ICMUB - News

Vendredi 1 juin 2007 : Conf du Dr Irena G. Stara, Institut de Chimie Organique et de Biochimie, Prague

Synthesis and properties of novel helical aromatics: azahelicenes and extended helicenes

Azahelicenes as well as extended helicenes belong to the larger family of helicenes, which are unique 3D aromatic systems consisting of all ortho fused benzene/pyridine units and exhibiting helical chirality.
Here, we report their first practical syntheses in a racemic or optically pure form. The preparation of 1-aza[6]helicene demonstrates the new strategy for the azahelicene synthesis. The key tetrahydroderivative intermediate was prepared using a general methodology for the synthesis of helicenes and their congeners based on [2+2+2] cycloisomerisation of aromatic triynes. It was subsequently aromatized to the desired azahelicene by reaction with MnO2. The racemate of 1-aza[6]helicene was resolved into single enantiomers on a preparative scale by co-crystallisation with a tartaric acid derivative. Mixing racemic 1-aza[6]helicene with silver triflate provided the homochiral (M,M)- and (P,P)-(azahelicene)2AgOTf complexes. The basicity and complexation studies are in progress.

The synthesis of the higher homologues of helicene represents a challenging task. Although [14]helicene, [15]heterohelicene and [17]heliphene were already prepared, their syntheses suffered from considerably low yields of the final cyclisation steps. The preparation of [11]helicene exemplifies the multicyclisation strategy, which uses [2+2+2] cycloisomerisation reaction mediated by Co(I) or Ni(0) complexes. rac-[11]Helicene was deposited on InSb (001) step edges and imaged by LT STM providing a preliminary image of the submolecular resolution. rac-[11]Helicene was resolved into enantiomers by means of chiral HPLC. The stereochemistry aspects will be discussed as well as the synthesis of other heterocyclic analogues in a racemic and nonracemic form.

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Contact : Sylvain Jugé

ICMUB (UMR 5260) - University of Burgundy - Dijon (France)