|Bioconjugation; Radiochemistry; Nanoscience;
|The main approach of medicinal chemistry during the process of
drug discovery or lead optimization is to synthesize compounds or
libraries of compounds. Due to this reason, the loom of this concept
in this field is attracted for rapid generation of new molecules. Click
chemistry, a term coined by K. Barry Sharpless is a way to generate
substances rapidly and reliably by joining small modular units together.
Click chemistry is an approach to develop a set of powerful, selective,
and modular building blocks, such as azide and alkyne that work for
both small and large scales . Its applications are increasingly in all
aspects of drug discovery in medicinal chemistry, such as for generating
lead compounds through combinatorial methods, in-situ applications
for target templating, and in bioconjugation reactions. There are many
reactions, which comprises the click universe due to its high reliability,
specificity and selectivity. The Husigen 1,3-dipolar cycloaddition
(Scheme 1) of alkynes to azides for 1,4-disubstituted-1,2,3- triazole
copper catalyzed reaction is mild, efficient and powerful linking
reaction requiring no protecting groups and no purification lead to
high degree of dependability, complete specificity and biocompatibility
of the reactants  (Scheme 1).
|In history and even now a day lead discovery and optimization
had aided by combinatorial methods and high throughput screening
to generate library of test compounds for screening. However, due to
unreliability and new discoveries revealed click chemistry as a modular
for the synthesis of drug-like molecules that can accelerate the drug
discovery process by utilizing a few practical and reliable reactions. It
is a new type of chemistry that able to synthesize complex molecule
in a efficient manner. It makes use of few chemical reactions for the
synthesis and designing of new building blocks. There are two important
features that makes click reaction as a novel approach for clinical and
preclinical studies. First, in click reactions neither the reactant nor
the product functional groups interact with functional biomolecules.
Second, the reaction continues with ease under mild conditions, easy
to perform, uses only readily available reagents, and is insensitive to
oxygen and water. In fact, in several instances water is the ideal reaction
solvent, providing the best yields and highest rates. Reaction work-up
and purification uses benign solvents and avoids chromatography .
|It is a powerful and attractive technology alternative to
conventional technology proven it to be highly fulfilled in different fields such as selectivity, stereo specificity, yield and biocompatibility
and a novel approach for a wide range of application in upcoming
discoveries. Many of the reactions was remain untouched due to
different reasons one of the example of such kind of reaction is azidealkyne
cycloaddition reaction discovered by Huisgen in 1963, which is
ignored for past few years due to high temperatures and pressures and
was revived by in 2001 by employing Cu(I) as a catalyst. They give the
criteria for successful click chemistry by stating that “the reaction must
be modular, wide in scope, give consistently high yields, generate only
inoffensive byproducts that can be removed by non-chromatographic
methods, and be stereo-specific (but not necessarily enantioselective).
To fulfill this criteria, there should be a variety of starting material,
easy to perform, insensitive to oxygen and water and use only readily
available reagents and should proceed at room temperature. Reaction
conditions, work-up and product isolation should be smooth and
simple. Thus, this methodology allows rapid construction of molecules
with efficiency, versatility and selectivity .
|Click reactions click on carbon-heteroatom bond formation but
the reaction is irreversible. For lead discovery and optimization, click
reaction provides a mean of rapid exploration of library of chemicals
and for later, it helps in providing rapid SAR profiling through
generation of number of molecules. The main characteristic of this
reaction is to provide novel structure that might not resemble the
existing pharmacophores. The research based on click chemistry is
very fast because it avoids the screening of library of compounds [4-6].
Exharatingly it have been concluded that greater drug discovery can
be achieved with limited reactions as it is not the number of reactions
that makes an importance but it is the ability of reaction to compete
with variations in nature of their components. Click chemistry was
now being use increasingly in various fields of research ranging from
lead discovery and optimization to tagging of biological systems, such
as protein, nucleotides, and whole organism. The applications of this
approach have been review in this article.
|Nucleophilic ring opening and Husigen 1, 3-diploar cycloaddition
are the best known and extensively studied click reaction till date.
Azides and alkynes are the building blocks for the preparation of
potential drug candidates. They are the least reactive functional group
in organic chemistry and have been termed bioorthogonal because of
their stability and inertness towards the functional groups found in
biological molecules. This reaction has been termed the “cream of the
crop” of click reactions .
|Drug discovery based on Natures secondary metabolites is very slow and complex synthesis and thereby, click chemistry provides faster lead
discovery and optimization. This concept of Click Chemistry has been
twisted into convenient, resourceful and reliable two-step coupling
procedures of two molecules A and B [1,8], that are widely used in
biosciences [9-11], drug discovery  and material science [1,13].
Today, there are many researches going on based on click reactions
due to its wide scope, modularity and simplicity.
Universe of Click Chemistry
|Different click reactions applied in different field of science and
yielded extremely reliable process for the synthesis of building block
and compound libraries. The various reactions is classified as:
|1. Cycloaddition reaction of such as 1,3-dipolar cycloaddition
reaction  and hetero Diels Alder reaction [15,16].
|2. Nucleophilic substitution reaction specifically rings opening
reaction of strained heterocyclic electrophiles such as epoxides,
aziridines, azridiniumions, and episulfoniumions .
|3. Carbonyl chemistry of the non-aldol type such as formation
of ureas, thioureas, aromatic heterocyles, oxime ethers, hydrazones and
|4. Addition to carbon-carbon multiple bonds particularly
oxidation reaction such as epoxidation , hydroxylation,
dihydroxylation , aziridination  and sulfenyl halide 
addition also Michael addition of Nu-H reactants.
|5. Thiol-ene clicks reactions.
|6. Azide-Phosphine (Staudinger ligation) reaction.
Principles of Click Chemistry
1,3-Dipolar cycloaddition reaction of azides and alkynes
|In floral of click reactions the most accepted reaction till date,
which was found to be 1,3-dipolar cycloaddition reaction also known
as Husigen cycloaddition between an azide and a terminal alkynes
affording 1,2,3-triazole (Scheme 2). Dimorth in early 1900 but the
generality, first reported the formation of triazoles with cycloaddition
reaction of azide and acetylene and mechanism of these cycloaddition
is not fully realized till 1960’s. The reactions were discovering at the
beginning of 20th century but the prospective of this reaction were
unveiled in 1960 by Husigen. Due to some drawbacks of this reaction
such as formation of mixture of 1, 4 and 1, 5-regiomers, which were
uneasy to separate using chromatographic methods and transformation,
requires heating and long reaction time for completion  therefore,
in 2002, two groups Sharpless and Meldal independently reported that
copper salts were able to accelerate this reaction upto 10 million times
1(a) Copper catalyzed Azide Alkyne cycloaddition (CuAAC)
|Copper catalyzed alkyne-azide reaction (Scheme 3) features an
enormous rate acceleration of 107 to 108 compared to uncatalyzed
reaction. More importantly, this reaction succeeds over a broad
temperature range, directs the formation of only one of the two
regiomers, mainly the 1,4-regiomers insensitive to aqueous conditions
and pH range over 4-12 and tolerates a broad range of functional
groups. No chromatography is required for the separation or isolation
of products. The standard catalytic system uses copper (II) salts or
copper (I) salt in the presence of reducing agent e.g., sodium ascorbate
or metallic copper prevents the formation of oxidative homocoupling
products . A mixture of tert-butanol and water was use as a solvent because it rectified the use of base to form copper acetylide species. The
reactions where aqueous conditions cannot be used, organic solvents
e.g., THF, toluene, DCM in the presence of stochiometric amount of
copper (I) salts e.g., CuI , Cu(CH3CN)4PF6 , CuBr(PPh3)4 or
CuIP(OEt)3 [22,23] and an excess of a base, usually a tertiary amine
(e.g., TEA, DIPEA) can be used. Till date research is going on for the
improvement of better copper-based catalyzed reaction, e.g., copperin-
charcoal have been developed as simple, inexpensive and efficient
heterogeneous catalyst for triazole formation. This development is
proving advantageous as it can easily remove by filtration and can reuse
in another cycloaddition without losing its potency. This discovery
led to renaissance of Husigen cycloaddition in synthetic chemistry
including polymer chemistry and biochemistry .
|Although the copper found to be efficient, catalyst but still
further investigations are proceeding in order to get the potential
products. Therefore, a number of additives have been investigated
which increases the efficiency of reaction and reduction of copper
concentration such as (tris-(benzyl-triazolylmethyl) amine) (TBTA)
, triethylamine hydrochloride  and the water-soluble sulfonated
bathopenantroline. They found to be very efficient as was found in
bioconjugation of expensive and precious substrates  (Scheme 3).
1(b) Ruthenium catalyzed alkyne- azide cycloaddition
|In the search of more valuable catalyst the ruthenium catalyzed
cycloaddition reaction found to be more fruitful as compared to copper
catalyzed reaction as it can yield 1, 5-disubstituted 1, 2, 3-traizoles
regioselectively. The classical Husigen 1, 3-dipolar cycloaddition often
gives mixtures of regiomers, whereas the copper catalyzed reaction
allows the synthesis of 1, 4-disubstituted regiomers selectively.
However, later developed ruthenium-catalyzed reaction gives the
opposite regioselectivity with the formation of 1, 5-disubstituted
triazoles (Scheme 4) . The reaction of 1, 3-dipolar cycloaddition
between alkynes and azide using the Cp*RuCl (PPh3)2 catalyst was
reported in 2005 . Another catalyst was pentamethylcyclopentane
diol ruthenium chloride complexes, which also has the capability to
catalyze the cycloaddition of azides and alkynes regioselectively leading
to 1, 5-disubstituted 1, 2, 3-traizoles. It allows the reaction between
internal alkynes and appears to proceed via oxidative coupling of
azide and alkyne to give a six-membered ruthenacycle followed by
reductive elimination to form triazole product. However, some other methods have also been reported for the synthesis of 1, 5-disubstituted
triazoles products by the reaction of bromomagnesium acetylenes and
organic azides. Substituted trimethylsilyl acetylenes have also been
reported to direct the cycloaddition affording the 1, 5-regioisomers. It
found that trimethylsilyl group probably controls the selectivity by a
combination of 2 factors: the ability to stabilize a positive charge on
acetylene at β-carbon position in transition state and stearic hindrance
that prevents the reverse orientation. Density functional calculations
support this mechanism and shows that reductive elimination is a ratedetermining
step . Therefore, these catalyzed reactions comply
fully with definition of click chemistry and put focus on azide-alkyne
cycloaddition as a prototype click reaction (Scheme 4).
1(c) Toulenesulfonyl cyanide as catalyst
|Toulenesulfonyl cyanide has been use in the presence of different
unhindered azide yield 1, 5-disubstituite tetrazole in good yield (Scheme
5). The substituted tetrazole can be functionalizing by the replacement
of sulfonyl group with various nucleophiles. The reaction was expand
by the use of acyl cyanides, cyanoformates and cyanoformides with
azide substrates generating acyltetrazoles e.g., reaction of benzoyl
cyanide with various azides at 120°C gives cycloaddition product
regioselectively in high yield  (Scheme 5).
Non transition metal click reaction
|Copper makes the reaction efficient but some instances reported
in-vitro copper-induced degradation of viruses or oligonucleotides
and in-vivo copper ions found to be highly toxic for living life form.
In 2008, Dondoni reported metal free cyclo addition reaction and their
correlation with click chemistry including Diels-Alder reactions, and
thio-alkene radical addition reaction  Scheme 6.
Non-azide 1, 3-cycloaddition reaction as click reaction
|Many of the alkynes engaged in non-azide 1, 3-cycloaddition reaction with electron deficient cases are usually being the most reactive.
Interestingly, these reactions yield five-membered heterocycles. The
sequence started with addition of hydrazine to aziridinium intermediate
generated from (a) The resulting cyclic hydrazide (b) then undergoes
condensation with aromatic aldehydes to give azomethine ylides c
which react with a variety of components to give cycloadducts. These
reactions provide the opportunities to create unique combinatorial
libraries with different array of functionalities .
Nucleophilic ring opening
|The ring opening of three-membered heterocycles is generally
facilitated by the release of strain  and so these compounds have
been termed as “spring loaded electrophiles” . Nucleophilic opening
of epoxides, aziridines (Scheme 7), cyclic sulfates, and episulfonium
and aziridinium ions can lead to wide variety of library of compounds.
However, epoxides and aziridines are the most common substrates for
ring opening and their specificity can be useful for formation of large
variety of compounds. Regioselective opening of aziridines is primarily
substrate controlled by the opening of unsymmetrical aziridines using
TMS azide . Benzyl substitution is more reactive than primary and
secondary-carbon substitution, which obeys normal SN2 behavior.
However, library of compound have been synthesize through variation
of the nitrogen substituent (Scheme 7).
|Regioselective opening of oxirane ring (Scheme 8) can be
continuing in the same way as that of nucleophilic opening of diepoxide
with benzylamine. Therefore, nucleophilic ring opening reactions
of oxiranes and aziridines can be yigh yielding, stereospecific, and
regioselective fulfilling the criteria of click reactions  (Scheme 8).
Triazole ring as bioesters
|Bioesters are the substituent or groups that have similar physical
or chemical properties and hence similar biological action. As CuAAC
has made possible the insertion of triazole moiety relatively easy, so
biosteric potential of this moiety is important to establish to broaden
the use of the reaction in improvement of target compounds. Biosteric
replacement may help to decrease toxicity or to change activity
spectra. The 1, 4-disubstituted triazole moiety mimics Z-amide bond
as the carbonyl oxygen of amide bond is similar to the 3-nitrogen of
the moiety, C (5)-H bond can act as a hydrogen donor and carbonyl
carbon is electronically similar to amide N-H bond (Figure 1). Since,
some properties such as dipole moment, hydrogen bonding donor and
acceptor are more marked than amide bond with enhanced peptide
mimicry. The difference between two exist in the distance between
the substituent linked by two atoms in amide and three atoms in
a triazole ring, with an overall increase of about 1.1 Å. However, 1,
5-substitution pattern mimics E-bond. Therefore, it has suggested that
triazole ring has use as bioester of amide bond . Including this, it
has also been suggested that the triazole moiety may act as bioesters
of acyl-phosphate and trans olefionic group. The substitution of trans
olefinic group with a triazole moiety in resveratrol proved to be more
rewarding (Scheme 8). Resveratrol has been shown to have a number of
potentially interesting biological activities but its usefulness is limited
because its shows its properties at high micromolar concentration,
making it difficult to predict which are the targets responsible for single
effects. So, an approach of click chemistry has utilized to synthesize
triazole analogues via click chemistry. This applicability was not limited
to resveratrol but also possible to maintain in esterogenic activity in
diethylstilbesterol analogues [39,40].
Applications of Click chemistry
Click in oligonucleotide lipids and sugars
|Click chemistry had a large impact on lipid and carbohydrate
chemistry and an efficient strategy for the synthesis of oligonucleotides
for applications such as labeled carbohydrate ONs, fluorescent ONs,
and multi modified ONs . Many instances have been existed
where click triazole formation has generated to improve the biological
profile of this biomolecules. The cyclic decapeptide an antibiotic have
coupled to different sugar moieties to improve its safety profile. Two
of the synthesized triazole grafted glycopeptides showed six fold better
therapeutic index compared to tyrocidine.
|The 1, 3-dipolar cycloaddition reaction has become an efficient
method and found its application in preparation of high order
molecular conjugates, such as glycoprotein, neoglycoconjugates ,
protein oligonucleotides  and DNA-peptide conjugates .
Wang et al. has prepared flourscent labeled oligonucleotide found to
be an efficient primer with high specificity in a Sanger dioxy DNAsequencing
reaction to produce DNA-sequencing fragments .
Similarly, Baccaro et al. carried out synthesis of two-azide modified
analogs of thymidine and incorporated them into DNA by using primer
extension reaction and polymerase chain reaction (PCR) conjugated to Biotin by using Staudinger ligation and found it to be less sterically
hindered . Staudinger ligation reaction was use to remodel cell
surfaces in mice by Prescher et al. Mice were injected with an azide
carbohydrate derivative (Man-NAz) and then with a phosphinederived
peptide conjugate and the reaction progress was monitored by
flow cytometry .
|CuAAC chemistry has implied to synthesize amino acidsaccharide
by using carbohydrate and amino acid derived azides
and alkynes as building blocks. The idea behind this strategy was the
finding that oligosaccharides covalently attached top proteins facilitate
the development of vaccines against pathogenic bacteria. Danishefsky
et al. combined three-tumor associated carbohydrate antigens on a
single molecule polypeptide scaffold by using strategy of multiple click
triazole formation. This linkage of sugars to poorly soluble drugs via
click catalyzed reaction might be exploited to improve pharmacokinetic
profile e.g., ferrocene derivatives that have antimalarial activity
solubility profile was improved by coupling it to sugars using click
Click in bioconjugation
|Another classical extension of this click chemistry is in the
development of bifunctional molecules. Bioconjugation covers a wide
range of science between molecular biology and chemistry. It involves
the attachment of synthetic labels to bimolecular building blocks such
as fusing two or more proteins together or linking a carbohydrate
with a peptide . The challenge for this reaction is the ability to
perform both in vitro as well as in vivo experiments. Tornoe was the
first who demonstrated the use of click chemistry in bioconjugation
for the preparation of peptidotriazole solid-state synthesis and the
idea behind this was to set up an efficient synthetic method to prepare
triazole derivatives for potential biologic targets . Click chemistry
continues to attract attention for the labeling of proteins and living
organism. One of the examples was demonstrated by Wang when a
fluorescent dye was attached to cow pea mosaic virus. Another instance
was established by Link modified Eschericia coli with an azide-bearing
outer membrane protein C (OmpC) followed by biotinylated by
reacting with a biotin alkyne derivative under copper-catalyzed click
chemistry conditions . Deiters et al. developed a method to genetic
encode proteins of Saccharomyces cerevisiae with azide- or acetylenebased
synthetic amino acids .
|The application of CuAAC to generate fluorogenic compounds
(Scheme 9) have broadly evaluated, as it is a powerful method to track
biomolecules in the cell. Examples include fluorescent DNA probes an
important tool has generated by this method .
|The approach of click chemistry represents a new bioconjugation
strategy that can be used to conveniently various ligands to the surface
of performed liposomes. The CuAAC under mild condition can be
considered to be efficient and chemoselective reaction in this strategy.
Frisch et al. demonstrated the application of click reaction to the
conjugation, in a single step of unprotected alpha-1-thiomannosyl ligands, functional with azide group to liposomes containing a terminal
alkyne-functionalized lipid anchor. The use of water-soluble Cu (I)
chelator, act as a catalyst such as phenathrlinedisulfonate gives an
excellent yield. Limitation of CuAAC reaction in this conjugation is
restricted to liposomes made of saturated phospholipids .
Polymer therapeutics and polymer chemistry
|Polymer therapeutics is an approach to treat various diseases. In this
aspect polymer is a broad term include polymeric drug, polymer-drug
conjugates, polymer protein, polymer-protein conjugates, polymeric
micelles, and multi-component polyplexes and they all utilizes the
property of water-soluble polymers for improved drug, protein, or
gene delivery . Biocompatible polymers such as PEG(Polyethylene
glycol), if conjugated with proteins and peptides has proved to be very
efficacy in increasing bioavailability and so it can be said that these
polymers play an important role in drug delivery and formulations
[55,56]. Yet, polymer chemistry is required to meet the demand for
versatile pharmaceutical properties. Schlaad et al. reported an excellent
work on the polymer chemistry.
|Many of the important reactions such as 1,4-benzethiol and
1,4-diethynylbenzene in the synthesis of block copolymer, linear
multifunctional copolymer, dendrimer, polymer network, and polymer
analogs have been described in previously published articles. So, it is
revealed that this new “tool in the box” may prove as one of the new
stratagem of polymer therapeutic development.
Click reactions on linear polymers
|Three main types of controlled radical polymerization are:
|1. Atom transfer radical polymerization (ATRP).
|2. Stable free radical polymerization (SFRP).
|3. Nitroxide mediated polymerization (NMP).
|4. Reversible addition fragmentation chain transfer (RAFT).
|The above mentioned methods have been extensively used in
the synthesis of Block copolymers which have many pharmaceutical
applications including in the formulations of various nanocapsules,
nanospheres, polymersomes . As click chemistry gives efficient
reaction product and had a tolerance to variety of chemicals therefore,
it has been used to form block copolymers from homopolymers
with azide and alkyne end fuctionalities. Van amp et al. amphiphilic
copolymer structures by combination of ATRP and HDC reaction.
Wherein, Opsteen et al. carried out synthesis of polystyrene (PS),
poly (tert-butyl acrylate) (PtBA), poly (methyl acrylate) (PMA) block
copolymers using click chemistry .
|This combination of polymer chemistry and click chemistry
proved to be very advantageous as it gives lead to synthesis of different
polymeric materials which was previously inaccessible to syntheise
with traditional methods. Other complex polymer structures are also
accessible to synthesize with this technique which proved to be very
efficient as polymer therapeutics. Example continues as amphiphilic
block copolymers provided application for the synthesis of polymeric
micelles as delivery vehicle for therapeutics, imaging or diagnostic
Activity Based Protein Profiling (ABPP)
|Approach of click chemistry has applied in other field of drug
discovery such as target identification by activity based protein profiling
and to understand the functions of proteins in their natural setting diphenylphosphaneincluding
their regulation. This strategy would allow the visualization
of proteins expressed at low level and the information would indicate
the activity more than of abundance. This technique utilizes active-site
directed chemical probes with broad target selectivity to label active
proteins with various enzyme classes and allows the discovery of new
targets . These sites directed probes are composed of two different
elements; the molecule that brings selectively to the binding and the
label that have visualized and contains a functional group that covalently
react with specific classes of enzymes such as serine hydrolases. These
two components has been pre-assembled and added to cell extracts,
and, indeed, that have done to profile a number of enzymes .
Previously, ABPP conducted in vitro because the bulky chemical tags
inhibited cellular uptake and caused the enzyme to profile outside their
natural biological environment. However, the discovery of coppercatalysed
click reaction provided the enzyme to be profiled in-vivo.
The use of a phenyl sulfonate ester reactive group allowed the in-vivo
profiling of glutathione S-transferase, aldehydes dehydrogenases, and
enoyl CoA hydratases . The small azide-groups are easily take by
the cells and covalently labels active proteins. Addition of the alkyne
cycloaddition reaction under copper catalysis after lysing the cells
tags the enzyme for detection and isolation. Therefore, the use of click
chemistry in ABPP resulted in the isolation of several enzymes in breast
cancer cell lines .
Click in peptide chemistry and cyclisation
|There are many articles defining the application of click chemistry
in peptide synthesis. The clickable functional groups are easy to
incorporate in peptides and are stable to oxidative conditions. Due
to relative planarity, strong dipole moment, and hydrogen bonding
ability of triazole it bear its physicochemical resemblance to the amide
bond. Therefore, the triazole linkage has found its applicability in
the field of peptide science . Meldal et al.  has demonstrated
the compatibility of protected amino acid side chain with CuAAC
reaction. A few numbers of amino acids has assembled on a solid phase
resin and then attached different combination of protected amino
acids (AA3) modified with alkyne group. The resulting peptides were
detached off the resin to give triazole containing amino acid in high
yields. Various protecting group found to be stable and compatible to
click reactions . Click chemistry found its application in cyclisation
of peptides to increase its potency and in-vivo half-life by locking its
confirmation. Click reaction was found to be exploited in a number of
different peptide cyclization reactions such as the preparation of novel
heterodetic cyclopeptides by an intramolecular side-to-side chain
click reaction, forming a 1,4-disubstituted [1,2,3] triazolyl-containing
bridge; Cu(I)- and Ru (II)-mediated “click” cyclization of tripeptides
[69-71] (Scheme 10).
Target Guided Synthesis and in-situ click chemistry
|Kinetic target guided synthesis (TGS) and in-situ click chemistry
are among unconventional drug strategies having the potential to
develop the libraries of compounds. TGS is the use of small molecules
building blocks that was assembles by specifically targeted enzymes
to synthesize their own inhibitors. Only building blocks that interact
with active site of protein will be in close proximity to interact with one another and from potent inhibitors . Mocks discovery of a dramatic
rate acceleration of the azide-alkyne cycloaddition by sequestering the
two components inside a host structure, prompted Sharpless et al. to
investigate a paradigm for drug discovery, which was dependent on
irreversible target guided synthesis of high affinity inhibitors from
reagents that are inert under physiological conditions. Installation of
the azide –alkyne within organic building blocks allowed the use of
Husigen [3+2] cycloaddition to discover a Femtomolar inhibitor of
acetylcholinesterase (AChE) . Their inhibitors have been employing
for a century in variety of therapeutic regimen to investigate the role of
acetylcholine in neurotransmission. Acetylcholine esterase hydrolyses
acetylcholine, a neurotransmitter, and plays an important role in the
central nervous system. The active site is located at the base of a narrow
20 Å gorge lined with aromatic amino acids and second binding site
at the rim of the gorge near to the enzyme surface . The design
of the building block based on the tacrine and phenanthridinium, two
known specific site ligands. This cycloaddition reaction is opted in
this study due to several reasons i.e., reaction is bioorthogonal, and is
extremely slow at room temperature. Out of all possible azide-alkyne
cycloaddition products, only the 1,5-traizole was formed from the
combination of A(a) and D(e) (TZ2PA6 syn 6 triazole) in the presence
of AChE. This inhibitor with a 100-fold greater affinity and a subpicomolar
constant dissociation constant has potency greater than all
known non-covalent organic AChE inhibitors [75,76] (Scheme 11).
Click in radiopharmaceuticals
|Radiochemistry is no exception, as the canonical Cu(I)-
catalyzed azide-alkyne cycloaddition, strain-promoted azide-alkyne
cycloaddition, inverse electron demand Diels-Alder reaction, and other
types of bioorthogonal click ligations have had a significant impact on
the synthesis and development of radiopharmaceuticals. Click chemistry
had a vast application in the preparation of imaging agents for SPECT
and PET, including small molecules, peptides, and proteins labeled
with radionuclides such as 18F, 64Cu, 111In, and 99mTc. CuAAC has
been used in click chemistry for labelling both peptidioc and small
molecule reaction tracers bearing the positron emiiting radio halogen
18F (half-life, 109.8 min) using 18F-labeled alkyne- or azide-bearing
building blocks . Various researchers have shown that this reaction
of CuAAC is a great approach for the construction of radiotracers.
Few examples include Gaeta et al. produce a high-affinity, 18F-labeled
subtype A g-aminobutyric acid receptor ligand in 7% uncorrected
yield over 2 steps and a specific activity of 0.9 GBq/mmol using the
ligation between 2-18F fluoroethylazide and a diphenylphosphane bearing precursor . In similar work, Carroll et al. reacts thioesterbased
phosphane precursors and 2-18F-fluoroethylazide to create
the 18F-labeled products in greater than 95% radiochemical yield for
the single ligation step . Pretze et al. in contrast, have moved the
18F to the phosphane, enabling the radiolabeling of an array of azidefunctionalized
targets. In their work, an 18F-fluoroacyl-functionalized
phosphane was synthesized and reacted with azide-containing model
compounds to produce 18F-labeled products in 30%-35% decaycorrected
radiochemical yields in 1 pot over 2 steps. Finally, the
Staudinger ligation have recently been applied to pre-targeted PET
imaging by Vugts et al. who ultimately concluded, however, that this
reaction is not suitable for in vivo pretargeting, likely because of the
relatively slow kinetics of the ligation and the in vivo oxidation of the
phosphane-based radioligand .
|From the above review it was concluded that click chemistry was
considered as a stringent and powerful tool and an important aid to
medicinal chemist that define a set of dependable transformations that
can be employed to design and synthesise novel compounds. Although
variations of the reaction exist, and probably better reactions will
described and applied in the future, the general principle of efficiency,
versatility and selectivity will maintained if not improved. For example,
there is the need to synthesize NH-1, 2, 3-triazole derivatives using
the CuAAC approach, to expand the versatility of this reaction, as
click chemistry with sodium azide was not optimize. The bioisosteric
potential of the triazole ring and Huisgen 1, 3-dipolar cycloaddition
cycloaddition is an ideal condition. Finally, it is crucial to highlight
that click chemistry has been shown to be able to help bridge the gap
between chemistry and biology, becoming a true interdisciplinary
reaction which can be utilized in future prospects for the construction
of novel pharmacophores. Indeed, click chemistry can directly link
chemistry to biology (such as in the ABPP assay) and can use biology
for creating tailored synthesis.
Future Prospects and Application of Click Chemistry
|Click chemistry is a stringent criterion that used to construct
novel pharmacophores to facilitate and in many other applications
in the future. Strategies using click chemistry have shown significant
advantages over traditional synthetic techniques for the modular, rapid,
clean and efficient synthesis of various biomolecules. Click chemistry
was fulfill with many conditions such as insensitivity to oxygen,
inhibition and high yields with basic purification procedures can lead
to photochemical and thermal initiation. It has many other wider
applications such as synthesis of 1,4-substituted triazoles, modification
of peptide function with triazoles, modification of natural products
and pharmaceuticals, drug discovery, macrocyclizations using Cu(I)
catalyzed triazole couplings, modification of DNA and nucleotides by
triazole ligation, supramolecular chemistry: calixarenes, rotaxanes, and
catenanes, dendrimer design, carbohydrate clusters and carbohydrate
conjugation by Cu(1) catalyzed triazole ligation reactions, polymers,
material science, nanotechnology 21, and Bioconjugation, for example,
azidocoumarin. The versatility of the reaction can be expand by synthesis
of NH-1,2,3-triazole derivatives. This reaction of click chemistry made
the use of azidomethylpivalate to the desired derivative of NH-1, 2,
3-triazole that is amenable for further derivatization. β-tosylethylazide
can react with alkynes to give the desired product. It is conclude that
click chemistry has high potential if exploited appropriately. It links
various types of chemistry with biology and can tailor various useful synthesis in future.
Figures at a glance
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