Visible-Light-Promoted Radical Acylation/Cyclization Of Alkynoates With Aldehydes For The Synthesis Of 3-Acylcoumarins

Coumarins,a kind of compounds with the structure of cis-hydroxy cinnamate lactone,are widely found in nature.The structure makes coumarins outstanding in aromaticity and fluorescence.To benefit from that,coumarin derivatives retain a high degree of importance in pharmaceuticals,agrochemical industries,and cosmetics.These growing needs have stimulated interest in the design and efficient synthesis of highly functionalized coumarins.The synthesis of coumarins has been greatly developed with the development of chemical field,from classical reactions,transition metal catalytic reactions,to the application of radical reactions and the continuous research of photocatalytic reaction.More and more coumarin derivatives were synthesized by chemical synthesis,which greatly promoted the research of coumarin derivatives.Herein,a visible-light-promoted radical acylation/cyclization of alkynoates for the synthesis of 3-acylcoumarins was studided in this paper,with mainly several parts:Based on the studies of radical reactions for the synthesis of coumarin derivatives,a variety of experiments were carried out to find the best conditions for synthesis of3-acylcoumarins.Indeed,the desired acylation/cyclization product was obtained in up to 80%yield under visible light irradiation at room temperature,with tris（2,2-bipyridine）ruthenium dichloride as the photoredox catalyst,tert-butyl hydrogen peroxide as oxidant,dipotassium hydrogen phosphate as the base,and methyl cyanide as the solvent.With the optimum reaction conditions in hand,various aldehydes and alkynoate derivatives were investigated successively.Both aromatic aldehydes and aliphatic aldehydes were better compatible under the standard conditions,compared to other’s studies.Alkynoates with different substituents also worked well under the condition.Finally,353-acylcoumarins were completed,providing moderate to good yields,with the analysis data of ¹H NMR,¹³C NMR and HRMS.In order to explore the reaction mechanism,control experiments were carried out.On the basis of the results,the previous investigations,a possible mechanism for the photoreaction is proposed as the reaction involves readical initiation,radical addition/5-exo cyclization and ester migration to afford the product.