4- Hydroxythiazoles, tautomerism

The same tautomerism as that described for hydroxythiazoles is observed for mercap-tothiazoles (see Sections 4.19.2.3.5(i) and (ii)). 2-Mercaptothiazole exists predominantly in the A4-thiazoline-2-thione form (213) whereas 2-phenylthiazole-5-thiol exists in the thiol form (214). 

Hydroxy derivatives are expected to show the same tautomeric preferences for the oxo form in ortho and para positions to the nitrogen as in the parent monocycle. Thus, for the 2-hydroxythiazole (395) comparison of the spectroscopic data with the data for its AT-methyl and O-methyl derivatives is consistent with the oxo form the same preference has been shown for its [5,4-c] isomer (68CJC691). 

Thiazolidinethiones, chemistry and biological activity 81CRV175. Thiazolinones, tautomeric equilibrium with hydroxythiazoles 80T2023. 

The tautomerism of hydroxythiazoles has been discussed in Section 3.06.4.4. The reactivity of hydroxythiazoles is well-documented in CHEC-I <84CHEC-I(4)285>. 2-Methoxy benzothiazole (184) has been found to react with allenylmagnesium bromide (185) to produce 2-allenylbenzothiazole (188), the formation of which can be rationalized in terms of a Sgi mechanism involving the attack of the allenic form of the Grignard reagent followed by the elimination of methanol and isomerization of the intermediate (187) (Scheme 44) <86S638>. 

Thiazolin-4-one derivative 29 in hydrazino-hydrazono tautomeric equilibrium is synthesized by cyclization of l,2-diaza-l,3-butadiene 27 with aryl thioamide 28. Subsequent hydrolytic removal of the NH-Boc-hydrazo protecting group provided 5-acetyl-4-hydroxythiazole derivative 30a, which undergoes a-bromination to give a-bromomethyl ketone 30b. This bromide is used to prepare polyfunctionalized 4,5 -bithiazol-4 -ol derivatives via the Hantzsch thiazole synthesis <04SL2681>. 

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