Synthesis and X-ray analysis of new [5]helicenes - HMO calculations on thephotocyclization of the stilbene precursors[1]

The syntheses of the new pentahelicenes 5, 11, 17, 21, and 28 with various substituents are described. In the case of 2,13-dicyano-[5]helicene (11) op tical resolution was achieved by HPLC using a column packed with gamma-cycl odextrin. However, the enantiomers racemized within a few hours. On the oth er hand, the enantiomers of 28 turned out to be stable after separation on triacetylcellulose using MPLC. The crystal structures of 11, 17, and 21 wer e solved and indicated the typical distortions which are expected for helic enes. The model of the sum of free valence numbers was applied in order to rationalize the reactivity pattern of the photochemical phenanthrene cycliz ation.