FeCl3/SiO2-catalyzed bis-indolylation of acetals and ketals: a highly atom-economical approach to the selective deprotection of protected carbohydrates†

A simple and green catalytic system is developed for the synthesis of 3,3′-bisindolyl(methanes) (BIMs) using cyclic/acyclic acetals as the carbon source for the bridging residue between two indole motifs. The reaction occurred under mild and benign conditions using FeCl₃/SiO₂ as a heterogeneous catalyst without the requirement of any toxic organic solvents. The ready availability and recyclability of the catalytic system allows the reaction to be highly efficient, resulting in very good BIM products. DFT studies were also performed to establish the proposed mechanism and preferential formation of unsymmetrical bisindolylmethanes using equimolar amounts of different indoles. The present protocol is also extended to the bisindolylation-induced selective cleavage of protected carbohydrates to diols in a 100% carbon-preservation and maximized atom-economical manner.