An Easy and Versatile Approach for the Regioselective De-O-benzylation of Protected Sugars Based on the I2/Et3SiH Combined System

Mild method for the synthesis of α-glycosyl chlorides: A convenient protocol for quick one-pot glycosylation

A simple and efficient protocol for the preparation of α-glycosyl chlorides within 15-30 min is described which employs a stable, cheap, and commercially available Trichloroisocyanuric acid (TCCA) as non-toxic chlorinating agent along with PPh3. This process involved a wide range of substrate scope and is well-suited with labile hydroxyl protecting groups such as benzyl, acetyl, benzoyl, isopropylidene, benzylidene, and TBDPS (tert-butyldiphenylsilyl) groups. This process is operationally simple, mild conditions and obtained good yields with excellent α selectivity. Moreover, a multi-catalyst one-pot glycosylation can be carried out to transform the glycosyl hemiacetals directly to a various O-glycosides in high overall yields without the need for separation or purification of the α-glycosyl chloride donors.

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.

Intramolecular Metal-Free C(sp3)-H Activation Enables a Selective Mono O-Debenzylation of Fully Protected Aminosugars

Carbamate-bearing benzylated aminosugars undergo an I₂/I(III)-promoted intramolecular hydrogen atom transfer (IHAT) followed by a nucleophilic attack to provide polycyclic structures. Thus, suitably positioned benzyl ethers are surgically oxidized into the corresponding mixed N/O-benzylidene acetals, which can be conveniently deprotected under mild acidic conditions to grant access to selectively O-deprotected aminosugars amenable for further derivatization. The scope of this strategy has been proven with a series of furanosic and pyranosic scaffolds. Preliminary mechanistic studies, including Hammett LFER and KIE analyses, support a reaction pathway with nucleophilic cyclization as the rate-determining step.

Glucose-based spiro-oxathiazoles as in vivo anti-hyperglycemic agents through glycogen phosphorylase inhibition

The design of glycogen phosphorylase (GP) inhibitors targeting the catalytic site of the enzyme is a promising strategy for a better control of hyperglycaemia in the context of type 2 diabetes. Glucopyranosylidene-spiro-heterocycles have been demonstrated as potent GP inhibitors, and more specifically spiro-oxathiazoles. A new synthetic route has now been elaborated through 1,3-dipolar cycloaddition of an aryl nitrile oxide to a glucono-thionolactone affording in one step the spiro-oxathiazole moiety. The thionolactone was obtained from the thermal rearrangement of a thiosulfinate precursor according to Fairbanks' protocols, although with a revisited outcome and also rationalised with DFT calculations. The 2-naphthyl substituted glucose-based spiro-oxathiazole 5h, identified as one of the most potent GP inhibitors (K_i = 160 nM against RMGPb) could be produced on the gram-scale from this strategy. Further evaluation in vitro using rat and human hepatocytes demonstrated that compound 5h is a anti-hyperglycaemic drug candidates performing slightly better than DAB used as a positive control. Investigation in Zucker fa/fa rat model in acute and subchronic assays further confirmed the potency of compound 5h since it lowered blood glucose levels by ∼36% at 30 mg kg^-1 and ∼43% at 60 mg kg^-1. The present study is one of the few in vivo investigations for glucose-based GP inhibitors and provides data in animal models for such drug candidates.

Direct and Regioselective Di-α-fucosylation on the Secondary Rim of β-Cyclodextrin

A straightforward glycosylation method is described to regio- and stereoselectively introduce two α-l-fucose moieties directly to the secondary rim of β-cyclodextrin. Using NMR and MS fragmentation studies, the nonasaccharide structure was determined, which was also visualized using molecular dynamics simulations. The reported glycosylation method proved to be robust on gram-scale, and may be generally applied to directly glycosylate β-cyclodextrins to make well-defined multivalent glycoclusters.

Synthesis of carbohydrate building blocks via regioselective uniform protection/deprotection strategies

Discussed herein is the synthesis of partially protected carbohydrates by manipulating only one type of a protecting group for a given substrate. The first focus of this review is the uniform protection of an unprotected starting material in a way that only one (or two) hydroxyl group remains unprotected. The second focus involves regioselective partial deprotection of uniformly protected compounds in a way that only one (or two) hydroxyl group becomes liberated.

"One-Pot" Protection, Glycosylation, and Protection-Glycosylation Strategies of Carbohydrates

Carbohydrates, which are ubiquitously distributed throughout the three domains of life, play significant roles in a variety of vital biological processes. Access to unique and homogeneous carbohydrate materials is important to understand their physical properties, biological functions, and disease-related features. It is difficult to isolate carbohydrates in acceptable purity and amounts from natural sources. Therefore, complex saccharides with well-defined structures are often most conviently accessed through chemical syntheses. Two major hurdles, regioselective protection and stereoselective glycosylation, are faced by carbohydrate chemists in synthesizing these highly complicated molecules. Over the past few years, there has been a radical change in tackling these problems and speeding up the synthesis of oligosaccharides. This is largely due to the development of one-pot protection, one-pot glycosylation, and one-pot protection-glycosylation protocols and streamlined approaches to orthogonally protected building blocks, including those from rare sugars, that can be used in glycan coupling. In addition, new automated strategies for oligosaccharide syntheses have been reported not only for program-controlled assembly on solid support but also by the stepwise glycosylation in solution phase. As a result, various sugar molecules with highly complex, large structures could be successfully synthesized. To summarize these recent advances, this review describes the methodologies for one-pot protection and their one-pot glycosylation into the complex glycans and the chronological developments associated with automated syntheses of oligosaccharides.

Large scale synthesis and regioselective protection schemes of ethyl 2-azido-2-deoxy-1-thio-α-d-cellobioside for preparation of heparin thiodisaccharide building blocks

Crystalline acetylated ethyl 2-azido-2-deoxy-1-thio-α-d-cellobioside has been prepared on a multigram scale from cellobiose in an overall yield of 23% with no chromatography required and converted after deacetylation into the 4',6'-O-benzylidene and 4',6'-O-benzylidene-6-O-TBDMS protected derivatives. Applying a number of regioselective benzylation methods on these gave access to a variety of regioselectively protected derivatives, both mono-ols (2'- and 3-OH), diols (2',6-, 2',3-, and 3,6-di-OH), and triols (2',3,6- and 2',3',3-tri-OH). A number of these derivatives were further processed by benzoylation followed by removal or opening of the benzylidene acetal and selective oxidation of the exposed primary alcohol to give heparin building block intermediates comprising a range of possible sulfation patterns.

Reductive Amination of Acetals by Anilines in the Presence of Triethylsilane and Iodine

A mild and efficient method for N-alkylation of aromatic amines with various acetals such as aryl, alkyl, cyclic and acyclic acetals was developed. A number of aromatic amines bearing electron-donating or electron-withdrawing substituents were directly alkylated by acetals with excellent yields. The method uses a catalytic amount of I2 and triethylsilane as the hydride source without a metal present. Monoalkylation with excellent chemoselectivity was observed.

One-pot synthesis of N-acetyl- and N-glycolylneuraminic acid capped trisaccharides and evaluation of their influenza A(H1 N1) inhibition

Human lung epithelial cells natively offer terminal N-acetylneuraminic acid (Neu5Ac) α(2→6)-linked to galactose (Gal) as binding sites for influenza virus hemagglutinin. N-Glycolylneuraminic acid (Neu5Gc) in place of Neu5Ac is known to affect hemagglutinin binding in other species. Not normally generated by humans, Neu5Gc may find its way to human cells from dietary sources. To compare their influence in influenza virus infection, six trisaccharides with Neu5Ac or Neu5Gc α(2→6) linked to Gal and with different reducing end sugar units were prepared using one-pot assembly and divergent transformation. The sugar assembly made use of an N-phthaloyl-protected sialyl imidate for chemoselective activation and α-stereoselective coupling with a thiogalactoside. Assessment of cytopathic effect showed that the Neu5Gc-capped trisaccharides inhibited the viral infection better than their Neu5Ac counterparts.

Is an acyl group at O-3 in glucosyl donors able to control α-stereoselectivity of glycosylation? The role of conformational mobility and the protecting group at O-6

The stereodirecting effect of a 3-O-acetyl protecting group, which is potentially capable of the remote anchimeric participation, and other protecting groups in 2-O-benzyl glucosyl donors with flexible and rigid conformations has been investigated. To this aim, an array of N-phenyltrifluoroacetimidoyl and sulfoxide donors bearing either 3-O-acetyl or 3-O-benzyl groups in combination with 4,6-di-O-benzyl, 6-O-acyl-4-O-benzyl, or 4,6-O-benzylidene protecting groups was prepared. The conformationally flexible 3-O-acetylated glucosyl donor protected at other positions with O-benzyl groups demonstrated very low or no α-stereoselectivity upon glycosylation of primary or secondary acceptors. On the contrary, 3,6-di-O-acylated glucosyl donors proved to be highly α-stereoselective as well as the donor having a single potentially participating acetyl group at O-6. The 3,6-di-O-acylated donor was shown to be the best α-glucosylating block for the primary acceptor, whereas the best α-selectivity of glycosylation of the secondary acceptor was achieved with the 6-O-acylated donor. Glycosylation of the secondary acceptor with the conformationally constrained 3-O-acetyl-4,6-O-benzylidene-protected donor displayed under standard conditions (-35°C) even lower α-selectivity as compared to the 3-O-benzyl analogue. However, increasing the reaction temperature essentially raised the α-stereoselectivities of glycosylation with both 3-O-acetyl and 3-O-benzyl donors and made them almost equal. The stereodirecting effects of protecting groups observed for N-phenyltrifluoroacetimidoyl donors were also generally proven for sulfoxide donors.

o-Xylylene protecting group in carbohydrate chemistry: application to the regioselective protection of a single vic-diol segment in cyclodextrins

A systematic study of the suitability of α,α'-dibromo-o-xylene as a reagent for cyclic o-xylylene protection of vic-diols in different monosaccharide substrates is reported. The installation of this protecting group, formally equivalent to a di-O-benzylation reaction, proceeds with good regioselectivity toward 1,2-trans-diequatorial diol systems in pyranose and furanose rings. Initially, the benzyl ether-type derivative of the more acidic hydroxyl is preferentially formed. Subsequent intramolecular etherification toward the equatorial-oriented vicinal OH is kinetically favored. The methodology has been implemented for the simultaneous protection of the secondary O-2 and O-3 positions of a single d-glucopyranosyl unit in cyclic oligosaccharides of the cyclodextrin (CD) family (cyclomaltohexa-, -hepta-, and -octaose; α, β, and γCD).

Regioselective monodeprotection of peracetylated carbohydrates

This protocol describes the regioselective deprotection of single hydroxyls in peracetylated monosaccharides and disaccharides by enzymatic or chemoenzymatic strategies. The introduction of a one-pot enzymatic step by using immobilized biocatalysts obviates the requirement to carry out tedious workups and time-consuming purifications. By using this straightforward protocol, different per-O-acetylated glycopyranosides (mono- or disaccharides, 1-substituted or glycals) can be transformed into a whole set of differentially monodeprotected 1-alcohols, 3-alcohols, 4-alcohols and 6-alcohols in high yields. These tailor-made glycosyl acceptors can then be used for stereoselective glycosylation for oligosaccharide and glycoderivative synthesis. They have been successfully used as building blocks to synthesize tailor-made di- and trisaccharides involved in the structure of lacto-N-neo-tetraose and precursors of the tumor-associated carbohydrate antigen T and the antitumoral drug peracetylated β-naphtyl-lactosamine. We are able to prepare a purified monoprotected carbohydrate in between 1 and 4 d. With this protocol, the small library of monodeprotected products can be synthesized in 1-2 weeks.