Chiral solvent-induced asymmetric synthesis. Part 2. Photosynthesis of optically enriched hexahelicenes

Abstract

Irradiation of 2-styrylbenzo[c]phenanthrene (1) in eleven different, chiral solvents gave rise to the formation of non-racemic hexahelicene with optical yields of 0.2–2.0%. The role of the chiral solvents is ascribed to their influence on the equilibrium between enantiomeric conformations of cis-(1), which leads to an excess of one of them. The magnitude and sign of the effect can be related to the size and position of large apolar residues at the chiral centre in the solvent molecules. Similar experiments were done with some other precursors of hexahelicene and some of its derivatives. It is concluded that the solvent behaves as a matrix, in which the photoreaction takes place.