Acylated Leuco Thiazine Dyes

There are cationic thiazine dyes (3 to 5) and neutral thiazinone dyes exemplified by Methylene Violet (6). Like leuco Methylene Blue, leuco Methylene Violet is too air sensitive to be isolated and therefore requires acylation. 

The leucos 7a-d are described as useful in printing ink for preventing forgery,1 whereas 7e-g are used in pressure sensitive copying paper.2 

The leucos 7h-o are claimed in electrolytic recording paper using a process coined electrochromic recording which is an irreversible electrooxidation of the leuco dye to regenerate Methylene Blue, not to be confused with reversible electrochromic display. The process consists of passing an electrical pulse through a substrate containing the leuco dye and 

Instead of using Methylene Blue, Azure B is reduced and acylated.4 The resulting leucos 8a-t are also described as useful in electrolytic recording.5 

The leucos lla-d are claimed to give blue gray images in electrolytic recording instead of the purple color of Methylene Violet from which they are derived.7 

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Phenazine leucos are generally more reactive and more susceptible to air oxidation than the thiazines and oxazines. Incorporation of electron-withdrawing groups on the acyl substituent at the 10-position of the leuco dye can provide a substantial improvement in the thermal and light stability of the leuco form and it is found that in general the stronger the electron-withdrawing character of the acyl substituents the more stable the leuco is.18... 

As in the case of thiazine and oxazine leuco dyes described earlier, the reductive acylation of the phenazine dye 52 results in the acylation of the exocyclic amino group.18 The phenazine leuco obtained 53 retains the exocyclic amide group on oxidation resulting in the acylated phenazine dye 54, the color of which is different from the one intended. 

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