Remarks on the Synthesis of Pure Mesomorphic Compounds

Abstract

The effect of impurities in sterol derivatives upon the mesomorphic behavior cannot be estimated or extrapolated from measured data. Therefore, it is imperative to use pure materials for meaningful measurements. All possible sources for impurities in mesomorphic sterol derivatives are assessed, and methods feasible for the detection of contaminents at levels of 1% or less are described. Thin layer chromatography (TLC) is the common method for purity determination of mesomorphic sterol derivatives. Most impurities such as unreacted starting materials and side products can be detected by this method. This technique also allows comparison of identical materials which were synthesized by independent and different reaction paths, and is demonstrated for the homologous series of 5α-cholestanyl n-alkyl carbonates. However, TLC is not sensitive enough to detect small amounts of impurities, which are very similar in structure and behavior to that of the mesomorphic compound. Therefore, a more sensitive analytical method for the detection of impurities in sterol derivatives is needed. Gas chromatography is an analytical approach which offers a big increase in sensitivity over TLC. Unfortunately, mesomorphic sterol derivatives are not suited for direct gas chromatographic analysis. Nevertheless, this method is essential for the quality control of starting materials. Furthermore, the detection of homologues in mesomorphic compounds is possible by suitable modification of gas chromatographic procedures. For example, a transesterification was adapted for esters of sterols and is demonstrated in detail for cholesteryl hexadecanoate. Other methods for the analysis of carbonates and thiocarbonates of sterols are proposed.