Mild Method for 2-Naphthylmethyl Ether Protecting Group Removal Using a Combination of 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and β-Pinene

Effects of Heparan Sulfate Trisaccharide Containing Oleanolic Acid in Attenuating Hyperphosphorylated Tau-Induced Cell Dysfunction Associated with Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia, marked by progressive brain degeneration and cognitive decline. A major pathological feature of AD is the accumulation of hyperphosphorylated tau (p-tau) in the form of neurofibrillary tangles (NFTs), which leads to neuronal death and neurodegeneration. P-tau also induces endoplasmic reticulum (ER) stress and activates the unfolded protein response, causing inflammation and apoptosis. Additionally, p-tau spreads in the brain through interactions with heparan sulfate (HS) proteoglycans, promoting aggregation and internalization. Targeting the tau-HS interaction offers a potential therapeutic strategy for AD. We present a novel HS mimetic with a lipophilic oleanolic acid linker and a sulfated trisaccharide, which shows strong cytoprotective effects against p-tau. Moreover, this compound alleviates p-tau-induced ER stress and inflammation. Molecular docking studies indicate that the conjugation of oleanolic acid enhances binding between the ligand and tau protofilament cores, facilitating protective interactions. These findings provide a foundation for the development of novel HS mimetics, enabling further investigation of tau-HS interactions in AD and other tauopathies.

Iriomoteolide-1a and -1b: Structure Elucidation by Integrating NMR Spectroscopic Analysis, Theoretical Calculation, and Total Synthesis

The structure of iriomoteolide-1a, a marine macrolide with potent cytotoxic activity against human cancer cells, has been under scrutiny for more than a decade since the first total synthesis of the proposed structure was achieved by Horne. Here we disclose the correct structure of iriomoteolide-1a. Given a huge number of possible stereoisomers, we adopted an integrated strategy toward the structure elucidation of iriomoteolide-1a: (1) NMR spectroscopic analysis/molecular mechanics-based conformational analysis for configurational reassignment of the macrolactone domain; (2) model synthesis for validating the reassigned configuration of the macrolactone domain; (3) GIAO NMR calculation/DP4+ analysis of side chain stereoisomers; and (4) total synthesis of the most likely structure. Moreover, the correct structure of iriomoteolide-1b, a natural congener, was also determined by an integration of NMR spectroscopic analysis, GIAO NMR calculation/DP4+ analysis, and total synthesis.

Synthesis of Salmonella enteritidis Antigenic Tetrasaccharide Repeating Unit by Employing Cationic Gold(I)-Catalyzed Glycosylation Involving Glycosyl N-1,1-Dimethylpropargyl Carbamate Donors

Synthesis of an antigenic tetrasaccharide repeating unit of the O-polysaccharide of Salmonella enteritidis lipopolysaccharide has been accomplished. Those four monosaccharides were assembled stereoselectively by employing our recently developed cationic gold(I)-catalyzed glycosylation methodology involving various glycosyl N-1,1-dimethylpropargyl carbamate donors. The newly formed α-anomeric stereochemical configuration was controlled by the axial C2-OBz of the glycosyl donors via anchimeric assistance.

Esterification and Etherification of Aliphatic Alcohols Enabled by Catalytic Strain-Release of Donor-Acceptor Cyclopropane

We herein disclose a highly efficient protocol for the esterification and etherification of alcohols, leveraging a Sc(OTf)3-catalyzed ring-strain release event in the meticulously designed, chromatographically stable mixed anhydrides or benzyl esters that incorporate an intramolecular donor-acceptor cyclopropane (DAC). This versatile method facilitates the straightforward functionalization of sugar, terpene, and steroid alcohols under mild acidic conditions, as showcased by the single-catalyst-driven, dual protection of sugar diol.

Total Synthesis of a Conjugation-Ready Tetrasaccharide Repeating Unit of Vibrio cholerae O:3 O-antigen Polysaccharide

Herein, we report the first total synthesis of the tetrasaccharide repeating unit of Vibrio cholerae O:3 O-antigen polysaccharide. The highly complex tetrasaccharide contains rare amino sugars such as d-bacillosamine and l-fucosamine, highly labile sugar ascarylose, and higher carbon sugar d-d-heptose. Stereoselective glycosylation of the notoriously reactive ascarylose with d-d-heptose, poor nucleophilicity of the axial C4-OH of l-fucosamine, and amide coupling are the key challenges encountered in the total synthesis, which was completed via a longest linear sequence of 23 steps in 4.2% overall yield.

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.

Oxidative Deprotection of Benzyl Protecting Groups for Alcohols by an Electronically Tuned Nitroxyl-Radical Catalyst

The oxidative deprotection of benzyl (Bn) groups using nitroxyl-radical catalyst 1 and co-oxidant phenyl iodonium bis(trifluoroacetate) (PIFA) is reported. This catalyst is highly active for the oxidation of benzylic ethers because of the electronic tuning on account of the electron-withdrawing ester groups next to the catalytically active center. This catalytic system promotes deprotections at ambient temperature and has a broad substrate scope, including substrates possessing hydrogenation-sensitive functional groups, while the deprotection hardly proceeds when using well-known nitroxyl-radical catalysts such as 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO). The 1/PIFA system also promotes the deprotection of several benzylic protecting groups, including 2-naphthylmethyl (NAP) and 4-methylbenzyl (MBn) groups. Catalyst 1 was also effective for the direct synthesis of ketones and aldehydes from Bn ethers via deprotected alcohols using an excess of the co-oxidant PIFA.

Rational Design and Expedient Synthesis of Heparan Sulfate Mimetics from Natural Aminoglycosides for Structure and Activity Relationship Studies

Heparan sulfate (HS) contains variably repeating disaccharide units organized into high- and low-sulfated domains. This rich structural diversity enables HS to interact with many proteins and regulate key signaling pathways. Efforts to understand structure-function relationships and harness the therapeutic potential of HS are hindered by the inability to synthesize an extensive library of well-defined HS structures. We herein report a rational and expedient approach to access a library of 27 oligosaccharides from natural aminoglycosides as HS mimetics in 7-12 steps. This strategy significantly reduces the number of steps as compared to the traditional synthesis of HS oligosaccharides from monosaccharide building blocks. Combined with computational insight, we identify a new class of four trisaccharide compounds derived from the aminoglycoside tobramycin that mimic natural HS and have a strong binding to heparanase but a low affinity for off-target platelet factor-4 protein.

Orthogonal Glycosylation with Phosphate Acceptors for Expeditious Synthesis of Bacterial Inner Core Oligosaccharides

We report a practical one-pot glycosylation strategy for synthesis of bacterial inner core oligosaccharides that composed of unavailable L-glycero-D-manno and D-glycero-D-manno-heptopyranose components. The glycosylation method features a new orthogonal glycosylation procedure; whereby a phosphate acceptor is coupled with a thioglycosyl donor producing a disaccharide phosphate, which can be engaged in another orthogonal glycosylation procedure to couple with a thioglycosyl acceptor. The phosphate acceptors used in above one-pot procedure are directly prepared from thioglycosyl acceptors via the in-situ phosphorylation. Such phosphate acceptor preparation protocol eliminates the traditional protection and deprotection procedures. Based on the new one-pot glycosylation strategy, two partial inner core structures of Yersinia pestis lipopolysaccharide and Haemophilus ducreyi lipooligosaccharide were acquired.

A Bioactive Synthetic Outer-Core Oligosaccharide Derived from a Klebsiella pneumonia Lipopolysaccharide for Bacteria Recognition

There is an urgent need for new treatment options for carbapenem-resistant Klebsiella pneumoniae (K. pneumoniae), which is a common cause of life-threatening hospital- and community-acquired infections. Prophylactic or therapeutic vaccination may offer an approach to control these infections, however, none has yet been approved for human use. Here, we report the chemical synthesis of an outer core tetra- and pentasaccharide derived from the lipopolysaccharide of K. pneumoniae. The oligosaccharides were equipped with an aminopentyl linker, which facilitated conjugation to the carrier proteins CRM197 and BSA. Mice immunized with the glycoconjugate vaccine candidates elicited antibodies that recognized isolated LPS as well as various strains of K. pneumoniae. The successful preparation of the oligosaccharides relied on the selection of monosaccharide building blocks equipped with orthogonal hydroxyl and amino protecting groups. It allowed the differentiation of three types of amines of the target compounds and the installation of a crowded 4,5-branched Kdo moiety.

Synthesis of the Tetrasaccharide Glycone Part of Tetrocarcin A

A de novo synthesis of the tetrasaccharide fragment of tetrocarcin A is described. The key feature of this approach is highlighted by the regio- and diastereoselective Pd-catalyzed hydroalkoxylation of ene-alkoxyallenes with an unprotected l-digitoxose glycoside. The subsequent reaction with digitoxal in combination with chemoselective hydrogenation generated the target molecule.

Protecting group principles suited to late stage functionalization and global deprotection in oligosaccharide synthesis

Chemical synthesis is a powerful tool to access homogeneous complex glycans, which relies on protecting group (PG) chemistry. However, the overall efficiency of chemical glycan assembly is still low when compared to oligonucleotide or oligopeptide synthesis. There have been many contributions giving rise to collective improvement in carbohydrate synthesis that includes PG manipulation and stereoselective glycoside formation and some of this chemistry has been transferred to the solid phase or adapted for programmable one pot synthesis approaches. However, after all glycoside bond formation reactions are completed, the global deprotection (GD) required to give the desired target OS can be challenging. Difficulties observed in the removal of permanent PGs to release the desired glycans can be due to the number and diversity of PGs present in the protected OSs, nature and structural complexity of glycans, etc. Here, we have reviewed the difficulties associated with the removal of PGs from densely protected OSs to obtain their free glycans. In particularly, this review focuses on the challenges associated with hydrogenolysis of benzyl groups, saponification of esters and functional group interconversion such as oxidation/reduction that are commonly performed in GD stage. More generally, problems observed in the removal of permanent PGs is reviewed herein, including benzyl, acyl (levulinoyl, acetyl), N-trichloroacetyl, N-2,2,2-trichloroethoxycarbonyl, N-phthaloyl etc. from a number of fully protected OSs to release the free sugar, that have been previously reported in the literature.

Synthesis of Oligosaccharides Resembling the Streptococcus suis Serotype 18 Capsular Polysaccharide as a Basis for Glycoconjugate Vaccine Development

Here we report the first total synthesis of several oligosaccharides resembling the capsular polysaccharide of swine pathogen S. suis serotype 18 repeating unit [→3)-d-GalNAc(α1-3)[d-Glc(β1-2)]-d-GalA4OAc(β1-3)-d-GalNAc(α1-3)-d-BacNAc4NAc(α1→]_n. Access to the pentasaccharide repeating unit antigen proved to be very challenging due to the poor reactivity in the context of the trisaccharide. The challenge was overcome by the creation of a galacturonic acid in a late stage of the synthesis.

Glycosyl Sulfonates Beyond Triflates

While glycosyl triflates are frequently invoked as intermediates in many chemical glycosylation reactions, the chemistry of other glycosyl sulfonates remains comparatively underexplored. Given the reactivity of sulfonates can span several orders of magnitude, this represents an untapped resource for the development of stereoselective glycosylation reactions. This personal account describes our laboratories efforts to take advantage of this reactivity to develop β-specific glycosylation reactions. Initial investigations led to the development of 2-deoxy-sugar tosylates as highly selective donors for β-glycoside synthesis, an approach which has been used to great success by our group and others for the construction of deoxy-sugar oligosaccharides and natural products. Subsequent studies demonstrate that "matching" the reactivity of the sulfonate to that of the sugar donor leads to highly selective SN 2-glycosylations with a range of substrates.

Synthesis of Pentasaccharide Repeating Unit Corresponding to the Cell Wall O-Polysaccharide of Salmonella enterica O55 Strain Containing a Rare Sugar 3-Acetamido-3-deoxy-d-fucose

A pentasaccharide repeating unit corresponding to the cell wall O-antigen of Salmonella enterica O55 containing a rare sugar, 3-acetamido-3-deoxy-d-fucose has been synthesized as its p-methoxyphenyl glycoside using a sequential stereoselective glycosylation strategy. A suitably functionalized 3-azido-3-deoxy-d-fucose thioglycoside derivative was prepared in very good yield and used in the stereoselective glycosylation reaction. Functionalized monosaccharide intermediates were prepared judiciously and stereoselectively assembled to get the desired pentasaccharide derivative in excellent yield.

Chemoenzymatic Synthesis of Complex N-Glycans of the Parasite S. mansoni to Examine the Importance of Epitope Presentation on DC-SIGN recognition

The importance of multivalency for N-glycan-protein interactions has primarily been studied by attachment of minimal epitopes to artificial multivalent scaffold and not in the context of multi-antennary glycans. N-glycans can be modified by bisecting GlcNAc, core xylosides and fucosides, and extended N-acetyl lactosamine moieties. The impact of such modifications on glycan recognition are also not well understood. We describe here a chemoenzymatic methodology that can provide N-glycans expressed by the parasitic worm S. mansoni having unique epitopes at each antenna and containing core xyloside. NMR, computational and electron microscopy were employed to investigate recognition of the glycans by the human lectin DC-SIGN. It revealed that core xyloside does not influence terminal epitope recognition. The multi-antennary glycans bound with higher affinity to DC-SIGN compared to mono-valent counterparts, which was attributed to proximity-induced effective concentration. The multi-antennary glycans cross-linked DC-SIGN into a dense network, which likely is relevant for antigen uptake and intracellular routing.

Discovery of Oligosaccharide Antigens for Semi-Synthetic Glycoconjugate Vaccine Leads against Streptococcus suis Serotypes 2, 3, 9 and 14*

Streptococcus suis bacteria are one of the most serious health problems for pigs and an emerging zoonotic agent in humans working in the swine industry. S. suis bacteria express capsular polysaccharides (CPS) a major bacterial virulence factor that define the serotypes. Oligosaccharides resembling the CPS of S. suis serotypes 2, 3, 9, and 14 have been synthesized, glycans related to serotypes 2 and 9 were placed on glycan array surfaces to screen blood from infected pigs. Lead antigens for the development of semi-synthetic S. suis serotypes 2 and 9 glycoconjugate veterinary vaccines were identified in this way.

Convergent Synthesis of Tetrasaccharide Fragment of Cervimycin K

A new synthetic approach toward oligosaccharides consisting only of 2,3,6-trideoxypyranoglycosides is reported. The key feature is highlighted by the convergent approach that allows the introduction of the aglycon moiety in the late stage of the synthesis. As an illustrative example, the tetrasaccharide portion of cervimycin K was prepared as cyclohexyl glycoside.

Triethylamine-methanol mediated selective removal of oxophenylacetyl ester in saccharides

A highly selective, mild, and efficient method for the cleavage of oxophenylacetyl ester protected saccharides was developed using triethylamine in methanol at room temperature. The reagent proved successful against different labile groups like acetal, ketal, and PMB and also generated good yields of the desired saccharides bearing lipid esters. Further, we also observed DBU in methanol as an alternative reagent for the deprotection of acetyl, benzoyl, and oxophenylacetyl ester groups.

Synthesis of monophosphoryl lipid A using 2-naphtylmethyl ethers as permanent protecting groups

Lipid A, which is a conserved component of lipopolysaccharides of gram-negative bacteria, has attracted considerable interest for the development of immuno-adjuvants. Most approaches for lipid A synthesis rely on the use of benzyl ethers as permanent protecting groups. Due to the amphiphilic character of lipid A, these compounds aggregate during the hydrogenation step to remove benzyl ethers, resulting in a sluggish reaction and by-product formation. To address this problem, we have developed a synthetic approach based on the use of 2-naphtylmethyl ether (Nap) ethers as permanent protecting group for hydroxyls. At the end of a synthetic sequence, multiple of these protecting groups can readily be removed by oxidation with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). Di-allyl N,N-diisopropylphosphoramidite was employed to install the phosphate ester and the resulting allyl esters were cleaved using palladium tetrakistriphenylphosphine. The synthetic strategy allows late stage introduction of different fatty acids at the amines of the target compound, which is facilitated by Troc and Fmoc as orthogonal amino-protecting groups.

Synthesis of monophosphorylated lipid A precursors using 2-naphthylmethyl ether as a protecting group

Lipid A, the hydrophobic domain of lipopolysaccharide (LPS), is a strong immunostimulator and therefore a valuable target for the development of novel immunomodulators. Various lipid A derivatives have been chemically synthesized in order to reduce toxicity while retaining the immunostimulatory activity. In this work, we describe a novel approach to the frequently problematic synthesis of monophosphorylated mono- and disaccharide lipid X using a combination of established chemistry and a novel 2-naphthylmethyl ether (Nap) protecting group for “permanent” protection of hydroxy groups. Of particular note is the fact that the key Nap protecting group is able to remain in the molecule until the final global deprotection step. Our synthetic strategy is not only efficient in regards to the yield of the various chemical transformations, but also robust in regards to the potential application of this route to the production of other lipid A analogs.

Reagent-Controlled α-Selective Dehydrative Glycosylation of 2,6-Dideoxy Sugars: Construction of the Arugomycin Tetrasaccharide

The first synthesis of the tetrasaccharide fragment of the anthracycline natural product Arugomycin is described. A reagent controlled dehydrative glycosylation method involving cyclopropenium activation was utilized to synthesize the α-linkages with complete anomeric selectivity. The synthesis was completed in 20 total steps, and in 2.5% overall yield with a longest linear sequence of 15 steps.

Modulating Heparanase Activity: Tuning Sulfation Pattern and Glycosidic Linkage of Oligosaccharides

Heparanase cleaves polymeric heparan sulfate (HS) molecules into smaller oligosaccharides, allowing for release of angiogenic growth factors promoting tumor development and autoreactive immune cells to reach the insulin-producing β cells. Interaction of heparanase with HS chains is regulated by specific substrate sulfation sequences. We have synthesized 11 trisaccharides that are highly tunable in structure and sulfation pattern, allowing us to determine how heparanase recognizes HS substrate and selects a favorable cleavage site. Our study shows that (1) N-SO₃^- at +1 subsite and 6-O-SO₃^- at -2 subsite of trisaccharides are critical for heparanase recognition, (2) addition of 2-O-SO₃^- at the -1 subsite and of 3-O-SO₃^- to GlcN unit is not advantageous, and (3) the anomeric configuration (α or β) at the reducing end is crucial in controlling heparanase activity. Our study also illustrates that the α-trisaccharide having N- and 6-O-SO₃^- at -2 and +1 subsites inhibited heparanase and was resistant toward hydrolysis.

Total Synthesis of the Lipid-Anchor-Attached Core Trisaccharides of Lipoteichoic Acids of Streptococcus pneumoniae and Streptococcus oralis Uo5

Herein we report an efficient total synthesis of lipid-anchor-appended core trisaccharides of lipoteichoic acids of Streptococcus pneumoniae and Streptococcus oralis Uo5. The key features include the expedient synthesis of the rare sugar 2,4,6-trideoxy-2-acetamido-4-amino-d-Galp building block via one-pot sequential S_N2 reactions and the α-selective coupling of d-thioglucoside with the diacyl glycerol acceptor to construct a common disaccharide acceptor, which was utilized in the total synthesis of target molecules 1 and 2.

Total Synthesis of the Congested, Bisphosphorylated Morganella morganii Zwitterionic Trisaccharide Repeating Unit

Zwitterionic polysaccharides (ZPSs) activate T-cell-dependent immune responses by major histocompatibility complex class II presentation. Herein, we report the first synthesis of a Morganella morganii ZPS repeating unit as an enabling tool in the synthesis of novel ZPS materials. The repeating unit incorporates a 1,2-cis-α-glycosidic bond; the problematic 1,2-trans-galactosidic bond, Gal-β-(1 → 3)-GalNAc; and phosphoglycerol and phosphocholine residues which have not been previously observed together as functional groups on the same oligosaccharide. The successful third-generation approach leverages a first in class glycosylation of a phosphoglycerol-functionalized acceptor. To install the phosphocholine unit, a highly effective phosphocholine donor was synthesized.

Reagent Controlled Direct Dehydrative Glycosylation with 2-Deoxy Sugars: Construction of the Saquayamycin Z Pentasaccharide

The first synthesis of the pentasaccharide fragment of the angucycline antibiotic saquayamycin Z is described. By using our sulfonyl chloride mediated reagent controlled dehydrative glycosylation, we are able to assemble the glycosidic linkages with high levels of anomeric selectivity. The total synthesis was completed in 25 total steps, and in 2.5% overall yield with a longest linear sequence of 15 steps.

Synthesis of the Hexasaccharide Fragment of Landomycin A Using a Mild, Reagent-Controlled Approach

The synthesis of the hexasaccharide fragment of landomycin A is reported. Using p-toluenesulfonyl chloride mediated dehydrative glycosylation, we constructed the deoxy-sugar linkages in a stereoselective fashion without the need for temporary prosthetic groups to control selectivity. Through this approach, the hexasaccharide was obtained in 28 steps and 8.9% overall yield, which is an order of magnitude higher than that of previously reported approaches.

Introducing Oxo-Phenylacetyl (OPAc) as a Protecting Group for Carbohydrates

A series of oxo-phenylacetyl (OPAc)-protected saccharides, with divergent base sensitivity profiles against benzoyl (Bz) and acetyl (Ac) were synthesized, and KHSO₅/AcCl in methanol was identified as an easy, mild, selective, and efficient deprotecting reagent for their removal in the perspective of carbohydrate synthesis. Timely monitoring of AcCl reagent was supportive in both sequential and simultaneous deprotecting of OPAc, Bz, and Ac. The salient feature of our method is the orthogonal stability against different groups, its ease to generate different valuable acceptors using designed monosaccharides, and use of OPAc as a glycosyl donar.

Secondary amine salt catalyzed controlled activation of 2-deoxy sugar lactols towards alpha-selective dehydrative glycosylation

A new organocatalytic glycosylation method exploiting the lactol functionality has been disclosed. The catalytic generation of glycosyl oxacarbenium ions from lactols under forcible conditions via weakly Brønsted-acidic, readily available secondary amine salts affects the diastereoselective glycosylation of 2-deoxypyranoses and furanoses. This operationally simple iminium catalyzed activation of 2-deoxy hemi-acetals is a potential alternative to the existing cumbersome methods that need specialized handling. The mechanisms for this unique transformation and kinetic/thermodynamic effects have been discussed based on both experimental evidence and theoretical studies.

Assembly of glycoamino acid building blocks: a new strategy for the straightforward synthesis of heparan sulfate mimics

A new strategy that enables a modular straightforward synthesis of heparan sulfate oligosaccharide mimics by the assembly of simple glycoamino acid building blocks is described. The coupling between units is readily carried out by an amidation reaction. Several glycoamino acid oligomers were prepared and their interaction with the FGF2 protein was analyzed.

Synthesis of the Non-Reducing Hexasaccharide Fragment of Saccharomicin B

A synthesis of the nonreducing end hexasaccharide of saccharomicin B, α-l-Eva-(1→4)-α-l-Eva-(1→4)-α-l-Dig-(1→4)-α-l-Eva-(1→4)-α-l-Dig-(1→4)-β-d-Fuc, has been developed. Selective glycosylations of l-digitoxose (l-Dig) using AgPF₆/TTBP-mediated thioether activation and l-4-e pi-vancosamine (l-Eva) using Tf₂O/DTBMP-mediated sulfoxide activation produced the hexasaccharide as a single diastereomer in very good yield. This hexasaccharide is properly functionalized to serve as a glycosyl donor for the total synthesis of saccharomicin B.

Stereospecific Synthesis of the Saccharosamine-Rhamnose-Fucose Fragment Present in Saccharomicin B

A synthetic route has been developed for constructing the d-saccharosamine-l-rhamnose-d-fucose (Sac-Rha-Fuc) trisaccharide fragment present in the antibacterial natural product saccharomicin B. The Sac monosaccharide was synthesized through a modified nine step procedure starting from d-rhamnal in 23% overall yield. 1- O-TBS Sac donors were used to construct the β-linked Sac-Rha disaccharide. This disaccharide was coupled to a Fuc acceptor under BSP/Tf2O conditions to afford a trisaccharide properly functionalized for elaboration to saccharomicin B.

Reagent-Controlled Synthesis of the Branched Trisaccharide Fragment of the Antibiotic Saccharomicin B

A concise synthesis of a branched trisaccharide, α-l-Dig-(1 → 3)-[α-l-Eva-(1 → 4)]-β-d-Fuc, corresponding to saccharomicin B, has been developed via reagent-controlled α-selective glycosylations. Starting from the d-fucose acceptor, l- epi-vancosamine was selectively installed using 2,3-bis(2,3,4-trimethoxyphenyl)cyclopropene-1-thione/oxalyl bromide mediated dehydrative glycosylation. Following deprotection, l-digitoxose was installed using the AgPF6/TTBP thioether-activation method to produce the trisaccharide as a single α-anomer. This highly functionalized trisaccharide can potentially serve as both a donor and an acceptor for the total synthesis of the antibiotic saccharomicin B.

Challenges in the Conversion of Manual Processes to Machine-Assisted Syntheses: Activation of Thioglycoside Donors with Aryl(trifluoroethyl)iodonium Triflimide

The steps needed to adapt a stable iodonium promoter for use in automated fluorous-assisted solution-phase oligosaccharide synthesis are described. Direct adaptation of the originally reported batch procedure resulted in the formation of an orthoester or protecting group transfer to the glycosyl acceptor. Fortunately, the addition of inexpensive β-pinene as an acid scavenger avoided both of these side reactions. The utility of this newly developed protocol was applied to the automated solution-phase synthesis of a β-glucan fragment.

Hydrogenolytic cleavage of naphthylmethyl ethers in the presence of sulfides

With the aid of a series of model thioether or thioglycoside containing polyols protected with combinations of benzyl ethers and 2-naphthylmethyl ethers it is demonstrated that the latter are readily cleaved selectively under hydrogenolytic conditions in the presence of the frequently catalyst-poisoning sulfides. These results suggest the possibility of employing 2-naphthylmethyl ethers in place of benzyl ethers in synthetic schemes when hydrogenolytic deprotection is anticipated in the presence of thioether type functionality.