Synthesis of Core M3 Matriglycan Constituents via an Additive-Controlled 1,2-cis-Xylopyranosylation with O-Xylosyl Imidates as Donors

A highly stereoselective strategy for 1,2-cis-xylopyranoside bond formation was established via a preactivation-based, additive-modulated trichloroacetimidate glycosidation strategy. The current protocol is mild, practical, and successful with various xylopyranosyl donors and glycosyl acceptors, including acceptors that are reported to be less reactive due to steric hindrance. The utility of this method was demonstrated with the facile assembly of matriglycan constituent tetra- and hexasaccharides.

Preparation of Tea Aroma Precursor Glycosides: An Efficient and Sustainable Approach via Chemical Glycosidation

Tea aroma precursor glycosides are plant-derived natural products with great economic value. However, the preparation of these glycosides remains largely overlooked in the past decades. Herein, we report a mild, efficient, and sustainable chemocatalytic procedure for the production of tea aroma precursor glycosides. During the study of the glycosidation, the catalysts were found to be decisive in the product formation favoring different reaction pathways; in addition, the influence of molecular sieves was elucidated. With regard to these findings, the serious problem of the competing orthoester formation side reaction was successfully overcome with low catalyst loading (1 mol %) and the use of 5 Å molecular sieves, leading to the preparation of a variety of tea aroma precursor β-d-glucopyranosides and β-primeverosides on a gram scale in high yields in an economical way. Taken together, the current approach features catalytic glycosidation with non-toxic and low-cost catalysts, demonstrates highly favorable greenness and sustainability, and promises industrial production of tea aroma precursor glycosides.

Regioselective benzoylation of unprotected β-glycopyranosides with benzoyl cyanide and an amine catalyst – application to saponin synthesis

Regioselective protection of trans-trans triol and tetrol moieties in carbohydrates was achieved with BzCN as the benzoylating agent and amine catalysts. The protocols are useful for the chemical synthesis of oligosaccharides and saponins.

The 2,2-Dimethyl-2-(ortho-nitrophenyl)acetyl (DMNPA) Group: A Novel Protecting Group in Carbohydrate Chemistry

The 2,2-dimethyl-2-(ortho-nitrophenyl)acetyl (DMNPA) group was introduced to synthetic carbohydrate chemistry as a protecting group (PG) for the first time. Benefiting from a unique chemical structure and novel deprotection conditions, the DMNPA group can be cleaved rapidly and mutually orthogonal to other PGs. Orchestrated application of the DMNPA group with other PGs led to the highly efficient synthesis of the glycan of thornasterside A.

Diversity-Oriented Synthesis of Steviol Glycosides

With cheap and easily available mixtures of steviol glycosides as starting materials, a practical method for steviol acquisition has been developed, on the basis of which a facile, diversity-oriented, and economic protocol for the synthesis of structurally defined steviol glycosides was established. The novel approach is featured by the highly efficient glycosylation of sterically hindered and acid-sensitive steviol via orchestrated application of Yu glycosylation, Schmidt glycosylation, and PTC glycosylation. Hence, these high-intensity sweeteners and potential lead compounds for drug development are now readily accessible.

Regioselective One-Pot Benzoylation of Triol and Tetraol Arrays in Carbohydrates

Protection of 2,3,4- O-unprotected α-galacto- and α-fucopyranosides with BzCN and DMAP/DIPEA as the base afforded directly and regioselectively the 3- O-unprotected derivatives. The rationale for these studies was to take advantage of the eventual cooperativity of the "cyanide effect" and "the alkoxy group mediated diol effect". This way, even the totally unprotected α-galactopyranosides could be regioselectively transformed into the corresponding 2,4,6- O-protected derivatives. The great utility of these building blocks was demonstrated in efficient trisaccharide syntheses.

Acid-Base Catalysis in Glycosidations: A Nature Derived Alternative to the Generally Employed Methodology

Inverting glycosyltransferases enforce in the active site an intramolecular, acid-base catalyzed glycosidation that, due to proximity of the donor anomeric carbon and the acceptor hydroxyl group, follows an S_N2-type reaction. Spacers, tethering donor and acceptor via nonreacting functional groups, led in intramolecular glycosidations to excellent yields and, independent of the donor anomeric configuration, to either the α- or the β-anomer. The requirement of a demanding protecting group pattern confines the application of this efficient method. Only the method where the 2-hydroxyl group of a mannopyranosyl donor is tethered via an acetal spacer to the reacting acceptor functional group is used for β-mannopyranoside synthesis. The most elegant method, tethering donor and acceptor covalently to the spacer via the leaving group and the reacting functional group, was so far not as efficient as hoped. This method is very efficient when donor and acceptor are temporarily assembled through a hydrogen-bond facilitating a stretched hexagon-like transition state. This follows from the stereoselective O-glucopyranosyl trichloroacetimidate transformation into O-glucopyranosyl phosphate with dibenzyl phosphoric acid as acceptor that can be regarded as A═B-C-H acceptor type. Generalizing this concept to the use of alcohols as acceptors requires reversible generation of an A-B-C-H adduct where A-H represents the acceptor (RO-H) and B═C a catalyst that has to fulfill several criteria. Among these criteria are low affinity to nitrogen, avoiding glycosyl donor activation in the absence of acceptor, and high affinity to oxygen in order to generate the A-B-C-H adduct with increased proton acidity. Thus, hydrogen-bond mediated self-assembly of donor and acceptor and concomitant donor activation via a transition state is available, which enforces an acid-base catalyzed S_N2-type reaction. It could be shown that PhBF₂, Ph₂BF, and PhSiF₃ are such catalysts that fulfill the desired four functions: reversible adduct formation with the acceptor, hydrogen-bond mediated tethering of this adduct with the donor, and acid- and base-catalysis of the glycosidation. Also Lewis acidic metal salts, particularly the dimeric gold(III) chloride, turned out to exhibit excellent B═C type catalyst properties. Worth mentioning in this context is the ability of gold(III) chloride to regioselectively activate diols. As thioureas have high affinity to anions and also to neutral compounds through strong hydrogen bonds, their binding to alcohols and concomitant activation of O-glycosyl trichloroacetimidates was of interest. Yet, even the acidic N,N'-bis[3,5-bis(trifluoromethyl)phenyl]-thiourea was unable to catalyze glycosidations. However, as a cocatalyst to acids, thiourea exerts a strong effect that, based on NMR studies, leads first to a hydrogen-bond mediated catalyst-cocatalyst-acceptor complex. This complex activates the donor in an intramolecular, acid-base catalyzed reaction that is again closely related to the action of inverting glycosyltransferases. Thus, from O-(α-glycosyl) trichloroacetimidates, good yields of the inversion products, that is, the β-glycosides, are obtained. This novel conceptual approach to glycosidation revealed that for retention of configuration in addition a catalytic nucleophile is required that enables formation of the α-glucoside from the α-trichloroacetimidate. Preliminary studies with a catalyst possessing this 5-fold function, that is, adduct formation with the acceptor, hydrogen-bonding between the reactants, acid and base catalysis, and a catalytic nucleophile as part of a chiral framework supporting facial selection, exhibited good chances for final success in this endeavor.

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.

2-Nitro-thioglycosides: α- and β-selective generation and their potential as β-selective glycosyl donors

Michael-type addition of thiolates to 2-nitro-D-glucal or to 2-nitro-D-galactal derivatives readily provides 2-deoxy-2-nitro-1-thioglycosides. Kinetic and thermodynamic reaction control permitted formation of either the α- or preferentially the β-anomers, respectively. Addition of achiral and chiral thiourea derivatives to the reaction mixture increased the reaction rate; the outcome is substrate-controlled. The 2-deoxy-2-nitro-1-thioglycosides are excellent glycosyl donors under arylsulfenyl chloride/silver triflate (ArSCl/AgOTf) activation, and they provide, anchimerically assisted by the nitro group, mostly β-glycosides.

Human L-ficolin recognizes phosphocholine moieties of pneumococcal teichoic acid

Human L-ficolin is a soluble protein of the innate immune system able to sense pathogens through its fibrinogen (FBG) recognition domains and to trigger activation of the lectin complement pathway through associated serine proteases. L-Ficolin has been previously shown to recognize pneumococcal clinical isolates, but its ligands and especially its molecular specificity remain to be identified. Using solid-phase binding assays, serum and recombinant L-ficolins were shown to interact with serotype 2 pneumococcal strain D39 and its unencapsulated R6 derivative. Incubation of both strains with serum triggered complement activation, as measured by C4b and C3b deposition, which was decreased by using ficolin-depleted serum. Recombinant L-ficolin and its FBG-like recognition domain bound to isolated pneumococcal cell wall extracts, whereas binding to cell walls depleted of teichoic acid (TA) was decreased. Both proteins were also shown to interact with two synthetic TA compounds, each comprising part structures of the complete lipoteichoic acid molecule with two PCho residues. Competition studies and direct interaction measurements by surface plasmon resonance identified PCho as a novel L-ficolin ligand. Structural analysis of complexes of the FBG domain of L-ficolin and PCho revealed that the phosphate moiety interacts with amino acids previously shown to define an acetyl binding site. Consequently, binding of L-ficolin to immobilized acetylated BSA was inhibited by PCho and synthetic TA. Binding of serum L-ficolin to immobilized synthetic TA and PCho-conjugated BSA triggered activation of the lectin complement pathway, thus further supporting the hypothesis of L-ficolin involvement in host antipneumococcal defense.

Equivalence of students' scores on timed and untimed anatomy practical examinations

Untimed examinations are popular with students because there is a perception that first impressions may be incorrect, and that difficult questions require more time for reflection. In this report, we tested the hypothesis that timed anatomy practical examinations are inherently more difficult than untimed examinations. Students in the Doctor of Physical Therapy program at Thomas Jefferson University were assessed on their understanding of anatomic relationships using multiple-choice questions. For the class of 2012 (n = 46), students were allowed to circulate freely among 40 testing stations during the 40-minute testing session. For the class of 2013 (n = 46), students were required to move sequentially through the 40 testing stations (one minute per item). Students in both years were given three practical examinations covering the back/upper limb, lower limb, and trunk. An identical set of questions was used for both groups of students (untimed and timed examinations). Our results indicate that there is no significant difference between student performance on untimed and timed examinations (final percent scores of 87.3 and 88.9, respectively). This result also held true for students in the top and bottom 20th percentiles of the class. Moreover, time limits did not lead to errors on even the most difficult, higher-order questions (i.e., items with P-values < 0.70). Thus, limiting time at testing stations during an anatomy practical examination does not adversely affect student performance.

Chemical synthesis of bacterial lipoteichoic acids: an insight on its biological significance

During infections caused by Gram-negative bacteria, lipopolysaccharide (LPS, endotoxin) has a dominant role leading to fulminant pro-inflammatory reactions in the host. As there is no LPS in Gram-positive bacteria, other microbial cell wall components have been identified to be the causative agent for the pro-inflammatory activity since also Gram-positive bacterial infections lead to comparable clinical symptoms and reactions. On search for the "Gram-positive endotoxin" a widely accepted hypothesis has been raised in that the lipoteichoic acids (LTAs) serve as pathogen-associated molecular patterns (PAMPs) during Gram-positive sepsis, although the amount necessary for a pro-inflammatory in vitro response is several orders of magnitude higher than that for LPS. Therefore, LTA cannot be considered to be "the (endo)toxin of Gram-positive bacteria". Although LPS and LTA show structural relatedness (amphiphilic, negatively charged glycophospholipids), they are structurally quite different from each other and one might expect that they are also recognized by different receptors of the innate immune system, the so called toll-like receptors 4 and 2 (TLR4 and TLR2), respectively. Based on their chemical structure, the LTAs were classified into four types (type I-IV) of which we have carefully investigated the LTA of Staphylococcus aureus (type I), Lactococcus garvieae (type II) and Streptococcus pneumoniae (type IV). Hence, these LTAs have been synthesized in our group and biologically evaluated with respect to their potency to activate cytokines in transiently TLR2/CD14-transfected human endothelial kidney cells (HEK 293) or human macrophages and whole blood cells. Although LTA of type I and IV are structurally quite different, especially in their hydrophilic moiety, they originally were believed to interact with the same receptor (TLR2). Hence, the chemical syntheses leading to structurally defined, non-contaminated stimuli have a major impact on the outcome and interpretation of these biological studies of the innate immune system. With this material, it became evident that synthetic LTA from S. aureus and S. pneumoniae are not recognized by TLR2. Instead, another receptor of the innate immune system, the lectin pathway of the complement, known since many years to interact with LTA in quite a specific way, has gained increasing attractivity. With the help of synthetic LTA we obtained first evidences that this receptor is indeed the pathogen recognition receptor (PRR) for LTA.

Binding of Toxoplasma gondii glycosylphosphatidylinositols to galectin-3 is required for their recognition by macrophages

We showed that the production of tumor necrosis factor (TNF) α by macrophages in response to Toxoplasma gondii glycosylphosphatidylinositols (GPIs) requires the expression of both Toll-like receptors TLR2 and TLR4, but not of their co-receptor CD14. Galectin-3 is a β-galactoside-binding protein with immune-regulatory effects, which associates with TLR2. We demonstrate here by using the surface plasmon resonance method that the GPIs of T. gondii bind to human galectin-3 with strong affinity and in a dose-dependent manner. The use of a synthetic glycan and of the lipid moiety cleaved from the GPIs shows that both parts are involved in the interaction with galectin-3. GPIs of T. gondii also bind to galectin-1 but with a lower affinity and only through the lipid moiety. At the cellular level, the production of TNF-α induced by T. gondii GPIs in macrophages depends on the expression of galectin-3 but not of galectin-1. This study is the first identification of a galectin-3 ligand of T. gondii origin, and galectin-3 might be a co-receptor presenting the GPIs to the TLRs on macrophages.

Innate-like control of human iNKT cell autoreactivity via the hypervariable CDR3beta loop

T-cell receptor variability gives rise to a functional hierarchy of human invariant Natural Killer T-cells through a powerful effect on CD1d binding affinity, which is independent of CD1d ligands.

Invariant Natural Killer T cells (iNKT) are a versatile lymphocyte subset with important roles in both host defense and immunological tolerance. They express a highly conserved TCR which mediates recognition of the non-polymorphic, lipid-binding molecule CD1d. The structure of human iNKT TCRs is unique in that only one of the six complementarity determining region (CDR) loops, CDR3β, is hypervariable. The role of this loop for iNKT biology has been controversial, and it is unresolved whether it contributes to iNKT TCR:CD1d binding or antigen selectivity. On the one hand, the CDR3β loop is dispensable for iNKT TCR binding to CD1d molecules presenting the xenobiotic alpha-galactosylceramide ligand KRN7000, which elicits a strong functional response from mouse and human iNKT cells. However, a role for CDR3β in the recognition of CD1d molecules presenting less potent ligands, such as self-lipids, is suggested by the clonal distribution of iNKT autoreactivity. We demonstrate that the human iNKT repertoire comprises subsets of greatly differing TCR affinity to CD1d, and that these differences relate to their autoreactive functions. These functionally different iNKT subsets segregate in their ability to bind CD1d-tetramers loaded with the partial agonist α-linked glycolipid antigen OCH and structurally different endogenous β-glycosylceramides. Using surface plasmon resonance with recombinant iNKT TCRs and different ligand-CD1d complexes, we demonstrate that the CDR3β sequence strongly impacts on the iNKT TCR affinity to CD1d, independent of the loaded CD1d ligand. Collectively our data reveal a crucial role for CDR3β for the function of human iNKT cells by tuning the overall affinity of the iNKT TCR to CD1d. This mechanism is relatively independent of the bound CD1d ligand and thus forms the basis of an inherent, CDR3β dependent functional hierarchy of human iNKT cells.

Our immune system uses randomly modified T-cell receptors (TCRs) to adapt its discriminative capacity to rapidly changing pathogens. The T-cell receptor (TCR) has six flexible, variable peptide loops that make contact with antigens presented to them on the surface of other cells. Invariant Natural Killer T-cells (iNKT) are regulatory T-cells with a unique type of TCR (iNKT-TCR) that recognizes lipid antigens presented by specific MHC-like molecules known as CD1d. In human iNKT-TCRs, only one of the six loops, CDR3beta, is variable. By comparing how different human iNKT clones bind and react to different CD1d-lipid complexes we uncover the existence of a hierarchical order of the human iNKT cell repertoire in which strongly CD1d-binding clones are autoreactive while weak CD1d-binding clones are non-autoreactive. Direct measurements of iNKT-TCR binding to CD1d using surface plasmon resonance recapitulated this hierarchy at the protein level. The data show that variation in the CDR3beta loop conveys dramatic differences in human iNKT TCR affinity that are independent of the CD1d bound ligand. Thus the CDR3beta loop provides the structural basis for the functional hierarchy of the human iNKT repertoire. We postulate that during the life-course, CDR3beta-dependent asymmetrical activation of different human iNKT clones leads to a bias in the iNKT repertoire, and this could result in age-dependent defects of iNKT-mediated immune regulation in later life.

Commensal bacteria regulate Toll-like receptor 3-dependent inflammation after skin injury

The normal microflora of the skin includes staphylococcal species that will induce inflammation when present below the dermis but are tolerated on the epidermal surface without initiating inflammation. Here we reveal a previously unknown mechanism by which a product of staphylococci inhibits skin inflammation. This inhibition is mediated by staphylococcal lipoteichoic acid (LTA), and acts selectively on keratinocytes triggered through Toll-like receptor (TLR) 3. The significance of this is seen by observations that TLR3 activation is required for normal inflammation after injury, and that keratinocytes require TLR3 to respond to RNA from damaged cells with the release of inflammatory cytokines. Staphylococcal LTA inhibits both inflammatory cytokine release from keratinocytes and inflammation triggered by injury through a TLR2-dependent mechanism. These findings show for the first time that the skin epithelium requires TLR3 for normal inflammation after wounding and that the microflora can modulate specific cutaneous inflammatory responses.

Nonglycosidic agonists of invariant NKT cells for use as vaccine adjuvants

Based on the crystal structures of human alpha-GalCer-CD1d and iNKT-alpha-GalCer-CD1d complexes, nonglycosidic analogues of alpha-GalCer were synthesized. They activate iNKT cells resulting in dendritic cell maturation and the priming of antigen-specific T and B cells. Therefore, they are attractive adjuvants in vaccination strategies for cancer and infectious diseases.

2-nitroglycals as powerful glycosyl donors: application in the synthesis of biologically important molecules

[Reaction: see text]. The biological significance of oligosaccharides and glycoconjugates is profound and wide-ranging. For example, the mucins have attracted attention because of their role in fundamental cellular processes such as fertilization, parasitic infection, inflammation, immune defense, cell growth, and cell-cell adhesion. Increased expression of mucins is implicated in malignant transformation of cells. Antifreeze glycoproteins also are of interest because they are important for the survival of many marine teleost fishes that live in polar and subpolar waters. The synthesis of glycoconjugates requires methods for glycoside bond formation, the most difficult aspect of which is the assembly of monosaccharide building blocks. This Account discusses a valuable addition to the repertoire of methods for glycoconjugate synthesis: an approach that involves 2-nitroglycal concatenation. For a long time, methods for glycosylation via glycosyl donor generation required either an anomeric oxygen exchange reaction or anomeric oxygen retention. In the case of an anomeric oxygen exchange reaction, activation of the glycosyl donors demands a promoter in at least equimolar amounts. However, anomeric oxygen retention, such as base-catalyzed formation of O-glycosyl trichloroacetimidates, can be activated by catalytic amounts of acid or Lewis acid. Alternatively, glycals, which are readily available from sugars, can be an attractive starting material for glycoside bond formation. Their nucleophilic character at C-2 permits reactions with oxygen, nitrogen, and sulfur electrophiles that under high substrate stereocontrol generally lead to three-membered rings; ring opening under acid catalysis furnishes the corresponding glycosides, whichdepending on the electrophile Xare also employed for 2-deoxyglycoside synthesis. Glycals also can be transformed into derivatives that have at C-2 an electron-withdrawing group and are amenable to Michael-type addition. A good example are 2-nitroglycals. In this case, glycoside bond formation is achieved under base catalysis and leads to 2-deoxy-2-nitroglycosides. These intermediates are readily converted into 2-amino-2-deoxyglycosides, which are constituents of almost all glycoconjugates. This 2-nitroglycal concatenation has been extensively investigated with 2-nitrogalactal derivatives. When alcohols are used as nucleophiles and strong bases used as catalysts, the result is primarily or exclusively the alpha-galacto-configured adducts. Some studies show that weaker bases may lead to preferential formation of the beta-galacto-configured products instead. The reaction was very successfully extended to other nucleophiles and also to other 2-nitroglycals that undergo base-catalyzed stereoselective Michael-type additions. Thus, 2-nitroglycals are versatile synthons in glycoconjugate and natural-products synthesis, and it is foreseeable that many more applications will be based on these readily available and highly functionalized skeletons.

Cutting edge: nonglycosidic CD1d lipid ligands activate human and murine invariant NKT cells

Invariant NKT cells (iNKT cells) recognize CD1d/glycolipid complexes. We demonstrate that the nonglycosidic compound threitolceramide efficiently activates iNKT cells, resulting in dendritic cell (DC) maturation and the priming of Ag-specific T and B cells. Threitolceramide-pulsed DCs are more resistant to iNKT cell-dependent lysis than alpha-galactosylceramide-pulsed DCs due to the weaker affinity of the human iNKT TCR for CD1d/ threitolceramide than CD1d/alpha-galactosylceramide complexes. iNKT cells stimulated with threitolceramide also recover more quickly from activation-induced anergy. Kinetic and functional experiments showed that shortening or lengthening the threitol moiety by one hydroxymethylene group modulates ligand recognition, as human and murine iNKT cells recognize glycerolceramide and arabinitolceramide differentially. Our data broaden the range of potential iNKT cell agonists. The ability of these compounds to assist the priming of Ag-specific immune responses while minimizing iNKT cell-dependent DC lysis makes them attractive adjuvants for vaccination strategies.