Study for Highly Stereocontrolled Reactions by Liquid Crystalline Mesophases

点击翻译按钮获取中文摘要

Photodimerizations and thermal cycloadditions in thermotropic liquid crystalline media were explored in order to show the utility of liquid crystals in organic reaction and chemical significance of the liquid crystalline phases was clarified.1. A well documented photodimerization of uracils seemed to be a suitable reaction for the present study, and provided information regarding highly preferred orientations of solutes in liquid crystalline media. Thus methyluracils were irradiated with a high pressure HG-lamp in isotropic and anisotropic media including frozen solutions. The photodimer distribution and conversion yields were determined as a function of temperature and solvent phase. Smectic liquid crystalline medium significantly enhanced the stereoselectivity and the rate in photodimerization, in strong contrast to the isotropic phase reaction which gave poor dimer-selectivity and yield.2. Alkyl and aryl cholesteryl fumarates were prepared to serve as a liquid crystalline dienophile in Diels-Alder reaction. Some of them showed cholesteric liquid crystial phases in temperature ranges suitable to the cycloaddition. The reaction of n-hexyl cholesteryl cholesteryl fumarate in mesomorphic phase as a dienophile with cyclopentadiene gave the optically active cycloadducts in 10% enantiomeric excess. On the other hand, the reaction of methyl cholesteryl fumarate with 2,6-dimethoxyanthracene gave 2,6-dimethoxy-9,10-dihydro-9,10-ethano-anthracene-11(syn),12(syn)-dicarboxylic acid and the 11(anti),12(anti)-isomer in a ratio of 96 : 4 (92% d.e.). These results clearly showed that a cholesteric mesophase could stereochemically control the diels-Alder reaction to a considerable extend.