K. Barry Sharpless

The growing impact of click chemistry on drug discovery

Click chemistry is a modular approach that uses only the most practical and reliable chemical transformations. Its applications are increasingly found in all aspects of drug discovery, ranging from lead finding through combinatorial chemistry and target-templated in situ chemistry, to proteomics and DNA research, using bioconjugation reactions. The copper-(I)-catalyzed 1,2,3-triazole formation from azides and terminal acetylenes is a particularly powerful linking reaction, due to its high degree of dependability, complete specificity, and the bio-compatibility of the reactants. The triazole products are more than just passive linkers; they readily associate with biological targets, through hydrogen bonding and dipole interactions.

Click‐Chemie: diverse chemische Funktionalität mit einer Handvoll guter Reaktionen

Betrachtet man die in der Natur am haufigsten vorkommenden Verbindungen, so fallt auf, dass die Bildung von Kohlenstoff-Heteroatom-Bindungen gegenuber der von Kohlenstoff-Kohlenstoff-Bindungen deutlich bevorzugt ist. Da zum einen Kohlendioxid die Basisverbindung der Natur ist und andererseits das Medium naturlicher Reaktionen zumeist Wasser ist, uberrascht dies sicherlich nicht. Nucleinsauren, Proteine und Polysaccharide sind polymere Kondensationsprodukte kleiner Untereinheiten, die durch Kohlenstoff-Heteroatom-Bindungen verknupft sind. Sogar die etwa 35 Baueinheiten, aus denen diese essentiellen Verbindungen bestehen, enthalten nicht mehr als sechs aufeinander folgende C-C-Bindungen, sieht man einmal von den drei aromatischen Aminosauren ab. Mit der Natur als Vorbild richteten wir unser Interesse auf die Entwicklung leistungsfahiger, gut funktionierender und selektiver Reaktionen fur die effiziente Synthese neuartiger nutzlicher Verbindungen sowie kombinatorischer Bibliotheken mittels Heteroatomverknupfungen (C-X-C). Diese Synthesestrategie nennen wir „Click-Chemie“. Click-Chemie ist durch eine Auswahl einiger weniger nahezu idealer Reaktionen charakterisiert, mit all ihren Grenzen und Moglichkeiten. In diesem Beitrag werden zum einen die strengen Kriterien, die Reaktionen erfullen mussen, um die Bezeichnung „Click-Chemie“ zu verdienen, definiert, zum anderen werden Beispiele fur molekulare Strukturen gegeben, die mit dieser spartanischen, aber dennoch leistungsfahigen Synthesestrategie leicht hergestellt werden konnen.

Searching for new reactivity (Nobel lecture).

The processes for the selective oxidation of olefins have long been among the most useful tools for day-to-day organic synthesis. Herein, the focus is on the asymmetric-epoxidation (AE) and asymmetric-dihydroxylation (AD) reactions developed by Sharpless and co-workers. The reactions have a wide scope, are simple to run, and involve readily available starting materials. Ligand-accelerated catalysis is crucial to these reactions and might be the agent for uncovering more catalytic processes. In addition to the selectivity benefits of catalysis, the phenomenon of turnover (amplification) raises its potential impact. The author and his co-workers developed small, highly enantioselective catalysts that were unfettered by the “lock-and-key” selectivity of Natures enzymes, and tolerant of substrates throughout the entire range of olefin substitution patterns.

A simplified procedure for the stereospecific transformation of 1,2-diols into epoxides

Abstract A simple, ‘one-pot’ procedure for the conversion of vicinal diols into epoxides via halohydrin ester intermediates has been developed. This method tolerates a wide range of functionality including acid sensitive functional groups. The transformation proceeds via a, usually highly regioselective, nucleophilic opening of a cyclic acetoxonium intermediate, generated from a cyclic orthoacetate and Me3SiCl, acetyl bromide or acetyl chloride/NaI to form 1-acetoxy-2-chloride, 1-acetoxy-2-bromide or 1-acetoxy-2-iodide intermediates, respectively. No epimerization occurs, even with benzylic substrates. Subsequent base mediated methanolysis furnishes epoxides in excellent overall yield. Application of this method led to an efficient formal synthesis of the leukotriene antagonist SKF 104353, commencing with the highly enantioselective cis-dihydroxylation of methyl 3-[2-(8-phenyloctyl)phenyl]propenoate.

Cyclic sulfates containing acid-sensitive groups and chemoselective hydrolysis of sulfate esters

Abstract Diols containing acid-sensitive functionalities such as acetonide and silyloxy groups are efficiently converted to the corresponding cyclic sulfates via formation of the cyclic sulfites in the presence of a base and oxidation of the isolated sulfites using catalytic RuO4. Reactions of these cyclic sulfates with nucleophiles such as azide and benzoate provide sulfates, which can be successfully hydrolyzed by using a catalytic amount of sulfuric acid and 0.5∼1.0 eq of water in THF.

Total synthesis of the L-hexoses

Enantiomerically pure polyhydroxylated natural products are synthesized by using a reiterative two-carbon extension cycle consisting of four steps. The generality and efficiency of this methodology are demonstrated in the total synthesis of all eight L-hexoses.

Sulfur(VI) fluoride exchange (SuFEx): another good reaction for click chemistry.

Aryl sulfonyl chlorides (e.g. Ts-Cl) are beloved of organic chemists as the most commonly used S(VI) electrophiles, and the parent sulfuryl chloride, O2 S(VI) Cl2 , has also been relied on to create sulfates and sulfamides. However, the desired halide substitution event is often defeated by destruction of the sulfur electrophile because the S(VI) Cl bond is exceedingly sensitive to reductive collapse yielding S(IV) species and Cl(-) . Fortunately, the use of sulfur(VI) fluorides (e.g., R-SO2 -F and SO2 F2 ) leaves only the substitution pathway open. As with most of click chemistry, many essential features of sulfur(VI) fluoride reactivity were discovered long ago in Germany.6a Surprisingly, this extraordinary work faded from view rather abruptly in the mid-20th century. Here we seek to revive it, along with John Hyatts unnoticed 1979 full paper exposition on CH2 CH-SO2 -F, the most perfect Michael acceptor ever found.98 To this history we add several new observations, including that the otherwise very stable gas SO2 F2 has excellent reactivity under the right circumstances. We also show that proton or silicon centers can activate the exchange of SF bonds for SO bonds to make functional products, and that the sulfate connector is surprisingly stable toward hydrolysis. Applications of this controllable ligation chemistry to small molecules, polymers, and biomolecules are discussed.

Mechanistic Studies on the Cu-Catalyzed Three-Component Reactions of Sulfonyl Azides, 1-Alkynes and Amines, Alcohols, or Water: Dichotomy via a Common Pathway

Combined analyses of experimental and computational studies on the Cu-catalyzed three-component reactions of sulfonyl azides, terminal alkynes and amines, alcohols, or water are described. A range of experimental data including product distribution ratio and trapping of key intermediates support the validity of a common pathway in the reaction of 1-alkynes and two distinct types of azides substituted with sulfonyl and aryl(alkyl) groups. The proposal that bimolecular cycloaddition reactions take place initially between triple bonds and sulfonyl azides to give N-sulfonyl triazolyl copper intermediates was verified by a trapping experiment. The main reason for the different outcome from reactions between sulfonyl and aryl(alkyl) azides is attributed to the lability of the N-sulfonyl triazolyl copper intermediates. These species are readily rearranged to another key intermediate, ketenimine, into which various nucleophiles such as amines, alcohols, or water add to afford the three-component coupled products: amidines, imidates, or amides, respectively. In addition, the proposed mechanistic framework is in good agreement with the obtained kinetics and competition studies. A computational study (B3LYP/LACV3P*+) was also performed confirming the proposed mechanistic pathway that the triazolyl copper intermediate plays as a branching point to dictate the product distribution.

Osmium-Catalyzed Dihydroxylation of Olefins in Acidic Media: Old Process, New Tricks

A screen of over 500 diversely functionalized additives in osmium-catalyzed dihydroxylation has uncovered that electron-deficient olefins are converted into the corresponding diols much more efficiently when the pH of the reaction medium is maintained on the acidic side. Further studies have identified citric acid as the additive of choice, for it allows preparation of very pure diols in yields generally exceeding 90%. As described here, a much wider range of olefin classes can now be successfully dihydroxylated. The process is experimentally simple, in most cases involving little more than dissolving the reactants in water or a water/tert-butyl alcohol mixture, stirring them, and filtering off the pure diol product.

Rapid Discovery and Structure−Activity Profiling of Novel Inhibitors of Human Immunodeficiency Virus Type 1 Protease Enabled by the Copper(I)-Catalyzed Synthesis of 1,2,3-Triazoles and Their Further Functionalization

Building from the results of a computational screen of a range of triazole-containing compounds for binding efficiency to human immunodeficiency virus type 1 protease (HIV-1-Pr), a novel series of potent inhibitors has been developed. The copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), which provides ready access to 1,4-disubstituted-1,2,3-triazoles, was used to unite a focused library of azide-containing fragments with a diverse array of functionalized alkyne-containing building blocks. In combination with direct screening of the crude reaction products, this method led to the rapid identification of a lead structure and readily enabled optimization of both azide and alkyne fragments. Replacement of the triazole with a range of alternative linkers led to greatly reduced protease inhibition; however, further functionalization of the triazoles at the 5-position gave a series of compounds with increased activity, exhibiting Ki values as low as 8 nM.