Expanding Glycomic Investigations through Thiol-Derivatized Glycans.

N-(2-thioethyl)-2-aminobenzamide (TEAB), a novel glycan auxiliary, was synthesized and its utility was evaluated. The auxiliary was conjugated to glycans by reductive amination with the water-stable reagent 2-picoline borane complex. Glycan products, which ranged from 1 to 7 linked hexoses, were all isolated in yields ranging from 60% to 90% after purification by reverse-phase chromatography. The novel conjugate introduces a convenient, shelf-stable thiol directly onto the desired free glycans with purification advantages and direct modification with efficient reactions through alkenes, halides, epoxides, disulfides, and carboxylates in yields of 49% to 93%. Subsequently, a thiol-selective modification of the BSA protein was used to generate a neoglycoprotein with a bifunctional PEG-maleimide linker. To further illustrate the utility of a thiol motif, 2-thiopyridine activation of a thiol-containing support facilitated the covalent chromatographic purification of labeled glycans in yields up to 63%. Finally, initial proof of concept of implementation in a light printed microarray was explored and validated through FITC-labeled concanavalin A binding. In conclusion, the thiol-functionalized glycans produced greatly expand the diversity of bioconjugation tools that can be developed with glycans and enable a variety of biological investigations.

Development of a multifunctional neoglycoside auxiliary for applications in glycomics research.

A novel, multifunctional, tetrazine-containing neoglycoside auxiliary has been synthesized in three steps and 28% overall yield. The oxyamine was conjugated with unprotected carbohydrates under aqueous conditions (pH = 4.7), with DMF as a cosolvent, to provide neoglycosides in yields ranging between 51% and 68%. This auxiliary displayed broad advantages in the isolation and purification of complex carbohydrate mixtures, compatibility during extension by glycosyltransferases, and direct conjugation to chemical probes. Furthermore, the auxiliary can be removed in 96% yield under acidic conditions (0.25% TFA in H2O) that leave glycosidic linkages intact. Thereby, the tetrazine-containing neoglycoside auxiliary can serve to facilitate future glycomics investigations.

Asymmetric Synthesis of Stereogenic Phosphorus P(V) Centers Using Chiral Nucleophilic Catalysis.

Organophosphates have been widely used in agrochemistry, as reagents for organic synthesis, and in biochemistry. Phosphate mimics possessing four unique substituents, and thereby a chirality center, are useful in transition metal catalysis and as nucleotide therapeutics. The catalytic, stereocontrolled synthesis of phosphorus-stereogenic centers is challenging and traditionally depends on a resolution or use of stochiometric auxiliaries. Herein, enantioenriched phosphorus centers have been synthesized using chiral nucleophilic catalysis. Racemic H-phosphinate species were coupled with nucleophilic alcohols under halogenating conditions. Chiral phosphonate products were synthesized in acceptable yields (33-95%) and modest enantioselectivity (up to 62% ee) was observed after identification of an appropriate chiral catalyst and optimization of the solvent, base, and temperature. Nucleophilic catalysis has a tremendous potential to produce enantioenriched phosphate mimics that could be used as prodrugs or chemical biology probes.

Transformation of aldehydes into nitriles in an aqueous medium using O-phenylhydroxylamine as the nitrogen source.

The conversion of an aldehyde into a nitrile can be efficiently performed using O-phenylhydroxylamine hydrochloride in buffered aqueous solutions. The reported method is specifically optimized for aqueous-soluble substrates including carbohydrates. Several reducing sugars including monosaccharides, disaccharides, and silyl-protected saccharides were transformed into cyanohydrins in high yields. The reaction conditions are also suitable for the formation of nitriles from various types of hydrophobic aldehyde substrates. Furthermore, cyanide can be eliminated from cyanohydrins, analogous to the Wohl degradation, by utilizing a readily-removed weakly basic resin as a promoter.

Synthesis, structure and photophysical properties of a 2D network with gold dicyanide donors coordinated to aza[5]helicene viologen acceptors.

A recently synthesized photoluminescent organic acceptor, 5,10-dimethyl-5,10-diaza[5]helicene is shown to react with dicyanoaurate anions to form a 2D network N,N-dimethylaza[5]helicene dicyanoaurate. The structure of the synthesized complex was investigated via X-ray crystallography showing the presence of [Au(CN)2]- dimers and monomers within the helicene framework. Photophysical measurements between 298 K and 10 K indicate quenching of the [Au(CN)2]- anion by 5,10-dimethyl-5,10-diaza[5]helicene via an electron transfer. A Stern-Volmer and Rehm-Weller analysis shows that this is a result of quenching from transfer of an electron from [Au(CN)2]- anions to 5,10-dimethyl-5,10-diaza[5]helicene as opposed to resonance energy transfer. DFT calculations were performed to support the assignment of an electron transfer.

Structural analyses of NudT16-ADP-ribose complexes direct rational design of mutants with improved processing of poly(ADP-ribosyl)ated proteins.

ADP-ribosylation is a post-translational modification that occurs on chemically diverse amino acids, including aspartate, glutamate, lysine, arginine, serine and cysteine on proteins and is mediated by ADP-ribosyltransferases, including a subset commonly known as poly(ADP-ribose) polymerases. ADP-ribose can be conjugated to proteins singly as a monomer or in polymeric chains as poly(ADP-ribose). While ADP-ribosylation can be reversed by ADP-ribosylhydrolases, this protein modification can also be processed to phosphoribosylation by enzymes possessing phosphodiesterase activity, such as snake venom phosphodiesterase, mammalian ectonucleotide pyrophosphatase/phosphodiesterase 1, Escherichia coli RppH, Legionella pneumophila Sde and Homo sapiens NudT16 (HsNudT16). Our studies here sought to utilize X-ray crystallographic structures of HsNudT16 in complex with monomeric and dimeric ADP-ribose in identifying the active site for binding and processing free and protein-conjugated ADP-ribose into phosphoribose forms. These structural data guide rational design of mutants that widen the active site to better accommodate protein-conjugated ADP-ribose. We identified that several HsNudT16 mutants (Δ17, F36A, and F61S) have reduced activity for free ADP-ribose, similar processing ability against protein-conjugated mono(ADP-ribose), but improved catalytic efficiency for protein-conjugated poly(ADP-ribose). These HsNudT16 variants may, therefore, provide a novel tool to investigate different forms of ADP-ribose.

Synthesis of dimeric ADP-ribose and its structure with human poly(ADP-ribose) glycohydrolase.

Poly(ADP-ribosyl)ation is a common post-translational modification that mediates a wide variety of cellular processes including DNA damage repair, chromatin regulation, transcription, and apoptosis. The difficulty associated with accessing poly(ADP-ribose) (PAR) in a homogeneous form has been an impediment to understanding the interactions of PAR with poly(ADP-ribose) glycohydrolase (PARG) and other binding proteins. Here we describe the chemical synthesis of the ADP-ribose dimer, and we use this compound to obtain the first human PARG substrate-enzyme cocrystal structure. Chemical synthesis of PAR is an attractive alternative to traditional enzymatic synthesis and fractionation, allowing access to products such as dimeric ADP-ribose, which has been detected but never isolated from natural sources. Additionally, we describe the synthesis of an alkynylated dimer and demonstrate that this compound can be used to synthesize PAR probes including biotin and fluorophore-labeled compounds. The fluorescently labeled ADP-ribose dimer was then utilized in a general fluorescence polarization-based PAR-protein binding assay. Finally, we use intermediates of our synthesis to access various PAR fragments, and evaluation of these compounds as substrates for PARG reveals the minimal features for substrate recognition and enzymatic cleavage. Homogeneous PAR oligomers and unnatural variants produced from chemical synthesis will allow for further detailed structural and biochemical studies on the interaction of PAR with its many protein binding partners.

Stereospecific ring expansion of chiral vinyl aziridines.

In this report, it is demonstrated that chiral vinyl aziridines can be stereospecifically ring expanded. This synthetic approach allows controlled access to chiral 2,5-cis- or 2,5-trans-3-pyrroline products from starting materials with the appropriate aziridine geometry. Twenty three ring expansion examples, most of which feature a stereospecific cyclization, are presented.

An efficient oxidative dearomatization-radical cyclization approach to symmetrically substituted bicyclic guttiferone natural products.

Detailed in this communication is an efficient synthetic approach towards the guttiferone family of natural products. Oxidatively unraveling a para-quinone monoketal followed by consecutive 5-exo radical cyclizations provides the bicyclic core. An additional strength of this approach is a late stage asymmetric desymmetrization of an advanced symmetric intermediate.

Creative approaches towards the synthesis of 2,5-dihydro- furans, thiophenes, and pyrroles. One method does not fit all!

A single method is never sufficient, which is why there is a great need for developing many diverse and creative approaches towards every chemical substrate class. This statement is supported by the contents of this perspective in addition to providing the reader with a helpful synthetic roadmap for selecting a suitable method for the building blocks being discussed. Detailed in this review are eight different synthetic approaches that provide access to valuable 2,5-dihydro- furan, thiophene and pyrrole building blocks. Each approach is briefly presented and its limits discussed. The strengths and weaknesses of each approach are further highlighted with a graphical table summary at the end. This summary clearly drives home the point that for every chemical substrate class we need many good methods in order to provide access to every member of each class.

Lewis acid catalyzed [1,3]-sigmatropic rearrangement of vinyl aziridines.

This paper details the copper-catalyzed ring expansion of vinyl aziridines to 3-pyrrolines. Broad substrate scope (24 examples) using tosyl- and phthalimide-protected vinyl aziridine substrates is observed. Cu(hfacac)2 was determined to be superior to all other catalysts tested.