Marine Sponge/H3PO4: As a Naturally Occurring Chiral Catalyst for Solvent-free Fischer-Indole Synthesis

Background: A new and efficient method have been developed for the synthesis of different indole derivatives from various ketones, having at least one hydrogen atom attached to each of their α-carbon atoms, and hydrazines in solvent-free conditions, using marine sponge/H3PO4 as a naturally occurring chiral catalyst. Objectives: This study recommended the use of marine sponge/H3PO4 as a naturally occurring chiral catalyst for preparation of phenylhydrazones from ketones having one α-hydrogen and subsequent cyclisation of the products to indoles. Materials and Methods: The reaction was carried out by mixing the phenylhydrazine, ketone, and marine sponge/H3PO4 powder in mortar and pestle; the mixture was ground at room temperature in an appropriate time until TLC show the completion of the reaction. The product extracted by CH2Cl2 and evaporation of solvent yields the products. Results: In this research work, several indoles are synthesized using phenylhydrazine and aliphatic or aromatic ketone as starting materials, in the presence of marine sponge/H3PO4 powder as a natural catalyst under solvent-free condition. Conclusions: We found marine sponge/H3PO4 to be an effective catalyst for indolisation of phenylhydrazones from ketones having α-hydrogens in solvent-free conditions.