Patent ID: 8440821

Claim:
A method for the synthesis of a high purity diaminophenothiazinium compound (DAPTC) of the following formula: wherein: each of R 1 and R 9 is independently selected from: —H; C 1-4 alkyl; C 2-4 alkenyl; and halogenated C 1-4 alkyl; each of R 3NA and R 3NB is independently selected from: C 1-4 alkyl; C 2-4 alkenyl; and halogenated C 1-4 alkyl; each of R 7NA and R 7NB is independently selected from: C 1-4 alkyl; C 2- 4 alkenyl; and halogenated C 1-4 alkyl; and X is one or more anionic counter ions to achieve electrical neutrality; wherein the diaminophenothiazinium compound (DAPTC) is characterised by: a purity of greater than 98%; less than 1% Azure B as impurity; less than 0.15% Azure A as impurity; less than 0.15% Azure C as impurity; and less than 0.05% Methylene Violet Bernthsen (MVB) as impurity which method comprises at least the following steps, in order: purifying (PUR) a corresponding acylated reagent compound (ARC) of the following formula wherein R 10 is independently saturated aliphatic C 1-5 alky, phenyl, p-methoxyphenyl, or p-nitrophenyl: deacylating (DAC) said acylated reagent compound (ARC) to give a corresponding deacylated compound of the following formula optionally purifying (PUR DAC-OX ) said deacylated compound; oxidizing (OX) said deacylated compound at a temperature below 10° C. to give said diaminophenothiazinium compound (DAPTC); and purifying (PUR OX ) said diaminophenothiazinium compound (DAPTC) by recrystallization, wherein said recrystallization includes a step of adjusting the pH of the mixture to be about 0.5 to 2.5.