Glycosyl-Acceptor-Derived Borinic Ester-Promoted Direct and β-Stereoselective Mannosylation with a 1,2-Anhydromannose Donor

Designed Mannosylerythritol Lipid Analogues Exhibiting Both Selective Cytotoxicity Against Human Skin Cancer Cells and Recovery Effects on Damaged Skin Cells

Mannosylerythritol lipids (MELs) are a class of amphipathic molecules bearing a hydrophilic 4-O-β-D-mannopyranosyl-D-erythritol skeleton. Here, we designed and synthesized four kinds of MEL analogues R-MEL-A ([2R,3S]-erythritol type), S-mannosylthreitol lipid (MTL)-A ([2S,3S]-threitol type), R-MTL-A ([2R,3R]-threitol type), and α-S-MEL-A ([2S,3R]-erythritol type) using our previously reported boron-mediated aglycon delivery (BMAD) method and a neighboring group assisted glycosylation method. The selective cytotoxicity of the target compounds against cancer cells was evaluated, with R-MTL-A showing the highest selective cytotoxicity against human skin squamous carcinoma HSC-5 cells. Our findings suggest that R-MTL-A induces necrosis-like cell death against HSC-5 cells by decreasing cell membrane fluidity. R-MTL-A also exhibits an efficient recovery effect on damaged skin cells, indicating that R-MTL-A has potential as a lead compound for new cosmeceuticals with both cancer cell-selective toxicity and recovery effects on damaged skin cells.

Exploring the Dual Functions of Distal Acyl Group Direction in Various Nucleophilic Environments

A universal glycosylation strategy could significantly simplify glycoside synthesis. One approach to achieving this goal is through acyl group direction for the corresponding 1,2-, 1,3-, 1,4-, or 1,6-trans glycosylation; however, this approach has been challenging for glycosidic bonds that require distal equatorial-acyl group direction. We developed an approach in weakly nucleophilic environments for selective 1,4-trans glycosylation directed by the equatorial-4-O-acyl group. Here, we explored this condition in other distal acyl groups and found that, besides 1,n-trans direction, acyl groups also mediated hydrogen bonding between acyl groups and alcohols. The latter showed a diverse effect and classified the acyl group direction into axial and equatorial categories. Corresponding glycosylation conditions were distinguished as guidance for acyl group direction from either category. Hence, acyl group direction may serve as a general glycosylation strategy.

Use of Novel Homochiral Thioureas Camphor Derived as Asymmetric Organocatalysts in the Stereoselective Formation of Glycosidic Bonds

We synthesized six new camphor-derived homochiral thioureas 1-6, from commercially available (1R)-(-)-camphorquinone. These new compounds 1-6 were evaluated as asymmetric organocatalysts in the stereoselective formation of glycosidic bonds, with 2,3,4,6-tetra-O-benzyl-D-glucopyranosyl and 2,3,4,6-tetra-O-benzyl-D-galactopyranosyl trichloroacetimidates as donors, and several alcohols as glycosyl acceptors, such as methanol, ethanol, 1-propanol, 1-butanol, 1-octanol, iso-propanol, tert-butanol, cyclohexanol, phenol, 1-naphtol, and 2-naphtol. Optimization of the asymmetric glycosylation reaction was achieved by modifying reaction conditions such as solvent, additive, loading of catalyst, temperature, and time of reaction. The best result was obtained with 2,3,4,6-tetra-O-benzyl-D-galactopyranosyl trichloroacetimidates, using 15 mol% of organocatalyst 1, in the presence of 2 equiv of MeOH in solvent-free conditions at room temperature for 1.5 h, affording the glycosidic compound in a 99% yield and 1:73 α:β stereoselectivity; under the same reaction conditions, without using a catalyst, the obtained stereoselectivity was 1:35 α:β. Computational calculations prior to the formation of the products were modeled, using density functional theory, M06-2X/6-31G(d,p) and M06-2X/6-311++G(2d,2p) methods. We observed that the preference for β glycoside formation, through a stereoselective inverted substitution, relies on steric effects and the formation of hydrogen bonds between thiourea 1 and methanol in the complex formed.

Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.

β-l-Rhamnosylation and β-d-Mannosylation Mediated by 4-O-Ester Groups in a Weakly Nucleophilic Environment

eq-4-O-Acyl group directed β-rhamnosylation and β-mannosylation are achieved in a carborane or BARF anion formed weakly nucleophilic environment with the assistance of a 2,3-orthocarbonate group. The 4-O-acyl group plays a critical role in directing the β-selectivity, and the weakly coordinating anion is essential to amplify this direction. The orthocarbonate group could be readily removed with 1,3-propanediol in the presence of BF3·Et2O.

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.

Boron-mediated aglycon delivery (BMAD) for the stereoselective synthesis of 1,2-cis glycosides

1,2-cis Glycosides are frequently found in biologically active natural products, pharmaceutical compounds, and highly functional materials. Therefore, elucidating the role of mechanism of their biological activities will help clarify the structure-activity relationships of these diverse compounds and create new lead compounds for pharmaceuticals by modifying their structures. However, unlike 1,2-trans glycosides, the stereoselective synthesis of 1,2-cis glycosides remains difficult due to the nonavailability of neighboring group participation from the 2-O-acyl functionalities of the glycosyl donors. In this context, we recently developed organoboron-catalyzed 1,2-cis-stereoselecitve glycosylations, called boron-mediated aglycon delivery (BMAD) methods. In this review article, we introduce the BMAD methods and several examples of their application to the synthesis of biologically active glycosides.

Recent Advances in Stereoselective Chemical O-Glycosylation Reactions

Carbohydrates involving glycoconjugates play a pivotal role in many life processes. Better understanding toward glycobiological events including the structure–function relationship of these biomolecules and for diagnostic and therapeutic purposes including tailor-made vaccine development and synthesis of structurally well-defined oligosaccharides (OS) become important. Efficient chemical glycosylation in high yield and stereoselectivity is however challenging and depends on the fine tuning of a protection profile to get matching glycosyl donor–acceptor reactivity along with proper use of other important external factors like catalyst, solvent, temperature, activator, and additive. So far, many glycosylation methods have been reported including several reviews also. In the present review, we will concentrate our discussion on the recent trend on α- and β-selective glycosylation reactions reported during the past decade.

Synthetic neoglycoconjugates of hepta- and nonamannoside ligands for eliciting oligomannose-specific HIV-1-neutralizing antibodies

Oligomannose-type glycans on the spike protein of HIV-1 constitute relevant epitopes to elicit broadly neutralizing antibodies (bnAbs). Herein we describe an improved synthesis of α- and β-linked hepta- and nonamannosyl ligands that, subsequently, were converted into BSA and CRM 197 neoglycoconjugates. We assembled the ligands from anomeric 3-azidopropyl spacer glycosides from select 3-O-protected thiocresyl mannoside donors. Chain extensions were achieved using 4+3 or 4+5 block synthesis of thiocresyl and trichloroacetimidate glycosyl donors. Subsequent global deprotection generated the 3-aminopropyl oligosaccharide ligands. ELISA binding data obtained with the β-anomeric hepta- and nonamannosyl conjugates with a selection of HIV-1 bnAbs showed comparable binding of both mannosyl ligands by Fab fragments yet lesser binding of the nonasaccharide conjugate by the corresponding IgG antibodies. These results support previous observations that a complete Man 9 structure might not be the preferred antigenic binding motif for some oligomannose-specific antibodies and have implications for glycoside designs to elicit oligomannose-targeted HIV-1-neutralizing antibodies.

Stereoselective β-Mannosylation via Anomeric O-Alkylation with L-Sugar-Derived Electrophiles

A total synthesis of the trisaccharide repeat unit of Salmonella serogroup E1 O-antigen is reported. This synthesis features a key β-mannosylation reaction via cesium carbonate-mediated anomeric O-alkylation of a partially protected D-mannose with an L-fucose-derived electrophile for the first time.

Zinc(II) Iodide-Directed β-Mannosylation: Reaction Selectivity, Mode, and Application

A direct, efficient, and versatile glycosylation methodology promises the systematic synthesis of oligosaccharides and glycoconjugates in a streamlined fashion like the synthesis of medium to long-chain nucleotides and peptides. The development of a generally applicable approach for the construction of 1,2-cis-glycosidic bond with controlled stereoselectivity remains a major challenge, especially for the synthesis of β-mannosides. Here, we report a direct mannosylation strategy mediated by ZnI₂, a mild Lewis acid, for the highly stereoselective construction of 1,2-cis-β linkages employing easily accessible 4,6-O-tethered mannosyl trichloroacetimidate donors. The versatility and effectiveness of this strategy were demonstrated with successful β-mannosylation of a wide variety of alcohol acceptors, including complex natural products, amino acids, and glycosides. Through iteratively performing ZnI₂-mediated mannosylation with the chitobiosyl azide acceptor followed by site-selective deprotection of the mannosylation product, the novel methodology enables the modular synthesis of the key intermediate trisaccharide with Man-β-(1 → 4)-GlcNAc-β-(1 → 4)-GlcNAc linkage for N-glycan synthesis. Theoretical investigations with density functional theory calculations delved into the mechanistic details of this β-selective mannosylation and elucidated two zinc cations' essential roles as the activating agent of the donor and the principal mediator of the cis-directing intermolecular interaction.

Stereoselective O-Glycosylations by Pyrylium Salt Organocatalysis

Despite many years of invention, the field of carbohydrate chemistry remains rather inaccessible to non-specialists, which limits the scientific impact and reach of the discoveries made in the field. Aiming to increase the availability of stereoselective glycosylation chemistry for non-specialists, we have discovered that several commercially available pyrylium salts catalyze stereoselective O-glycosylations of a wide range of phenols and alkyl alcohols. This catalytic reaction utilizes trichloroacetimidates, an easily accessible and synthetically proven electrophile, takes place under air and only initiates when all three reagents are mixed, which should provide better reproducibility by non-specialists. The reaction exhibits varying degrees of stereospecificity, resulting in β-selective glycosylations from α-trichloroacetimidates, whilst an α-selective glycosylation proceeds from β-trichloroacetimidates. A mechanistic study revealed that the reaction likely proceeds via an S_N 2-like substitution on the protonated electrophile.

Self-Assembling Properties and Recovery Effects on Damaged Skin Cells of Chemically Synthesized Mannosylerythritol Lipids

Mannosylerythritol lipids (MELs), which are one of the representative sugar-based biosurfactants (BSs) produced by microorganisms, have attracted much attention in various fields in the sustainable development goals (SDGs) era. However, they are inseparable mixtures with respect to the chain length of the fatty acids. In this study, self-assembling properties and structure-activity relationship (SAR) studies of recovery effects on damaged skin cells using chemically synthesized MELs were investigated. It was revealed, for the first time, that synthetic and homogeneous MELs exhibited significant self-assembling properties to form droplets or giant vesicles. In addition, a small difference in the length of the fatty acid chains of the MELs significantly affected their recovery effects on the damaged skin cells. MELs with medium or longer length alkyl chains exhibited much higher recovery effects than that of C18-ceramide NP.

Stereoselective β-mannosylations and β-rhamnosylations from glycosyl hemiacetals mediated by lithium iodide

Stereoselective β-mannosylation is one of the most challenging problems in the synthesis of oligosaccharides. Herein, a highly selective synthesis of β-mannosides and β-rhamnosides from glycosyl hemi-acetals is reported, following a one-pot chlorination, iodination, glycosylation sequence employing cheap oxalyl chloride, phosphine oxide and LiI. The present protocol works excellently with a wide range of glycosyl acceptors and with armed glycosyl donors. The method doesn't require conformationally restricted donors or directing groups; it is proposed that the high β-selectivities observed are achieved via an S_N2-type reaction of α-glycosyl iodide promoted by lithium iodide.

Merging Reagent Modulation and Remote Anchimeric Assistance for Glycosylation: Highly Stereoselective Synthesis of α-Glycans up to a 30-mer

The efficient synthesis of long, branched, and complex carbohydrates containing multiple 1,2-cis glycosidic linkages is a long-standing challenge. Here, we report a merging reagent modulation and 6-O-levulinoyl remote anchimeric assistance glycosylation strategy, which is successfully applied to the first highly stereoselective synthesis of the branched Dendrobium Huoshanense glycans and the linear Longan glycans containing up to 30 contiguous 1,2-cis glucosidic bonds. DFT calculations shed light on the origin of the much higher stereoselectivities of 1,2-cis glucosylation with 6-O-levulinoyl group than 6-O-acetyl or 6-O-benzoyl groups. Orthogonal one-pot glycosylation strategy based on glycosyl ortho-alkynylbenzoates and ortho-(1-phenylvinyl)benzoates has been demonstrated in the efficient synthesis of complex glycans, precluding such issues as aglycon transfer inherent to orthogonal one-pot synthesis based on thioglycosides.

Synthesis of a Pentasaccharide Repeating Unit of Lipopolysaccharide Derived from Virulent E. coli O1 and Identification of a Glycotope Candidate of Avian Pathogenic E. coli O1

Avian pathogenic Escherichia coli (APEC) is a common bacterial pathogen infecting chickens, resulting in economic losses to the poultry industry worldwide. In particular, APEC O1, one of the most common serotypes of APEC, is considered problematic due to its zoonotic potential. Therefore, many attempts have been made to develop an effective vaccine against APEC O1. In fact, the lipopolysaccharide (LPS) O-antigen of APEC O1 has been shown to be a potent antigen for inducing specific protective immune responses. However, the detailed structure of the O-antigen of APEC O1 is not clear. The present study demonstrates the first synthesis of a pentasaccharide repeating unit of LPS derived from virulent E. coli O1 and its conjugate with BSA. ELISA tests using the semi-synthetic glycoconjugate and the APEC O1 immune chicken serum revealed that the pentasaccharide is a glycotope candidate of APEC O1, with great potential as an antigen for vaccine development.

Diboron-Catalyzed Regio- and 1,2-cis-α-Stereoselective Glycosylation of trans-1,2-Diols

Regio- and 1,2-cis-α-stereoselective glycosylations were investigated using 1,2-anhydroglucose donors and trans-1,2-diol sugar acceptors in the presence of a diboron catalyst. The reactions proceeded smoothly to provide the corresponding 1,2-cis-α-glycosides with consistently very high stereoselectivity and were regioselectivity controlled by the protecting groups of the acceptor. The present glycosylation method was applied successfully to the efficient synthesis of α-1,3-glucan pentasaccharide.

Highly Selective β-Mannosylations and β-Rhamnosylations Catalyzed by Bis-thiourea

We report highly β-selective bis-thioureas-catalyzed 1,2-cis-O-pyranosylations employing easily accessible acetonide-protected donors. A wide variety of alcohol nucleophiles, including complex natural products, glycosides, and amino acids were β-mannosylated and β-rhamnosylated successfully using an operationally simple protocol under mild and neutral conditions. Less nucleophilic acceptors such as phenols were also glycosylated efficiently in excellent yields and with high β-selectivities.

Diastereoselective desymmetric 1,2-cis-glycosylation of meso-diols via chirality transfer from a glycosyl donor

Chemical desymmetrization reactions of meso-diols are highly effective for the precise and efficient synthesis of chiral molecules. However, even though enzyme-catalyzed desymmetric glycosylations are frequently found in nature, there is no method for highly diastereoselective desymmetric chemical glycosylation of meso-diols. Herein, we report a highly diastereoselective desymmetric 1,2-cis-glycosylation of meso-diols found in myo-inositol 1,3,5-orthoesters using a boronic acid catalyst based on predictions of regioselectivity by density functional theory (DFT) calculations. The enantiotopic hydroxyl groups of the meso-diols are clearly differentiated by the stereochemistry at the C2 position of the glycosyl donor with excellent regioselectivities. In addition, the present method is successfully applied to the synthesis of core structures of phosphatidylinositolmannosides (PIMs) and glycosylphosphatidylinositol (GPI) anchors, and common β-mannoside structures of the LLBM-782 series of antibiotics.

Enzyme-catalyzed desymmetric glycosylations are often found in nature, however the corresponding chemical methods are lacking. Here, the authors report a highly diastereoselective desymmetric 1,2-cis-glycosylation of meso-diols found in myo-inositol 1,3,5-orthoesters using a boronic acid catalyst.

Stereoselective Phenylselenoglycosylation of Glycals Bearing a Fused Carbonate Moiety toward the Synthesis of 2-Deoxy-β-galactosides and β-Mannosides

A phenylselenoglycosylation reaction of glycal derivatives mediated by diphenyl diselenide and phenyliodine(III) bis(trifluoroacetate) under mild conditions is described. Stereoselective glycosylation has been achieved by installing fused carbonate on those glycals. 3,4-O-Carbonate galactals and 2,3-O-carbonate 2-hydroxyglucals are converted into corresponding glycosides in good yields with excellent β-selectivity, resulting in 2-phenylseleno-2-deoxy-β-galactosides and 2-phenylseleno-β-mannosides which are good precursors of 2-deoxy-β-galactosides and β-mannosides, respectively.

Perfluorophenylboronic acid-catalyzed direct α-stereoselective synthesis of 2-deoxygalactosides from deactivated peracetylated d-galactal

Perfluorophenylboronic acid 1c catalyzes the direct stereoselective addition of alcohol nucleophiles to deactivated peracetylated d-galactal to give 2-deoxygalactosides in 55-88% yield with complete α-selectivity. The unprecedented results reported here also enable the synthesis of disaccharides containing the 2-deoxygalactose moiety directly from the deactivated peracetylated d-galactal. This convenient and metal-free glycosylation method works well with a wide range of alcohol nucleophiles as acceptors and tolerates a range of functional groups without the formation of the Ferrier byproduct and without the need for a large excess of nucleophiles or additives. The method is potentially useful for the synthesis of a variety of α-2-deoxygalactosides.

Recent Developments in Stereoselective Chemical Glycosylation

Because of their pivotal biological functions, attention to sugars and glycobiology has grown rapidly in recent decades, leading to increased demand for homogeneous oligosaccharides. The stereoselective preparation of oligosaccharides by chemical means remains challenging and continues to be a vivid research area for organic chemists. In the past decade, new approaches and reinvestigated traditional methods have transformed the field. These developments include novel catalyses, various types of glycosylation modulators and the use of photochemical energy to facilitate glycosylation. This Minireview presents a brief overview of the latest trends in chemical glycosylation, with emphasis on the stereoselective synthetic protocols developed in the past decade.

Stereoselective β-Mannosylation with 2,6-Lactone-bridged Thiomannosyl Donor by Remote Acyl Group Participation

Stereoselective β-mannosylation has been recognized as one of the greatest challenges of carbohydrate chemistry. Herein, we described a practical method for stereoselective construction of β-mannosides by using a 2,6-lactone-bridged thiomannosyl donor through the remote acyl-group participation as well as the steric effect of O-4 substituent. The two effects are enabled through the conversion of a regular mannopyranosyl ⁴ C₁ conformation into a 2,6-lactone bridged conformation. The lactone donor could be readily prepared in three steps on a gram scale and the β-mannosylation proceeded smoothly with high stereoselectivity for primary, secondary and tertiary alcohol acceptors. In addition, this strategy was successfully applied to the synthesis of a naturally occurring trisaccharide.

Stereoselective organocatalyzed glycosylations - thiouracil, thioureas and monothiophthalimide act as Brønsted acid catalysts at low loadings

Thiouracil catalyzes stereoselective glycosylations with galactals in loadings as low as 0.1 mol%. It is proposed that in these glycosylations thiouracil, monothiophthalimide, and the previously reported catalyst, Schreiner's thiourea, do not operate via a double H-bonding mechanism but rather by Brønsted acid/base catalysis. In addition to the synthesis of 2-deoxyglycosides and glycoconjugates, we report the first organocatalytic synthesis of 1,1'-linked trehalose-type sugars.

Recent advances in reagent-controlled stereoselective/stereospecific glycosylation

The formation of O-glycosidic linkage is arguably one of the most important topics in glycoscience due to the prevalence of O-glycosides in nature. Great efforts have been devoted to this field by many carbohydrate chemists to develop stereoselective/stereospecific glycosylation methodologies. Although glycosyl donor- and acceptor-controlled strategies have significantly progressed, the tedious design and pre-synthesis of substrates could not be avoided. On the other hand, reagent-controlled glycosylation can overcome these challenges and produce the desired selectivity by only altering external factors such as concentration, reagents or other reaction conditions. This mini-review discusses selected recent novel methodologies on reagent-mediated stereo-controlled glycosylation in the last decade, classified by the types of glycosyl donors.

The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1-SN2 Interface

A critical review of the state-of-the-art evidence in support of the mechanisms of glycosylation reactions is provided. Factors affecting the stability of putative oxocarbenium ions as intermediates at the SN1 end of the mechanistic continuum are first surveyed before the evidence, spectroscopic and indirect, for the existence of such species on the time scale of glycosylation reactions is presented. Current models for diastereoselectivity in nucleophilic attack on oxocarbenium ions are then described. Evidence in support of the intermediacy of activated covalent glycosyl donors is reviewed, before the influences of the structure of the nucleophile, of the solvent, of temperature, and of donor-acceptor hydrogen bonding on the mechanism of glycosylation reactions are surveyed. Studies on the kinetics of glycosylation reactions and the use of kinetic isotope effects for the determination of transition-state structure are presented, before computational models are finally surveyed. The review concludes with a critical appraisal of the state of the art.

Stereodivergent Mannosylation Using 2- O-( ortho-Tosylamido)benzyl Group

We report a novel strategy for obtaining both anomers from a single mannosyl donor equipped with a C2- o-TsNHbenzyl ether (2- O-TAB) by switching reaction conditions. In particular, the formation of various β-mannosides was achieved with high selectivity by using a mannosyl phosphite in the presence of ZnI₂.

Regioselective O-Glycosylation of Nucleosides via the Temporary 2',3'-Diol Protection by a Boronic Ester for the Synthesis of Disaccharide Nucleosides

Disaccharide nucleosides, which consist of disaccharide and nucleobase moieties, have been known as a valuable group of natural products having multifarious bioactivities. Although chemical O-glycosylation is a commonly beneficial strategy to synthesize disaccharide nucleosides, the preparation of substrates such as glycosyl donors and acceptors requires tedious protecting group manipulations and a purification at each synthetic step. Meanwhile, several research groups have reported that boronic and borinic esters serve as a protecting or activating group of carbohydrate derivatives to achieve the regio- and/or stereoselective acylation, alkylation, silylation, and glycosylation. In this article, we demonstrate the procedure for the regioselective O-glycosylation of unprotected ribonucleosides utilizing boronic acid. The esterification of 2',3'-diol of ribonucleosides with boronic acid makes the temporary protection of diol, and, following O-glycosylation with a glycosyl donor in the presence of p-toluenesulfenyl chloride and silver triflate, permits the regioselective reaction of the 5'-hydroxyl group to afford the disaccharide nucleosides. This method could be applied to various nucleosides, such as guanosine, adenosine, cytidine, uridine, 5-metyluridine, and 5-fluorouridine. This article and the accompanying video represent useful (visual) information for the O-glycosylation of unprotected nucleosides and their analogs for the synthesis of not only disaccharide nucleosides, but also a variety of biologically relevant derivatives.

Systematic and Stereoselective Total Synthesis of Mannosylerythritol Lipids and Evaluation of Their Antibacterial Activity

The total synthesis of the 20 homogeneous members of mannosylerythritol lipids (MELs) with different alkyl chain lengths was effectively and systematically accomplished from a strategically designed common key intermediate that was stereoselectively constructed by the borinic acid catalyzed β-mannosylation reaction. In addition, their antibacterial activities against Gram-positive bacteria were evaluated. Our results demonstrated that not only the length of the alkyl chains but also the pattern of Ac groups on the mannose moiety were important factors for antibacterial activity.

Boronic-Acid-Catalyzed Regioselective and 1,2- cis-Stereoselective Glycosylation of Unprotected Sugar Acceptors via SNi-Type Mechanism

Regio- and 1,2- cis-stereoselective chemical glycosylation of unprotected glycosyl acceptors has been in great demand for the efficient synthesis of natural glycosides. However, simultaneously regulating these selectivities has been a longstanding problem in synthetic organic chemistry. In nature, glycosyl transferases catalyze regioselective 1,2- cis-glycosylations via the S_Ni mechanism, yet no useful chemical glycosylations based on this mechanism have been developed. In this paper, we report a highly regio- and 1,2- cis-stereoselective S_Ni-type glycosylation of 1,2-anhydro donors and unprotected sugar acceptors using p-nitrophenylboronic acid (10e) as a catalyst in the presence of water under mild conditions. Highly controlled regio- and 1,2- cis-stereoselectivities were achieved via the combination of boron-mediated carbohydrate recognition and the S_Ni-type mechanism. Mechanistic studies using the KIEs and DFT calculations were consistent with a highly dissociative concerted S_Ni mechanism. This glycosylation method was applied successfully to the direct glycosylation of unprotected natural glycosides and the efficient synthesis of a complex oligosaccharide with minimal protecting groups.