Photoinduced Thiol-ene Coupling Research Articles

Selected Research Topics of the Dondoni Group over the Last Two Decades (2000–2020)

From a selection of research topics carried out in our laboratory during the last twenty years it becomes apparent that our main target was the discovery of new or improved synthetic methods together with new properties. Our efforts were made with the aim of being of some utility to other fields of research, with particular emphasis to glycobiology and heterocyle-based bioorganic chemistry. We performed new chemistry mainly in the field of carbohydrate manipulations taking as a primary rule the simplicity and efficiency manners. Toward this end, modern synthetic tools and approaches were employed such as heterocyle-based transformations, multicomponent reactions, organocatalysis, click azide–alkyne cycloadditions, reactions in ionic liquids, click photoinduced thiol-ene coupling, and click sulfur–fluoride exchange chemistry. With these potent methodologies in hand, the syntheses of carbohydrate containing amino acids up to proteins glycosylation were performed.1 Heterocyclic Glycoconjugates and Amino Acids2 Triazole-Linked Oligonucleotides: Application of Click CuAAC3 Non-Natural Glycosyl Amino Acids4 Non-Natural Oligosaccharides5 Calixarene-Based Glycoclusters6 Carbohydrate-Based Building Blocks7 Homoazasugars and Aza-C-disaccharides8 Synthesis of Glycodendrimers9 Peptide and Protein Glycoconjugates10 Conclusions

Stereoselective Thioconjugation by Photoinduced Thiol-ene Coupling Reactions of Hexo- and Pentopyranosyl d- and l-Glycals at Low-Temperature-Reactivity and Stereoselectivity Study.

A comprehensive optimization and mechanistic study on the photoinduced hydrothiolation of different d‐ and l‐ hexo‐ and pentoglycals with various thiols was performed, at the temperature range of RT to −120 °C. Addition of thiols onto 2‐substituted hexoglycals proceeded with complete 1,2‐cis‐α‐stereoselectivity in all cases. Hydrothiolation of 2‐substituted pentoglycals resulted in mixtures of 1,2‐cis‐α‐ and ‐β‐thioglycosides of varying ratio depending on the configuration of the reactants. Hydrothiolation of unsubstituted glycals at −80 °C proceeded with excellent yields and, except for galactal, provided the axially C2‐S‐linked isomers with high selectivity. Cooling was always beneficial to the efficacy, increased the yields and in most cases significantly raised the stereoselectivity. The suggested mechanism explains the different conformational preferences of the intermediate carbon‐centered radicals, which is a crucial factor in the stereoselectivity of the reactions.

In-Cell Synthesis of Bioorthogonal Alkene Tag S-Allyl-Homocysteine and Its Coupling with Reprogrammed Translation.

In this study, we report our initial results on in situ biosynthesis of S-allyl-l-homocysteine (Sahc) by simple metabolic conversion of allyl mercaptan in Escherichia coli, which served as the host organism endowed with a direct sulfhydration pathway. The intracellular synthesis we describe in this study is coupled with the direct incorporation of Sahc into proteins in response to methionine codons. Together with O-acetyl-homoserine, allyl mercaptan was added to the growth medium, followed by uptake and intracellular reaction to give Sahc. Our protocol efficiently combined the in vivo synthesis of Sahc via metabolic engineering with reprogrammed translation, without the need for a major change in the protein biosynthesis machinery. Although the system needs further optimisation to achieve greater intracellular Sahc production for complete protein labelling, we demonstrated its functional versatility for photo-induced thiol-ene coupling and the recently developed phosphonamidate conjugation reaction. Importantly, deprotection of Sahc leads to homocysteine-containing proteins—a potentially useful approach for the selective labelling of thiols with high relevance in various medical settings.

Enabling Directional Sequence-Control via Step-Growth Polymerization of Heterofunctionalized Precision Macromonomers.

The synthesis of periodic copolymers with a regularly recurring sequence in one direction along the polymeric backbone is presented, applying a step-growth polymerization of heterofunctionalized precision macromonomers derived from solid phase synthesis (SPS) via photoinduced thiol-ene coupling (TEC). Heterofunctional macromonomers with monomer sequence-control of the AB type present a terminal alkene and a terminal thiol group carrying a photolabile protecting group to avoid uncontrolled polymerization by self-initiation. As protecting group, 3,4-methylenebisoxy-6-nitrobenzyl is attached onto the thiol via its bromide derivative directly on solid support. The protected heterofunctionalized macromonomer is polymerized in a two-step procedure, first cleaving the photolabile group and subsequent polymerization of the macromonomer via TEC, giving a high molecular weight polymer with of 23.8kDa corresponding to a of 10 with one directional sequence-control due to their consistent head-to-tail linkage.

Sequence-Controlled High Molecular Weight Glyco(oligoamide)–PEG Multiblock Copolymers as Ligands and Inhibitors in Lectin Binding

A synthesis toward sequence-controlled multiblock glycopolymers, presenting a mannopyranoside (Man) glyco(oligoamide) block followed by a poly(ethylene glycol) (PEG) (Mn of 6 kDa) block, is shown. Therefore, monodisperse and sequence-defined glyco(oligoamide) macromonomers derived from solid phase synthesis (SPS) are polymerized with dithiol-functionalized PEG via thiol–ene coupling (TEC) in a step-growth fashion. For the polymerization, a novel building block introducing a norbornene moiety is developed which is used for end-functionalization of the glyco(oligoamide) macromonomers. As a highly reactive alkene moiety in photoinduced TEC, this gives access to Xn of up to 45. A total of 12 glyco(oligoamide)–PEG multiblock copolymers with maximum Mn of 200 kDa are obtained and subjected to a series of purification steps decreasing overall dispersity. In different binding studies toward model lectin Concanavalin A, despite their high number of Man ligands, we see rather weak binding of glycopolymers that w...

Sequence-Controlled Glycopolymers via Step-Growth Polymerization of Precision Glycomacromolecules for Lectin Receptor Clustering.

A versatile approach for the synthesis of sequence-controlled multiblock copolymers, using a combination of solid phase synthesis and step-growth polymerization by photoinduced thiol-ene coupling (TEC) is presented. Following this strategy, a series of sequence-controlled glycopolymers is derived from the polymerization of a hydrophilic spacer macromonomer and different glycomacromonomers bearing between one to five α-d-Mannose (Man) ligands. Through the solid phase assembly of the macromonomers, the number and positioning of spacer and sugar moieties is controlled and translates into the sequence-control of the final polymer. A maximum M̅n of 16 kDa, corresponding to a X̅n of 10, for the applied macromonomers is accessible with optimized polymerization conditions. The binding behavior of the resulting multiblock glycopolymers toward the model lectin Concanavalin A (ConA) is studied via turbidity assays and surface plasmon resonance (SPR) measurements, comparing the ability of precision glycomacromolecules and glycopolymers to bind to and cross-link ConA in dependence of the number of sugar moieties and overall molecular weight. The results show that there is a clear correlation between number of Man ligands and Con A binding and clustering, whereas the length of the glycooligomer- or polymer backbone seems to have no effect.

Design of S-Allylcysteine in Situ Production and Incorporation Based on a Novel Pyrrolysyl-tRNA Synthetase Variant.

The noncanonical amino acid S-allyl cysteine (Sac) is one of the major compounds of garlic extract and exhibits a range of biological activities. It is also a small bioorthogonal alkene tag capable of undergoing controlled chemical modifications, such as photoinduced thiol-ene coupling or Pd-mediated deprotection. Its small size guarantees minimal interference with protein structure and function. Here, we report a simple protocol efficiently to couple in-situ semisynthetic biosynthesis of Sac and its incorporation into proteins in response to amber (UAG) stop codons. We exploited the exceptional malleability of pyrrolysyl-tRNA synthetase (PylRS) and evolved an S-allylcysteinyl-tRNA synthetase (SacRS) capable of specifically accepting the small, polar amino acid instead of its long and bulky aliphatic natural substrate. We succeeded in generating a novel and inexpensive strategy for the incorporation of a functionally versatile amino acid. This will help in the conversion of orthogonal translation from a standard technique in academic research to industrial biotechnology.

Soft hydrogels from tetra-functional PEGs using UV-induced thiol–ene coupling chemistry: a structure-to-property study

In this work, photo-induced thiol–ene coupling (TEC) was used to produce well-defined poly(ethylene glycol) (PEG)-based hydrogels. PEGs of four different molecular weights (2k, 6k, 10k, and 20k) were functionalized with G1-allyl dendrons using anhydride chemistry to produce tetra-functional TEC crosslinkable PEGs. The tetra-functional PEGs were subsequently crosslinked with a tri-functional thiol in ethanol to form hydrogels. The synthesized hydrogels were characterized with respect to swelling behaviour, rheological properties and hydrolytic degradation. It was found that the molecular weight of the PEG chain greatly influences the final properties of the hydrogel, where a higher molecular weight of PEG gives an increased weight swelling ratio from 240% for PEG-2k hydrogels to 1400% for PEG-20k hydrogels, as well as decreased elastic moduli, with Young's moduli ranging from 106 MPa to 6 MPa, for PEG-2k and PEG-20k hydrogels, respectively. It was also found that the hydrolytic stability in alkaline conditions (pH 10) decreased when the molecular weight of PEG in the hydrogels increased.

Systematic study on free radical hydrothiolation of unsaturated monosaccharide derivatives with exo- and endocyclic double bonds

Exo- and endocyclic double bonds of glycals and terminal double bonds of enoses were reacted with various thiols by irradiation with UV light in the presence of a cleavable photoinitiator. The photoinduced radical-mediated hydrothiolation reactions showed highly varying overall conversions depending not only on the substitution pattern and electron-density of the double bond but also on the nature and substitution pattern of the thiol partner. Out of the applied thiols thiophenol, producing the highly stabilized thiyl radical, exhibited the lowest reactivity toward each type of alkene. In most cases, the hydrothiolations took place with full regio- and stereoselectivities. Successful addition of 1,2 : 3,4-di-O-isopropylidene-6-thio-α-d-galactopyranose to a 2,3-unsaturated N-acetylneuraminic acid derivative, providing a (3 → 6)-S-linked pseudodisaccharide, demonstrated that the endocyclic double bond of Neu5Ac-2-ene, bearing an electron-withdrawing substituent, shows sufficient reactivity in the photoinduced thiol-ene coupling reaction.

Silica-supported 5-(pyrrolidin-2-yl)tetrazole: development of organocatalytic processes from batch to continuous-flow conditions

5-(Pyrrolidin-2-yl)tetrazole functionalized silica prepared by photoinduced thiol–ene coupling is packed into a short stainless steel column. The resulting packed-bed microreactor is conveniently heated to perform environmentally benign continuous-flow aldol reactions with good stereoselectivities, complete conversion efficiencies, and long term stability of the packing material.

Exhaustive glycosylation, PEGylation, and glutathionylation of a [G4]-ene(48) dendrimer via photoinduced thiol-ene coupling.

We report in this paper the use of free-radical thiol-ene coupling (TEC) for the introduction of carbohydrate, poly(ethylene glycol), and peptide fragments at the periphery of an alkene functional dendrimer. Four different sugar thiols including glucose, mannose, lactose and sialic acid, two PEGylated thiols and the natural tripeptide glutathione were reacted with a fourth generation alkene functional dendrimer [G4]-ene(48) upon irradiation at λ(max) 365 nm. In all cases, the (1)H NMR spectra of the crude reaction mixture revealed the complete disappearance of alkene proton signals indicating the quantitative conversion of all 48 alkene groups of the dendrimer. With one exception only, all dendrimer conjugates were isolated in high yields (70-94%), validating the high efficiency of multiple TEC reactions on a single substrate. All isolated and purified compounds were analyzed by MALDI-TOF spectrometry and gave spectra consistent with the assigned structure.

A New Ligation Strategy for Peptide and Protein Glycosylation: Photoinduced Thiol–Ene Coupling

Photoclick with proteins: Two cysteine containing peptides and the natural protein bovine serum albumin (BSA) were glycosylated (see scheme) by an alkenyl glycoside upon irradiation with visible light in the presence of a sensitizer (click thiol–ene coupling). The hyperglycosylation of BSA involved the cysteine free SH and two SH groups derived from selective photocleavage of a cystine residue.

ChemInform Abstract: Photoinduced Thiol—Ene Coupling as a Click Ligation Tool for Thiodisaccharide Synthesis.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Single and dual glycoside clustering around calix[4]arene scaffolds via click thiol–ene coupling and azide–alkyne cycloaddition

We present the first synthesis of calix[4]arene-based S-glycoclusters via photoinduced multiple thiol-ene coupling of tetra- and octa-allyl calix[4]arenes with peracetylated glucosyl thiol (67-88% yields). Moreover we describe the dual clustering at the upper and lower rim of a calix[4]arene with two different sugars (galactose and glucose) via sequential copper(i)-catalyzed azide-alkyne cycloaddition and photoinduced thiol-ene coupling.