This invention pertains generally to the field of chemical synthesis and purification, and more specifically to methods of synthesizing and purifying certain 3,7 diamino-phenothiazin-5-ium compounds (referred to herein as diaminophenothiaziniumcompounds) including Methythioninium Chloride (MTC) (also known as Methylene Blue). In one embodiment, the method comprises the steps of, in order: nitrosylation (NOS); nitrosyl reduction (NR); thiosulfonic acid formation (TSAF); oxidative coupling (OC); Cr(VI) reduction (CR); isolation and purification of zwitterionic intermediate (IAPOZI); ring closure (RC); chloride salt-formation (CSF); one of: sulphide treatment (ST); dimethyldithiocarbamate treatment (DT); carbonate treatment (CT); ethylenediaminetetraacetic acid treatment (EDTAT); organic extraction (OE); and recrystallization (RX). The present invention also pertains to the resulting (high purity) compounds, compositions comprising them (e.g., tablets, capsules), and their use in methods of inactivating pathogens, and methods of medical treatment and diagnosis, etc., for example, for tauopathies, Alzheimer's disease (AD), skin cancer, melanoma, viral diseases, bacterial diseases, or protozoal diseases.

The present disclosure provides carbazole-dioxazine pigment of Formula I; ##STR00001## wherein, R is C.sub.1-C.sub.8 alkyl group, preferably R is an ethyl group, R.sub.1 is a chloro group, and R.sub.2 is a NHC(O)Ph group. The pigment is synthesized from 3-nitro-N-ethylcarbazole. The first step is nitration of 3-nitro-N-ethylcarbazole to form 3,6-dinitro-N-ethylcarbazole which is reduced to form 3-amino-6-nitro-N-ethylcarbazole. This amino compound is benzoylated to form 3-benzamido-6-nitro-N-ethylcarbazole which is then hydrogenated to form 6-amino-3-benzamido-N-ethylcarbazole. Condensation of 6-amino-3-benzamido-N-ethylcarbazole with chloranil provides an intermediate. Cyclization of the intermediate in the presence of a cyclization reagent provides the compound of Formula-I. The pigment has wavelength of maximum absorption (.sub.max) at 571 nm in dimethylformamide as a solvent.

The present disclosure provides carbazole-dioxazine pigment of Formula I; ##STR00001## wherein, R is C.sub.1-C.sub.8 alkyl group, preferably R is an ethyl group, R.sub.1 is a chloro group, and R.sub.2 is a NHC(O)Ph group. The pigment is synthesized from 3-nitro-N-ethylcarbazole. The first step is nitration of 3-nitro-N-ethylcarbazole to form 3,6-dinitro-N-ethylcarbazole which is reduced to form 3-amino-6-nitro-N-ethylcarbazole. This amino compound is benzoylated to form 3-benzamido-6-nitro-N-ethylcarbazole which is then hydrogenated to form 6-amino-3-benzamido-N-ethylcarbazole. Condensation of 6-amino-3-benzamido-N-ethylcarbazole with chloranil provides an intermediate. Cyclization of the intermediate in the presence of a cyclization reagent provides the compound of Formula-I. The pigment has wavelength of maximum absorption (.sub.max) at 571 nm in dimethylformamide as a solvent.

The present disclosure relates to a benzoyl substituted carbazole-dioxazine pigment and its preparation. The process involves benzoylation of 3-nitro-N-ethylcarbazole in monochlorobenzene using benzoylchloride and ferric chloride to yield 3-nitro-6-benzoyl-N-ethyl carbazole, which on catalytic hydrogenation and subsequent condensation with chloranil and cyclization yields benzoyl substituted carbazole-dioxazine pigment.