Thiazole
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Thiazole, or 1,3-thiazole, is an organic chemical compound with the empirical formula C3H3NS. Its structure is a 5-membered ring, in which two of the vertices of the ring are nitrogen and sulfur, and the other three are carbons The Chemistry of Heterocycles : Structure, Reactions, Syntheses, and Applications Theophil Eicher, Siegfried Hauptmann ISBN 3527307206.
It is a clear to pale yellow flammable liquid with boiling point 116-118 °C and pyridine-like odor. Its CAS number is [288-47-1][link] and its SMILES structure is N1=CSC=C1.
Thiazole is used for manufacturing biocides, fungicides, pharmaceuticals, and dyes.
Thiazoles and thiazolium salts
Thiazoles are a class of organic compounds related to azoles with a common thiazole functional group.The thiazole moiety is a crucial part of vitamin B1 (thiamine) and epothilone. Other important thiazoles are Benzothiazoles for example the firefly chemical luciferin.
When the amino atom is alkylated the resulting thiazolium salt is a salt. Thiazolium salts are catalysts in the Stetter reaction and the Benzoin condensation. Thiazole dyes are used for dying cotton.
Oxazoles are related compound with sulfur replaced by oxygen. Thiazoles are well represented in biomolecules, oxazoles are not.
Organic synthesis
Various laboratory methods exist for the organic synthesis of thiazoles.- The Hantzsch thiazole synthesis (1889) is a reaction between haloketones and thioamides. For example 2,4-dimethylthiazole is synthesized from acetamide, phosphorus pentasulfide and chloroacetone 2,4-dimethylthiazole George Schwarz Organic Syntheses, Coll. Vol. 3, p.332 (1955); Vol. 25, p.35 (1945) [Link].
- In an adaptation of the Robinson-Gabriel synthesis a 2-acylamino-ketones reacts with phosphorus pentasulfide
- In the Cook-Heilbron synthesis an α-aminonitrile reacts with carbon disulfide.
- Certain thiazoles can be accessed though application of the Herz reaction.
Organic reactions
Thiazoles are characterized by larger pi-electron delocalization than the corresponding oxazoles and have therefore greater aromaticity. This is evidenced by the position of the ring protons in proton NMR (between 7.27 and 8.77 ppm) clearly indicating a strong diamagnetic ring current.The calculated pi-electron density marks C5 as the primary electrophilic site and C2 as the nucleophilic site.
The reactivity of a thiazole can be summarized as follows:
- Deprotonation at C2, the negative charge on this position is stabilized as an ylide. Grignard reagents and organolithium compounds react at this site replacing the proton.
- 2-(trimethylsiliyl)thiazole Organic Syntheses, Coll. Vol. 9, p.52 (1998); Vol. 72, p.21 (1995). [Article] (with a trimethylsilyl group in the 2-position) is a stable substitute and reacts with a range of electrophiles such as aldehydes, acyl halides and ketenes.
- Alkylation at nitrogen forms a thiazolium salt.
- Electrophilic aromatic substitution at C5 requires activating groups such as a methyl group in this bromination:
- Nucleophilic aromatic substitution often requires an electrofuge at C2 such as chlorine with
- Organic oxidation at nitrogen gives the thiazole N-oxide. Many oxidizing agents exist such as mCPBA, a novel one is hypofluorous acid prepared from fluoride and water in acetonitrile. Some of the oxidation takes place at sulfur leading to a sulfoxide Easy access to the family of thiazole N-oxides using HOF·CH3CN Elizabeta Amir and Shlomo Rozen Chemical Communications, 2006, 2262 - 2264
- Thiazoles are formyl synthons. Conversion of R-thia to the R-CHO aldehyde takes place with respectively methyl iodide (N-methylation), organic reduction with sodium borohydride and hydrolysis with mercury chloride in water.
References
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