Carbohydrate-Based Lactones: Synthesis and Applications

Organocatalytic Asymmetric Approach to γ,δ-Functionalization of 3-Cyano-4-styrylcoumarins via Bifunctional Catalysis

A novel organocatalytic reaction cascade between 3-cyano-4-styrylcoumarins and 2-mercaptoacetophenones is described. It is based on stereocontrolled functionalization of cyanocoumarins proceeding in a sequence of thia-Michael/aldol/annulation reactions. This highly diastereo- and enantioselective reaction is realized by employing enantioselective bifunctional catalysis and exhibits a broad substrate scope and excellent functional group tolerance. The synthetic application involves the transformation of the imidoester group, thus opening access to biologically relevant coumarin and δ-lactone-fused products.

Carbohydrate structure-activity relations of Au-catalysed base-free oxidations: gold displaying a platinum lustre

This paper describes the base-free gold-catalysed oxidation of four different carbohydrates in a packed bed plug flow reactor. The influence of the carbohydrate structure on the catalytic activity and selectivity was investigated by comparing two neutral sugars (glucose (Glc) and galactose (Gal), both with primary alcohols at C6), with their sugar-acid analogues (glucuronic acid (GlcA) and galacturonic acid (GalA), both with carboxylic acids at C6). The orientation of OH-groups at the C4-position (equatorial in Glc/GlcA and axial in Glc/GlcA), and the C6-functionality (primary alcohols in Gal/Glc and carboxylic acids in GalA/GlcA) has a profound influence on the catalytic activity. When the OH-groups are in an axial position their reactivity was higher compared to the OH-groups in the equatorial position for both the sugars and the sugar acids. In addition the reactivity of carbohydrates over Au-catalysts under base-free conditions is different compared to alkaline conditions, and is more in line with a Pt-catalysed dehydrogenation mechanism.

Gold mimicking Platinum in base-free catalytic carbohydrate oxidations.

Synthesis of glycoconjugates utilizing the regioselectivity of a lytic polysaccharide monooxygenase

Polysaccharides from plant biomass are the most abundant renewable chemicals on Earth and can potentially be converted to a wide variety of useful glycoconjugates. Potential applications of glycoconjugates include therapeutics and drug delivery, vaccine development and as fine chemicals. While anomeric hydroxyl groups of carbohydrates are amenable to a variety of useful chemical modifications, selective cross-coupling to non-reducing ends has remained challenging. Several lytic polysaccharide monooxygenases (LPMOs), powerful enzymes known for their application in cellulose degradation, specifically oxidize non-reducing ends, introducing carbonyl groups that can be utilized for chemical coupling. This study provides a simple and highly specific approach to produce oxime-based glycoconjugates from LPMO-functionalized oligosaccharides. The products are evaluated by HPLC, mass spectrometry and NMR. Furthermore, we demonstrate potential biodegradability of these glycoconjugates using selective enzymes.

Flavin Nitroalkane Oxidase Mimics Compatibility with NOx/TEMPO Catalysis: Aerobic Oxidization of Alcohols, Diols, and Ethers

Biomimetic flavin organocatalysts oxidize nitromethane to formaldehyde and NO_x-providing a relatively nontoxic, noncaustic, and inexpensive source for catalytic NO₂ for aerobic TEMPO oxidations of alcohols, diols, and ethers. Alcohols were oxidized to aldehydes or ketones, cyclic ethers to esters, and terminal diols to lactones. In situ trapping of NO_x and formaldehyde suggest an oxidative Nef process reminiscent of flavoprotein nitroalkane oxidase reactivity, which is achieved by relatively stable 1,10-bridged flavins. The metal-free flavin/NO_x/TEMPO catalytic cycles are uniquely compatible, especially compared to other Nef and NO_x-generating processes, and reveal selectivity over flavin-catalyzed sulfoxide formation. Aliphatic ethers were oxidized by this method, as demonstrated by the conversion of (-)-ambroxide to (+)-sclareolide.

Synergistic effect of Fusarium lateritium LP7 and Trichoderma viride LP5 promotes ethoxylated oleyl-cetyl alcohol biodegradation

This study evaluated the growth and metabolic activity of consortium and pure cultures Fusarium lateritium LP7 and Trichoderma viride LP5 in response to the presence of 0.5% ethoxylated oleyl-cetyl alcohol (EOCA) in the liquid Czapek-Dox medium. The effectiveness of mentioned cultures was monitored according to the following parameters: biomass dry weight (BDW), pH, quantity of free and total organic acids, proteolytic activity and the qualitative composition of carbohydrates, during 19 days. The biodegrading efficiency was determined spectrophotometrically. The BDW of consortium was significantly stimulated by EOCA (16.59%) whereas biomass of LP7 was significantly inhibited (30.61%). The EOCA had influence on decrease in pH value of the media of LP5 and consortium, and pH changes were correlated with the amount of excreted organic acids. The alkaline protease activities of consortium, LP7 and LP5 retained 73%, 62.2% and 49.5% activity respectively in the presence of EOCA. Consortium has shown the best biodegradation capacity up to 82% of EOCA. The pure cultures were less effective in biodegradation and removed approximately 65% (LP7) and 60% (LP5) of EOCA after 19 days. In brief, the synergistic interaction between pure cultures enhances capacity to reduce EOCA in environment and influences production of some biotechnological useful metabolites.

Solvent issues in the Baylis-Hillman reaction of 5-hydroxymethyl furfural (HMF) and 5-glucosyloxymethyl furfural (GMF). Towards no-solvent conditions

The possibility to apply solventless conditions for the Baylis-Hillman reaction of 5-hydroxymethyl furfural (HMF) and its glucosylated analog, glucosyloxymethyl furfural (GMF) has been investigated. This study shows that highly functionalized adducts can be obtained in fair to good yields, under the conditions combining the renewability of the substrates, the straightforwardness of the strategy, and the lowered cost and toxicity of the solvent conditions. The issue of the polarity of the furanic substrate is addressed by comparing HMF with furfural and GMF.

Multivalent Effect in Glycosidase Inhibition: The End of the Beginning

Glycosidases are ubiquitous enzymes involved in a diversity of key biological processes such as energy uptake or cell wall degradation. The design of specific glycosidase inhibitors has been therefore the subject of intense research efforts in academia and pharmaceutical industry. However, until recently, the study of the impact of multivalency on glycosidase inhibition was almost completely neglected. The following account will review our ten year journey on the design of multivalent glycomimetics within our research group, from the discovery of the first strong multivalent effect in glycosidase inhibition to the high-resolution crystal structures of Jack bean α-mannosidase in complex with the multimeric inhibitor displaying the largest binding enhancements reported so far.

An organocatalytic cis-selective approach to bicyclic δ-lactones

A new, cis-selective, approach for the synthesis of bicyclic δ-lactones bearing a fused cyclohex-2-en-1-one moiety is described. The strategy utilizes a cascade reactivity of cyclic 1,3-diketones and 3-arylidenefuran-2(3H)-ones with the butenolide-ring-opening reaction enabling the construction of the δ-lactone framework. It benefits from broad scope, high enantioselectivity and excellent cis-diastereoselectivity.

A glucose-targeted mixed micellar formulation outperforms Genexol in breast cancer cells

Breast cancer represents the top cancer among women, accounting 521.000 deaths per year. Development of targeted nanomedicines to breast cancer tissues represents a milestone to reduce chemotherapy side effects. Taking advantage of the over-expression of glucose (Glu) membrane transporters in breast cancer cells, we aim to expand the potential of a paclitaxel (PTX)-loaded mixed micellar formulation based on polyvinyl caprolactam-polyvinylacetate-polyethylene glycol graft copolymer (Soluplus®) and D-α-tocopheryl polyethylene glycol 1000 succinate (TPGS) by its surface decoration with Glu moieties. The glycopolymer (Soluplus(Glu)) was obtained by microwave-assisted ring opening reaction of δ-gluconolactone initiated by Soluplus®. The glycosylation was confirmed by ¹H NMR and by agglutination assays employing Concanavalin A. The hydrodynamic diameter of Soluplus(Glu) micelles was characterized by dynamic light scattering (100.3±3.8nm) as well as the critical micellar concentration value (0.0151% w/v). Then, a mixed micelle formulation employing Soluplus®, Soluplus(Glu) and TPGS (3:1:1wt ratio) loaded with PTX (4mg/mL) was developed as a multifunctional nanocarrier. Its in vitro anticancer performance in MCF-7 (1.6-fold) and MDA-MB-231 (14.1-fold) was significantly enhanced (p<0.05) versus the unique commercially available micellar-based PTX-nanoformulation (Genexol®). Furthermore, the in vitro PTX cellular uptake assays revealed that the drug intracellular/cell content was significantly (p<0.05) higher for the Glu-containing mixed micelles versus Genexol® after 6h of incubation with MCF-7 (30.5-fold) and MDA-MB-231 (5-fold). Overall, results confirmed the potential of our Glu-decorated mixed colloidal formulation as an intelligent nanocarrier for PTX-targeted breast cancer chemotherapy.

Reduction of sugar lactones to hemiacetals with lithium triethylborohydride

Reduction of ribono-1,4-lactones and gulono-1,4-lactone as well as ribono-1,5-lactone and glucono-1,5-lactones with LTBH (1.2 equiv.) in CH2Cl2 at 0 °C for 30 min provided the corresponding pentose or hexose hemiacetals in high yields. Commonly used in carbohydrate chemistry protecting groups such as trityl, benzyl, silyl, acetals and to some extent acyls are compatible with this reduction.

S-Ribosylhomocysteine Analogues Modified at the Ribosyl C-4 Position

4-C-Alkyl/aryl-S-ribosylhomocysteine (SRH) analogues were prepared by coupling of homocysteine with 4-substituted ribofuranose derivatives. The diastereoselective incorporation of the methyl substituent into the 4 position of the ribose ring was accomplished by addition of methylmagnesium bromide to the protected ribitol-4-ulose yielding the 4-C-methylribitol in 85% yield as single 4R diastereomer. The 4-C hexyl, octyl, vinyl, and aryl ribitols were prepared analogously. Chelation controlled addition of a carbanion to ketones from the (Si-face) was responsible for the observed stereochemical outcome. Oxidation of the primary alcohol of the 4-C ribitols with the catalytic amount of tetrapropylammonium perruthenate in the presence of N-methylmorpholine N-oxide produced 4-C-alkylribono-1,4-lactones in high yields. Mesylation of the latter compounds at the 5-hydroxyl position and treatment with a protected homocysteine thiolate afforded protected 4-C-alkyl/aryl-SRH analogues as the lactones. Reduction with lithium triethylborohydride and successive global deprotections with TFA afforded 4-C-alkyl/aryl SRH analogues. These analogues might impede the S-ribosylhomocysteinase(LuxS)-catalyzed reaction by preventing β-elimination of a homocysteine molecule, and thus depleting the production of quorum sensing signaling molecule AI-2.

Glucosyloxymethylfurfural (GMF): a creative renewable scaffold towards bioinspired architectures

Glucosyloxymethylfurfural (GMF) is the glucosylated analogue of hydroxymethylfurfural (HMF), and is obtained in one step from the very available disaccharide isomaltulose. This account gives an overview on the preparation and the uses of GMF towards architectures containing a carbohydrate moiety and shows that rather elaborated targets can be synthesized from GMF in very short sequences. A special focus is made on carbon–carbon formation on the aldehyde group leading to new biobased acrylic derivatives by the Baylis–Hillman reaction.

Gluconic acid from biomass fast pyrolysis oils: specialty chemicals from the thermochemical conversion of biomass

Fast pyrolysis of biomass to produce a bio-oil followed by catalytic upgrading is a widely studied approach for the potential production of fuels from biomass. Because of the complexity of the bio-oil, most upgrading strategies focus on removing oxygen from the entire mixture to produce fuels. Here we report a novel method for the production of the specialty chemical, gluconic acid, from the pyrolysis of biomass. Through a combination of sequential condensation of pyrolysis vapors and water extraction, a solution rich in levoglucosan is obtained that accounts for over 30% of the carbon in the bio-oil produced from red oak. A simple filtration step yields a stream of high-purity levoglucosan. This stream of levoglucosan is then hydrolyzed and partially oxidized to yield gluconic acid with high purity and selectivity. This combination of cost-effective pyrolysis coupled with simple separation and upgrading could enable a variety of new product markets for chemicals from biomass.

Dehydrogenative α-oxygenation of ethers with an iron catalyst

Selective α-oxidation of ethers under aerobic conditions is a long-pursued transformation; however, a green and efficient catalytic version of this reaction remains challenging. Herein, we report a new family of iron catalysts capable of promoting chemoselective α-oxidation of a range of ethers with excellent mass balance and high turnover numbers under 1 atm of O2 with no need for any additives. Unlike metalloenzymes and related biomimetics, the catalyst produces H2 as the only byproduct. Mechanistic investigations provide evidence for an unexpected two-step reaction pathway, which involves dehydrogenative incorporation of O2 into the ether to give a peroxobisether intermediate followed by cleavage of the peroxy bond to form two ester molecules, releasing stoichiometric H2 gas in each step. The operational simplicity and environmental friendliness of this methodology affords a useful alternative for performing oxidation, while the unique ability of the catalyst in oxygenating a substrate via dehydrogenation points to a new direction for understanding metalloenzymes and designing new biomimetic catalysts.

Photochemical conversion of the o-nitrobenzyl-C-glucoside to a sugar lactone

A new family of activated glycosidic compounds has been designed and synthesized: (2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl)-2-nitrophenylmethane (1). It is stable in conditions commonly used for synthesis, and it can be converted to a sugar lactone derivative merely by photoirradiation (λ=365 nm): 2,3,4,6-tetra-O-acetyl-D-glucono-1,5-lactone (2). A mechanism for the reaction is proposed. The photochemical conversion of 1 in the presence of methanol has also been demonstrated, giving rise to methyl 2,3,4,6-tetra-O-acetyl-D-gluconate (3).