The Skaggs Institute
for Chemical Biology
Click Chemistry: the Power of Simplicity
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,
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.
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
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.
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
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.
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.
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
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.
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.