Thiazole-2-thiones, tautomerism

Tautomerism of the A-2-thiazoline-5-thiones has not been investigated intensively. A recent report shows that 2-phenylthiazo e-5-thiols exist in the thiol form in both polar and nonpolar solvents (563). This behavior is in contrast with that of corresponding thiazolones. Addition reactions involve only the exocyclic sulfur atom, and thiazole-5-thiols behave as typical heteroaromatic thiols towards unsaturated systems, giving sulfides (1533) (Scheme 80) (563),... 

Thiazolidine-2-thione, 4-hydroxy-synthesis, 6, 314 Thiazolidinethiones tautomerism, 6, 273 1,2,4-thiazoles from, 5, 776 Thiazolidine-2-thiones reactions... 

Thiazoles, vinyl-radical polymerization, 6, 278 Thiazole-5-sulfonic acid, 2-amino-synthesis, 6, 255-256 Thiazole-2-sulfonic acids reactions, 5, 104 6, 291 Thiazole-5-sulfonic acids synthesis, 6, 255 Thiazole-5-thione, 2-phenyl-tautomerism, 6, 249 Thiazolethiones reactivity, 6, 250 Thi azole-2-thiones reactions, 5, 102 tautomerism, 5, 367 Thiazolidine, 2-alkyl-occurrence, 6, 327 Thiazolidine, 2-arylimino-X-ray analysis, 6, 238 Thiazolidine, 4-imino-mesoionic didehydro derivative nomenclature, 1, 34 nomenclature, 1, 34... 

Cyclization occurs directly through catalysis by the acid liberated when a pyridine-2(lH)-thione is heated with an a-halo acid ester. The most convenient method for preparing the thiazole, however, seems to be the cyclization of (2-pyridinethio)acetic acids in acetic anhydride in the presence of pyridine. Without base catalysis the reaction is slow, which suggests a mixed anhydride intermediate. Mixed anhydride formation with ethyl chlorofor-mate in pyridine, or carboxyl activation by DCC in pyridine, gives the mesoionic product. The cyclization reaction and the chemical stability of the thiazole are adversely affected by a pyridine 6-substituent. The initially formed acylpyridinium salt (407) undergoes rapid tautomerization to the aromatic thiazole form equilibrium between the forms (407) and (408) is verified by rapid deuteration at C-2 (R1 = H) in AcOH-d (81H(15)1349). 

Thione-thiol tautomerism has been investigated in several compounds. One of the best known examples is the tautomerism in 2-mercaptobenzo-thiazole, XXVII, which has been investigated in detail24- 1. The spectrum of the compound in alcohol is similar to that of the N-methyl derivative and not to that of the S-methyl derivative, thus indicating that the thione-form is predominant. The thione-thiol tautomerism in derivatives of mercapto-thiazoles82 and mercaptotetrazoles32 has been investigated. 

Recently, however, there has been increased interest in N-oxides for the generation of hydroxyl radicals in aqueous solution. This process has been reported for some nitrated N-oxides but is most often observed for tautomeric N-oxides, where the N-hydroxy form is significant. " "" The main application is in photobiology, as specific sources of hydroxyl radicals for DNA damage, both strand break and base modifications. N-Hydroxy-2-pyridone and 2-pyridinethione, - N-hydroxy-4-(4-chlorophenyl)thiazole-2(3H)-thione, " and pyrimido[5,4-g ]pteridinetetraone N-oxide (for which the path followed depends on conditions) " and related N-oxides " have been used for this purpose. 

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