Protecting Group Strategy Research Articles

Synthesis and Initial Characterization of a Selective, Pseudo-irreversible Inhibitor of Human Butyrylcholinesterase as PET Tracer.

The enzyme butyrylcholinesterase (BChE) represents a promising target for imaging probes to potentially enable early diagnosis of neurodegenerative diseases like Alzheimer's disease (AD) and to monitor disease progression in some forms of cancer. In this study, we present the design, facile synthesis, in vitro and preliminary ex vivo and in vivo evaluation of a morpholine‐based, selective inhibitor of human BChE as a positron emission tomography (PET) tracer with a pseudo‐irreversible binding mode. We demonstrate a novel protecting group strategy for 18F radiolabeling of carbamate precursors and show that the inhibitory potency as well as kinetic properties of our unlabeled reference compound were retained in comparison to the parent compound. In particular, the prolonged duration of enzyme inhibition of such a morpholinocarbamate motivated us to design a PET tracer, possibly enabling a precise mapping of BChE distribution.

Glycosylation and Protecting Group Strategies Towards the Synthesis of Saponins and Bacterial Oligosaccharides: A Personal Account.

Carbohydrates and their conjugates are not only involved in important biological processes but are also regarded as promising therapeutics and prophylactics. Over the last century, several glycosylation methodologies, glycosyl donors, and protecting groups have been developed and some of them have found broad synthetic applications in carbohydrate chemistry. In this Personal Account, we describe how glycosylation and protecting group strategies have been implemented in our as well as in other research groups as to synthesize bioactive glycans, more specifically naturally occurring lupane-type saponins as well as oligosaccharides related to Burkholderia species.

Synthesis of a phosphomolybdate with a tetranuclear vanadium core by installing vanadium atoms in a lacunary template using the protecting group strategy.

Vanadium-containing phosphomolybdates have outstanding catalytic and electrochemical properties. However, their traditional one-pot synthesis in aqueous media generates a mixture of randomly distributed isomers and undesirable impurities, which has hampered the in-depth study of the catalysis and reaction mechanism of these compounds. Here, the selective synthesis of a tetranuclear vanadium-containing phosphomolybdate, [PV3Mo9O40(VO)]3-, was achieved for the first time by using a pyridine-protected [A-α-PMo9O31(py)3]3- (py = pyridine) as the template.

A protecting group strategy to access stable lacunary polyoxomolybdates for introducing multinuclear metal clusters

Although metal-containing polyoxomolybdates (molybdenum oxide clusters) exhibit outstanding catalytic properties, their precise synthetic method has not yet been developed. This is mainly because the very low stability of the multivacant lacunary polyoxomolybdates limited their use as synthetic precursors. Here, we present a “protecting group strategy” in polyoxometalate synthesis and successfully develop an efficient method for synthesising multinuclear metal-containing polyoxomolybdates using pyridine as a protecting group for unstable trivacant lacunary Keggin-type polyoxomolybdate [PMo9O34]9−. Specifically, tetranuclear cubane- and planar-type manganese clusters were selectively synthesised in the polyoxomolybdates using the present method. The importance of this work is that, in addition to being the first practical way of utilizing multivacant lacunary polyoxomolybdates as precursors, this new “protecting group strategy” will make it possible to produce polyoxometalates with unexplored structures and properties.

Chemical Synthesis of Saponins.

Saponins are a large family of amphiphilic glycosides of steroids and triterpenes found in plants and some marine organisms. By expressing a large diversity of structures on both sugar chains and aglycones, saponins exhibit a wide range of biological and pharmacological properties and serve as major active principles in folk medicines, especially in traditional Chinese medicines. Isolation of saponins from natural sources is usually a formidable task due to the microheterogeneity of saponins in Nature. Chemical synthesis can provide access to large amounts of natural saponins as well as congeners for understanding their structure-activity relationships and mechanisms of action. This article presents a comprehensive account on chemical synthesis of saponins. First highlighted are general considerations on saponin synthesis, including preparation of aglycones and carbohydrate building blocks, assembly strategies, and protecting-group strategies. Next described is the state of the art in the synthesis of each type of saponins, with an emphasis on those representative saponins having sophisticated structures and potent biological activities.

Indolonaphthyridine: A Versatile Chromophore for Organic Electronics Inspired by Natural Indigo Dye.

Organic electronics is an exciting field of research offering innovative technologies from roll-to-roll inkjet-printed solar cells to foldable displays for cellphones and televisions. These functional devices exploit the flexible nature of conjugated organic materials, both polymeric and molecular, to absorb and emit light and to facilitate transport of charge carriers. A major driving force of development within the field is the creation of novel high-performance building blocks, providing a fruitful and ever-growing library of materials for tailored applications. Most of these building blocks contain chromophores that are entirely synthetic, yet there exist many naturally occurring building blocks, which have been relatively overlooked, despite their innate high stability and inexpensive nature. Indigo is the most produced dye worldwide and has one of the richest histories of all known textile dyes, dating before 4000 BC. Indigo's superior photostability has been linked to fast, favorable deactivation pathways following light absorption. But through one straightforward reaction, the chromophore of indigo can be transformed to a new chromophore with remarkable optoelectronic properties.In this Account, we discuss this chromophore, indolonaphthyridine, and give an overview of our research into the synthesis and optoelectronics properties of functional organic electronic materials derived from it. The unit's strong, fused planar construction contains bis-imide functional groups in similarity to the field-favorite diketopyrrolopyrrole, and similarly requires solubilizing with long alkyl chains, the installation of which is nontrivial and achieved using a protecting group strategy. Our solubilized indolonaphthyridine monomer allows us to copolymerize it with simple archetypal comonomers (thiophene, benzothiadiazole, etc.), in contrast to the other research groups working on the chromophore, who employ complex alkylated comonomer units. We discovered materials with extraordinary performance in organic photovoltaics, affording power conversion efficiencies up to 4.1% in the near-IR region of the spectrum. In organic field-effect transistors, the copolymers exhibited ambipolar transport and notable n-type mobilities up to 3.1 cm2/(V s), well above the benchmark set by silicon (1 cm2/(V s)). The strong absorption in the near-IR allowed us to explore the use of the polymers as contrast agents in photoacoustic imaging, an emerging technique capable of achieving deep tissue penetration without the need for ionizing radiation, while maintaining high contrast and high accuracy responses. Finally, we discuss an exciting aspect of the photophysics of molecular indolonaphthyridine: its ability to undergo singlet fission. Moreover, most singlet fission materials exhibit poor ambient stability; however our molecular indolonaphthyridines exhibit superior stability. It is our hope that this Account showcases the remarkable potential of this relatively unexplored, versatile chromophore and leads to wider adoption in the future.

Post‐Assembly Photomasking of Potassium Acyltrifluoroborates (KATs) for Two‐Photon 3D Patterning of PEG‐Hydrogels

AbstractChemical ligation reactions of functional groups that can be masked with two‐photon labile protecting groups provide a powerful technology for the three‐dimensional patterning of molecules – including proteins – onto hydrogel scaffolds. In order to utilize readily prepared hydrogels constructed by the potassium acyltrifluoroborate (KAT)‐hydroxylamine amide formation ligation for two‐photon patterning, we have developed a unique post‐polymerization protecting group strategy through the reaction of KATs and dithiols in water and deprotection by two‐photon excitation. After precise 3D spatially confined light irradiation, the unprotected KATs undergo ligations with hydroxylamine‐functionalized superfolder GFP and sulforhodamine B for the composition of three‐dimensional patterns.

Solution-Phase Fmoc-Based Peptide Synthesis for DNA-Encoded Chemical Libraries: Reaction Conditions, Protecting Group Strategies, and Pitfalls.

Peptide drug discovery has shown a resurgence since 2000, bringing 28 non-insulin therapeutics to the market compared to 56 since its first peptide drug, insulin, in 1923. While the main method of discovery has been biological display-phage, mRNA, and ribosome-the synthetic limitations of biological systems has restricted the depth of exploration of peptide chemical space. In contrast, DNA-encoded chemistry offers the synergy of large numbers and ribosome-independent synthetic flexibility for the fast and deeper exploration of the same space. Hence, as a bridge to building DNA-encoded chemical libraries (DECLs) of peptides, we have developed substrate-tolerant amide coupling reaction conditions for amino acid monomers, performed a coupling screen to illustrate such tolerance, developed protecting group strategies for relevant amino acids and reported the limitations thereof, developed a strategy for the coupling of α,α-disubstituted alkenyl amino acids relevant to all-hydrocarbon stapled peptide drug discovery, developed reaction conditions for the coupling of tripeptides likely to be used in DECL builds, and synthesized a fully deprotected DNA-decamer conjugate to illustrate the potency of the developed methodology for on-DNA peptide synthesis.

A Shortcut to the Synthesis of Peptide Thioesters.

Peptide thioesters serve as fundamental building blocks for the synthesis of proteins and cyclic peptides. Classically, methods to synthesize thioesters have been based on acid-labile amino-protecting groups for which final side-chain deprotection required the use of hazardous hydrogen fluoride (HF). Alternative protection schemes based on base-labile amino-protecting groups have become preferred methods but are not suitable due to the lability of thioester bonds toward bases. In this method, we employ a trifluoracetic acid/trimethylsilyl bromide (TFA/TMSBr) protocol using a hydroxymethyl resin obviating the need for HF. TFA/TMSBr is volatile enough to be easily removed yet less hazardous than HF, making it more practical for general peptide chemists. We describe optimized cleavage procedures and appropriate protecting group schemes and discuss in situ neutralization protocols. The method is relatively simple, straightforward, and easily scalable, allowing the facile preparation of alkyl and aryl thioesters.

Novel polymer crafted sugar thiacrown ether and its applications in recovery of metal ions

Polymeric macrocycles are important as complexation agents in chemistry. The multivalent binding of heteroatoms inside the macrocycle to metals and organic pollutants has gained much interest. A protecting group strategy was applied to synthesise functionalized crown ether based on glucose. The 4,6-position was protected by benzylidene, and arms were introduced on the remaining hydroxyls in 2- and 3-positions. Cyclization was achieved through sodium sulfide in ethanol. The selective cleavage of the benzylidene protecting group left the free hydroxyl group on C-6 position. The propargyl group was introduced and attached the macrocycles to polyvinyl azide by click reaction. The compounds were identified by nuclear magnetic resonance, infrared spectroscopy and mass spectrometry, and the morphology and thermal stability of the polymer were characterized via field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray powder diffraction and differential scanning calorimetry. The removal of metal ions Co, Cd and Pb was performed by atomic absorption spectrometry in different experimental conditions. The results indicated that synthesised crown ether can effectively removal such metals and has highly sufficient recyclability.

Effects of N-Terminal Residues on the Assembly of Constrained β-Hairpin Peptides Derived from Aβ.

This paper describes the synthesis, solution-phase biophysical studies, and X-ray crystallographic structures of hexamers formed by macrocyclic β-hairpin peptides derived from the central and C-terminal regions of Aβ, which bear "tails" derived from the N-terminus of Aβ. Soluble oligomers of the β-amyloid peptide, Aβ, are thought to be the synaptotoxic species responsible for neurodegeneration in Alzheimer's disease. Over the last 20 years, evidence has accumulated that implicates the N-terminus of Aβ as a region that may initiate the formation of damaging oligomeric species. We previously studied, in our laboratory, macrocyclic β-hairpin peptides derived from Aβ16-22 and Aβ30-36, capable of forming hexamers that can be observed by X-ray crystallography and SDS-PAGE. To better mimic oligomers of full length Aβ, we use an orthogonal protecting group strategy during the synthesis to append residues from Aβ1-14 to the parent macrocyclic β-hairpin peptide 1, which comprises Aβ16-22 and Aβ30-36. The N-terminally extended peptides N+1, N+2, N+4, N+6, N+8, N+10, N+12, and N+14 assemble to form dimers, trimers, and hexamers in solution-phase studies. X-ray crystallography reveals that peptide N+1 assembles to form a hexamer that is composed of dimers and trimers. These observations are consistent with a model in which the assembly of Aβ oligomers is driven by hydrogen bonding and hydrophobic packing of the residues from the central and C-terminal regions, with the N-terminus of Aβ accommodated by the oligomers as an unstructured tail.

Sustainable Peptide Synthesis Enabled by a Transient Protecting Group.

The growing interest in synthetic peptides has prompted the development of viable methods for their sustainable production. Currently, large amounts of toxic solvents are required for peptide assembly from protected building blocks, and switching to water as a reaction medium remains a major hurdle in peptide chemistry. We report an aqueous solid‐phase peptide synthesis strategy that is based on a water‐compatible 2,7‐disulfo‐9‐fluorenylmethoxycarbonyl (Smoc) protecting group. This approach enables peptide assembly under aqueous conditions, real‐time monitoring of building block coupling, and efficient postsynthetic purification. The procedure for the synthesis of all natural and several non‐natural Smoc‐protected amino acids is described, as well as the assembly of 22 peptide sequences and the fundamental issues of SPPS, including the protecting group strategy, coupling and cleavage efficiency, stability under aqueous conditions, and crucial side reactions.

Sustainable Peptide Synthesis Enabled by a Transient Protecting Group

AbstractThe growing interest in synthetic peptides has prompted the development of viable methods for their sustainable production. Currently, large amounts of toxic solvents are required for peptide assembly from protected building blocks, and switching to water as a reaction medium remains a major hurdle in peptide chemistry. We report an aqueous solid‐phase peptide synthesis strategy that is based on a water‐compatible 2,7‐disulfo‐9‐fluorenylmethoxycarbonyl (Smoc) protecting group. This approach enables peptide assembly under aqueous conditions, real‐time monitoring of building block coupling, and efficient postsynthetic purification. The procedure for the synthesis of all natural and several non‐natural Smoc‐protected amino acids is described, as well as the assembly of 22 peptide sequences and the fundamental issues of SPPS, including the protecting group strategy, coupling and cleavage efficiency, stability under aqueous conditions, and crucial side reactions.

Prevention of aspartimide formation during peptide synthesis using cyanosulfurylides as carboxylic acid-protecting groups

Although peptide chemistry has made great progress, the frequent occurrence of aspartimide formation during peptide synthesis remains a formidable challenge. Aspartimide formation leads to low yields in addition to costly purification or even inaccessible peptide sequences. Here, we report an alternative approach to address this longstanding challenge of peptide synthesis by utilizing cyanosulfurylides to mask carboxylic acids by a stable C–C bond. These functional groups—formally zwitterionic species—are exceptionally stable to all common manipulations and impart improved solubility during synthesis. Deprotection is readily and rapidly achieved under aqueous conditions with electrophilic halogenating agents via a highly selective C–C bond cleavage reaction. This protecting group is employed for the synthesis of a range of peptides and proteins including teduglutide, ubiquitin, and the low-density lipoprotein class A. This protecting group strategy has the potential to overcome one of the most difficult aspects of modern peptide chemistry.

Protecting-Group-Mediated Diastereoselective Synthesis of C4'-Methylated Uridine Analogs and Their Activity against the Human Respiratory Syncytial Virus.

Adjusting the protecting group strategy, from an alkyl ether to a bidentate ketal at the carbohydrate backbone of uridine, facilitates a switchable diastereoselective α- or β-C4'/C5'-spirocyclopropanation. Using these spirocyclopropanated nucleosides as key intermediates, we synthesized a variety of C4'-methylated d-ribose and l-lyxose-configured uridine derivatives by a base-mediated ring-opening of the spirocyclopropanol moiety. Investigations of antiviral activity against the human respiratory syncytial virus were carried out for selected derivatives, showing moderate activity.

Ferrier Carbocyclization-Mediated Synthesis of Enantiopure Azido Inositol Analogues.

Azide-modified inositol (InoAz) analogues are valuable as inhibitors and have shown promise as metabolic chemical reporters (MCRs) for labeling inositol-containing glycoconjugates in eukaryotic cells and potentially in mycobacteria, but the synthesis of enantiomerically pure InoAz analogues via traditional approaches is challenging. As a complementary route, here we investigated the application of the Ferrier carbocyclization reaction to the synthesis of enantiopure InoAz analogues starting from readily available azido glucosides. Using this approach combined with a para-methoxybenzyl protecting group strategy, 3-azido-3-deoxy- and 4-azido-4-deoxy-d-myo-inositol were efficiently synthesized. 5-Azido-5-deoxy-d-myo-inositol was inaccessible due to an unusual β-elimination reaction, wherein the azide anion acted as the leaving group. The reported strategy is expected to facilitate continued development of synthetic InoAz analogues as inhibitors or MCRs of inositol-containing glycoconjugates in eukaryotic and mycobacterial systems.

Multifunctional Building Blocks Compatible with Photoredox-Mediated Alkylation for DNA-Encoded Library Synthesis.

DNA-encoded library (DEL) technology has emerged as a novel interrogation modality for ligand discovery in the pharmaceutical industry. Given the increasing demand for a higher proportion of C(sp3)-hybridized centers in DEL platforms, a photoredox-mediated cross-coupling and defluorinative alkylation process is introduced using commercially available alkyl bromides and structurally diverse α-silylamines. Notably, no protecting group strategies for amines are necessary for the incorporation of a variety of amino-acid-based organosilanes, providing crucial branching points for further derivatization.

General Fmoc‐Based Solid‐Phase Synthesis of Complex Depsipeptides Circumventing Problematic Fmoc Removal

Development of an Fmoc‐based solid‐phase depsipeptide methodology has been hampered by base‐promoted fragmentation and diketoperazine formation upon Fmoc group elimination. Such a strategy would be a useful tool given the number of commercially available Fmoc‐protected residues. Herein we report that the addition of small percentages of organic acids to the Fmoc‐removal cocktail proves effective to circumvent these drawbacks and most importantly, allowed the development of an exclusively solid‐phase stepwise methodology to prepare a highly complex depsipeptide with multiple and consecutive esters bonds. Alongside, the optimal protecting group scheme for residue incorporation, which is not as straightforward as it is for traditional peptide synthesis, was explored. The developed stepwise strategy proved effective for the synthesis of a highly complex cyclodepsipeptide, being comparable to the yields obtained when using traditional combined chemistry approaches.

Comparison of disaccharide donors for heparan sulfate synthesis: uronic acids vs. their pyranose equivalents.

Late oxidation of hexose based building blocks or the use of uronic acid containing building blocks are two complementary strategies in the synthesis of glycosaminoglycans, the latter simplifiying the later stages of the process. Here we report the synthesis and evaluation of various disaccharide donors-uronic acids and their pyranose equivalents-for the synthesis of heparan sulfate, using an established protective group strategy. Hexose based "imidate" type donors perform well in the studied glycosylations, while their corresponding uronate esters fall short; a uronate ester thioglycoside performs equal to, if not better than, a hexose thioglycoside equivalent.

The key entity of a DCAR agonist, phosphatidylinositol mannoside Ac1PIM1: its synthesis and immunomodulatory function.

Ac1PIM1 is a potential biosynthetic intermediate for phosphatidylinositol mannosides (PIMs) from Mycobacterium tuberculosis. We achieved the first synthesis of Ac1PIM1 by utilizing an allyl-type protecting group strategy and regioselective phosphorylation of inositol. A very potent agonist of an innate immune receptor DCAR, which is better than previously known agonists, is demonstrated.