Abstract
AbstractStarting with natural monochiral (−)‐(S)‐2‐methyl‐1‐butanol (1a) generated by alcoholic fermentations, we have synthesized the new monochiral heterocyclic iminium salt (+)‐(3R,15S)‐3‐sec‐butyl‐1,2,3‐trimethylindoleninium tetrafluoroborate (11) in an eight‐step reaction sequence. Important steps of this sequence are the zeolite‐catalyzed Fischer synthesis of the monochiral indole 6a from the phenylhydrazone of ketone 5a and the separation of the diastereomeric salts 9 and 10 by fourfold fractional recrystallization from ethanol. With the new quaternary iminium salt 11, new monochiral dinuclear tri‐ (13) and pentamethinium cyanine dyes (14) as well as the trinuclear [2.2.2]heptamethinediium 17 and [1.1.1]tetramethinium cyanine dyes 18 have been synthesized. The absolute configuration of the four stereogenic centers in the cyanine dye 14 has been confirmed by means of an X‐ray structural analysis. Spectroscopical and chiroptical properties of all new cyanine dyes have been determined.
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