Photoinduced [3+2] Cycloaddition of Carbenes and Nitriles: A Versatile Approach to Oxazole Synthesis

Sustainable Synthesis through Catalyst-Free Photoinduced Cascaded Bond Formation

The beginning of photochemical reactions revolutionized synthetic chemistry through sustainable practices. This review explores cutting-edge developments in leveraging light-induced processes for generating cascaded C-C and C-hetero bonds without catalysts. Significantly, catalyst-free photoinduced methodologies have garnered considerable attention, especially in the creation of varied heterocyclic frameworks for drug design and the synthesis of natural products. The article delves into underlying mechanisms, addresses limitations, and evaluates various methodologies, emphasizing the potential of photocatalyst and transition metal-free photochemical reactions to enhance sustainability. Divided into two sections, it covers recent strides in C-C and C-heteroatom and multiple C-heteroatom bond formation reactions.

Synthesis of 2,4-Disubstituted Oxazoles and Thiazoles via Brønsted Acid-Catalyzed Cyclization of α-diazoketones with Amides

A novel method is described for the synthesis of 2,4-disubstituted oxazole and thiazole derivates via the coupling of α-diazoketones with (thio)amides or thioureas using trifluoromethanesulfonic acid (TfOH) as a catalyst. This protocol is characterized by mild reaction conditions, metal-free, and simplicity and also features good functional group tolerance, good to excellent yields, and a broad substrate scope with more than 40 examples. Experimental studies suggest a mechanism involving 2-oxo-2-phenylethyl trifluoromethanesulfonate as the key intermediate.

Advances in heterocycle synthesis through photochemical carbene transfer reactions

Heterocyclic skeletons are commonly found in various bioactive molecules and pharmaceutical compounds, making them crucial in areas such as medicinal chemistry, materials science, and the realm of natural product synthesis. In recent years, the rapid advancements of visible light methodologies in organic synthesis have shown promising potential for the development of light-induced carbene transfer reactions. This is particularly significant as most organic molecules do not absorb visible light. Free carbene, known for its high activity, is frequently utilized for insertion reactions or cyclopropanation reactions. This review focuses on the photochemical strategy for the construction of heterocyclic skeletons, specifically highlighting the methods that employ visible light-promoted carbene transfer reactions.