Synthesis of the First Chiral Bisindolyl Monomethinium Cyanine Dyes

Abstract

Abstract The synthesis of the chiral monomethine cyanine dyes 8 and 9 starting with the achiral indole 1 is described. Key intermediates are the new chiral Fischer bases R ‐ and S ‐ 4 , which were separated after diastereomeric salt formation. The conformation of the dyes 8 and 9 is twisted di‐ Z , according to NOE and dynamic NMR spectroscopy. The free energy difference between oppositely twisted conformations is ca. 2.5 kJ mol −1 .