The Skaggs Institute
for Chemical Biology
Click Chemistry: Diversity and Function From a Few Reactions
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
During the past
decade, the laborious process of discovering and optimizing compounds that may be
useful as drugs has been aided by combinatorial chemistry, which allows generation
of large collections of test compounds for screening. However, in order to produce
useful chemically diverse libraries of compounds, individual reactions involved
in combinatorial syntheses must be supremely reliable. Click chemistry is an approach
to synthesis that we have developed during the past 5 years. Our goal is to facilitate
the discovery process through 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 linkage reaction with complementary sites in other blocks.
click chemistry, searches for function are restricted to molecules that are easy
to make. For the discovery of possibly useful compounds, 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 chemistrybased searches are fast, because they avoid the
regions of the chemical universe that are difficult to access. They are wide-ranging,
because of the use of strongly driven, highly selective reactions of broad scope,
allowing the use of a much greater diversity of building blocks. 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 and, through the choice of appropriate building blocks, can
provide derivatives or mimics of traditional pharmacophores, drugs,
and natural products. However, the real power of click chemistry is the ability
to generate novel structures that might not necessarily resemble known biologically
Click chemistry is both enabled and constrained
by reliance on a few nearly perfect reactions, and this characteristic raises concerns
about limitations on the access of click chemistry to chemical diversity. A computational
study suggests that the pool of druglike compounds is 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 most 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 combinatorial chemistry to bioconjugation strategies in proteomics; in polymer
chemistry; and in materials science.
Inhibitors of HIV Type 1 Protease Via in Situ Click Chemistry
Although click chemistry is a powerful
way to rapidly assemble 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 accelerate
the discovery cycle by engaging 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, at least under physiologic
conditions. Furthermore, 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 and carbonic anhydrase. However, in these studies, researchers
used building blocks with high affinity for the target. Not having such luxury in
our attempts to produce inhibitors of HIV type 1 protease, we were pleased to find
that the enzyme selectively formed an inhibitor from components that had only weak
binding to the target (Fig. 1) when mixtures of azide- and alkyne-containing fragments
were incubated with the protease for 2448 hours. Encouraged by these results,
we are continuing our collaboration with J.H. Elder and A.J. Olson, Scripps Research,
to rapidly identify potent inhibitors of HIV protease mutants.
|Fig. 1. Formation of an HIV type 1 protease inhibitor.
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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.
Narayan, S., Fokin, V.V., Sharpless, K.B. Organic synthesis in aqueous suspension: chemistry on water. In: Organic Reactions
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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,
Whiting, M., Fokin, V.V. Copper-catalyzed reaction cascade: direct conversion of alkynes to N-sulfonyl azetidin-2-imines.
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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.
Whiting, M., Tripp, J., Lin, Y.-C., Lindstrom, W., Olson, A.J., Elder, J.H., Sharpless, K.B., Fokin, V.V. 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. J. Med. Chem. 49:7697, 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.