Flüssigkristalline Kohlenhydratdithioacetale / Liquid Crystalline Carbohydrate Dithioacetals

Abstract

Abstract S,S-dialkyl acetals of hexanal and numerous aldoses, tripodal in structure, have been synthesized, characterized and investigated by optical microscopy and D.S.C. Most of these geminal dithioacetals show a complicated melting behaviour and are thermotropic liquid crystal-line, exhibiting only one smectic-like phase of identical type. This mesophase is not miscible with the SAd -Phase of other amphiphilic sugar-based liquid crystals. As shown in a homologues series of D-glucose S,S-dialkylacetal derivatives clearing points are rising as function of growing number n of carbon atoms in their S-alkyl chains. With constant n given a similar trend is observed by varying the length of the polar, hydrophilic sugar units. Regarding stereochemistry the dithioderivatives 5 and 12 of the pentose or hexose series, respectively, with a configuration related to that of D-mannose exhibit relatively the most stable mesophase. However, the most stable mesophase of all investigated sugar dithioderivatives is exhibited by the heptose S,S-dialkylacetal 13. Surprisingly, derivative 10 of L-rhamnose (6-desoxy-L-mannose) - the terminal hydroxyl group in the sugar chain of mannose is missing! -showed to be non-mesomorphic, demonstrating that a terminal hydroxyl group of these sugar derivatives seems to be essential for exhibiting a mesophase. On the other hand, as shown with the dithioderivative 9 of 2-desoxy-D-glucose in comparison to the derivative 8e and 12 of D-glucose or D-mannose, respectively, their clearing points seem to be less, although differently effected if an internal hydroxyl group - for instance here at C-2 - has been stripped off. Melting behaviour as well as thermomesogenic properties of the presented amphiphilic carbo-hydrate liquid crystals in relationship to their molecular structure are discussed in detail.