The Skaggs Institute
for Chemical Biology

Scientific Report 2007

Click Chemistry: the Power of Simplicity

K.B. Sharpless, V.V. Fokin, M. Ahlquist, B. Boren, A. Chanda, S. Chang, A. Feldman, J. Fotsing, N. Grimster, J. Hein, J. Kalisiak, L. Krasnova, S.-W. Kwok, K. Nagai, S. Pitram, J. Raushel, A. Salameh, J. Tripp, C. Valdez, X. Wang, T. Weide, M. Whiting

In recent years, the laborious process of discovering and optimizing compounds that may be useful as drugs has been aided by combinatorial chemistry, which enables generation of large collections of test compounds for screening. However, in order to produce molecules that likely will aid in identifying new and useful functions, individual reactions involved in combinatorial syntheses must be supremely reliable. The click chemistry philosophy embodies an attitude that function matters most and that tools that enable researchers to achieve function are to be prized. Thus, click chemistry relies on the use of a few nearly perfect chemical reactions for the synthesis and assembly of specially designed building blocks. These building blocks have a high built-in energy content that drives a spontaneous, selective, and irreversible linking reaction with complementary sites in the reactive partners.

The power of click chemistry lies in the ability to rapidly generate novel structures that may not necessarily resemble known biologically active compounds. For discovering compounds that may be useful as drugs, this strategy provides a means for the rapid exploration of the chemical space. For optimization of compounds, the method enables rapid structure-activity profiling through generation of analog libraries. Click chemistry does not replace existing methods for drug discovery; rather it complements and extends them. It works well in conjunction with structure-based design and combinatorial chemistry techniques.

Click chemistry is both enabled and constrained by its reliance on a few nearly perfect reactions, and this characteristic raises concerns about limitations on the access of click chemistry to chemical diversity. However, the pool of druglike compounds may be as large as 10⁶³. Currently, only a few million compounds that fulfill these criteria are known, implying that only an infinitesimal part of the potential medicinal chemistry universe has been explored so far.

These facts have staggering implications for drug discovery. First and foremost, most molecules with useful properties remain to be discovered. Second, the majority of useful new compounds likely will be found in unconventional structure space. Thus, with click chemistry, we have the interesting proposition that greater diversity can be achieved with fewer reactions, because it is not the number of reactions that is important, but the reach of the reactions, which is determined by the tolerance to variations in the nature of their components. Click chemistry approaches have already proved themselves in biomedical research, ranging from synthetic chemistry to bioconjugation strategies, polymer chemistry, and materials science.

In Situ Click Chemistry

Although click chemistry allows rapid assembly of diverse collections of molecules, further evolution of the molecules is traditionally achieved by iterative cycles of screening for biological activity and synthetic modification. Can direct involvement of the target, usually a specific receptor or enzyme, in the selection and evolution of possible drug candidates accelerate this drug discovery cycle? The aim of in situ click chemistry is to engage an enzyme in the selection and covalent assembly of its own best fitting inhibitor. Although the concept has been previously tested by several researchers, in situ click chemistry is unique because it relies on the completely bioorthogonal 1,3-dipolar cycloaddition of organic azides and alkynes. This highly exergonic reaction produces 5-membered nitrogen heterocycles, 1,2,3-triazoles, which are exceedingly stable to acidic and basic hydrolysis and to severe reduction-oxidation conditions. At the same time, the triazoles produced can actively participate in hydrogen-bonding, dipole-dipole, and p-stacking interactions.

Even though both azides and alkynes are energetic species, their reactivity profiles are quite narrow, and despite the large thermodynamic driving force for cycloaddition, the high kinetic barrier effectively hides the reactants until they are brought into proximity by a biological template. These features allow the target to sample numerous combinations of building blocks but synthesize only the best binders.

The efficacy of in situ click chemistry has already been demonstrated by the discovery of novel, highly potent inhibitors of acetylcholinesterase, carbonic anhydrase, and HIV protease. During the past year, we extended the approach to more challenging targets that do not have 2 distinct binding sites. Among these are the enzyme β-secretase, which is involved in the progression of Alzheimer's disease, and nicotinic acetylcholine receptors, the family of ligand-gated ion channels responsible for key events in neurotransmission.

Organic Reactions on Water

During the development of click chemistry, we noticed that many click reactions proceed optimally in pure water, particularly when the organic reactants are insoluble in the water phase. A growing number of examples illustrate a remarkable phenomenon: a substantial rate acceleration occurs when insoluble reactants are stirred in aqueous suspension, denoted here as on water conditions (Fig. 1), suggesting that the venerable assumption that substances do not interact unless dissolved can be distinctly counterproductive. The use of water as the only supporting medium has other advantages, including ease of product isolation and, above all, safety, thanks to its high heat capacity and unique redox stability. We are currently studying the mechanistic underpinnings of the reactions at the water-organic interfaces.

Fig. 1. The practical nature of the on-water phenomenon is illustrated by successive photographs of the reaction of β-pinene with diethyl azodicarboxylate on water. Left, Reactants are shown immediately after being mixed together on the water layer before stirring is started. Middle, Midway in the reaction, the lightening color indicates consumption of the orange diethyl azodicarboxylate. Right, At the end of the reaction, the product, which is colorless, settles at the bottom.

Publications

Fokin, V.V, Wu, P. Epoxides and aziridines in click chemistry. In: Aziridines and Epoxides in Organic Synthesis. Yudin, A.K. (Ed.). Wiley-VCH, Weinheim, Germany, 2006, p. 443.

Hirose, T., Sunazuka, T., Noguchi, Y., Yamaguchi, Y., Hanaki, H., Sharpless, K.B., Omura, S. Rapid 'SAR' via click chemistry: an alkyne-bearing spiramycin is fused with diverse azides to yield new triazole antibacterial candidates: Heterocycles 69:55, 2006.

Kade, M., Vestberg, R., Malkoch, M., Wu, P., Fokin, V.V., Finn, M.G. Sharpless, K.B., Hawker, C. A covalently bonded layer-by-layer assembly of dendrimers by 'click' chemistry. Polymer Prepr. 47: 376, 2006.

Narayan, S., Fokin, V.V., Sharpless, K.B. Chemistry 'on water': organic synthesis in aqueous suspension. In: Organic Synthesis in Water: Principles, Strategies and Applications. Lindstrom, U.M. (Ed.). Blackwell, Oxford, England, 2007, p. 350.

Sharpless, K.B., Manetsch, R. In situ click chemistry: a powerful means for lead discovery. Expert Opin. Drug Discov. 1:525, 2006.

Wu, P., Fokin, V.V. Catalytic azide-alkyne cycloadditions: reactivity and applications. Aldrichim. Acta 40:7, 2007.

Yoo, E.J., Ahlquist, M., Kim, S.H., Bae, I., Fokin, V.V., Sharpless, K.B., Chang, S. Copper-catalyzed synthesis of N-sulfonyl-1,2,3-triazoles: controlling selectivity. Angew. Chem. Int. Ed. 46:1730, 2007.