Oxazole and thiazole comprise an important type of aromatic heterocyclic compounds with a five-membered structure, which is a conjugated ring consisting three carbon atoms and one nitrogen atom, oxazole contains another oxygen atom while thiazole contains the sulfur atom (Fig 1). They are widely abundant in various natural substances, numerous biologically active compounds have been demonstrated that the oxazole or thiazole moiety is the major structural framework, offering potential application in medicinal synthesis. It is noteworthy that oxazole and thiazole exhibit a strong toxicity to organisms, they are flammable and will generate toxic smoke through combustion. So every care should be taken during their use and storage.
Fig.1. Oxazole and thiazole compounds
Unmodified oxazole and thiazole are slightly water-soluble, because the polar five-membered rings bring stable conjugated structure and limit their dissolution, but the oxygen atom can form hydrogen bond with water molecule which slightly promote the solubility. Oxazole and thiazole possess relative low boiling point and melting point thus they are commonly presented as colorless liquid. Their chemical property is relatively stable and they are not easy to be reduced and react with nitric acid. However, thiazole can react with liquid bromine, sulfuric acid, acetic acid via electrophilic substitution reaction (Fig 2).
Oxazole and thiazole compounds have been detected in various natural medicinal molecules, motivating the synthesis of artificial derivatives to fabricate biologic precursors, which possess excellent pharmacological and physiological activities, such as antibacterial, antidiabetic, antiparasitic, anti-tumor and anti-inflammatory behavior, etc. For example, Various alkaloids with thiazole motif have been fabricated (Fig 3).
Fig.3. Chemical structure of antibacterial benzothiazole derivatives. (Prod. Rep. 2003, 20, 584-605.)
Inspired by the antibacterial activity in medicinal precursors, oxazole and thiazole derivatives can be applied as fungicides, insecticide, herbicide and so on. a variety of thiazole compounds are designed and characterized for further commercial manufacture.
It is well known that a larger conjugated constitution of compund will promote the red shift of the absorption to visible region, thus the color of the thiazole dye can be designed via expanding the degree of conjugation. Cyanine dye is a significant instance of thiazole dye fused with benzene ring, which can be classified as symmetric or asymmetric dyes according to the structure (Fig 4). Moreover, Cyanine dye can be utilized for intercalation and groove binding with DNA.
Fig.4. Chemical structure of thiazole cyanine dyes. (Heterocycl. Commun. 2013, 19, 1-11.)