Scalable total synthesis of natural vanillin-derived glucoside ω-esters

The first total synthesis of vanilloloside, calleryanin, and a series of naturally occurring ω-esters of vanilloloside was realized through direct glycosylation of vanillin-based aglycones or late-stage derivatization of vanilloloside. All aglycones and their fragments were synthesized from vanillin as the sole aromatic precursor. Subsequently, these intermediates were used to construct various vanillin-derived glucoside ω-esters using a mild acidic deacetylation as the key synthetic step, providing the final products in the total yields of 10-50% and general purity of >95%. Additionally, the first operationally simple and sustainable synthesis of litseafoloside B was realized on large scale, avoiding the use of toxic solvents and reagents, providing an attractive alternative to isolation of this and other similar compounds from plant sources.

Design, Synthesis, and Anticancer Activity of a Selenium-Containing Galectin-3 and Galectin-9N Inhibitor

Galectins are soluble β-D-galactoside-binding proteins whose implication in cancer progression and disease outcome makes them prominent targets for therapeutic intervention. In this frame, the development of small inhibitors that block selectively the activity of galectins represents an important strategy for cancer therapy which is, however, still relatively underdeveloped. To this end, we designed here a rationally and efficiently novel diglycosylated compound, characterized by a selenoglycoside bond and the presence of a lipophilic benzyl group at both saccharide residues. The relatively high binding affinity of the new compound to the carbohydrate recognition domain of two galectins, galectin 3 and galectin 9, its good antiproliferative and anti-migration activity towards melanoma cells, as well as its anti-angiogenesis properties, pave the way for its further development as an anticancer agent.

First Total Synthesis of Gaylussacin and Its Stilbene Derivatives

Gaylussacin (1), a stilbene glucoside, has been isolated from Pentarhizidium orientale and is used in Korean folk medicine. Although it was first isolated in 1972, the synthesis of gaylussacin has never been reported. Herein, we report the first total synthesis of gaylussacin in six steps with an overall yield of 23.8%, as well as the synthesis of its derivatives. Structurally, gaylussacin contains a carboxylic acid and a glycoside along with a free phenol on the same benzene ring, making selective functionalization for the synthesis of 1 difficult. Heck cross-coupling was employed as a key step to introduce the stilbene moiety. Glycosylation followed by global deprotection provided natural product 1.

Synthesis and characterization of a small library of bisglucosides: Influence of the nature of the diol/diphenol used in O-glucosylation

In this paper, the synthesis of different bisglucosides is investigated through the reaction of two acetylated glucose units with a diol (or diphenol) in order to develop a versatile molecular platform for the future development of bio-based polymers. A panel of five diols and one diphenol is initially used in order to examine the influence of their chemical skeleton on the reaction yield and both nature and proportion of formed species. Reaction products are identified using ¹H and ¹³C NMR spectroscopies completed by MALDI-TOF MS technique. The nucleophilicity of these dihydroxy compounds is identified as being the main factor that governs the reaction characteristics. In particular, the best selectivity is obtained with the use of hydroquinone. Inversely, by-products (oligomers, deacetylated compounds) are observed with the diols defined by higher nucleophilicity despite the choice of stereoselective pathway using acyl protecting groups.

Divergent Synthesis of Natural Benzyl Salicylate and Benzyl Gentisate Glucosides

Herein is reported the first total synthesis of benzyl salicylate and benzyl gentisate glucosides present in various plant species, in particular the Salix genus, such as Populus balsamifera and P. trichocarpa. The method permits the synthesis of several natural phenolic acid derivatives and their glucosides starting from salicylic or gentisic acid. The divergent approach afforded access to three different acetylated glucosides from a common synthetic intermediate. The key step in the total synthesis of naturally occurring glycosides-the selective deacetylation of the sugar moiety-was achieved in the presence of a labile benzyl ester group by employing mild deacetylation conditions. The protocol permitted synthesis of trichocarpine (4 steps, 40% overall yield), isotrichocarpine (3 steps, 51% overall yield), trichoside (6 steps, 40% overall yield), and deoxytrichocarpine (3 steps, 42% overall yield) for the first time (>95% purity). Also, the optimized mild deacetylation conditions allowed synthesis of 2-O-acetylated derivatives of all four glycosides (5-17% overall yield, 90-95% purity), which are rare plant metabolites.

Investigation of the H-bond-mediated aglycone delivery reaction in application to the synthesis of β-glucosides

In an attempt to refine the H-bond-mediated Aglycone Delivery (HAD) glycosylation reaction reported herein is the synthesis of β-glucosides using an ethylthio glucoside donor equipped with the remote 6-O-picoloyl substituent. Upon examining various aliphatic, aromatic, and carbohydrate acceptors, it was determined that both electronic and steric factors may greatly affect the stereoselectivity of the HAD reaction with this donor.

Application of a Janus aglycon with dual function in benzyl-free synthesis of spacer-armed oligosaccharide fragments of polysaccharides from rhizobacterium Azospirillum brasilense sp7

Both protective and pre-spacer features of 4-(2-chloroethoxy)phenyl (CEP) aglycon, which belong to the class of Janus aglycons, were engaged in a benzyl-free synthesis of oligosaccharide fragments of polysaccharides from rhizobacterium Azospirillum brasilense sp7. Introduction of α-1,4-linked L-fucose residue was performed using 3,4-di-O-benzoyl-2-O-triisopropylsilyl-α-L-fucopyranosyl N-phenyltrifluoroacetimidate in excellent stereoselectivity and high yields. The obtained deprotected di-, tri- and tetrasaccharides contain 4-(2-azidoethoxy)phenyl (AEP) spacer aglycon, which allows straightforward preparation of neoglycoconjugates that will be used for the study of the role of lipopolysaccharide of rhizobacterium A. brasilense sp7 in plant-microbe symbiosis. The intermediate protected oligosaccharide building blocks with cleavable CEP/AEP aglycons have a strong potential for further application in the synthesis of more complex oligosaccharides.

An Improved Helferich Method for the α/β-Stereoselective Synthesis of 4-Methylumbelliferyl Glycosides for the Detection of Microorganisms

An improved Helferich method is presented. It involves the glycosylation of 4-methyl-umbelliferone with glycosyl acetates in the presence of boron trifluoride etherate combined with triethylamine, pyridine, or 4-dimethylaminopyridine under mild conditions, followed by deprotection to give fluorogenic 4-methylumbelliferyl glycoside substrates. Due to the use of base, the glycosylation reaction proceeds more easily, is uncommonly α- or β-stereoselective, and affords the corresponding products in moderate to excellent yields (51%-94%) under appropriate conditions.

LiCl-mediated preparation of functionalized benzylic indium(III) halides and highly chemoselective palladium-catalyzed cross-coupling in a protic cosolvent

Sensitive functional groups such as COR, CHO, or CH(2)OH can be present in benzylic indium reagents prepared by the direct insertion of indium in the presence of LiCl. These reagents undergo palladium-catalyzed cross-coupling reactions in the presence of a protic cosolvent after activation with iPrMgCl x LiCl (see scheme). Remarkable chemoselectivities are achieved by using various electrophiles containing NH or OH groups.

Sialylation using N-glycolylneuraminyl phosphite donors to synthesize Neu5Gc-containing glycans

Efficient sialylations using N-glycolylneuraminic acid (Neu5Gc) phosphite donors having an acetyl or benzyl group on the glycolyl moiety are described in the synthesis of Neu5Gc-containing glycans. Both phosphite donors 1 and 2 were readily coupled with primary and secondary acceptor alcohols in propionitrile at -78 degrees C to provide the desired glycosides with good alpha-selectivities.

Investigation of the specificity of an α-l-arabinofuranosidase using C-2 and C-5 modified α-l-arabinofuranosides

The synthesis of three novel glycosyl donors presenting the same scaffold as alpha-L-arabinofuranose but modified at the C-2 or C-5 positions has been achieved. Furthermore, chemoenzymatic syntheses using the alpha-L-arabinofuranosidase AbfD3 and these unnatural furanosides were investigated. The use of the novel p-nitrophenyl furanoside donors revealed that AbfD3 can perform transglycosylation with the C-5 deoxygenated donor but not with the C-2 modified one. These results emphasize the vital role for OH-2 in AbfD3 substrate recognition.

Effects of oxygen–sulfur substitution on glycosaminoglycan-priming naphthoxylosides

Three series of sulfur-containing analogs to the selectively antiproliferative 2-(6-hydroxynaphthyl) beta-D-xylopyranoside were synthesized and their biological properties investigated. A short, general route to hydroxynaphthyl disulfides from dihydroxynaphthalenes was developed to utilize the disulfide bond as a sulfur-selective protecting group to enable the orthogonal protection of hydroxyls and thiols. The results indicate that hydrophobic, uncharged oxygen-sulfur substituted naphthoxylosides are taken up by cells and initiate priming of GAG chains to a greater extent compared to the oxygen analogs. No correlation between priming ability and antiproliferative activity was observed.

Evaluation of fluorescently labeled xylopyranosides as probes for proteoglycan biosynthesis

A new fluorescent analog to the antiproliferative 2-(6-hydroxynaphthyl)-beta-d-xylopyranoside has been synthesized and tested on a T24 cell line. The new analog was efficiently uptaken by the T24 cells but did not initiate priming of GAG chains. The results are similar to other fluorescently labeled analogs and we propose that these compounds are too large and unpolar to efficiently function as GAG-primers.

Synthesis and biology of bis-xylosylated dihydroxynaphthalenes

The 10 analogous bis-xylosylated dihydroxynaphthalenes have been synthesized and their chemical and biological properties investigated. The yield of the xylosylation reactions can be correlated to the electrostatic potential, and thus to the nucleophilicity, for the oxygen atoms of the dihydroxynaphthalenes. The bis-xylosylated compounds were more stable compared to the mono-xylosylated ones. They initiate priming of glycosaminoglycan chains to less extent but the priming proceeds in two directions. Contrary to the mono-xylosylated analogs, the tested compounds did not show any antiproliferative properties.

Novel glycosylated [Lys7]-dermorphin analogues: synthesis, biological activity and conformational investigations

Syntheses of the [Lys(7)]- and [Hyp(6),Lys(7)]-dermorphin analogues in which either Tyr(5) or Hyp(6) are O-glucosylated are described. For comparison, the carbohydrate-free peptides have also been prepared. Structural investigations by FT-IR and CD measurements were carried out on the synthetic analogues and some preliminary pharmacological experiments were also performed. The biological potency of the glucosylated analogues was compared with that of the micro-opioid receptor agonist dermorphin in GPI preparations. Glucosylation of either Tyr(5) or Hyp(6) reduces the potency of both [Lys(7)]-dermorphin and [Hyp(6),Lys(7)]-dermorphin. The effect induced by the Tyr(5) glucosylation is quite strong and the potency of both peptides is reduced by about 150 times. A similar but less dramatic effect is induced by the glucosylation of the Hyp(6) residue, and the potency of the parent peptide is reduced by about 15 times. The presence of acetyl groups on the sugar hydroxyl functions further reduces the agonistic potency of the glucosylated analogues. The analgesic potency of [Hyp(6),Lys(7)]-, [Hyp(betaGlc)(6),Lys(7)]- and [Tyr(betaGlc)(5),Lys(7)]-dermorphin were also tested in vivo by the tail-flick test. The glucosylated hydroxyproline-containing analogue is 8-10 times less active than the parent peptide, but its analgesic effect lasts significantly longer.

Aromatic O-glycosylation

Carbohydrates carrying an aromatic aglycon are important natural products and thus key synthetic targets. However, due to the electron-withdrawing properties of aromatic rings, phenols are difficult to glycosylate. This review covers the most common carbohydrate donors used for aromatic O-glycosylation (anomeric acetates, halides, trichloroacetimidates and thioglycosides) as well as some less common donors. The scope of the review is to give practical examples of aromatic O-glycosylations and to offer guidelines for glycosylation of typical aromatic residues. Anomeric acetates or trichloroacetimidates, activated under acidic conditions, are preferred for electron rich aromatic aglycons, while glycosyl halides, activated using basic conditions, are preferred for electron deficient aromatic residues.

Adding water to sugar: A spectroscopic and computational study of α- and β-phenylxyloside in the gas phase

The gas phase structures of phenyl alpha- and beta-d-xylopyranoside (alpha- and beta-pXyl) and their mono-hydrates have been investigated using a combination of resonant two-photon ionization (R2PI), ultra-violet hole-burning and resonant infrared ion dip spectroscopy, coupled with density functional theory (DFT) and ab initio computation. The hole-burning experiments indicate the population of a single conformer only, in each of the two anomers. Their experimental and calculated infrared spectra are both consistent with a conformational assignment corresponding to the computed global minimum configuration. All three OH groups are oriented towards the oxygen atom (O1) on the anomeric carbon atom to form an all trans(ttt) counter-clockwise chain of hydrogen bonds. The mono-hydrates, alpha- and beta-pXyl(H(2)O) each populate two distinct structures in the molecular beam environment, with the water molecule inserted between OH4 and OH3 or between OH3 and OH2 in alpha-pXyl(H2O), and between OH2 and O1 in either of two alternative orientations, in beta-pXyl(H2O). In all of the mono-hydrated xyloside complexes, the water molecule inserts into the weakest link of the sugar molecules' hydrogen-bonded chain of hydroxy groups, creating a single extended chain, strengthened by co-operativity. The all-trans configuration of the xylose moiety is retained and the mono-hydrate structures correspond to those calculated to lie at the lowest relative energies.

Facile synthesis of flavonoid 7-O-glycosides

Highly regioselective removal of the 7-O-acyl groups of the peracylated flavones, isoflavones, and flavonols (PhSH, imidazole, NMP) followed by effective glycosylation with glycosyl trifluoroacetimidates (BF(3).Et(2)O) and cautious deprotection of the acyl groups under basic conditions afforded the desired 7-O-flavonoid glycosides in satisfactory yields.