Stereochemistry-dependent thermotropic liquid crystalline phases of monosaccharide-based amphiphiles.

Conformational rigidity controls the bulk self-assembly and liquid crystallinity from amphiphilic block molecules to copolymers. The effects of block stereochemistry on the self-assembly have, however, been less explored. Here, we have investigated amphiphilic block molecules involving eight open-chain monosaccharide-based polyol units possessing different stereochemistries, derived from D-glucose, D-galactose, L-arabinose, D-mannose and L-rhamnose (allylated monosaccharides t-Glc*, e-Glc*, t-Gal*, e-Gal*, t-Ara*, e-Ara*, t-Man*, and t-Rha*), end-functionalized with repulsive tetradecyl alkyl chain blocks to form well-defined amphiphiles with block molecule structures. All compounds studied showed low temperature crystalline phases due to polyol crystallization, and smectic (lamellar) and isotropic phases upon heating in bulk. Hexagonal cylindrical phase was additionally observed for the composition involving t-Man*. Cubic phases were observed for e-Glc*, e-Gal*, e-Ara*, and t-Rha* derived compounds. Therein, the rich array of WAXS-reflections suggested that the crystalline polyol domains are not ultra-confined in spheres as in classic cubic phases but instead show network-like phase continuity, which is rare in bulk liquid crystals. Importantly, the transition temperatures of the self-assemblies were observed to depend strongly on the polyol stereochemistry. The findings underpin that the stereochemistry in carbohydrate-based assemblies involves complexity, which is an important parameter to be considered in material design when developing self-assemblies for different functions.

Phase-selective low molecular weight organogelators derived from allylated d-mannose.

In the last decades, synthesis and design of low molecular weight organogelators has gained increasing attention due to their versatile use in, for example, cosmetics, biomedicine and oil spill remediation. In this work, three potential gelators have been prepared from allylated d-mannose. Both the gelators and the corresponding gels formed were thoroughly characterized by crystallography, FTIR spectroscopy, SEM, rheometry and NMR spectroscopy, in solution and in solid state. The results showed that two of the compounds phase-selectively form gels with hydrocarbon solvents. The most promising gelator compound is alkene terminated, with the unsaturated end functionality not critical for gel formation, tentatively providing the possibility for customizing the gelation properties by further chemical modification. Alternatively, the alkene group could possibly be utilized as a linker for future coupling to carrier materials or surfaces to further increase the mechanical strength of the gel.

Improved synthesis and application of conjugation-amenable polyols from d-mannose.

A series of polyhydroxyl sulfides and triazoles was prepared by reacting allyl and propargyl d-mannose derivatives with selected thiols and azides in thiol-ene and Huisgen click reactions. Conformational analysis by NMR spectroscopy proved that the intrinsic rigidity and linear conformation of the mannose derived polyol backbone is retained in the final click products in solution. Single crystal X-ray structure determination of one of the compounds prepared further verified that the linear conformation of the polyol segment is also retained in the solid state. In addition, an improved method for direct Barbier-type propargylation of unprotected d-mannose is reported. The new reaction protocol, involving tin-mediated propargylation in an acetonitrile-water mixture, provides access to multigram quantities of the desired, valuable alkyne polyol without relying on protecting group manipulations or chromatographic purification.

Thermal, Spectroscopic, and Crystallographic Analysis of Mannose-Derived Linear Polyols.

The major diastereomer formed in the Barbier-type metal-mediated allylation of d-mannose has previously been shown to adopt a perfectly linear conformation, both in solid state and in solution, resulting in the formation of hydrogen-bonded networks and subsequent aggregation from aqueous solution upon stirring. Here, a comprehensive study of the solid state structure of both the allylated d-mannose and its racemic form has been conducted. The binary melting point diagram of the system was determined by differential scanning calorimetry analysis, and the obtained results, along with structure determination by single crystal X-ray diffraction, confirmed that allylated mannose forms a true racemate. Further examination by powder X-ray diffraction and CP MAS 13C NMR spectroscopy revealed polymorphism both in the pure enantiomer and in the racemate. In addition, the propargylated and hydrogenated analogues of allylated d-mannose were prepared and subjected to thermal and spectroscopic analyses. The crystal structure of the propargylated compound was successfully determined, showing a linear molecular conformation similar to that found for allylated d-mannose. Both new compounds likewise display aggregation behavior in water, further verifying that the low-energy linear conformation plays a significant role in this unusual behavior of these rodlike mannose derivatives.