Highly Active Chromium Complexes Supported by Constrained Schiff-Base Ligands for Cycloaddition of Carbon Dioxide to Epoxides.

Novel constrained Schiff-base ligands (inden) were developed based on the well-known salen ligands. Chromium complexes supported by the constrained inden ligands were successfully synthesized and used as catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide (CO2). The catalyst having tert-butyl (tBu) groups as substituents in combination with tetrabutylammonium bromide (TBAB) as a cocatalyst exhibited very high catalytic activity with a turnover frequency of up to 14800 h-1 for the conversion of CO2 and propylene oxide into propylene carbonate exclusively at 100 °C and 300 psi of CO2 under solvent-free conditions. The catalyst was found to be highly active for various epoxide substrates to produce terminal cyclic carbonates in 100% selectivity.

Templated Oligosaccharide Synthesis: The Linker Effect on the Stereoselectivity of Glycosylation.

A new method for intramolecular oligosaccharide synthesis that is conceptually related to the general molecular clamp approach is introduced. Exceptional α-selectivity has been achieved in a majority of applications. Unlike other related concepts, this approach is based on the bisphenol A template, which allows one to connect multiple building blocks to perform templated oligosaccharide synthesis with complete stereoselectivity. This principle was demonstrated by the synthesis of an α,α-linked trisaccharide.

Synthesis of the highly glycosylated hydrophilic motif of extensins.

Extensin, the structural motif of plant extracellular matrix proteins, possesses a unique highly glycosylated, hydrophilic, and repeating Ser1Hyp4 pentapeptide unit, and has been proposed to include post-translational hydroxylation at proline residue and subsequent oligo-L-arabinosylations at all of the resultant hydroxyprolines as well as galactosylation at serine residue. Reported herein is the stereoselective synthesis of one of the highly glycosylated motifs, Ser(Galp1)-Hyp(Araf4)-Hyp(Araf4)-Hyp(Araf3)-Hyp(Araf1). The synthesis has been completed by the application of 2-(naphthyl)methylether-mediated intramolecular aglycon delivery to the stereoselective construction of the Ser(Galp1) and Hyp(Araf(n)) fragments as the key step, as well as Fmoc solid-phase peptide synthesis for the backbone pentapeptide.

Crystal structure of glycoside hydrolase family 127 ß-l-arabinofuranosidase from Bifidobacterium longum.

Enzymes acting on ß-linked arabinofuranosides have been unknown until recently, in spite of wide distribution of ß-l-arabinofuranosyl oligosaccharides in plant cells. Recently, a ß-l-arabinofuranosidase from the glycoside hydrolase family 127 (HypBA1) was discovered in the newly characterized degradation system of hydroxyproline-linked ß-l-arabinooligosaccharides in the bacterium Bifidobacterium longum. Here, we report the crystal structure of HypBA1 in the ligand-free and ß-l-arabinofuranose complex forms. The structure of HypBA1 consists of a catalytic barrel domain and two additional ß-sandwich domains, with one ß-sandwich domain involved in the formation of a dimer. Interestingly, there is an unprecedented metal-binding motif with Zn(2+) coordinated by glutamate and three cysteines in the active site. The glutamate residue is located far from the anomeric carbon of the ß-l-arabinofuranose ligand, but one cysteine residue is appropriately located for nucleophilic attack for glycosidic bond cleavage. The residues around the active site are highly conserved among GH127 members. Based on biochemical experiments and quantum mechanical calculations, a possible reaction mechanism involving cysteine as the nucleophile is proposed.

Preparation of p-nitrophenyl ß-l-arabinofuranoside as a substrate of ß-l-arabinofuranosidase.

Synthesis of p-nitrophenyl ß-l-arabinofuranoside 1 as the substrate for novel ß-l-arabinofuranosidase has been achieved by using both our inter- and intra-molecular glycosylation methodologies. Although the intermolecular glycosylation with l-Araf donors 3 and 4 resulted in a mixture of both α- and ß-isomers, NAP ether-mediated IAD with 3 and 6 afforded the desired ß-l-arabinofuranoside stereospecifically which was confirmed by NMR analysis on the (3)JH1-H2 coupling constant and (13)C chemical shift of C1. As expected, 1 has been revealed to be an efficient substrate in the biological study of a novel ß-arabinofuranosidase such as HypBA1 with higher apparent affinity compared with other reported substrates.

Stereoselective synthesis of Arabidopsis CLAVATA3 (CLV3) glycopeptide, unique protein post-translational modifications of secreted peptide hormone in plant.

The unique hydroxylproline (Hyp)-linked O-glycan modification is a common process in hydroxyproline-rich glycoproteins (HRGPs). The modification occurs through post-translational hydroxylation at 4-position of proline residues some of which are followed by O-glycosylation at the resulting Hyp which is also found in some secreted peptide hormones such as CLAVATA3 (CLV3) of Arabidopsis thaliana plants. An active mature CLV3 is a tridecapeptide linked to ß-L-Araf-(1→2)-ß-L-Araf-(1→2)-ß-L-Araf at a Hyp residue in the center of the peptide sequence such as Arg-Thr-Val-Hyp-Ser-Gly-Hyp(L-Arafn)-Asp-Pro-Leu-His-His-His (n = 3). We report here the synthesis of the secreted and modified CLV3 glycopeptide with all glycoforms (Araf0-3CLV3) of A. thaliana plants. A highly stereoselective ß-arabinofuranosylation of Hyp derivatives as the key step of the synthesis of CLV3 glycopeptide was achieved by NAP ether-mediated IAD, which was effectively applied to the synthesis of oligoarabinosylated hydroxylproline [Hyp(L-Araf1-3)] derivatives. Fmoc-solid phase peptide synthesis was carried out using COMU as the coupling reagent for the introduction of [Hyp(L-Araf0-3)] derivatives as well as further elongation to the CLV3 glycopeptides.

Chemical synthesis of ß-arabinofuranosyl containing oligosaccharides derived from plant cell wall extensins.

Extensins are plant-derived glycoproteins that are densely modified by oligo-arabinofuranosides linked to hydroxyproline residues. These glycoproteins have been implicated in many aspects of plant growth and development. Here, we describe the chemical synthesis of a tetrameric ß(1-2)-linked arabinofuranoside that is capped by an α(1-3)-arabinofuranoside and a similar trisaccharide lacking the capping moiety. The challenging ß(1-2)-linked arabinofuranosides were installed by using an arabinofuranosyl donor protected with 3,5-O-(di-tert-butylsilane) and a C-2 2-methylnaphthyl (Nap) ether. It was found that the cyclic silane-protecting group of the glycosyl donor greatly increased ß-anomeric selectivity. It was, however, imperative to remove the silane-protecting group of an arabinosyl acceptor to achieve optimal anomeric selectivities. The anomeric linker of the synthetic compounds was modified by a biotin moiety for immobilization of the compounds to microtiter plates coated with streptavidine. The resulting microtiter plates were employed to screen for binding against a panel of antibodies elicited against plant cell wall polysaccharides.

Glycosidation of thioglycosides in the presence of bromine: mechanism, reactivity, and stereoselectivity.

Elaborating on previous studies by Lemieux for highly reactive "armed" bromides, we discovered that ß-bromide of the superdisarmed (2-O-benzyl-3,4,6-tri-O-benzoyl) series can be directly obtained from the thioglycoside precursor. When this bromide is glycosidated, α-glycosides form exclusively; however, the yields of such transformations may be low due to the competing anomerization into α-bromide that is totally unreactive under the established reaction conditions.

On orthogonal and selective activation of glycosyl thioimidates and thioglycosides: application to oligosaccharide assembly.

Discrimination among S-thiazolinyl (STaz), S-benzoxazolyl (SBox), and S-ethyl anomeric leaving groups was achieved by fine-tuning activation conditions. Preferential glycosidation of a certain leaving group is determined neither by the strength of the activating reagent nor by the stability of the leaving group itself; instead, the type of activation plays the key role. The activation conditions established herein were applied to a sequential five-step synthesis of a hexasaccharide using six monosaccharide building blocks equipped with six different leaving groups.

Expeditious oligosaccharide synthesis via selective, semi-orthogonal, and orthogonal activation.

Traditional strategies for oligosaccharide synthesis often require extensive protecting and/or leaving group manipulations between each glycosylation step, thereby increasing the total number of synthetic steps while decreasing the efficiency of the synthesis. In contrast, expeditious strategies allow for the rapid chemical synthesis of complex carbohydrates by minimizing extraneous chemical manipulations. Oligosaccharide synthesis by selective activation of one leaving group over another is one such expeditious strategy. Herein, the significant improvements that have recently emerged in the area of the selective activation are discussed. The development of orthogonal strategy further expands the scope of the selective activation methodology. Surveyed in this article, are representative examples wherein these excellent innovations have been applied to the synthesis of various oligosaccharide sequences.

Glycosyl alkoxythioimidates as complementary building blocks for chemical glycosylation.

It is reported that S-glycosyl O-methyl phenylcarbamothioates (SNea carbamothioates) have a fully orthogonal character in comparison to S-benzoxazolyl (SBox) glycosides. This complete orthogonality was revealed by performing competitive glycosylation experiments in the presence of various promoters. The results obtained indicate that SNea carbamothioates have a very similar reactivity profile to that of glycosyl thiocyanates, yet are significantly more stable and tolerate selected protecting group manipulations. These features make the SNea carbamothioates new promising building blocks for further utilization in oligosaccharide synthesis.

On the stereoselectivity of glycosidation of thiocyanates, thioimidates, and thioglycosides.

Comparative side-by-side glycosylation studies demonstrated that glycosyl thiocyanates, thioimidates, and thioglycosides provide comparative stereoselectivities in glycosylations. Very high α-stereoselectivity that was previously recorded for glycosyl thiocyanates can be achieved, but only if glycosyl acceptors are equipped with electron-withdrawing acyl substituents. Partially benzylated glycosyl acceptors provided relatively modest stereoselectivity, which was on a par with other common glycosyl donors. Accordingly, thioimidates and thioglycosides showed high stereoselectivity similarly to that of thiocyanates with different classes of acylated primary and secondary glycosyl acceptors.

Coordination chemistry approach to the long-standing challenge of stereocontrolled chemical glycosylation.

This study clearly demonstrates that a multi-dentate metal coordination to the leaving group, along with O-5 and/or a protecting group at O-6, has a strong effect on the stereoselectivity of chemical glycosylation.

Unexpected orthogonality of S-benzoxazolyl and S-thiazolinyl glycosides: application to expeditious oligosaccharide assembly.

Thorough mechanistic studies of the alkylation pathway for the activation of glycosyl thioimidates have led to the development of the "thioimidate-only orthogonal strategy". Discrimination among S-thiazolinyl (STaz) and S-benzoxazolyl (SBox) anomeric leaving groups was achieved by fine-tuning of the activation conditions. Preferential glycosidation of a certain thioimidate is not simply determined by the strength of activating reagents; instead, the type of activation--direct vs indirect--comes to the fore and plays the key role.

Silver(I) tetrafluoroborate as a potent promoter for chemical glycosylation.

We have identified silver tetrafluoroborate (AgBF(4)) as an excellent promoter for the activation of various glycosyl donors including glycosyl halides, trichloroacetimidates, thioimidates, etc. Easy handling and no requirement for azeotropic dehydration prior to application makes AgBF(4) especially beneficial in comparison to the commonly used AgOTf. Selective activation of glycosyl halides or thioimidates over thioglycosides or n-pentenyl glycosides, including simple sequential one-pot syntheses, has been also demonstrated. Versatility of glycosyl thioimidates was further explored by converting these intermediates into a variety of other classes of glycosyl donors.

Molecular cloning and sequence analysis of alboaggregin B.

Green pit viper (Trimeresurus albolabris) venom contains a variety of C-type lectin-like proteins (CLPs) causing platelet aggregation and consumptive thrombocytopenia in biting victims. Alboaggregin B (AL-B), a heterodimeric glycoprotein (Gp) Ib-binding protein, was purified from the venom, but there is no reported cDNA sequence and the platelet agglutination mechanism is poorly understood. The full-length AL-B beta clone was obtained from T. albolabris venom gland cDNA library. AL-B alpha was, later, derived using 3'-RACE based on the conserved sequence. In this study, purified AL-B dimer agglutinated human platelets with the EC(50) of 180 nM and was completely inhibited by anti GpIb antibody. MALDI ToF mass spectroscopy found no glycosylation. The peptide mass fingerprints were matched with deduced amino acid sequences of cloned genes. AL-B alpha and beta contained 156 and 146 amino acid, respectively, including 23-residue signal peptides. AL-B beta showed the conserved hydrophilic patches, putative sites for GpIb binding. Furthermore, there was another conserved motif (SRTY) exclusively in platelet-agglutinating AL-B, TSV-GPIb-BP and Mamushigin. We propose that these three CLPs may function as bivalent adhesive proteins linking two GpIb molecules on adjacent platelets.

Hemagglutinating activity of Curcuma plants.

Crude proteins obtained by Mg/NP-40 extraction from Thai medicinal plants of the Curcuma species exhibited agglutination activity against rabbit erythrocytes. A crude extract from Salingalinthong, a Thai Curcuma specie, exhibited the strongest hemagglutinating activity, 2 x 10(-5) mg/ml.