Block Synthesis of Streptococcus pneumoniae Type 14 Capsular Polysaccharide Structures*

Synthesis of a thioglycoside tetrasaccharide block, β‐D‐Galp‐(1→4)‐β‐DGlcp‐(1→6)‐[β‐D‐Galp‐(1→4)]‐β‐D‐GlcNPhthp‐(1→SEt, corresponding to the repeating unit of Streptococcus pneumoniae serotype 14 CPS is described. Coupling of this block with a spacer followed by removal of an isopropylidene acetal yielded an acceptor, which was elongated with the donor block to give a protected dimer of the repeating unit. Iteration of this methodology yielded the trimer. Deprotection then produced an octa and a dodecasaccharide derivative ready for conjugation to proteins to afford immunoactive glycoconjugates.     

Synthesis of Monomeric and Dimeric Repeating Units of the Zwitterionic Type 1 Capsular Polysaccharide from Streptococcus pneumoniae

Zwitterionic polysaccharides (ZPSs) from Bacteroides fragilis and Streptococcus pneumoniae display unique T‐cell activities. The first synthesis of a hexasaccharide representing two repeating units of the zwitterionic capsular polysaccharide from S. pneumoniae type 1 (Sp1) is reported. Key elements of the approach are stereoselective construction of 1,4‐cis‐α‐galactose linkages based on a reactive trichloroacetimidate donor that incorporates a 6‐O‐acetyl group, which may contribute to the high α selectivity in glycosylation. After assembly of the fully protected hexasaccharide from five monosaccharide synthons 2 – 4 , 24 and 25 , selective deprotection of the primary hydroxyl groups of the four galactose residues followed by oxidation to the corresponding uronic acids provides hexasaccharide 19 . The trisaccharide counterpart 1 was synthesized in similar fashion from three synthons, 2 – 4 . This approach employed both conventional and dehydrative glycosylation methodologies and avoids the use of poorly reactive uronic acid derived glycosyl donors and acceptors.     

Chemical Synthesis and Immunological Evaluation of Helicobacter pylori Serotype O6 Tridecasaccharide O‐Antigen Containing a dd‐Heptoglycan

The development of glycoconjugate vaccines against Helicobacter pylori is challenging. An exact epitope of the H. pylori lipo‐polysaccharide (LPS) O‐antigens that contain Lewis determinant oligosaccharides and unique dd ‐heptoglycans has not yet been identified. Reported here is the first total synthesis of H. pylori serotype O6 tridecasaccharide O‐antigen containing a terminal Le^y tetrasaccharide, a unique α‐(1→3)‐, α‐(1→6)‐, and α‐(1→2)‐linked heptoglycan, and a β‐d ‐galactose connector, by an [(2×1)+(3+8)] assembly sequence. Seven oligosaccharides covering different portions of the entire O‐antigen were prepared for immunological investigations with a particular focus on elucidation of the roles of the dd ‐heptoglycan and Le^y tetrasaccharide. Glycan microarray analysis of sera from rabbits immunized with isolated serotype O6 LPS revealed a humoral immune response to the α‐(1→3)‐linked heptoglycan, a key motif for designing glycoconjugate vaccines for H. pylori serotype O6.     

Chemical Synthesis and Immunological Evaluation of Helicobacter pylori Serotype O6 Tridecasaccharide O-Antigen Containing a dd-Heptoglycan

The development of glycoconjugate vaccines against Helicobacter pylori is challenging. An exact epitope of the H. pylori lipo-polysaccharide (LPS) O-antigens that contain Lewis determinant oligosaccharides and unique dd -heptoglycans has not yet been identified. Reported here is the first total synthesis of H. pylori serotype O6 tridecasaccharide O-antigen containing a terminal Le^y tetrasaccharide, a unique α-(1→3)-, α-(1→6)-, and α-(1→2)-linked heptoglycan, and a β-d -galactose connector, by an [(2×1)+(3+8)] assembly sequence. Seven oligosaccharides covering different portions of the entire O-antigen were prepared for immunological investigations with a particular focus on elucidation of the roles of the dd -heptoglycan and Le^y tetrasaccharide. Glycan microarray analysis of sera from rabbits immunized with isolated serotype O6 LPS revealed a humoral immune response to the α-(1→3)-linked heptoglycan, a key motif for designing glycoconjugate vaccines for H. pylori serotype O6.     

Synthesis of a dimeric Lewis antigen and the evaluation of the epitope specificity of antibodies elicited in mice

The Lewis(y)-Lewis(x) heptasaccharide, modified by an artificial aminopropyl spacer, was synthesized by an approach that employed two orthogonally protected lactosamine building blocks. A p-(benzoyl)-benzyl glycoside was used as a novel anomeric protecting group, which could be selectively removed at a late stage in the synthesis, thus offering the benefit of enhanced flexibility. The artificial aminopropyl moiety was modified by a thioacetyl group, which allowed an efficient conjugation to keyhole limpet hemocyanin (KLH) that had been activated with electrophilic 3-(bromoacetamido)-propionyl groups. Mice were immunized with the Le(y)Le(x)-BrAc-KLH antigen. Analysis of the sera by ELISA established that a strong helper T-cell immune response was raised against the Le(y)Le(x) saccharide. Further ELISA analysis showed that the titer for monomeric Le(y) tetrasaccharide was tenfold lower whereas recognition of the Le(x) trisaccharide was negligible.     

Total Synthesis of the Escherichia coli O111 O‐Specific Polysaccharide Repeating Unit

The first total synthesis of the O‐antigen pentasaccharide repeating unit from Gram‐negative bacteria Escherichia coli O111 was achieved starting from four monosaccharide building blocks. Key to the synthetic approach was a bis‐glycosylation reaction to combine trisaccharide 10 and colitose 5 . The colitose building block ( 5 ) was obtained de novo from non‐carbohydrate precursors. The pentasaccharide was equipped at the reducing end with an amino spacer to provide a handle for subsequent conjugation to a carrier protein in anticipation of immunological studies.     

Chemical Synthesis and Immunological Evaluation of a Pentasaccharide Bearing Multiple Rare Sugars as a Potential Anti‐pertussis Vaccine

With the infection rate of Bordetella pertussis at a 60‐year high, there is an urgent need for new anti‐pertussis vaccines. The lipopolysaccharide (LPS) of B. pertussis is an attractive antigen for vaccine development. With the presence of multiple rare sugars and unusual glycosyl linkages, the B. pertussis LPS is a highly challenging synthetic target. In this work, aided by molecular dynamics simulation and modeling, a pertussis‐LPS‐like pentasaccharide was chemically synthesized for the first time. The pentasaccharide was conjugated with a powerful carrier, bacteriophage Qβ, as a vaccine candidate. Immunization of mice with the conjugate induced robust anti‐glycan IgG responses with IgG titers reaching several million enzyme‐linked immunosorbent assay (ELISA) units. The antibodies generated were long lasting and boostable and could recognize multiple clinical strains of B. pertussis, highlighting the potential of Qβ‐glycan as a new anti‐pertussis vaccine.     

Acceptor Reactivity in the Total Synthesis of Alginate Fragments Containing α‐L‐Guluronic Acid and β‐D‐Mannuronic Acid

The total synthesis of mixed‐sequence alginate oligosaccharides, featuring both β‐D ‐mannuronic acid (M) and α‐L ‐guluronic acid (G), is reported for the first time. A set of GM, GMG, GMGM, GMGMG, GMGMGM, GMGMGMG, and GMGGMG alginates was assembled using GM building blocks, having a guluronic acid acceptor part and a mannuronic acid donor side to allow the fully stereoselective construction of the cis‐glycosidic linkages. It was found that the nature of the reducing‐end anomeric center, which is ten atoms away from the reacting alcohol group in the key disaccharide acceptor, had a tremendous effect on the efficiency with which the building blocks were united. This chiral center determines the overall shape of the acceptor and it is revealed that the conformational flexibility of the acceptor is an all‐important factor in determining the outcome of a glycosylation reaction.     

On the Reach of Chemical Synthesis: Creation of a Mini‐Pipeline from an Academic Laboratory

In this retrospective, we recall some select cases of synergy between very challenging chemical synthesis and the identification of promising new candidates for pharmaceutics development. The progression from targets, often referred to as small molecules, to those of a size commonly associated with biologics (including glycoproteins) is also charted.     

Recent Advances in the Z/E Isomers of Tetraphenylethene Derivatives: Stereoselective Synthesis,AIE Mechanism,Photophysical Properties,and Application as Chemical Probes

Focused research on the Z/E isomers of tetraphenylethene (TPE) derivatives is scarce in comparison with the thousands of luminogens with AIE properties (AIEgens) that have been synthesized based on the TPE moiety. The similar chemical and physical properties of the Z/E isomers make them difficult to separate by using conventional chromatographic techniques. However, they can be isolated by introducing polar groups and the pure isomers exhibit very different photophysical properties, mechanochromism, and host–guest coordination, as well as assisting in deciphering the AIE mechanism. In this Minireview, we present an overview of the disagreement regarding the AIE mechanism between the restriction of intramolecular vibration and photoinduced Z/E isomerization. Then, we discuss the development of (Z)‐/(E)‐TPE derivatives, their use in host–guest detection, and their mechanoluminescence properties, with a focus on their photophysical characteristics. Finally, we explore the stereoselective synthesis of pure (Z)‐/(E)‐TPE derivatives.     

Chemoenzymatic Posttranslational Modification Reactions for the Synthesis of Ψ[CH2NH]‐Containing Peptides

The Ψ[CH₂NH] reduced amide bond is a peptide isostere widely used in the development of bioactive pseudopeptides. Reported here is a method of chemoenzymatic posttranslational modification for the synthesis of Ψ[CH₂NH]‐containing peptides converted from ribosomally expressed peptides. The posttranslational conversion composed of an enzymatic cyclodehydration and facile two‐step chemical reduction achieves deoxygenation of a specific amide bond present in a nonprotected peptide in water. This method generates the Ψ[CH₂NH] bond in peptides and is applicable to various peptide sequences, potentially enabling the preparation of a library of Ψ[CH₂NH]‐containing peptides.     

Chemical Synthesis of the 20 kDa Heme Protein Nitrophorin 4 by α‐Ketoacid‐Hydroxylamine (KAHA) Ligation

The chemical synthesis of the 184‐residue ferric heme‐binding protein nitrophorin 4 was accomplished by sequential couplings of five unprotected peptide segments using α‐ketoacid‐hydroxylamine (KAHA) ligation reactions. The fully assembled protein was folded to its native structure and coordinated to the ferric heme b cofactor. The synthetic holoprotein, despite four homoserine residues at the ligation sites, showed identical properties to the wild‐type protein in nitric oxide binding and nitrite dismutase reactivity. This work establishes the KAHA ligation as a valuable and viable approach for the chemical synthesis of proteins up to 20 kDa and demonstrates that it is well‐suited for the preparation of hydrophobic protein targets.     

Resonance‐Assisted Hydrogen Bonding as a Driving Force in Synthesis and a Synthon in the Design of Materials

Resonance‐assisted hydrogen bonding (RAHB), a concept introduced by Gilli and co‐workers in 1989, concerns a kind of intramolecular H‐bonding strengthened by a conjugated π‐system, usually in 6‐, 8‐, or 10‐membered rings. This Review highlights the involvement of RAHB as a driving force in the synthesis of organic, coordination, and organometallic compounds, as a handy tool in the activation of covalent bonds, and in starting moieties for synthetic transformations. The unique roles of RAHB in molecular recognition and switches, E/Z isomeric resolution, racemization and epimerization of amino acids and chiral amino alcohols, solvatochromism, liquid‐crystalline compounds, and in synthons for crystal engineering and polymer materials are also discussed. The Review can provide practical guidance for synthetic chemists that are interested in exploring and further developing RAHB‐assisted synthesis and design of materials.