Abstract
Symmetrical cyanine 6 was synthesized as shown in Scheme 1. The acid-catalysed condensation of 1,1,3,3-tetramethoxypropane (1) with aniline (2) afforded the polymethine precursor 3. Subsequent alkylation of 2,3,3-trimethylindolenine (4) with excess of 1-iodooctane led to the ammonium salt 5 in 75% yield. Next, the condensation reaction of polymethine 3 (1.0 equiv.) with 3H-indolium salt 5 (2.0 equiv.), under reflux in EtOH in the presence of AcONa, afforded the symmetrical cyanine dye 6 in 54% yield.
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