- New Methods for the Introduction of Substituents into Thiazoles
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New methods for the regioselective introduction of substituents into thiazoles have been developed using thiazole, 2-phenylthiazole and 4,5-dimethylthiazole as representative thiazoles.Improved halogenation methods, displacement of hydroxy groups in combination with dehalogenation at C-5 yield all eight possible 2-phenyl-4-halo-, 5-halo and 4,5-dihalo-thiazoles in which halogen is chlorine and bromine.Peracid oxidation of the thiazoles gives the corresponding thiazole 3-oxides.These are not activated towards halogenation but are deprotonated with sodium hydride.The anion formed react with electrophiles such as paraformaldehyde, 2,2-dimethylpropanal, 2,2-dimethylpropanoyl chloride, hexachloroethane, tetrabromomethane, and dimethyl disulfide leading to the introduction of carbon substituents, halogen, and methylthio groups.In these reactions, the reactivity of the thiazole ring positions decreases in the order 2 than 5 than 4.Monoselectivity is low when halogen and methylthio groups are introduced since these substituents enhance the acidity of adjacent ring protons. 2-Phenyl-4,5-dihalothiazole 3-oxides lose the 5-halogen when treated with sulfite ion.Trimethyloxonium tetrafluoroborate O-methylates thiazole 3-oxides.Thiazole N-oxides also react with acetyl chloride and phosphorus oxychloride to afford chlorothiazoles in a non-selective manner.Phosphorus trichloride deoxygenates thiazole 3-oxides.
- Begtrup, Mikael,Hansen, Lars Bo L.
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p. 372 - 383
(2007/10/02)
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- Azoles. Part 8. Metallation and Bromine -> Lithium Exchange Reactions of Polyhalogenothiazoles
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2,4-Dichloro- and 2,4-dibromo-thiazole were deprotonated at position-5 with LiN(iPr)2 in THF at -78 deg C and the resulting lithium compound was quenched with various reagents, to yield various trisubstituted thiazoles. 2,5-Dibromo-4-chlorothiazole reacted with n-butyllithium in THF at -78 deg C at position-5 and the resulting lithium derivative gave 2-bromo-4-chloro-5-substituted thiazoles when quenched with the appropriate reagent.Both the 2- and 5-bromine-atoms were reactive in diethyl ether. 2,5-Dibromothiazole failed to deprotonate at position-4 under various reaction conditions, whereas treatment of 2,4,5-tribromothiazole with 1 mole equivalent of n-butyllithium in THF at -90 deg C, followed by addition of dimethyl disulfide after 30 min, gave a high yield of the 2,5-bis(methylthio)-compound.The tribromo-compound was also treated with 1 mole equivalent of n-butyllithium or methyllithium under various reaction conditions and the products formed after hydrolysis were analysed by 1H NMR spectroscopy.The 5-bromine-atom is the most reactive and greater selectivity is obtained with methyllithium.
- Athmani, Salah,Bruce, Andrew,Iddon, Brian
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p. 215 - 219
(2007/10/02)
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