Explanation
Proteins are made from repeating amino acid units, and sugars are made from repeating monosaccharide units. The connections are carbon - hetero atom bonds C-X-C rather than carbon - carbon bonds. In addition, enzymes ensure that chemical processes can overcome large enthalpy hurdles by a series of reactions each requiring only a small energy step. Mimicking nature in organic synthesis is essential in the discovery of new pharmaceuticals given the large number of possible structures.
In 1996, Guida calculated the size of the pool of drug candidates at 1063, based on the presumption that a candidate consists of fewer than 30 non-hydrogen atoms, weighs less than 500 daltons, is made up of atoms of hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur, chlorine and bromine, is stable at room temperature, and does not react with oxygen and water. Click chemistry in combination with combinatorial chemistry, high-throughput screening and building chemical libraries speeds up new drug discoveries by making each reaction in a multistep synthesis fast, efficient and predictable.
Many of the Click chemistry criteria are subjective, and even if measurable and objective criteria could be agreed upon, it is unlikely that any reaction will be perfect for every situation and application. However, several reactions have been identified that fit the concept better than others:
- cycloadditions, such as the Huisgen 1,3-dipolar cycloaddition, in particular the Cu(I)-catalyzed stepwise variant, are often referred to simply as Click reactions
- thiol-ene click reactions
- Diels-Alder reaction and inverse electron demand Diels-Alder reaction
- cycloadditions between isonitriles (isocyanides) and tetrazines
- nucleophilic substitution especially to small strained rings like epoxy and aziridine compounds
- carbonyl-chemistry-like formation of ureas but not reactions of the aldol type due to low thermodynamic driving force.
- addition reactions to carbon-carbon double bonds like dihydroxylation or the alkynes in the thiol-yne reaction.
Read more about this topic: Click Chemistry
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