Conformational studies on helicenes VII. Synthesis and conformation of 5,6-dihydrohexahelicene†

Irradiation of 2,7-distyrylnaphthalene or 2-styrylbenzo[c]phenanthrene under completely oxygen-free conditions gives rise to the formation of 5,6-dihydrohexahelicene (2) as a consequence of H-shifts in the primary formed cyclization product. The conformation of 2 was analysed by NMR spectroscopy. From the chemical shifts of the aromatic protons, the coupling constants of the saturated moiety and the temperature independence of the spectrum it could be concluded that 2 occurs in one conformation, in which the axial proton at C(5) is directed towards the opposite ring. The resemblance of the spectra of 2 and l-phenylbenzo[c]phenanthrene affirms that this conformation is the less hindered one.