihydro-8,11-dimethyl-6H-cyclohepta[b]quinoxaline-5,6-sultone. H2O2 oxidation of 10a mainly gives 16a in addition to a small quantity of 6H-cyclohepta[b]quinoxalin-6-one. Compound 21a is stable under basic conditions, but 23 rearranges under alkaline conditions to give, among other unidentified products, N-methyl-N-[o-(methylamino)phenyl]benzamide and 5,10-dihydro-5,10-dimethyl-2-phenazinecarbaldehyde. Reactions of title compounds with alkali and hydrogen peroxide are compared with those of O- and S-analogues.