Scientific Report 2006

Chemistry

Powerful Click Processes for Organic Synthesis, Chemical Biology, and Materials Research

K.B. Sharpless, V.V. Fokin, M. Ahlquist, B. Boren, M. Cassidy, S. Chang, A. Feldman, J. Fotsing, R. Fraser, A. Grant, J. Hein, J. Kalisiak, L. Krasnova, S.-W. Kwok, Y. Liu, J. Loren, R. Manetsch, K. Nagai, S. Narayan, S. Pitram, L.K. Rasmussen, J. Raushel, S. Roeper, W. Sharpless, A. Sugawara, J. Tripp, X. Wang, J. Wassenaar, T. Weide, M. Whiting, P. Wu

The driving forces in our research are the discovery and understanding of chemical reactivity, the harbingers of all new reactions. Our main goal is to develop practical transformations that facilitate synthesis of novel molecules with desired functions and allow manipulation of complex biological systems at the molecular level.

Click Chemistry

Among the many factors that determine the success of a search for compounds with desired properties, 2 stand out: the degree of diversity of the building blocks that can be used and the speed with which synthesis, screening for the desired function, and lead optimization can be performed. The greater the variety of scaffolds and functional groups that can be used in the rapid construction of candidate compounds, the more likely it is that new and useful function will be discovered. Because of the enormous number of compounds to explore (the number of small druglike compounds may be as high as 10⁶⁴), the size of a given collection becomes much less important than the ability to rapidly probe the collection for a desired activity. However, many chemical methods often have restrictions such as limited scope, inaccessibility of starting materials, requirements for protecting groups, and difficult purifications. In addition, inert atmospheres and anhydrous solvents are usually required, a situation that makes these methods impractical for manipulating biological molecules in the molecules’ natural, aqueous environment.

In the past several years, we have sought to develop and use only the best reactions for the synthesis of functional molecules. The reactions that fulfill the most stringent criteria of usefulness and convenience have been grouped under the name click chemistry. Most click reactions form carbon-heteroatom bonds, are tolerant of water, and are often accelerated when water is used as the sole medium, even if the reagents are not soluble in water.

Copper-Catalyzed Cycloadditions

The copper-catalyzed 1,3-dipolar cycloaddition of azides and alkynes has emerged as a premiere click reaction that enables reliable assembly of complex molecules by means of the 1,2,3-triazole heterocycle. Although both alkynes and azides are highly reactive, their reactivity profiles are quite narrow, that is, “orthogonal” to an unusually broad range of reagents, solvents, and other functional groups. These features allow clean sequential transformations of broad scope without the need for protecting groups, even if the reactions are performed under physiologic conditions.

The 1,2,3-triazoles have the advantageous properties of high chemical stability (in general, being inert to severe hydrolytic, oxidizing, and reducing conditions, even at high temperatures), strong dipole moment, presence of aromatic groups, and the ability to accept hydrogen bonds. Thus, they can interact productively in several ways with biological molecules. For example, 1,2,3-triazoles can replace the amide bond in peptides, preventing proteolytic degradation of the peptides.

The fundamental thermal reaction, involving terminal or internal alkynes (Fig. 1, top), has been known for more than a century and has been thoroughly investigated. Although the process is strongly thermodynamically favored, it has a relatively high kinetic barrier that makes the reaction slow at room temperature for unactivated reactants and results in the formation of regioisomers. Copper(I) catalysis dramatically accelerates the reaction, by a factor of up to 10⁷, and regiospecifically produces only 1,4-disubstituted-1,2,3-triazoles (Fig. 1, bottom). Because of its experimental simplicity and unusually broad scope, this process has been used in a number of applications in synthesis, medicinal chemistry, molecular biology, and materials science.

Fig. 1. Thermal cycloaddition of azides and alkynes (top) requires prolonged heating and results in mixtures of both 1,4- and 1,5-regioisomers, whereas the copper-catalyzed 1,3-dipolar cycloaddition of azides and alkynes produces only 1,4-disubstituted-1,2,3-triazoles at room temperature (bottom).

Although a number of copper(I) complexes can be used to catalyze the reaction, we found that the catalyst is often better prepared in situ by reduction of copper(II) salts, which are readily available and are easier to handle than most copper(I) salts. As the reductant, ascorbic acid (vitamin C) or sodium ascorbate is excellent. Remarkably, even copper metal can be used as a source of the catalytic species, making the experimental procedure even simpler: a small piece of copper metal (wire or turning) is all that is added to the reaction mixture, which is then shaken or stirred for 12–48 hours. This protocol is particularly convenient in parallel synthesis, because triazole products are generally isolated in high yields and can often be submitted for screening without further purification.

Our studies of reactivity of sulfonyl azides in the copper-catalyzed cycloaddition of azides and alkynes resulted in the development of an experimentally simple catalytic procedure for the highly selective conversion of alkynes to N-sulfonyl azetidin-2-imines under mild conditions. This 3-component process is thought to proceed via initial reaction of in situ generated copper(I) acetylides with sulfonyl azides, resulting in transient (1-sulfonyltriazolyl) copper intermediates that upon extrusion of dinitrogen generate N-sulfonyl keteneimines. The azetidinimine products are remarkably stable in a wide range of reaction conditions, and other functional groups can be easily added (Fig. 2). This newly discovered reaction sequence rapidly produces densely functionalized azetidine derivatives from readily available terminal alkynes in just 2 or 3 simple steps and should be useful for exploring the usefulness of these 4-atom heterocycles.

Fig 2. Synthesis of azetidinimines from alkynes, sulfonyl azides, and imines.

Although useful because of its rate and functional group tolerance, the copper-catalyzed 1,3-dipolar cycloaddition of azides and alkynes cannot produce 1,5-disubstituted 1,2,3-triazoles, and it is not effective with internal alkynes. Therefore, the recent discovery of ruthenium(II) catalysts that are active in azide-alkyne cycloaddition and result in the formation of the complementary 1,5-regioisomers of 1,2,3-triazoles was a welcome advance. Pentamethyl cyclopentadienyl ruthenium(II) complexes are active and easy to handle and provide triazole products in good to excellent yields (Fig. 3). In addition, these catalysts are active with internal alkynes, allowing easy synthesis of fully substituted triazoles.

Fig. 3. Ruthenium-catalyzed synthesis of fully substituted 1,2,3-triazoles.

Synthesis of Polyfunctional Dendrimers

Dendrimers are highly ordered, regularly branched globular macromolecules of defined structure; dendrimers are ideal building blocks for creating bioactive macromolecules and nanomaterials. Previously, we exploited the high fidelity of the copper-catalyzed 1,3 dipolar cycloaddition of azides and alkynes in the efficient synthesis of dendrimers. We used procedures that involved little more than mixing stoichiometric quantities of reactants, stirring, and isolating dendrimer products.

In collaboration with M.G. Finn, Department of Chemistry, and C.J. Hawker, University of California, Santa Barbara, we have extended this approach to synthesize chemically heterogeneous dendrimers (Fig. 4) that have multiple recognition functions (such as 16 α-d-mannose units) and detection elements (2 coumarin-derived fluorescent chromophores). The performance of one such bifunctional dendrimer was evaluated in a standard hemagglutination assay with the mannose-binding protein concanvalin A and rabbit red blood cells. The dendrimer had 240-fold greater potency than monomeric mannose, a difference that translates to a 15-fold increase in activity per unit. Additionally, our preliminary experiments indicate that this dendrimer binds to the modified surface of Escherichia coli, producing detectable fluorescent signals. These results are a significant advance in dendrimer chemistry and illustrate an evolving synergy between organic chemistry and functional materials.

Fig. 4. Synthesis of polyfunctional dendrimers via copper-catalyzed 1,3-dipolar cycloaddition of azides and alkynes.

Studies of other applications, ranging from biology to materials science, are currently underway in our laboratories and in collaboration with M.G. Finn, P.K. Vogt, C.-H. Wong, J.H. Elder, and others at Scripps Research.

Publications

Bourne, Y., Radic, Z., Kolb, H.C., Sharpless, K.B., Taylor, P., Marchot, P. Structural insights into conformational flexibility at the peripheral site and within the active center gorge of AChE. Chem. Biol. Interact. 157-158:159, 2005.

Cassidy, M.P., Raushel, J., Fokin, V.V. Practical synthesis of amides from in situ generated copper(I) acetylides and sulfonyl azides. Angew. Chem. Int. Ed. 45:3154, 2006.

Hansen, T.V., Wu, P., Fokin, V.V. One-pot copper(I)-catalyzed synthesis of 3,5-disubstituted isoxazoles. J. Org. Chem. 70:7761, 2005.

Loren, J.C., Krasinski, A., Fokin, V.V., Sharpless, K.B., NH-1,2,3-triazoles from azidomethyl pivalate and carbamates: base-labile N-protecting groups. Synlett 2847, 2005, Issue 18.

Malkoch, M., Schleicher, K., Drockenmuller, E., Hawker, C.J., Russell, T.P., Wu, P., Fokin, V.V. Structurally diverse dendritic libraries: a highly efficient functionalization approach using click chemistry. Macromolecules 38:3663, 2005.

Meng, J.-C., Fokin, V.V., Finn, M.G. Kinetic resolution by copper-catalyzed azide-alkyne cycloaddition. Tetrahedron Lett. 46:4543, 2005.

Petasis, N.A., Akritopoulou-Zanze, I., Fokin, V.V., Bernasconi, G., Keledjian, R., Yang, R., Uddin, J., Nagulapalli, K.C., Serhan, C.N. Design, synthesis and bioactions of novel stable mimetics of lipoxins and aspirin-triggered lipoxins. Prostaglandins Leukot. Essent. Fatty Acids 73:301, 2005.

Radic, Z., Manetsch, R., Krasinski, A., Raushel, J., Yamauchi, J., Garcia, C., Kolb, H.C., Sharpless, K.B., Taylor, P. Molecular basis of interactions of cholinesterases with tight binding inhibitors. Chem. Biol. Interact. 157-158:133, 2005.

Rodionov, V.O., Fokin, V.V., Finn, M.G. Mechanism of the ligand-free Cu(I)-catalyzed azide-alkyne cycloaddition reaction. Angew. Chem. Int. Ed. 44:2210, 2005.

Whiting, M., Fokin, V.V. Copper-catalyzed reaction cascade: direct conversion of alkynes to N-sulfonylazetidin-2-imines. Angew. Chem. Int. Ed. 45:3157, 2006.

Whiting, M., Muldoon, J., Lin, Y.-C., Silverman, S.M., Lindstrom, W., Olson, A.J., Kolb, H.C., Finn, M.G., Sharpless, K.B., Elder, J.H., Fokin, V.V. Inhibitors of HIV-1 protease by using in situ click chemistry. Angew. Chem. Int. Ed. 45:1435, 2006.

Wu, P., Hilgraf, R., Fokin, V.V. Osmium-catalyzed olefin dihydroxylation and aminohydroxylation in the second catalytic cycle. Adv. Synth. Catal. 348:1079, 2006.

Wu, P., Malkoch, M., Hunt, J., Vestberg, R., Kaltgrad, E., Finn, M.G., Fokin, V.V., Sharpless, K.B., Hawker, C.J. Multivalent, bifunctional dendrimers prepared by click chemistry. Chem. Commun. (Camb.) 5775, 2005, Issue 46.

Zhang, L., Chen, X., Xue, P., Sun, H.H.Y., Williams, I.D., Sharpless, K.B., Fokin, V.V., Jia, G. Ruthenium-catalyzed cycloaddition of alkynes and organic azides. J. Am. Chem. Soc. 127:15998, 2005.