Synthesis and Cytotoxicity of Monomethylated Betulinic Acid 3-O-α-l-Rhamnopyranosides

Three original derivatives of the cytotoxic betulinic acid 3-O-α-l-rhamnopyranoside featuring a monomethylated rhamnoside residue were synthesized. An improved catalytic procedure was involved to functionalize the O-3 position of the monosaccharide in a site-selective fashion. The cytotoxicity of the novel compounds was evaluated in vitro to highlight the moderate impact of carbohydrate monomethylation on the biological activity of betulinic acid 3-O-α-l-rhamnopyranoside.

Catalytic Approaches to Chemo- and Site-Selective Transformation of Carbohydrates

Carbohydrates are a fundamental unit playing pivotal roles in all the biological processes. It is thus essential to develop methods for synthesizing, functionalizing, and manipulating carbohydrates for further understanding of their functions and the creation of sugar-based functional materials. It is, however, not trivial to develop such methods, since carbohydrates are densely decorated with polar and similarly reactive hydroxy groups in a stereodefined manner. New approaches to chemo- and site-selective transformations of carbohydrates are, therefore, of great significance for revolutionizing sugar chemistry to enable easier access to sugars of interest. This review begins with a brief overview of the innate reactivity of hydroxy groups of carbohydrates. It is followed by discussions about catalytic approaches to enhance, override, or be orthogonal to the innate reactivity for the transformation of carbohydrates. This review avoids making a list of chemo- and site-selective reactions, but rather focuses on summarizing the concept behind each reported transformation. The literature references were sorted into sections based on the underlying ideas of the catalytic approaches, which we hope will help readers have a better sense of the current state of chemistry and develop innovative ideas for the field.

Towards better syntheses of partially methylated carbohydrates?

We give an overview of the reported synthetic strategies towards partially methylated glycosides and discuss how better protocols could stem from catalytic site-selective transformations of carbohydrates and cleaner methylation reagents.

Synthesis and conformational analysis of vicinally branched trisaccharide β-d-Galf-(1 → 2)-[β-d-Galf-(1 → 3)-]-α-Galp from Cryptococcus neoformans galactoxylomannan

The synthesis of a vicinally branched trisaccharide composed of two d-galactofuranoside residues attached viaβ-(1 → 2)- and β-(1 → 3)-linkages to the α-d-galactopyranoside unit has been performed for the first time. The reported trisaccharide represents the galactoxylomannan moiety first described in 2017, which is the capsular polysaccharide of the opportunistic fungal pathogen Cryptococcus neoformans responsible for life-threatening infections in immunocompromised patients. The NMR-data reported here for the synthetic model trisaccharide are in good agreement with the previously assessed structure of galactoxylomannan and are useful for structural analysis of related polysaccharides. The target trisaccharide as well as the constituent disaccharides were analyzed by a combination of computational and NMR methods to demonstrate good convergence of the theoretical and experimental results. The results suggest that the furanoside ring conformation may strongly depend on the aglycon structure. The reported conformational tendencies are important for further analysis of carbohydrate-protein interaction, which is critical for the host response toward C. neoformans infection.

On the Search of a Silver Bullet for the Preparation of Bioinspired Molecular Electrets with Propensity to Transfer Holes at High Potentials

Biological structure-function relationships offer incomparable paradigms for charge-transfer (CT) science and its implementation in solar-energy engineering, organic electronics, and photonics. Electrets are systems with co-directionally oriented electric dopes with immense importance for CT science, and bioinspired molecular electrets are polyamides of anthranilic-acid derivatives with designs originating from natural biomolecular motifs. This publication focuses on the synthesis of molecular electrets with ether substituents. As important as ether electret residues are for transferring holes under relatively high potentials, the synthesis of their precursors presents formidable challenges. Each residue in the molecular electrets is introduced as its 2-nitrobenzoic acid (NBA) derivative. Hence, robust and scalable synthesis of ether derivatives of NBA is essential for making such hole-transfer molecular electrets. Purdie-Irvine alkylation, using silver oxide, produces with 90% yield the esters of the NBA building block for iso-butyl ether electrets. It warrants additional ester hydrolysis for obtaining the desired NBA precursor. Conversely, Williamson etherification selectively produces the same free-acid ether derivative in one-pot reaction, but a 40% yield. The high yields of Purdie-Irvine alkylation and the selectivity of the Williamson etherification provide important guidelines for synthesizing building blocks for bioinspired molecular electrets and a wide range of other complex ether conjugates.

Regioselective Sulfonylation/Acylation of Carbohydrates Catalyzed by FeCl3 Combined with Benzoyltrifluoroacetone and Its Mechanism Study

A catalytic amount of FeCl₃ combined with benzoyl trifluoroacetone (Hbtfa) (FeCl₃/Hbtfa = 1/2) was used to catalyze sulfonylation/acylation of diols and polyols using diisopropylethylamine (DIPEA) or potassium carbonate (K₂CO₃) as a base. The catalytic system exhibited high catalytic activity, leading to excellent isolated yields of sulfonylation/acylation products with high regioselectivities. Mechanism studies indicated that FeCl₃ initially formed [Fe(btfa)₃] (btfa = benzoyl trifluoroacetonate) with twice the amount of Hbtfa under basic conditions in the solvent acetonitrile at room temperature. Then, Fe(btfa)₃ and two hydroxyl groups of the substrates formed a five- or six-membered ring intermediate in the presence of the base. The subsequent reaction between the cyclic intermediate and a sulfonylation reagent led to the selective sulfonylation of the substrate. All key intermediates were captured in the high-resolution mass spectrometry assay, therefore demonstrating this mechanism for the first time.

S-Adamantyl Group Directed Site-Selective Acylation: Applications in Streamlined Assembly of Oligosaccharides

The site-selective functionalization of carbohydrates is an active area of research. Reported here is the surprising observation that the sterically encumbered adamantyl group directed site-selective acylation at the C2 position of S-glycosides through dispersion interactions between the adamantyl C-H bonds and the π system of the cationic acylated catalyst, which may have broad implications in many other chemical reactions. Because of their stability, chemical orthogonality, and ease of activation for glycosylation, the site-selective acylation of S-glycosides streamlines oligosaccharide synthesis and will have wide applications in complex carbohydrate synthesis.

Regio/Site-Selective Benzoylation of Carbohydrates by Catalytic Amounts of FeCl3

This work uncovered the regio/site-selective benzoylation of 1,2- and 1,3-diols and glycosides containing a cis-vicinal diol using a catalytic amount of FeCl3 with the assistance of acetylacetone. FeCl3 may initially form [Fe(acac)3] (acac = acetylacetonate) with excess acetylacetone in the presence of diisopropylethylamine (DIPEA) in acetonitrile at room temperature. Then, benzoylation was catalyzed by Fe(acac)3 with added benzoyl chloride in the presence of DIPEA under mild conditions as reported. This reaction produced selectivities and isolated yields similar to or slightly lower than the reaction using Fe(acac)3 as a catalyst in most cases. The result provides not only the green and convenient selective benzoylation method associated with the most inexpensive catalysts but also the possibility that the effects of various metal salts and ligands on the regioselective protection can be extensively investigated in future study to obtain the optimized catalytic system.

Synthesis of protected glucose derivatives from levoglucosan by development of common carbohydrate protecting group reactions under continuous flow conditions

Common carbohydrate protecting group reactions under continuous flow processes are reported in the context of producing partially-protected glucose building blocks from levoglucosan. Benzyl ether protection was demonstrated without the use of NaH using barium oxide, which, however, pointed to the need for forms of this catalyst not as susceptible to close packing under flow. Acylation conditions were developed under continuous flow in acetonitrile and avoiding pyridine. Ring-opening the derivatized levoglucosan with propanethiol was also demonstrated producing S-alkyl 2,4-di-O-benzyl-glucopyranoside building block in 2 rather than 12 steps in increased overall yield.

Diisopropylethylamine-triggered, highly efficient, self-catalyzed regioselective acylation of carbohydrates and diols

A diisopropylethylamine (DIPEA)-triggered, self-catalyzed, regioselective acylation of carbohydrates and diols is presented. The hydroxyl groups can be acylated by the corresponding anhydride in MeCN in the presence of a catalytic amount of DIPEA. This method is comparatively green and mild as it uses less organic base compared with other selective acylation methods. Mechanistic studies indicate that DIPEA reacts with the anhydride to form a carboxylate ion, and then the carboxylate ion could catalyze the selective acylation through a dual H-bonding interaction.

Regioselective alkylation of carbohydrates and diols: a cheaper iron catalyst, new applications and mechanism

We developed a much cheaper iron catalyst, Fe(dipm)₃, which has a good catalytic efficiency in regioselective alkylation of carbohydrates.