Preparation of Novel Unsymmetrical Bisindoles under Solvent-Free Conditions:  Synthesis, Crystal Structures, and Mechanistic Aspects

Indole aziridines and their hydroxyl derivatives have been used for the preparation of a small library of novel functionalized bisindoles. Diversification of these building blocks by solvent-free C−C-bond formation on solid support yielded annulated Hymenialdisine analogues under mild reaction conditions. Indoles as C-nucleophiles form potentially pharmacologically active bisindoles through an electrophilic aromatic substitution pathway in good to excellent yields. Further transformations of the indole aziridines with H-, N-, and O-nucleophiles demonstrate their versatility as key intermediates in diversity oriented synthesis. The hydroxyl precursor leads also to unsymmetrical bisindoles under similar reaction conditions. Important intermediates and final library compounds were confirmed by X-ray analysis. Theoretical studies on these systems show the possible cationic intermediate in the substitution pathway.