Thiazole, or 1,3-thiazole, is a 5-membered heterocyclic compound that contains both sulfur and nitrogen. The term 'thiazole' also refers to a large family of derivatives. Thiazole itself is a pale yellow liquid with a pyridine-like odor and the molecular formula C3H3NS.[2] The thiazole ring is notable as a component of the vitaminthiamine (B1).
Molecular and electronic structure
Thiazoles are members of the azoles, heterocycles that include imidazoles and oxazoles. Thiazole can also be considered a functional group when part of a larger molecule.
Being planar thiazoles are characterized by significant pi-electron delocalization and have some degree of aromaticity, moreso than the corresponding oxazoles. This aromaticity is evidenced by the 1H NMR chemical shift of the ring protons, which absorb between 7.27 and 8.77 ppm, indicating a strong diamagnetic ring current. The calculated pi-electron density marks C5 as the primary site for electrophilic substitution, and C2-H as susceptible to deprotonation.
Thiazoles are found in a variety of specialized products, often fused with benzene derivatives, the so-called benzothiazoles. In addition to vitamin B1, the thiazole ring is found in epothilone. Other important thiazole derivatives are benzothiazoles, for example, the firefly chemical luciferin. Whereas thiazoles are well represented in biomolecules, oxazoles are not. It is found in naturally occurring peptides, and utilised in the development of peptidomimetics (i.e. molecules that mimic the function and structure of peptides).[3]
Commercial significant thiazoles include mainly dyes and fungicides. Thifluzamide, Tricyclazole, and Thiabendazole are marketed for control of various agricultural pests. Another widely used thiazole derivative is the non-steroidal anti-inflammatory drug Meloxicam. The following anthroquinone dyes contain benzothiazole subunits: Algol Yellow 8 (CAS# [6451-12-3]), Algol Yellow GC (CAS# [129-09-9]), Indanthren Rubine B (CAS# [6371-49-9]), Indanthren Blue CLG (CAS# [6371-50-2], and Indanthren Blue CLB (CAS#[6492-78-0]). These thiazole dye are used for dyeing cotton.
Thiazoles are generally formed via reactions of cysteine, which provides the N-C-C-S backbone of the ring. Thiamine does not fit this pattern however. Several biosynthesis routes lead to the thiazole ring as required for the formation of thiamine.[5] Sulfur of the thiazole is derived from cysteine. In anaerobic bacteria, the CN group is derived from dehydroglycine.
Reactions
With a pKa of 2.5 for the conjugate acid, thiazoles are far less basic than imidazole (pKa =7).[6]
Deprotonation with strong bases occurs at C2-H. The negative charge on this position is stabilized as an ylide. Hauser bases and organolithium compounds react at this site, replacing the proton. 2-Lithiothiazoles are also generated by metal-halogen exchange from 2-bromothiazole.[7]
Oxidation at nitrogen gives the aromatic thiazole N-oxide; many oxidizing agents exist, such as mCPBA; a novel one is hypofluorous acid prepared from fluorine and water in acetonitrile; some of the oxidation takes place at sulfur, leading to non-aromatic sulfoxide/sulfone:[8] Thiazole N-oxides are useful in Palladium-catalysed C-H arylations, where the N-oxide is able to shift the reactivity to reliably favor the 2-position, and allows for these reactions to be carried out under much more mild conditions.[9]
Thiazoles can react in cycloadditions, but in general at high temperatures due to favorable aromatic stabilization of the reactant; Diels-Alder reactions with alkynes are followed by extrusion of sulfur, and the endproduct is a pyridine; in one study,[10] a very mild reaction of a 2-(dimethylamino)thiazole with dimethyl acetylenedicarboxylate (DMAD) to a pyridine was found to proceed through a zwitterionic intermediate in a formal [2+2]cycloaddition to a cyclobutene, then to a 1,3-thiazepine in a 4-electron electrocyclic ring opening and then to a 7-thia-2-azanorcaradiene in a 6-electron electrocyclic ring, closing before extruding the sulfur atom.
^Zoltewicz, J. A.; Deady, L. W. (1978). "Quaternization of Heteroaromatic Compounds: Quantitative Aspects". Advances in Heterocyclic Chemistry Volume 22. Vol. 22. pp. 71–121. doi:10.1016/S0065-2725(08)60103-8. ISBN9780120206223.
^Eicher, T.; Hauptmann, S. (2003). The Chemistry of Heterocycles: Structure, Reactions, Syntheses, and Applications. Wiley. ISBN978-3-527-30720-3.
^Alajarín, M.; Cabrera, J.; Pastor, A.; Sánchez-Andrada, P.; Bautista, D. (2006). "On the [2+2] Cycloaddition of 2-Aminothiazoles and Dimethyl Acetylenedicarboxylate. Experimental and Computational Evidence of a Thermal Disrotatory Ring Opening of Fused Cyclobutenes". J. Org. Chem.71 (14): 5328–5339. doi:10.1021/jo060664c. PMID16808523.
