Efficient Synthesis of 5,6-Dihydroxyindole Dimers, Key Eumelanin Building Blocks, by a Unified o-Ethynylaniline-Based Strategy for the Construction of 2-Linked Biindolyl Scaffolds

A unified convenient strategy for the synthesis of 5,6-dihydroxyindole-derived 2,7'-, 2,2'-, and 2,3'-biindolyls is reported, which is based on proper manipulation of key o-ethynylaniline precursors. By the same methodology 5,6-diacetoxy-7-iodoindole can also be obtained in good yield.