1,3,5,7-Tetraoxadecalin has been introduced as a novel central unit into the chemistry of calamitic liquid crystals. 2e,6e-Disubstituted derivatives 3a−h and 4b, g, i of the trans or cis fused heterobicyclus, respectively, have been synthesized. Whereas most of these tram-products (3c−h) are nematic or smectic A (new “rod-like mesogens”), the cis-products (4b, g, i) are not liquid crystalline. The configurations and conformations of all compounds have been elucidated by spectroscopic methods in particular by 1H and 13C NMR. In addition the structures of 3d, 3g and 4i as representatives of both series have been confirmed by X-ray analysis. The 1,3,5,7- tetraoxadecalin unit in the non-mesomorphic ds-products exists, as expected, in the “O-inside" conformation which has been established both by 1H NMR and X-ray studies. The relationship between the chemical structure and mesomorphic behaviour of these 1,3,5,7-tetraoxadecalin derivatives is discussed, as well as the most likely structural requirements for turning the non- mesomorphic cis-products into mesomorphic ones. The name “roof-like mesogens" is proposed for this predicted new type of liquid crystals with three-dimensional geometry.
Chemical Structure: Liquid Crystals