Structure Dependence of Cholesteric Mesophases IV. 17β-Alkyl Substituted Androst-5-en-3β-ols

Abstract The mesomorphic properties as functions of the length of the 17β-alkyl side chain were investigated on selected alkanoates of androst-5-en-3β-ol, pregn-5-en-3β-ol, 20-methylpregn-5-en-3β-ol, 24-norchol-5-en-3β-ol, chol-5-en-3β-ol, 25-methylchol-5-en-3β-ol and 27-norcholest-5-en-3β-ol. The alkanoates studied were generally the acetates and the octadecanoates. We found that only androst-5-en-3β-ol derivatives are unable to form cholesteric mesophases. Monotropic cholesteric mesophases were found in 3β-octadecanoyloxypregn-5-ene and the next three homologues; their corresponding acetates were not mesomorphic. With increasing chain length, starting with 3β-acetoxy-24-methylchol-5-ene, enantiotropic cholesteric mesophases were obtained. Additional smectic mesophases could not be observed. Thus, we could establish that the 17β-alkyl chain of cholesterol can be shortened considerably without impairing the mesomorphic behavior, since a 17β-ethyl group leads already to a cholesteric mesophase. As intermed...