No abstract

Terminally unsaturated and diastereochemically pure polyol derived from D-mannose shows spontaneous aggregation behavior in water solution. In order to study and clarify this unforeseen phenomenon, a conformational study based on NMR spectroscopy combined with ab initio structure analysis using the COSMO-solvation model was pursued. The results, together with X-2 ray diffraction studies, suggest a low energy linear conformation for this particular substrate both in solid states and in solution. For such small-sized acyclic carbohydrate derivatives, the linear conformation appears to be a key prerequisite for the unusual molecular self-assembly reported herein.

Heterogeneously catalyzed one‐pot reactions combining individual reaction steps into sequences enhance the efficiencies of overall chemical processes by reducing costs, time, and labor efforts. This minireview focuses on the production of chiral compounds by using dynamic kinetic resolution (DKR) and related sequential reactions, utilizing heterogeneous and immobilized metal catalysts and enzymes. Enzymes, especially lipases, often possess high chemical, stereo‐, and regiospecificities under mild reaction conditions. Supported transition‐metal catalysts are effective and widely used for various chemical transformations including racemization and hydrogenation reactions. This minireview discusses both the early and important applications, as well as modern state‐of‐the‐art processes, in which different catalyst combinations are utilized. Also catalyst compatibility issues, including catalyst inhibition and applicability are addressed with specific focus on the racemization catalysts applied in DKRs. Finally, some examples on the utilization of heterogeneous catalysts in flow reactors are presented.

The recently reported electro-catalytic reaction mechanism for the oxidation of cellulose is proposed to work also for the electro-oxidation of other polysaccharides (e.g. hemicelluloses). In this report, the electrochemical reactivity of some hemicelluloses (xylan, arabinoxylan, glucomannan, xyloglucan, and glucuronoxylan) in 1.3 M NaOH solution is described. The electrochemical property of each of the studied hemicelluloses at the Au electrode surface was characterized by cyclic voltammetry. Xylan, xyloglucan and glucuronoxylan were found to be electrochemically active. Electrochemical properties of xylan were further studied by electrochemical impedance spectroscopy. The electrochemical reaction products of xylan, obtained by extensive electrolysis of the xylan solution by cyclic voltammetry, were characterized by FTIR spectroscopy, (13)C-NMR spectroscopy, SEM and TEM. The structural studies suggest that the oxidized xylan is a functional material where some of the OH-groups have been oxidized to carboxyl groups making that part of the oxidation product soluble in water under ambient conditions (23 °C, pH 7). The other part remains insoluble in water and contains Au nanoparticles. This work indicates that other polysaccharides than cellulose can also be electrochemically oxidized.

β‐(1→2)‐Linked oligomannosides constitute an important class of carbohydrate structures located on the cell surface of several Candida species, including C. albicans. As a result of the immunostimulating properties of such compounds, the upscaling of their synthesis is relevant. In this paper, a highly stereoselective synthesis of β‐(1→2)‐linked oligomannosides was performed by further development of and modifications to the methodologies described earlier in the literature. In addition to the synthesis of fully deprotected β‐(1→2)‐linked mannobiose and mannotriose, some preliminary modifications to the oligosaccharide core, resulting in close analogues with biological potential, are presented. The fully deprotected products form potential targets for screening against C. albicans and may also result in new model structures for vaccine development.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Development of reaction sequences that utilize renewable resources, such as carbohydrates, as starting material is of topical interest. Particularly attractive are reaction protocols where unprotected mono-and oligosaccharides are applied. Herein, selected approaches for direct derivatization of unprotected carbohydrates are highlighted. Ultimately, such methodologies are also applicable for natural polysaccharides resulting in rapid development of novel functional biobased materials for future applications. 1 Introduction 2 Reacting Unprotected Carbohydrates as Aldehydes 3 Approaches for Selective Reactions Targeting the Nonanomeric Hydroxyl Groups on Unprotected Carbohydrates 4 Protecting-Group-Free Synthesis of Glycoclusters and Glycoconjugates 5 Conclusions

Kinetic resolution and dynamic kinetic resolution, in particular, provide practical tools for synthesis of enantiomerically pure compounds. In the present work, the resolution of rac‐2‐hydroxy‐1‐indanone and racemization of (S)‐2‐hydroxy‐1‐indanone were investigated. Immobilized lipase AK (Pseudomonas fluorescens) was found as the best enzyme catalyst for kinetic resolution and ruthenium supported on Al2O3 as a potential heterogeneous catalyst for racemization of the starting material. By combining the two reactions, a simple dynamic kinetic resolution of rac‐2‐hydroxy‐1‐indanone producing (R)‐1‐oxo‐indan‐2‐yl butanoate was developed. Under optimal conditions, the product was obtained in 92 % ee at approximately 90 % conversion.

No abstract