Chiral Phosphoric Acid Directed Regioselective Acetalization of Carbohydrate-Derived 1,2-Diols

Trivalent dialkylaminopyridine-catalyzed site-selective mono-O-acylation of partially-protected pyranosides

This work demonstrates trivalent tris-(3-N-methyl-N-pyridyl propyl)amine (1) catalyzing the site-selective mono-O-acylation of glycopyranosides. Different acid anhydrides were used for the acylation of monosaccharides, mediated by catalyst 1, at a loading of 1.5 mol%; the extent of site-selectivity and the yields of mono-O-acylation products were assessed. The reactions were performed between 2 and 10 h, depending on the nature of the acid anhydride, where the bulkier pivalic anhydride required a longer duration for acylation. The glycopyranosides are maintained as diols and triols, and from a set of experiments, the site-selectivity of acylations was observed to follow the intrinsic reactivities and stereochemistry of hydroxy functionalities. The trivalent catalyst 1 mediates the reactions with excellent site-selectivities for mono-O-acylation product formation in the studied glycopyranosides, in comparison to the monovalent N,N-dimethylamino pyridine (DMAP) catalyst. This study illustrates the benefits of the multivalency of catalytic moieties in catalysis.

Benzoxaborole Catalyst Embedded with a Lewis Base: A Highly Active and Selective Catalyst for cis-1,2-diol Modification

The regioselective modification of polyols allows rapid access to their derivatives, thereby accelerating the polyol-related biology and drug discovery. We previously reported that benzoxaborole is a potent catalyst for the regioselective modification of polyols containing a cis-1,2-diol structure. In this study, we developed a bifunctional benzoxaborole catalyst embedded with a Lewis base. Benzoxaborole and Lewis base groups were designed to cooperatively activate a substrate (cis-1,2-diol) and reactant (electrophile), respectively, hence lowering the reaction barrier for the cis-1,2-diol moiety. The bifunctional catalyst indeed exhibited a significantly higher catalytic activity and selectivity for cis-1,2-diol modifications rather than a benzoxaborole catalyst without a Lewis base group. Mechanistic analyses, using both experimental and theoretical methods, supported the design of our catalyst. The bifunctional catalyst reported herein would be a new tool for the straightforward synthesis of polyol derivatives.

Chiral Diselenophosphoric Acids for Ion Pair Catalysis: A Novel Approach to Enhance Both Proton Donating and Proton Accepting Properties

The activation of poorly reactive substrates via strong chiral acids is a central topic in asymmetric ion pair catalysis these days. Despite highly successful scaffolds such as N-triflylphosphoramides, these catalysts either lack C2-symmetry or provide multiple H-bond acceptor sites, leading to lower ee values for certain reactions. We present BINOL-based diselenophosphoric acids (DSA) as an extremely promising alternative. Using an intertwined approach of synthesis and NMR studies, we developed a synthetic approach to DSA with up to 98 % NMR yield. The obtained acids provide both very high proton donor and proton acceptor properties, a bifunctionality, which is key to catalytic applications. Indeed, first reactivity test proved the much higher acidity of DSA and its ability to initiate Mukaiyama-Mannich reaction and protodesilylation of silyl ethers. Together with their C2-symmetry, the single donor and single acceptor situation, the decreased tendency of self-association, and the straightforward synthesis with potential 3,3'-substitution, the DSA provide all features ideal for the further development of ion pair catalysis.

Development of Recyclable Polystyrene-Supported Phosphonic Acid Resins for Carbohydrate Immobilization and Glycosylation

This article describes the development of a recyclable polystyrene-based phosphonic acid resin and its use for the synthesis of immobilized glycosyl phosphonate donors and subsequent glycosylation reaction. This solid support was generated on a decagram scale from the commercially available Merrifield resin and subsequently functionalized via two different methods into eight different glycosylphosphonates. The resultant glycosylphosphonate-containing resins were obtained in 59-96% yields and were found to be bench-stable at room temperature. These donors could be activated using trifluoroborane etherate at 80 °C to react with various alcohol- and thiol-based acceptors to provide 17 different glycosides in good-to-excellent yields (53-98%). In addition, it was demonstrated that glycosylated resin could be recovered and recycled multiple times to regenerate immobilized glycosylphosphonate donors and could be subjected to on-resin glycan elongation.

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.

A synergistic Rh(I)/organoboron-catalysed site-selective carbohydrate functionalization that involves multiple stereocontrol

Site-selective functionalization is a core synthetic strategy that has broad implications in organic synthesis. Particularly, exploiting chiral catalysis to control site selectivity in complex carbohydrate functionalizations has emerged as a leading method to unravel unprecedented routes into biologically relevant glycosides. However, robust catalytic systems available to overcome multiple facets of stereoselectivity challenges to this end still remain scarce. Here we report a synergistic chiral Rh(I)- and organoboron-catalysed protocol, which enables access into synthetically challenging but biologically relevant arylnaphthalene glycosides. Our method depicts the employment of chiral Rh(I) catalysis in site-selective carbohydrate functionalization and showcases the utility of boronic acid as a compatible co-catalyst. Crucial to the success of our method is the judicious choice of a suitable organoboron catalyst. We also determine that exquisite multiple aspects of stereocontrol, including enantio-, diastereo-, regio- and anomeric control and dynamic kinetic resolution, are concomitantly operative.

New chemical processes to streamline carbohydrate synthesis

Carbohydrates hold potential for the future of therapeutic development due to their important role in essential biological processes. However, it is still challenging to produce homogenous materials, especially for non-mammalian sugars that are considered rare. Recent developments in this field have focused on catalytic methods, including organometallic and organocatalytic approaches to regioselective functionalization. Many approaches to glycosylations also utilize catalysts, increasingly in combination with photoredox conditions, to achieve stereoselectivity. Additionally, there have been significant advancements in the automation of glycosylation to synthesize oligosaccharides in less time and with fewer manually conducted steps by the user.

Electrochemical Synthesis of Glycosyl Fluorides Using Sulfur(VI) Hexafluoride as the Fluorinating Agent

This manuscript describes the electrochemical synthesis of 17 different glycosyl fluorides in 73-98% yields on up to a 5 g scale that relies on the use of SF6 as an inexpensive and safe fluorinating agent. Cyclic voltammetry and related mechanistic studies carried out subsequently suggest that the active fluorinating species generated through the cathodic reduction of SF6 are transient under these reductive conditions and that the sulfur and fluoride byproducts are effectively scavenged by Zn(II) to generate benign salts.

Studies of Catalyst-Controlled Regioselective Acetalization and Its Application to Single-Pot Synthesis of Differentially Protected Saccharides

This article describes studies on the regioselective acetal protection of monosaccharide-based diols using chiral phosphoric acids (CPAs) and their immobilized polymeric variants, (R)-Ad-TRIP-PS and (S)-SPINOL-PS, as the catalysts. These catalyst-controlled regioselective acetalizations were found to proceed with high regioselectivities (up to >25:1 rr) on various d-glucose-, d-galactose-, d-mannose-, and l-fucose-derived 1,2-diols and could be carried out in a regiodivergent fashion depending on the choice of chiral catalyst. The polymeric catalysts were conveniently recycled and reused multiple times for gram-scale functionalizations with catalytic loadings as low as 0.1 mol %, and their performance was often found to be superior to the performance of their monomeric variants. These regioselective CPA-catalyzed acetalizations were successfully combined with common hydroxyl group functionalizations as single-pot telescoped procedures to produce 32 regioisomerically pure differentially protected mono- and disaccharide derivatives. To further demonstrate the utility of the polymeric catalysts, the same batch of (R)-Ad-TRIP-PS catalyst was recycled and reused to accomplish single-pot gram-scale syntheses of 6 differentially protected d-glucose derivatives. The subsequent exploration of the reaction mechanism using NMR studies of deuterated and nondeuterated substrates revealed that low-temperature acetalizations happen via a syn-addition mechanism and that the reaction regioselectivity exhibits strong dependence on the temperature. The computational studies indicate a complex temperature-dependent interplay of two reaction mechanisms, one involving an anomeric phosphate intermediate and another via concerted asynchronous formation of an acetal, that results in syn-addition products. The computational models also explain the steric factors responsible for the observed C2 selectivities and are consistent with experimentally observed selectivity trends.

Exploiting non-covalent interactions in selective carbohydrate synthesis

Non-covalent interactions (NCIs) are a vital component of biological bond-forming events, and have found important applications in multiple branches of chemistry. In recent years, the biomimetic exploitation of NCIs in challenging glycosidic bond formation and glycofunctionalizations has attracted significant interest across diverse communities of organic and carbohydrate chemists. This emerging theme is a major new direction in contemporary carbohydrate chemistry, and is rapidly gaining traction as a robust strategy to tackle long-standing issues such as anomeric and site selectivity. This Review thus seeks to provide a bird's-eye view of wide-ranging advances in harnessing NCIs within the broad field of synthetic carbohydrate chemistry. These include the exploitation of NCIs in non-covalent catalysed glycosylations, in non-covalent catalysed glycofunctionalizations, in aglycone delivery, in stabilization of intermediates and transition states, in the existence of intramolecular hydrogen bonding networks and in aggregation by hydrogen bonds. In addition, recent emerging opportunities in exploiting halogen bonding and other unconventional NCIs, such as CH-π, cation-π and cation-n interactions, in various aspects of carbohydrate chemistry are also examined.

Chemical synthesis of the rare D-Fuc3NAc containing tetrasaccharide repeating unit of the O-antigenic polysaccharide from E. coli O74

Chemical synthesis of the tetrasaccharide repeating unit of the O-antigen from E. coli O74 is accomplished by a convergent [2 + 2] block synthesis strategy. The challenging rare D-Fuc3NAc has been prepared using DTBP and TIPST mediated deoxygenation reaction. Other monosaccharide synthons are prepared through rational protecting group manipulations and the stereoselective glycosylations are achieved either by the activation of thioglycoside or glycosyl trichloroacetimidate. The target tetrasaccharide is made in the form of its 2-aminoethyl glycoside to facilitate further glycoconjugate formation without affecting the anomeric stereochemistry.

Computational Design of Enhanced Enantioselectivity in Chiral Phosphoric Acid-Catalyzed Oxidative Desymmetrization of 1,3-Diol Acetals

A general method for the highly enantioselective desymmetrization of 2-alkyl-substituted 1,3-diols is presented. A combination of computational and experimental studies has been utilized to understand the origin of the stereocontrol of oxidative desymmetrization of 1,3-diol benzylideneacetals. DFT calculations demonstrate that the acetal protecting group is highly influential for high enantioselectivity, and a simple but effective new protecting group has been designed. The desymmetrization reactions proceed with high enantioselectivity for a variety of substrates. Moreover, the reaction conditions are also shown to be effective for desymmetrization of 2,2-dialkyl-substituted 1,3-diols, which provides chiral products bearing acyclic all-carbon quaternary stereocenters. The method has been applied to the formal synthesis of indoline alkaloids.

Experimental and Computational Studies on Regiodivergent Chiral Phosphoric Acid Catalyzed Cycloisomerization of Mupirocin Methyl Ester

This article presents a new strategy for achieving regiocontrol over the endo versus exo modes of cycloisomerizations of epoxide-containing alcohols, which leads to the formation of five- or six-membered cyclic ethers. Unlike traditional methods relying on achiral reagents or enzymes, this approach utilizes chiral phosphoric acids to catalyze the regiodivergent selective formations of either tetrahydrofuran- or tetrahydropyran-containing products. By using methyl ester of epoxide-containing antibiotic mupirocin as the substrate, it is demonstrated that catalytic chiral phosphoric acids (R)-TCYP and (S)-TIPSY could be used to achieve the selective formation of either the six-membered endo product (95:5 r.r.) or the five-membered exo product (77:23 r.r.), correspondingly. This cyclization was found to be unselective under the standard conditions involving various achiral acids, bases, or buffers. The subsequent mechanistic studies using state-of-the-art quantum chemical solutions provided the description of the potential energy surface, which is fully consistent with the experimental observations. Based on these results, highly detailed reaction paths are obtained and a concerted and highly synchronous mechanism is proposed for the formation of both exo and endo products.

Site-Selective and Stereoselective O-Alkylation of Glycosides by Rh(II)-Catalyzed Carbenoid Insertion

Carbohydrates are synthetically challenging molecules with vital biological roles in all living systems. Selective synthesis and functionalization of carbohydrates provide tremendous opportunities to improve our understanding on the biological functions of this fundamentally important class of molecules. However, selective functionalization of seemingly identical hydroxyl groups in carbohydrates remains a long-standing challenge in chemical synthesis. We herein describe a practical and predictable method for the site-selective and stereoselective alkylation of carbohydrate hydroxyl groups via Rh(II)-catalyzed insertion of metal carbenoid intermediates. This represents one of the mildest alkylation methods for the systematic modification of carbohydrates. Density functional theory (DFT) calculations suggest that the site selectivity is determined in the Rh(II)-carbenoid insertion step, which prefers insertion into hydroxyl groups with an adjacent axial substituent. The subsequent intramolecular enolate protonation determines the unexpected high stereoselectivity. The most prevalent trans-1,2-diols in various pyranoses can be systematically and predictably differentiated based on the model derived from DFT calculations. We also demonstrated that the selective O-alkylation method could significantly improve the efficiency and stereoselectivity of glycosylation reactions. The alkyl groups introduced to carbohydrates by OH insertion reaction can serve as functional groups, protecting groups, and directing groups.

Parallel Kinetic Resolution of Unsymmetrical Acyclic Aliphatic syn-1,3-Diols

Disclosed is a mild, reliable, and enantioselective catalytic parallel kinetic resolution of unsymmetrical acyclic aliphatic syn-1,3-diol derived acetals mediated by chiral phosphoric acid. This method provides stereoselective access to a variety of syn-1,3-diols as valuable building blocks with high enantioselectivity. Moreover, this mild system allows for site-selective protection of optically pure syn-1,3-diols in excellent regioselectivity.

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.

MOP and EE Protecting Groups in Synthesis of α- or β-Naphthyl-C-Glycosides from Glycals

The development of effective protection strategies is essential in the synthesis of complex carbohydrates and glycomimetics. This article describes a versatile four-stage protocol for the synthesis of α- or β-aryl-C-glycosides from unprotected d-glycals using two acetal protecting groups, ethoxyethyl and methoxypropyl, which are stable under harsh basic conditions and convenient for the C-1 metalation of glycals. Their stability was investigated in subsequent cross-coupling reactions with 1-iodonaphthalene followed by oxidative/reductive transformations to naphthyl-C-glycosides.

Chiral reagents in glycosylation and modification of carbohydrates

Carbohydrates play a significant role in numerous biological events, and the chemical synthesis of carbohydrates is vital for further studies to understand their various biological functions. Due to the structural complexity of carbohydrates, the stereoselective formation of glycosidic linkages and the site-selective modification of hydroxyl groups are very challenging and at the same time extremely important. In recent years, the rapid development of chiral reagents including both chiral auxiliaries and chiral catalysts has significantly improved the stereoselectivity for glycosylation reactions and the site-selectivity for the modification of carbohydrates. These new tools will greatly facilitate the efficient synthesis of oligosaccharides, polysaccharides, and glycoconjugates. In this tutorial review, we will summarize these advances and highlight the most recent examples.

Alkylative kinetic resolution of vicinal diols under phase-transfer conditions: a chiral ammonium borinate catalysis

Alkylative kinetic resolution of vicinal alcohols is realized by a cooperative chiral ammonium borinate catalysis.

Herein, we report the first alkylative kinetic resolution of vicinal alcohols realized by cooperative use of a chiral quaternary ammonium salt and an achiral borinic acid. In addition, a catalytic regioselective alkylation of a secondary alcohol in the presence of an unprotected primary one is presented, emphasizing the unique selectivity and potential of this ammonium borinate catalysis.

Regiodivergent Glycosylations of 6-Deoxy-erythronolide B and Oleandomycin-Derived Macrolactones Enabled by Chiral Acid Catalysis

This work describes the first example of using chiral catalysts to control site-selectivity for the glycosylations of complex polyols such as 6-deoxyerythronolide B and oleandomycin-derived macrolactones. The regiodivergent introduction of sugars at the C3, C5, and C11 positions of macrolactones was achieved by selecting appropriate chiral acids as catalysts or through introduction of stoichiometric boronic acid-based additives. BINOL-based chiral phosphoric acids (CPAs) were used to catalyze highly selective glycosylations at the C5 positions of macrolactones (up to 99:1 rr), whereas the use of SPINOL-based CPAs resulted in selectivity switch and glycosylation of the C3 alcohol (up to 91:9 rr). Additionally, the C11 position of macrolactones was selectively functionalized through traceless protection of the C3/C5 diol with boronic acids prior to glycosylation. Investigation of the reaction mechanism for the CPA-controlled glycosylations revealed the involvement of covalently linked anomeric phosphates rather than oxocarbenium ion pairs as the reactive intermediates.

Catalytic Site-Selective Acylation of Carbohydrates Directed by Cation-n Interaction

Site-selective functionalization of hydroxyl groups in carbohydrates is one of the long-standing challenges in chemistry. Using a pair of chiral catalysts, we now can differentiate the most prevalent trans-1,2-diols in pyranoses systematically and predictably. Density functional theory (DFT) calculations indicate that the key determining factor for the selectivity is the presence or absence of a cation-n interaction between the cation in the acylated catalyst and an appropriate lone pair in the substrate. DFT calculations also provided a predictive model for site-selectivity and this model is validated by various substrates.

Chiral phosphoric acid-catalyzed desymmetrizative glycosylation of 2-deoxystreptamine and its application to aminoglycoside synthesis

This work describes chiral phosphoric acid (CPA)-catalyzed desymmetrizative glycosylation ofmeso-diol derived from 2-deoxystreptamine.

Enhanced site-selectivity in acylation reactions with substrate-optimized catalysts on solid supports

A concept for site-selective acylation is presented, using substrate-optimized DMAP–peptide conjugates on a solid support.

Regioselective Acylation of Diols and Triols: The Cyanide Effect

Central topics of carbohydrate chemistry embrace structural modifications of carbohydrates and oligosaccharide synthesis. Both require regioselectively protected building blocks that are mainly available via indirect multistep procedures. Hence, direct protection methods targeting a specific hydroxy group are demanded. Dual hydrogen bonding will eventually differentiate between differently positioned hydroxy groups. As cyanide is capable of various kinds of hydrogen bonding and as it is a quite strong sterically nondemanding base, regioselective O-acylations should be possible at low temperatures even at sterically congested positions, thus permitting formation and also isolation of the kinetic product. Indeed, 1,2-cis-diols, having an equatorial and an axial hydroxy group, benzoyl cyanide or acetyl cyanide as an acylating agent, and DMAP as a catalyst yield at -78 °C the thermodynamically unfavorable axial O-acylation product; acyl migration is not observed under these conditions. This phenomenon was substantiated with 3,4-O-unproteced galacto- and fucopyranosides and 2,3-O-unprotected mannopyranosides. Even for 3,4,6-O-unprotected galactopyranosides as triols, axial 4-O-acylation is appreciably faster than O-acylation of the primary 6-hydroxy group. The importance of hydrogen bonding for this unusual regioselectivity could be confirmed by NMR studies and DFT calculations, which indicate favorable hydrogen bonding of cyanide to the most acidic axial hydroxy group supported by hydrogen bonding of the equatorial hydroxy group to the axial oxygen. Thus, the "cyanide effect" is due to dual hydrogen bonding of the axial hydroxy group which enhances the nucleophilicity of the respective oxygen atom, permitting an even faster reaction for diols than for mono-ols. In contrast, fluoride as a counterion favors dual hydrogen bonding to both hydroxy groups leading to equatorial O-acylation.

Site-Selective Acylations with Tailor-Made Catalysts

The acylation of alcohols catalyzed by N,N-dimethylamino pyridine (DMAP) is, despite its widespread use, sometimes confronted with substrate-specific problems: For example, target compounds with multiple hydroxy groups may show insufficient selectivity for one hydroxyl, and the resulting product mixtures are hardly separable. Here we describe a concept that aims at tailor-made catalysts for the site-specific acylation. To this end, we introduce a catalyst library where each entry is constructed by connecting a variable and readily tuned peptide scaffold with a catalytically active unit based on DMAP. For selected examples, we demonstrate how library screening leads to the identification of optimized catalysts, and the substrates of interest can be converted with a markedly enhanced site-selectivity compared with only DMAP. Furthermore, substrate-optimized catalysts of this type can be used to selectively convert "their" substrate in the presence of structurally similar compounds, an important requisite for reactions with mixtures of substances.

Synthesis and Biological Evaluation of Lactimidomycin and Its Analogues

The studies culminating in the total synthesis of the glutarimide-containing eukaryote translation elongation inhibitor lactimidomycin are described. The optimized synthetic route features a Zn(II)-mediated intramolecular Horner-Wadsworth-Emmons (HWE) reaction resulting in a highly stereoselective formation of the strained 12-membered macrolactone of lactimidomycin on a 423 mg scale. The presence of the E,Z-diene functionality was found to be key for effective macrocyclizations as a complete removal of these unsaturation units resulted in exclusive formation of the dimer rather than monocyclic enoate. The synthetic route features a late-stage installation of the glutarimide functionality via an asymmetric catalytic Mukaiyama aldol reaction, which allows for a quick generation of lactimidomycin homolog 55 containing two additional carbons in the glutarimide side chain. Similar to lactimidomycin, this analog was found to possess cytotoxicity against MDA-MB-231 breast cancer cells (GI50 =1-3 μM) using in vitro 2D and 3D assays. Although lactimidomycin was found to be the most potent compound in terms of anticancer activity, 55 as well as truncated analogues 50-52 lacking the glutarimide side-chain were found to be significantly less toxic against human mammary epithelial cells.