161. Modular synthesis of pH-sensitive fluorescent diaza[4]helicenes
     Antoine Wallabregue, Petr Sherin, Joyram Guin, Céline Besnard, Eric Vauthey, Jérôme Lacour
     Eur. J. Org. Chem. 2014, 6431-6438

Using an particularly-facile N-N bond cleavage reaction, configurationally stable diaza [4]helicenes are prepared in two steps. The synthetic procedure, that uses hydrazine NH₂NH₂, is general, modular and highly tolerant to functional groups. The resulting helical quinacridines are dyes that present absorption and emission properties that can be modulated as a function of pH; the pink quinacridine and green protonated forms (pKa ~9.0) displaying distinct optical features in the near-IR region. Single enantiomers were obtained by a chiral stationary phase HPLC resolution. Absolute configuration assignment was realized by ECD spectra and a rather high racemization barrier was measured (ΔG^‡ 30.7 ± 4.0 kcal.mol^-1 at 140 °C).

DOI : 10.1002/ejoc.201402863 

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