Structure Dependence of Cholesteric Mesophases III. Minor Changes within the 17?-side Chain of Cholesterol

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Abstract
Several modifications were made within the 17β-side chain of cholesterol to determine the influence of minor structural changes on the mesomorphic behavior. Alkyl substitution at C-24 leads to smectic mesophases as experienced with campesteryl alkanoates. Moving the methyl group to C-25 leads to a loss of mesomorphic properties as found in 3β-acetoxy-25-methylene group, or by hydrogen, alters only the stability of the cholesteric mesophases as shown with 3β-acetoxy-27-norcholest-5-en-25-one, 3β-acetoxycholesta-5, 25-diene, and 3β-acetoxy-27-norcholest-5-ene. However, the same modifications at C-20 lead to drastic changes in the mesomorphic properties. 3β-Acetoxycholest-5, 20-diene is both cholesteric and smectic, but no mesophases were observed in 3β-acetoxy-21-norcholest-5-en-20-one and 3β-acetoxy-21-norcholest-5-ene. 3β-Acetoxy-20-hydroxycholest-5-ene, 3β-octadecanoyloxy-20-azacholest-5-ene, and 3β-octadecanoyloxy-20, 25-diazacholest-5-ene, were also not mesomorphic.