A simple and efficient visible-light-induced methodology has been developed for the selective synthesis of dihydro-4H-furo[3,2-c]chromen-4-one derivatives via dehydrogenative coupling reaction. The methodology is highly useful for the synthesis of varieties of dihydro-4H-furo[3,2-c]chromen-4-ones from readily accessible reactants under oxygen atmosphere. Importantly, the additive-free approach has been also demonstrated for this transformation. The results of the mechanistic investigation showed that this dehydrogenative reaction proceeded through a radical pathway. Additionally, the synthesized dihydro-4H-furo[3,2-c]chromen-4-one has been transformed into different 4H-furo[3,2-c]chromen-4-ones through simple protocols.

Synthesis of pharmaceutically active heterocycles is always appealing as the majority of the widely used drugs contain heterocyclic moieties as their core structure. So, the straightforward construction of heterocycles from readily available/accessible reagents is one of the prime targets of the synthetic chemists. In this context, CÀ H functionalization has emerged as an effective tool for the designing and synthesis of various heterocyclic moieties as it offers a straightforward and step-economic pathway. On the other hand, the readily available/accessible conjugated carbonyls are well-known reagents for the construction of carbocycles and heterocycles over the years. However, the employment of CÀ H functionalization of the two CÀ H bonds of the conjugated alkene in carbocycle/heterocycle synthesis was not so explored. In the last decade, much focus has been paid on the synthesis of various pharmaceutically active heterocycles through CÀ H bond functionalization of α,β-unsaturated aldehydes/ketones. These protocols have been developed through either oxida-tive coupling of conjugated carbonyls with suitable coupling partners or intramolecular CÀ H bond functionalization of conjugated carbonyls. In this review, we will discuss all the methodologies developed for the synthesis of heterocycles employing intermolecular CÀ H bond functionalization of conjugated carbonyls. The mechanistic pathways and usefulness of the methodologies will be also highlighted.

A visible-light-induced cross-dehydrogenative methodology has been developed for the regioselective sulfenylation of pyrazolo[1,5-a]pyrimidine derivatives. Rose bengal, blue LEDs, KI, K 2 S 2 O 8 , and DMSO are all essential for this photocatalytic transformation. The protocol is applicable for the synthesis of a library of 3-(aryl/heteroaryl thio)pyrazolo[1,5-a]pyrimidine derivatives with broad functionalities. The selectivity and scalability of the methodology have been also demonstrated. Moreover, the efficiency of this strategy for sulfenylation of pyrazoles, indole, imidazoheterocycles, and 4-hydroxy coumarin has been proven. The mechanistic investigation revealed the radical-based mechanism and formation of diaryl disulfide as a key intermediate for this cross-dehydrogenative coupling reaction.

A one-pot methodology has been developed to synthesize 3-halo-pyrazolo­[1,5-a]­pyrimidine derivatives through the three-component reaction of amino pyrazoles, enaminones (or chalcone), and sodium halides. The use of easily accessible 1,3-biselectrophilic reagents like enaminones and chalcone offers a straightforward approach for the synthesis of 3-halo-pyrazolo­[1,5-a]­pyrimidines. The reaction proceeded through a cyclocondensation reaction between amino pyrazoles with enaminones/chalcone in the presence of K2S2O8 followed by oxidative halogenations by NaX-K2S2O8. Mild and environmentally benign reaction conditions, wide functional group tolerance, and scalability of the reaction are the attractive facet of this protocol. The combination of NaX-K2S2O8 is also beneficial for the direct oxidative halogenations of pyrazolo­[1,5-a]­pyrimidines in water.

A new method has been developed for the C-H selenylation of pyrazolo[1,5-a]pyrimidine derivatives under the irradiation of visible light. This photocatalyst-free strategy is applicable to a wide range of pyrazolo[1,5-a]pyrimidines with broad functionalities. The salient features of the method are mild reaction conditions, use of bench-stable oxidant, high regioselectivity and scalability.