A Radical Activation Strategy for Versatile and Stereoselective N-Glycosylation.

Previous N-glycosylation approaches have predominately involved acidic conditions, facing challenges of low stereoselectivity and limited scope. Herein, we introduce a radical activation strategy that enables versatile and stereoselective N-glycosylation using readily accessible glycosyl sulfinate donors under basic conditions and exhibits exceptional tolerance towards various N-aglycones containing alkyl, aryl, heteroaryl and nucleobase functionalities. Preliminary mechanistic studies indicate a pivotal role of iodide, which orchestrates the formation of a glycosyl radical from the glycosyl sulfinate and subsequent generation of the key intermediate, a configurationally well-defined glycosyl iodide, which is subsequently attacked by an N-aglycone in a stereospecific SN2 manner to give the desired N-glycosides. An alternative route involving the coupling of a glycosyl radical and a nitrogen-centered radical is also proposed, affording the exclusive 1,2-trans product. This novel approach promises to broaden the synthetic landscape of N-glycosides, offering a powerful tool for the construction of complex glycosidic structures under mild conditions.

Anion-Bridged Dual Hydrogen Bond Enabled Concerted Addition of Phenol to Glycal.

A noncovalent organocatalytic concerted addition of phenol to glycal is developed for the stereoselective and regioselective construction of biologically important phenolic 2-deoxyglycosides, featuring wide substrate tolerance. The method relies on an anion-bridged dual hydrogen bond interaction which is experimentally proved by Nuclear Magnetic Resonance (NMR), Ultraviolet and visible (UV-vis), and fluorescence analysis. Experimental evidence including kinetic analysis, Kinetic Isotope Effect (KIE) studies, linear free energy relationship, Hammett plot, and density functional theory (DFT) calculations is provided for a concerted mechanism where a high-energy oxocarbenium ion is not formed. In addition, the potential utility of this method is further demonstrated by the synthesis of biologically active glycosylated flavones. The benchmarking studies demonstrate significant advances in this newly developed method compared to previous approaches.

A Robust Copper-Catalyzed Cross-Coupling of Glycosyl Thiosulfonate and Boronic Acids Enables the Construction of Thioglycosides.

An efficient and stereoretentive copper-catalyzed cross-coupling of glycosyl thiosulfonate and boronic acid for the construction of thioglycosides is described. The good functional group compatibility of this method allows the preparation of many bioactive aryl/alkenyl thioglycosides, including the hSGLT1 inhibitor.

Tetrazine-induced activation of a trimethyl lock as a click-to-release system for protected doxorubicin.

We herein report a novel chemically triggered click-to-release system, that combines the trimethyl lock (TML) lactonization with the bioorthogonal inverse electron demand Diels-Alder (IEDDA) reaction of a vinyl ether and a tetrazine. Kinetic studies were carried out on a vinyl phenol model system with six tetrazines using NMR and UV/Vis spectroscopy, revealing that within the three step sequence the IEDDA reaction was rate-limiting. The reaction rates were enhanced by increasing the electrophilicity of the tetrazine, while balancing reactivity and stability of the tetrazines. The anticancer drug doxorubicin was conjugated to a vinyl-modified TML. Its subsequent liberation from vinyl-TML was triggered by dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate and followed quantitatively by NMR, thereby providing a proof-of-concept for the tetrazine/TML click-to-release system. In addition the applicability of the reaction under physiolgoical conditions could be shown.

Selective Bacterial Targeting and Infection-Triggered Release of Antibiotic Colistin Conjugates.

In order to render potent, but toxic antibiotics more selective, we have explored a novel conjugation strategy that includes drug accumulation followed by infection-triggered release of the drug. Bacterial targeting was achieved using a modified fragment of the human antimicrobial peptide ubiquicidin, as demonstrated by fluorophore-tagged variants. To limit the release of the effector colistin only to infection-related situations, we introduced a linker that was cleaved by neutrophil elastase (NE), an enzyme secreted by neutrophil granulocytes at infection sites. The linker carried an optimized sequence of amino acids that was required to assure sufficient cleavage efficiency. The antibacterial activity of five regioisomeric conjugates prepared by total synthesis was masked, but was released upon exposure to recombinant NE when the linker was attached to amino acids at the 1- or the 3-position of colistin. A proof-of-concept was achieved in co-cultures of primary human neutrophils and Escherichia coli that induced the secretion of NE, the release of free colistin, and an antibacterial efficacy that was equal to that of free colistin.

A robust and tunable halogen bond organocatalyzed 2-deoxyglycosylation involving quantum tunneling.

The development of noncovalent halogen bonding (XB) catalysis is rapidly gaining traction, as isolated reports documented better performance than the well-established hydrogen bonding thiourea catalysis. However, convincing cases allowing XB activation to be competitive in challenging bond formations are lacking. Herein, we report a robust XB catalyzed 2-deoxyglycosylation, featuring a biomimetic reaction network indicative of dynamic XB activation. Benchmarking studies uncovered an improved substrate tolerance compared to thiourea-catalyzed protocols. Kinetic investigations reveal an autoinductive sigmoidal kinetic profile, supporting an in situ amplification of a XB dependent active catalytic species. Kinetic isotopic effect measurements further support quantum tunneling in the rate determining step. Furthermore, we demonstrate XB catalysis tunability via a halogen swapping strategy, facilitating 2-deoxyribosylations of D-ribals. This protocol showcases the clear emergence of XB catalysis as a versatile activation mode in noncovalent organocatalysis, and as an important addition to the catalytic toolbox of chemical glycosylations.

A Multistage Halogen Bond Catalyzed Strain-Release Glycosylation Unravels New Hedgehog Signaling Inhibitors.

Halogen bonding (XB) has recently emerged as a promising noncovalent activation mode that can be employed in catalysis. However, methodologies utilizing XB remain rare, and the hydrogen-bonding (HB) catalysis congeners are more widespread in comparison. Herein, we demonstrate a remarkable case whereby employment of XB catalysis in strain-release glycosylation generates O, N-glycosides in excellent anomeric selectivity exceeding HB activation. Deeper investigation unraveled XB catalyst dependencies on multiple stages of the mechanism and a hitherto unknown XB-glycosyl acceptor activation. We present a proof of concept to interrogate sp3-rich glycosidic chemical space for novel biological activity, by integrating XB-catalyzed construction of a glycosidic compound collection, and evaluating these analogues via cell-based phenotypic screens. We show that XB-catalyzed strain-release glycosylation defines a new class of glycosides that inhibit the hedgehog signaling pathway through a nonsmoothened mode of action, opening new opportunities to combat acquired cancer resistance.

An ultra-low thiourea catalyzed strain-release glycosylation and a multicatalytic diversification strategy.

The utility of thiourea catalysis in selective glycosylation strategies has gained significant momentum lately due to its versatility in hydrogen bonding or anionic recognition activation modes. The use of these non-covalent interactions constitute a powerful means to construct glycosidic linkages as it mimics physiologically occurring glycosyltransferases. However, glycosyl donor activation through the currently employed catalysts is moderate such that, in general, catalyst loadings are rather high in these transformations. In addition, thiourea catalysis has not been well explored for the synthesis of furanosides. Herein, we demonstrate an ultra-low loadings stereoselective and stereospecific thiourea catalyzed strain-release furanosylation and pyranosylation strategy. Our ultra-low organocatalyzed furanosylation enables a multicatalytic strategy, which opens up a unique avenue towards rapid diversification of synthetic glycosides. In-situ NMR monitoring unravel insights into unknown reaction intermediates and initial rate kinetic studies reveal a plausible synergistic hydrogen bonding/Brønsted acid activation mode.

N-Trifluoromethylthio-dibenzenesulfonimide: A Shelf-Stable, Broadly Applicable Electrophilic Trifluoromethylthiolating Reagent.

The super electrophilicity of a shelf-stable, easily prepared trifluoromethylthiolating reagent N-trifluoromethylthio-dibenzenesulfonimide 7 was demonstrated. Consistent with the theoretical prediction, 7 exhibits reactivity remarkably higher than that of other known electrophilic trifluoromethylthiolating reagents. In the absence of any additive, 7 reacted with a wide range of electron-rich arenes and activated heteroarenes under mild conditions. Likewise, reactions of 7 with styrene derivatives can be fine-tuned by simply changing the reaction solvents to generate trifluoromethylthiolated styrenes or oxo-trifluoromethylthio or amino-trifluoromethylthio difunctionalized compounds in high yields.

Lewis Acid Mediated Trifluoromethylthio Lactonization/Lactamization.

A highly selective Lewis acid mediated trifluoromethylthio lactonization/lactamization of olefins is described. The reaction was proposed to proceed via a thiiranium ion intermediate, which was further attacked by the carboxylic acid or amide to generate the corresponding trifluoromethylthiolated lactone/lactam.

Shelf-stable electrophilic reagents for trifluoromethylthiolation.

Fluorine, which is the most electronegative element and has a small atomic radius, plays a key role in pharmaceutical, agrochemical, and materials sciences. One of the fluoroalkyl groups, the trifluoromethylthio group (CF3S-), has been well-recognized as an important structural motif in the design of lead compounds for new drug discovery because of its high lipophilicity (Hansch lipophilicity parameter π = 1.44) and strong electron-withdrawing properties, which could improve the drug molecule's cell-membrane permeability and enhance its chemical and metabolic stability. While classic methods for the preparation of trifluoromethylthiolated compounds typically involve halogen-fluorine exchange reactions of polyhalogenomethyl thioethers or trifluoromethylation of sulfur-containing compounds under harsh reaction conditions, an alternative but more attractive strategy is direct trifluoromethylthiolation of the substrate at a late stage by employing an electrophilic trifluoromethylthiolating reagent. Although several electrophilic trifluoromethylthiolating reagents have been reported previously, these reagents either require a strong Lewis acid/Brønsted acid as an activator or suffer from a toxic nature or limited substrate scope. To address these problems, in late 2011 we initiated a project with the aim to develop new, shelf-stable, and highly reactive electrophilic trifluoromethylthiolating reagents that could easily install the trifluoromethylthio group at the desired positions of the drug molecule at a late stage of drug development. Inspired by the broad reactivity of the hypervalent iodine reagent, we initially discovered a highly reactive trifluoromethylthiolating reagent, trifluoromethanesulfenate 1a. Structure-reactivity studies disclosed that the iodine atom of reagent 1a does not play an important role in this reagent's reactivity. Consequently, a simplified second-generation electrophilic reagent, trifluoromethanesulfenate 1b, was developed. In parallel, we developed another shelf-stable, highly reactive electrophilic reagent with a broad substrate scope, N-trifluoromethylthiosaccharin (2). In this Account, we mainly describe our discovery of these two different types of electrophilic trifluoromethylthiolating reagents, trifluoromethanesulfenates 1a and 1b and N-trifluoromethylthiosaccharin 2. Systematic studies showed that both types of reagents are highly reactive toward a wide range of nucleophiles, yet the substrate scopes of these two different types of reagents are complementary. In particular, reagents 1a and 1b are more reliable in transition-metal-catalyzed reactions such as copper-catalyzed trifluoromethylthiolation of aryl/vinyl/alkylboronic acids and silver-catalyzed decarboxylative trifluoromethylthiolation of aliphatic carboxylic acids as well as in the organocatalytic asymmetric trifluoromethylthiolation of ß-keto esters and oxindoles. Reagent 2 is more electrophilic than reagents 1a and 1b and is more efficient for direct trifluoromethylthiolation with nucleophiles such as alcohols, amines, thiols, and electron-rich arenes. The ease in preparation, broad scope, and mild reaction conditions make reagents 1a, 1b, and 2 very attractive as general reagents that allow rapid installation of the trifluoromethylthio group into small molecules.

Structure-reactivity relationship of trifluoromethanesulfenates: discovery of an electrophilic trifluoromethylthiolating reagent.

A family of electrophilic trifluoromethanesulfenates was prepared. Structure-reactivity relationship studies showed that the substituted groups on the aryl ring of the trifluoromethylthiolating reagent did not have an obvious influence on their reactivities. A simplified electrophilic trifluoromethylthiolating reagent 1c was then identified that can react with a wide range of nucleophiles such as Grignard reagents, arylboronic acids, alkynes, indoles, ß-ketoesters, oxindoles, and sodium sulfinates under mild reaction conditions. A variety of functional groups were tolerated under these conditions.

N-trifluoromethylthiosaccharin: an easily accessible, shelf-stable, broadly applicable trifluoromethylthiolating reagent.

A new, electrophilic trifluoromethylthiolating reagent, N-trifluoromethylthiosaccharin, was developed and can be synthesized in two steps from saccharin within 30 minutes. N-trifluoromethylthiosaccharin is a powerful trifluoromethylthiolating reagent and allows the trifluoromethylthiolation of a variety of nucleophiles such as alcohols, amines, thiols, electron-rich arenes, aldehydes, ketones, acyclic ß-ketoesters, and alkynes under mild reaction conditions.

Palladium-catalyzed trifluoromethylthiolation of aryl C-H bonds.

A method for monotrifluoromethylthiolation of arenes via palladium-catalyzed directed C-H bond activation was described. The reaction was compatible with a variety of functional groups. Initial mechanistic studies disclosed that the turnover limiting step of the catalytic cycle did not involve C-H activation.

Preparation of trifluorostyrenes via palladium-catalyzed coupling of arylboronic acids with chloro- and bromotrifluoroethylene.

A highly efficient and cost-effective method for the preparation of α,ß,ß-trifluorostyrene (TFS) and its derivatives is described. The method required only 0.2 mol % of Pd(dba)(2) and 0.4 mol % of P(t)Bu(3) and occurred to full conversion within 2.0 h. With this method, a wide range of arylboronic acids were efficiently incorporated to generate α,ß,ß-trifluorostyrene derivatives.