Synthesis of Pyranocoumarin\nthrough MCRs: Modulation\nof this Approach as a Function of Substituent Effect Forms the Furocoumarin\nand 4‑Hydroxy-3-styryl Coumarin as a Function of Solvent Effect

The synthesis of pyranocoumarin is\nreported from 4-hydroxycoumarin,\nstyrene oxide, and DMSO in the presence of <i>p</i>-TSA·H<sub>2</sub>O at 110 °C using a three-component reaction. Interestingly,\na CH<sub>2</sub> moiety from DMSO gets inserted in this reaction to\nform the pyranocoumarin ring. The plausible mechanism for this interesting\nreaction was proposed and validated using quantum chemical methods.\nInterestingly, when the reactions were performed with 7-methoxy-4-hydroxycoumarin\nand styrene oxides, having an electron-withdrawing group, the reaction\nfollowed a two-component pathway, and furocoumarins were obtained\nin good yields. Switching the solvent from DMSO to CH<sub>3</sub>CN\nfor the same reactants results in the exclusive formation of the novel\nmonomeric product 4-hydroxy-3-styryl-2<i>H</i>-chromen-2-one\n(<b>5</b>) instead of the expected furocoumarins. Interestingly,\none of the novel monomeric products, 4-hydroxy-3-styryl-2<i>H</i>-chromen-2-one, directly undergoes unexpected cyclization to provide\na more interesting and complex tricyclic product. The salient features\nof the present strategy are its easy handling, commercial availability\nof starting material, metal-free synthesis, inexpensive catalysts,\nenvironmentally benign, and high atom economy.