The influence of the chiral environment in the photosynthesis of enantiomerically enriched hexahelicene

Article abstract of DOI:10.1002/recl.19951141108

The enrichment of one of the enantiomers (P or M) of hexahelicene (3) synthesized by the photochemical cyclodehydrogenation of 2-styryl-benzophenanthrene (1) in chiral media has been (re)investigated.Four different chiral solvents, two cholesteric liquid crystals, two chiral crystals, all at several temperatures, and two chiral polymers were used as media.To avoid erroneous values from circular-dichroism (CD) measurements due to contributions of chiral side-products, the enantiomeric excess (ee) was determined by HPLC analysis.In general the observed ee is small ( < 7percent).In chiral solvents the ee increases with decreasing temperatures.In the liquid crystals the macroscopic helix of the cholesteric phase has a small but distinct effect on the ee, probably due to the preferred fitting of one enantiomer of Z-1 to the helix of the 'solvent'.In chiral crystals the ee is relatively larger.A peculiar effect is observed when 1 is irradiated in ethyl (S)-(+)-O-(4-phenylbenzoyl)lactate.Below -19 deg C the M enantiomer and above -14 deg C the P enantiomer of 3 is the preferred photoproduct.In the polymer triacetylcellulose, M-hexahelicene is formed in 4.5percent excess; in β-cyclodextrine only a very small amount of 3 is formed, due to the preferred photo-isomerization into E-1.