Visible-light-mediated synthetic approaches towards thiazoles: needs, recent advances, and future perspectives

Thiazoles constitute an important class of azoles containing S and N atoms at positions 1 and 3, respectively, and are present in many biological components, polymers, dyes, and pharmaceuticals. They possess a wide range of medicinal properties, such as anticancer, antibacterial, antioxidant, antimalarial, and anti-inflammatory properties. In the past decades, several methods, including Hantzsch^'s method, have been utilized to synthesize thiazoles. However, some of them are associated with disadvantages such as the use of strong bases or acids, highly volatile solvents, high temperatures, harsh reactive conditions, harmful effects on the environment, tedious work, the formation of by-products, and the generation of huge chemical wastes. To resolve these issues, many eco-friendly, greener, mild, low-cost, and efficient protocols have been developed. Visible-light, a universal source of sustainable energy, has become an important choice for synthetic chemists under non-hazardous reaction conditions with reliable workups. Recently, extensive studies have been conducted to develop milder, greener, and sustainable pathways for synthesizing thiazole derivatives. This review describes the importance of thiazoles, their structural and reactivity features, the advantages of greener protocols over traditional ones, the need for visible-light in heterocyclic synthesis, and recent advances in the development of light-mediated synthetic protocols for thiazoles in the past 15 years. The mechanistic aspects of the reactions involved in constructing these azoles are also discussed, along with prospects.