Thiocarboxylic acids, from thiols

Fukuyama [5] obtained the thioester 35 from the reaction of the thiocarboxylic acid anion 34 with the / -lactone compound 17. After acidic deprotection of the Boc group, the thiazoline ring is successfully closed in benzene under Dean-Stark conditions (35 —> 33) in yields between 60 and 80 %. Ehrler [6] and Pattenden [7] choose a more classical way. They close the thiazoline ring by condensation of the aminothiol hydrochloride 5 x HCl with a nitrile. The yields are generally modest (45 and 55 %) except in one case [6]. Installation of the necessary nitrile function hinders its repetitive use, as this procedure causes a considerable loss of material. Heathcock [8] and Ehrler [6 choose the thiol-amide 37 as a key compound. Titanium tetra-... 

DCC can be used to prepare 5-alkyl and 5-aryl thiocarboxylates (1) from carboxylic acids and thiols according to equation (5). This method has been successfully applied to the synAesis of thiol esters with sensitive substituents, e.g. 5-methyl thioacrylate, a natural product. In particular, N-protected amino acid and peptide 5-phenyl esters, which are useful building blocks in peptide synthesis, are obtained in excellent yields without racemization. N-Hydroxyphthalimide and DMAP have been used as cocatalysts to facilitate the reaction. The preparation of the Wittig reagent (5) by this route is shown in equation (6). 

Third in importance are the S-glucuro-nides formed from aliphatic and aromatic thiols (reaction 12 in Fig. 13.21) and from di-thiocarboxylic acids (reaction 13) such as di-ethyldithiocarbamic acid, a metabolite of dis-ulfiram. As for C-glucuronidation (reaction 14), this reaction has been seen in humans for 1,3-dicarbonyl drugs such as phenylbutazone and sulfinpyrazone (28) (Fig. 13.23). 

After irradiation, the HABI molecules decompose into lophyl radicals which subsequently abstract protons from a donor, e.g. thiols [BER 14] (Figure 4.9), heteroaromatic thiols, benzothiazoles, benzooxazoles, tertiary amines, alcohols, thiocarboxylic acids [BLA 09] or dyes [BEN 02], If used in a dental composite, they lead to improved compressive, yield and flexural strengths compared to conventional composites with CQ/DMABE. Furthermore, the sensitivity to ambient light could be decreased [BLA 09]. 

Methyl and S-aryP thiocarboxylic esters can also be prepared directly from carboxylic acids if thiocyanates or chlorothioformates are used instead of thiols (equations 11 and 12). 

A method of preparing alkanethiols and thiophenols by means of thio-carboxylic acids consists of converting, preferably, thioacetic or thiobenzoic acid by an alkyl (or aryl) halide into an 5-alkyl (or 5-aryl) thiocarboxylate and hydrolysing this by acid or alkali this method, and that described in the following paragraphs, have the advantage that the thiols can be obtained free from sulfide.268... 

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