Methylene blue as a photo‐redox catalyst employed for the synthesis of pyrano[2,3‐d]pyrimidine scaffolds via a single‐electron transfer/energy transfer pathway

Abstract

The radical tandem Knoevenagel‐Michael cyclocondensation reaction of barbituric acid/1,3‐dimethylbarbituric acid, malononitrile, and aryl aldehydes was used to establish a green tandem method for the metal‐free synthesis of pyrano[2,3‐d]pyrimidine scaffolds. At ambient temperature, photo‐excited state functions made from methylene blue (MB+) were used as single‐electron transfer and energy transfer catalysts in an aqueous solvent, utilizing visible light as a sustainable energy source. The purpose of this project is to enhance the use of a non‐metal cationic dye that is both affordable and widely available. Methylene blue is photochemically catalyzed with the least amount of catalyst feasible, yielding excellent yields, energy efficiency, and environmental friendliness, as well as excellent atom economy, time‐saving features, and ease of operation. As a result, a wide range of environmental and long‐term chemical properties can be obtained. The pyrano[2,3‐d]pyrimidine scaffolds' turnover number and turnover frequency have been calculated. Surprisingly, gram‐scale cyclization is conceivable, indicating that the technology could be used in industry.