Synthesis of Pyranocoumarin through MCRs: Modulation of this Approach as a Function of Substituent Effect Forms the Furocoumarin and 4-Hydroxy-3-styryl Coumarin as a Function of Solvent Effect

The synthesis of pyranocoumarin is reported from 4-hydroxycoumarin, styrene oxide, and DMSO in the presence of <i>p</i>-TSA·H₂O at 110 °C using a three-component reaction. Interestingly, a CH₂ moiety from DMSO gets inserted in this reaction to form the pyranocoumarin ring. The plausible mechanism for this interesting reaction was proposed and validated using quantum chemical methods. Interestingly, when the reactions were performed with 7-methoxy-4-hydroxycoumarin and styrene oxides, having an electron-withdrawing group, the reaction followed a two-component pathway, and furocoumarins were obtained in good yields. Switching the solvent from DMSO to CH₃CN for the same reactants results in the exclusive formation of the novel monomeric product 4-hydroxy-3-styryl-2<i>H</i>-chromen-2-one (<b>5</b>) instead of the expected furocoumarins. Interestingly, one of the novel monomeric products, 4-hydroxy-3-styryl-2<i>H</i>-chromen-2-one, directly undergoes unexpected cyclization to provide a more interesting and complex tricyclic product. The salient features of the present strategy are its easy handling, commercial availability of starting material, metal-free synthesis, inexpensive catalysts, environmentally benign, and high atom economy.