Solvent-Free Addition of Indole to Aldehydes: Unexpected Synthesis of Novel 1-[1-(1H-Indol-3-yl) Alkyl]-1H-Indoles and Preliminary Evaluation of Their Cytotoxicity in Hepatocarcinoma Cells

New 1-[1-(1H-indol-3-yl) alkyl]-1H-indoles, surprisingly, have been obtained from the addition of indole to a variety of aldehydes under neat conditions. CaO, present in excess, was fundamental for carrying out the reaction with paraformaldehyde. Under the same reaction conditions, aromatic and heteroaromatic aldehydes afforded only classical bis (indolyl) aryl indoles. In this paper, the role of CaO, together with the regiochemistry and the mechanism of the reaction, are discussed in detail. The effect of some selected 3,3′- and 1,3′-diindolyl methane derivatives on cell proliferation of the hepatoma cell line FaO was also evaluated.