Synthesis and mesomorphism of 6-Z-n-alkyl-alpha-D-galactopyranoses

Three homologous series of 6-Z-n-alkyl-alpha-D-galactopyranoses, where link ing 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 ef fect on the thermotropic liquid crystal transition temperatures studied. An enantiotropic smectic A* phase was found for each of the homologues prepar ed. The order of efficiency of the linking group Z in favouring liquid crys tal formation for the same homologues of the 6-O-n-acyl-alpha-D-galactopyra noses (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 approximate to OCO > O > OC3H6S. Th is correlates well with the order of polarisability of Z for the first of t he linking groups (Z = S, OCO, O). The low clearing point of the 6-O-(propy lene-[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 para llels with similar behaviour found for non-amphiphilic liquid crystals and is not well understood.