Role of hydroxyl group on the mesomorphism of alkyl glycosides: synthesis and thermal behavior of alkyl 6-deoxy-β-D-glucopyranosides

A homologous series of alkyl 6-deoxy-β-D-glucopyranoside amphiphiles was prepared, in an effort to identify the role of hydroxyl group in the mesomorphic behavior of alkyl glycosides. Synthesis was performed by a chlorination of the sugar moiety in alkyl β-D-glucopyranosides with methylsulfonyl chloride in DMF, followed by a metal mediated dehalogenation to secure alkyl 6-deoxy-β-D-glucopyranosides, wherein the alkyl chain length varied from C9 to C16. The mesomorphic behavior of these 6-deoxy alkyl glycosides was assessed using polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction method. Whereas the lower homologues exhibited a monotropic SmA phase till sub-ambient temperatures, the higher homologues formed a plastic phase. A partial interdigitized bilayer structure of SmA phase is inferred from experimental d-spacing and computationally derived lengths of the molecules. The results were compared with those of normal alkyl glucopyranosides, retained with hydroxyl groups at C-2-C-6 carbons, and alkyl 2-deoxy-glucopyranosides, devoid of a hydroxyl group at C-2 and the comparison showed important differences in the mesomorphic behavior