Carbohydrates as enantioinduction components in stereoselective catalysis

Iminosugar-Phosphines as Organocatalysts in the [3 + 2] Cycloaddition of Allenoates and N-Tosylimines

Iminosugar derivatives containing a pyrrolidine-phosphine moiety were prepared from carbohydrates and used as catalysts in the [3 + 2] cycloaddition reaction between alkyl allenoates and electron-deficient imines. The corresponding 1,2,3,5-tetrasubstituted pyrrolines were obtained in good yields and diastereoselectivities but with moderate enantiocontrol. The stereochemical outcome of the reaction depends on the substituent at the nitrogen atom and hydroxyl groups, the configuration of the stereogenic centers and the distance between the diphenylphosphine group and the pyrrolidine skeleton of the catalyst. The preparation of both enantiomers of the catalyst allowed the corresponding enantiomeric pyrrolines to be obtained with similar yields, diastereo- and enantioselectivities.

Galactopyranoside-Substituted N-Heterocyclic Carbene Gold(I) Complexes: Synthesis, Stability, and Catalytic Applications to Alkyne Hydration

A series of novel gold(I) complexes bearing galactopyranoside-based N-heterocyclic carbene ligands have been synthesized via transmetalation of the corresponding Ag(I) complex. Gold(I) complexes have been characterized by NMR, Fourier transform infrared (FTIR), and elemental analysis. An exhaustive NMR analysis shows that the complexes are not stable when hydroxyl groups are deprotected. Catalytic studies, using the alkyne hydration as a model reaction, indicate that the synthesized complexes are active and reusable.

Carbohydrate‐functionalized triazolylidene iridium complexes: hydrogenation catalysis in water with asymmetric induction

Two sets of carbohydrate‐NHC hybrid iridium complexes were synthesised in order to combine properties of carbohydrates and triazolylidene (trz) ligands in organometallic catalysis. One set features a direct trz linkage to the anomeric carbohydrate carbon, while the second set is comprised of an ethyl linker between the two functional units. Deprotection of the carbohydrate afforded hybrid complexes that efficiently catalyse the direct hydrogenation of ketones in water. The catalytic activity of the hybrid complexes was influenced by the pH of the aqueous medium and surpassed the activity of carbohydrate‐free or acetyl‐protected analogues (>90 % vs 13 % yield). While no enantiomeric induction was observed for the ethyl‐linked hybrids, a moderate enantiomeric excess (ee) was induced by the directly linked systems. Moreover, these carbohydrate‐trz hybrid complexes displayed mixed inhibitory activity towards a glycosidase from H. orenii that contain a glucose binding site.

Simultaneously colorimetric detection and effective removal of mercury ion based on facile preparation of novel and green enzyme mimic

In this work, an environmentally-friendly and cost-effective enzyme mimic was obtained by facile one-pot preparation of chitosan/Cu/Fe (CS/Cu/Fe) composite. This composite exhibited significantly enhanced oxidase-mimicking activity during catalyzing the oxidation of 3, 3', 5, 5'-tetramethylbenzidine (TMB). The CS/Cu/Fe composite was comprehensively characterized and the possible catalytic mechanism was reasonably explored and discussed. Benefiting from the thermal stability and the compatibility with carbohydrate, the CS/Cu/Fe composite was further integrated with agarose hydrogel to fabricate a portable analytical tube containing oxidase mimic. Based on the inhibition of the catalytic oxidation of TMB in the presence of cysteine, as well as the recovery of oxidase-like activity of CS/Cu/Fe due to the specific complexation of cysteine and mercury ion (Hg²⁺), the rapid colorimetric detection of Hg²⁺ was successfully carried out in the analytical tube. This colorimetric method showed good linear response to Hg²⁺ over the range from 40 nM to 8.0 μM with a detection limit of 8.9 nM. The method also revealed high selectivity and satisfactory results in recovery experiments of Hg²⁺ detection in tap water and lake water. Furthermore, it was found that the effective removal of Hg²⁺ could be realized in the analytical tube based on efficient Hg²⁺ adsorption by CS/Cu/Fe composite and agarose hydrogel. This study not only prepared a robust and low-cost enzyme mimic, but also proposed a smart strategy to simultaneously monitor and remove toxic Hg²⁺ from contaminated water.

Synthesis and Applications of Carbohydrate-Based Organocatalysts

Organocatalysis is a very useful tool for the asymmetric synthesis of biologically or pharmacologically active compounds because it avoids the use of noxious metals, which are difficult to eliminate from the target products. Moreover, in many cases, the organocatalysed reactions can be performed in benign solvents and do not require anhydrous conditions. It is well-known that most of the above-mentioned reactions are promoted by a simple aminoacid, l-proline, or, to a lesser extent, by the more complex cinchona alkaloids. However, during the past three decades, other enantiopure natural compounds, the carbohydrates, have been employed as organocatalysts. In the present exhaustive review, the detailed preparation of all the sugar-based organocatalysts as well as their catalytic properties are described.

Pseudo-enantiomeric carbohydrate-based N-heterocyclic carbenes as promising chiral ligands for enantiotopic discrimination

The practical synthesis of carbohydrate-based NHC-Rh complexes bearing C1 or C3 sterically differentiated positions, accessed by glycosylation or SNAr strategies, is reported. These catalysts exhibit pseudo-enantiomeric behaviour in the hydrosilylation of acetophenone. We show that steric bulk at C1 gives preference for (S)-phenyl-1-ethanol, while bulk at C3 leads to the (R)-enantiomer. These results represent the first example of pseudo-enantiomeric carbohydrate-based NHC ligands leading to enantiotopic discrimination.

C-Linked Glycomimetic Libraries Accessed by the Passerini Reaction

Carbohydrates and their conjugates are the most abundant natural products, with diverse and highly important biological roles. Synthetic glycoconjugates are versatile tools used to probe biological systems and interfere with them. In an endeavor to provide an efficient route to glycomimetics comprising structurally diverse carbohydrate units, we describe herein a robust, stereoselective, multicomponent approach. Isopropylidene-protected carbohydrate-derived aldehydes and ketones were utilized in the Passerini reaction, giving different glycosylated structures in high yields and diastereoselectivities up to 90:10 diastereomeric ratio (d.r). Access to highly valuable building blocks based on α-hydroxy C-glycosyl acids or more complex systems was elaborated by simple post-condensation methodologies.

Carbohydrate-functionalized N-heterocyclic carbene Ru(ii) complexes: synthesis, characterization and catalytic transfer hydrogenation activity

Triazolylidene NHCs decorated with a carbohydrate wingtip group were complexed to a ruthenium(ii) center. Deprotection of the carbohydrate in the metal complex affords a carbohydrate–NHC hybrid system for use as a transfer hydrogenation catalyst.

Discrimination of chiral copper(ii) complexes upon binding of galactonoamidine ligands

Chiral binuclear Cu(ii) complexes are differentiated upon binding top-methylbenzyl-d-galactonoamidine. A sequential binding model is elaborated reflecting the altered coordination sites.