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Abstract
Three homologous series of 6-Z-n-alkyl-alpha-D-galactopyranoses, where linking group Z represents either a carboxy group (OCO), a sulphur atom (S) or a propylthio group (C3H6S), have been synthesized starting from 1,2:3,4-di-O-isopropylidene-alpha-D-galactopyranose in two or three steps. The length of the terminal aliphatic chains has been varied systematically and the effect on the thermotropic liquid crystal transition temperatures studied. An enantiotropic smectic A* phase was found for each of the homologues prepared. The order of efficiency of the linking group Z in favouring liquid crystal formation for the same homologues of the 6-O-n-acyl-alpha-D-galactopyranoses (Z=OCO), 6-S-n-alkyl-6-thio-alpha-D-galactopyranoses (Z=S), 6-O-n-alkyl-alpha-D-galactopyranoses (Z=O) and 6-O-(propylene-[3'-S-n-alkyl])-alpha-D-galactopyranoses (OC3H6S), is S OCO>O>OC3H6S. This correlates well with the order of polarisability of Z for the first of the linking groups (Z=S, OCO, O). The low clearing point of the 6-O-(propylene-[3'-S-n-alkyl])-alpha-D-galactopyranoses may be due to the presence of a non-conjugated heteroatom in the terminal aliphatic chain. This has parallels with similar behaviour found for non-amphiphilic liquid crystals and is not well understoood.