Bunte salts reaction with nucleophiles

Restoring of the cysteine-SH is achieved by reduction, e.g. with 1,4-dithioerythrol (Cleland s reagent) or by sulfitolysis. Disulfides on reaction with nucleophiles (Nu) like rhodanide, cyanide or sulfite are split with formation of an SH- and an -SNu moiety. The thiosulfate compounds formed by SO3", so called Bunte salts were important in the insulin studies in Aachen (Helmut Zahn and colleagues). They react, like thiorhodanides, -S-SCN, with free thiols to form disulfides. 

Thio-D-xylopyranose has been the sugar of this type most thoroughly investigated. Its synthesis was reported in 1961, independently and simultaneously from three different laboratories. With sodium thiocyanate, l,2-0-isopropylidene-5-0-p-tolylsulfonyl-a-D-xylofuranose (211) gives a thiocyanate that reacts with sodium sulfide to give l,2-0-isopropylidene-5-thio-a-D-xylofuranose (212). A better synthesis is the treatment of 211 with sodium thiosulfate, followed by reduction of the resultant Bunte salt with sodium boro-hydride. The same compound 212 is obtained by nucleophilic replacement of the 5-sulfonyloxy group in 211 by treatment with potassium thioacetate followed by deacetylation to 212, or by the reaction of 211 with sodium a-toluenethioxide, followed by scission of the resultant S-benzyl compound with sodium in liquid ammonia. 

Sulhtes or bisulfite (depending on pH) are another important reducing agent for the disulfide bonds in commercial permanent waves. The reaction of sulhte with hair involves nucleophilic attack of sulfite ion on disulfide. This reaction produces one equivalent of mercaptan and one equivalent of Bunte salt [44]. 

Reese and Eyring [45] have demonstrated that the reaction of sulfite with hair is a pseudo-hrst-order reaction. In other words, the chemical reaction of sulfite with the disulfide bond of hair is slower than diffusion of sulfite into hair. Elsworth and Phillips [46, 47] and Volk [48] examined the sulfi-tolysis of keratin, demonstrating that the rate of cystine cleavage is optimal at acid pH. Wolfram and Underwood [49] found a broad optimum for cystine cleavage by sulhte at pH 4 to 6. The decrease in cystine cleavage at acidic pH (below pH 4) is due to a decrease in the concentration of the nucleophilic sulhte species. On the other hand, the decrease in cystine cleavage as pH is raised (alkaline pH) results from alkaline hydrolysis of the Bunte salt [50]. 

From -Chlorovinylmethine-immonium and Bunte Salts Types C and E). j8-Chloro-vinylmethine-immonium salts (9) react with alkali-metal thiocyanates in the presence of aromatic amines to give isothiazoles (10). The reaction is considered to proceed by a nucleophilic displacement to give eventually the jS-thiocyanovinyl aldehyde anils ArC(SCN)=CHCH=NR, which cyclize in situ. The reaction of (9) with sodium thiosulphate gives Bunte salts, which react with amines (R R NH) to give the stable j8-aminovinyl thioketones (11), and hence, by oxidative cycliza-tion, the isothiazolium salts (10 R = H) (Scheme 1). 

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