12 октября, 2015 
Malyar  |
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It is remarkable that more than a century after the discovery of tetrazoliums, they continue to have wide applications. In particular, the reductive ring opening reaction leading to formazans continues to receive a lot of attention. In 1993, there were 316 citations to tetrazolium salts in Chemical Abstracts, 23 of which were to their reduction. Yet, the same reaction that makes them a unique class of reduction indicator leuco dyes, may be a part of a broader phenomenon.
(236)
Recent European patents,645,646 drawing on much earlier work647-649 describe an assay for NADH based on its ability to reduce naphthotri — azolium salts (236) to the azo dye (237) in close similarity to tetrazolium salts.
It would seem likely that tetrazolium salts are only one class of a wider variety of redox indicators acting through reductive ring opening, and that many more examples will be discovered in the future that may open some new directions to the chemistry of tetrazolium salts.
[1] NaOH
Scheme 24
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