5- -2,5-diphenyltetrazolium salts

The reaction of sodium azide with N-aryl chloroimines, obtained from benzanilides and thionyl chloride, to form 1,5-disubstituted tetrazoles is catalysed by tetra-n-butyl-ammonium bromide (Scheme 5.26, Table 5.40) [18] in variable yields, but generally <85%. 5-Butyl-2,3-diphenyltetrazolium salts have also been used as catalysts [18, 19]. 1,5-Disubstituted tetrazoles are also obtained from a one-pot sequential reaction of carbodimides with sodium azide and an aroyl chloride in the presence of tetra-n-butylammonium chloride [20]. 5-Chlorotetrazoles are obtained from the catalysed reaction of aryldichloroisocyanides with sodium azide (Scheme 5.26) [21],... 

Poly-O-acetyl aldehydo-sugSLT A, A -diphenylformazans (786) are easily accessible starting materials and readily cyclizable with Pb(OAC)4 (53CB472, 53MI2) or NBS (57JCS3802 88MI7) to 5-(alditoM-yl)-2,5-diphenyltetrazolium salts (787) (Scheme 217). 

A colorimetric version of the method uses a tetrazolium salt. The oxidation of the NADH is coupled to the reduction of 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide by a diaphorase (EC 1.6.4.3) and a deep blue formazan develops. The absorbance is read at a wavelength between 540 and 600 nm. This is more sensitive than the ultraviolet method but is more prone to interference, especially from any reducing agents present in the sample, which will result in a positive error. 

Thiadiazolines were also obtained by a 1,5-dipolar ring reconstruction of mesoionic ylides. The bromine adduct of 2,3-diphenyltetrazolium-5-thiolate (181) reacts with the sodium salt of diethyl malonate to yield as the only product 5,5-bis(ethoxycarbonyl)-4-phenyl-2-phenylazo-2,3-dihydro-1,3,4-thiadiazole (182) (Scheme 33). The same type of product was obtained from reactions with ethyl cyanoacetate and ethyl acetoacetate <88BCJ2979>. 

Barltrop, J. and Owen, T. 1991. 5-(3-carboxymethoxyphenyl)-2-(4,5-dimethylthiazoly)-3-(4-sulfophenyl)tetra-zolium, inner salt (MTS) and related analogs of 3-(4,5-dimethylthiazolyl)-2,5-diphenyltetrazolium bromide (MTT) reducing to purple water-soluble formazans as cell viability indicators. Bioorg. Med. Chem. Lett. 1,611-614. 

This assay was developed and used in anti-HIV screening programs (Pauwels et al. 1988). The tetrazolium salt MTT [3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyltetrazolium bromide] is converted to dark blue formazan by living cells but not by dead cells or culture medium. These assays are carried out in 96-well plates. 

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