A Review of Recent Advances in the Green Synthesis of Azole- and Pyran-based Fused Heterocycles Using MCRs and Sustainable Catalysts

Nitrogen, oxygen and sulfur-containing fused heterocycles are of great importance because of their exciting and diverse biological activities. The construction of the carbonnitrogen and carbon-oxygen through a multicomponent reaction approach by using ecofriendly reusable heterogeneous catalysts are of significant importance as it opens avenues for the introduction of nitrogen and oxygen in organic molecules. Thus, green methodologies have gained particular significance in this field; today, green chemistry is considered a tool for introducing sustainable concepts at the fundamental level. This review emphasizes and discusses the current progress on the applications of eco-friendly, recyclable heterogeneous catalysts for the synthesis of different heterocyclic fused systems and their green protocols. We paid particular attention to the specific integration of carbon-nitrogen, and carbon-oxygen bond-forming fused heterocycles by a one-pot approach by evaluating the literature between 2012 and the middle of 2020. The efficiency of the catalyst is assessed in terms of reaction time, yield and possible reusability. The MCR and heterogeneous catalyst strategies have demonstrated broader scope, economical and viability for the green and sustainable processes in the field of synthetic organic chemistry.