It is a recent advance in drug discovery which involves an approach to chemical synthesis that enables the creation of a large number of compounds by putting chemical building blocks together in a given reaction sequence in every possible combination and screening them to get a “hit” – a compound which has the potential of becoming a future drug molecule. Thus in combinatorial technology there is a generation of diverse collection of molecules.

The development of combinatorial chemistry has its roots in peptide chemistry Bruce R .Meerifield won the Nobel prize in chemistry in 1984for his work on solid-phase synthesis. From 1972-1976, papers dealing with the development of solid-phase synthesis were published by Leznoff and Rapport. There seemed to be a lull, till in 1982-1985. When geysen and his associates developed an encoding strategy in which molecular tags are attached to beads or linker groups used in solid-phase synthesis. After the products have been assayed, the tags are cleaved and determined by mass spectroscopy (MS) to identity potential lead compounds. IN 1982 Arpad Furka is considered to be one of the fathers of combinatorial chemistry introduced the “split and pool” synthesis methods.

The 1984 Geysen article on pin technology for parallel synthesis led to Richard Houghten’s famous “tea-bag” technique for parallel synthesis of diazepines, published 1n1992 that the sleeping giant was awakened to become a powerful tool for drug discovery in the pharmaceutical industry.

This powerful new technology has begun to help pharmaceutical companies to find new drug candidates quickly, save significant money in preclinical development costs and ultimately change their fundamental approach to drug discovery.²

Principle of Combinatorial Chemistry:

Combinatorial chemistry is a technique by which large numbers of structurally distinct molecules may be synthesized at the same and screened for their pharmacological activity. The key of combinatorial chemistry is that a large range of analogues is synthesized using the same reaction conditions, the same reaction vessels in a systemic manner. In this way, the chemist can synthesize many hundreds or thousands of compounds in one time instead of preparing only a few by simple methodology.

In the past, chemists have traditionally made one compound at a time. For example compound A would have been reacted with compound B to give product AB, which would have been isolated after reaction work up and purification through crystallization, distillation, or chromatography or some other processes.

Fig no:1

In contrast to this approach, combinatorial chemistry offers the potential to make every combination of compound A₁ to A_n with compound B₁ to B_n.

Fig no:2

The basic principle of the combinatorial chemistry is to produce a large number of compounds at same time. The characteristic of combinatorial synthesis is that different compounds are synthesized simultaneously under identical reaction conditions in a systematic manner, so that ideally the products of all possible combinations of the starting materials will be obtained at once. The collection of these finally synthesized compounds is referred to as a “Combinatorial library.”

Combinatorial chemistry is one method of reducing the cost of the drug discovery in which the goal is to find new leads or proto-type compounds or to optimize and refine the structure activity relationships (SAR). After synthesis of new compounds by the use of combinatorial chemistry, the synthesized compounds to be tested biologically. Biological testing can also be automated, in a process called “High-throughput screening”, which can test tens to hundreds of synthesizing compounds, by which there is decrease in cost allotted for synthesize, isolate and testing of each compound.

The range of combinatorial techniques is highly diverse, and these products could be made individually in a parallel or in mixtures. Combinatorial chemistry is of two types: first one is Solid phase combinatorial chemistry and

Second is Solution phase combinatorial chemistry.^{4, 5}

SOLID PHASE COMBINATORIAL CHEMISTRY

Solid phase combinatorial chemistry, involves the synthesis of compounds by linking the substrates covalently to the solid supports (polymeric resins) of different forms such as beads, pins or chips. The compounds to be synthesized are not attached directly to the polymer molecules. They are usually attached by using a linker moiety as shown below.

Fig no: 3

In solid-phase organic synthesis, it’s easy to purify products by filtration, it’s possible to do mix-and-split synthesis (a technique used to make very large libraries), excess reagents can be used to drive reactions to completion, and synthesis can be automated easily.⁶ Solid phase chemistry has advantage over the solution-phase. First, in solid-phase synthesis, large excesses of reagents can be used to drive reactions to completion; these excess reagents can then be removed at the end of the reactions by filtration and washing. Second, because of easy separation of reagents and products, solid phase chemistry can be automated more easily than solution chemistry. Separation of compounds bound to the solid support from those in solution is accomplished by simple filtration.⁵

Fig no: 4

Examples for solid phase combinatorial synthesis:

1. Synthesis of 1, 4 benzodiazepines ^4,19

The choice of benzodiazepines was inspired because of the medicinal importance of these materials and their resemblance to peptides. Here the library was constructed by a combination of three reactants. In the synthesis 1, 4 benzodiazepines Fmoc are used as a common protecting group and detachment of solid support is done by tetrafluoroacetic acid as shown in fig no: 5

Fig no: 5

Solid supports used in Solid phase synthesis ^{7, 8, 9}

Generally solid support is a polymeric backbone that substrate is anchored. Most solid state combinatorial chemistry is conducted by using polymer beads ranging from 10 to 750 μm in diameter. The solid support must have the following characteristics for an efficient solid-phase synthesis:

1) Physical stability and of the right dimensions to allow for liquid handling and filtration;

2) Chemical inertness to all reagents involved in the synthesis;

3) An ability to swell under reaction conditions to allow permeation of solvents and reagents to the reactive sites within the resin;

4) Derivatization with functional groups to allow for the covalent attachment of an appropriate linker or first monomeric unit.¹⁰

The compounds to be synthesized are not attached directly to the polymer molecules. They are usually attached by using a linker moiety that enables attachment in a way that can be easily reversed without destroying the molecule that is being synthesized and allow some room for rotational freedom of the molecules attach to the polymer.

Types of solid supports that are used:

Polystyrene resins in this Polystyrene is cross linked with divinyl benzene (about 1% crosslinking).polystyrene resin are suitable for non polar solvents.

Tenta Gel resins Polystyrene in which some of the phenyl groups have polyethylene glycol (PEG) groups attached in the para position. The free OH groups of the PEG allow the attachment of compounds to be synthesized. PEG containing resins are suitable for use in polar solvents.

Polyacrylamide resins like super blue these resin swell better in polar solvent, since they contain amide bonds, more closely resemble biological materials.

Glass and ceramic beads these type of solid supports are used when high temperature and high pressure reaction are carried out.⁴

Linkers used in solid phase synthesis: ^4,
11-16

Linkers are the intermediate structure between resin and substrate. It support the attachment of a synthetic target, the polymer is usually modified by equipping it with a linker. Linker must be stable under the reaction conditions, but they must be susceptible to a cleavage. Some specialized linker have been developed to meet particular reaction or product conditions this type of linker is known as traceless linkers, it can be cleaved from the resin with no residual functionality left. This type of linkers allows the attachment of aryl and alkyl products that do not have OH or NH functionality example of these linker include silyl group (-Si (CH3)2) that is sensitive to acid and can be cleaved to give unsubstituted phenyl or alkyl product.

A new class of linkers was developed known as safety-catch linkers which is inert to synthetic condition and chemically transformed to allow final liberation of the product from the resin. Now a ultraviolet light sensitive protecting groups are used, like affymax group is used in the synthesis of carboxylic acid and carboxamide products. Some groups have used linkers that can only be cleaved by enzymes.^11-12 A novel linker possessing selenocyanate and masked carboxylic acid was developed for the solid-phase synthesis of dehydropeptides. This linker was used to demonstrate the synthesis of the model compound of RGD-conjugated dehydropeptide.¹³

Oxabicyclo norbornenes constitute a convenient and readily cleaved linker for solid-phase organic synthesis. A simple and inexpensive furfuryl-substituted resin has been shown to capture and release maleimide dienophiles under conditions compatible with intermediate synthetic steps.¹⁴ A new linker based on a chroman system is developed for the side-chain anchoring of Arg and other guanidine-containing molecules. The system is compatible with the Fmoc/tBu solid-phase strategy, because the release of the final product is achieved by treatment with TFA in the presence of scavengers.^15-16

Fig no: 6

Fig no: 7

Fig no: 8

Common Protecting groups used in solid phase synthesis: ^5,
17-18

Primary function of protecting group is to protect the portion of the molecule that is not covalently bound to the resin. Here the protection means in sense blocking the reactivity of that group. The protecting group must be stable to the reaction conditions of each coupling. After coupling is performed, the protecting group is removed to expose a new reactive site and synthesis continues in a repetitive fashion.

Example: -

For protecting the Amino functional Group

1. FMOC (Fluro methoxy carbonyl benzyl ester)

2. TBOC (Tertiary butyloxy carbonyl) etc.,

For protecting the carboxyl functional group

1. Benzyl ester

2. t-butyl ester

For protecting other functional groups

1. 2,2,5,7,8-pentamethylchroman-6-sulphonyl (Pmc) etc.,

The Solid Phase Synthesis can be done through two methods:

1. Parallel synthesis

2. Split Mix Synthesis

Parallel Synthesis:

The principle of parallel synthesis is the same as that applied by the house wives in the kitchen and the Tibetan monks in praying. Execution of the chemical reactions takes time and during that time not only one but a series of reactions can be realized. Each synthetic reaction is started in a different reaction vessel and all the necessary operations are executed in parallel. Below [Fig :9] shows the principle of the synthesis of five different trimers, for example tripeptides, in parallel.

Fig: 9 The Black, White and GrayCircles represent the building blocks, for example amino acids.

The five trimers are synthesized on solid support (P) in reaction vessels 1 to 5. At the end of the synthesis, each trimer is individually cleaved from the support and collected in one of the five vessels. By the above figure we can know that in parallel synthesis the number of reaction vessels is the same as the number of compounds to be prepared. The number of operations is practically the same as in the one by one synthesis of the same compounds since the solvents and reagents have to be serially transported into each reaction vessel. The main advantage is that the reaction time for the in synthesizing the multiple compounds are about the same as preparing a single one.

Generally parallel synthesis is done in the following manner as shown in table: 1 in which each cell represents a reaction vessel

Table:-1

Reagents - A Substrates-B	A₁	A₂	A₃	A₄
B₁	A₁ B₁	A₂ B₁	A₃ B₁	A₄ B₁
B₂	A₁ B₂	A₂ B₂	A₃ B₂	A₄ B₂
B₃	A₁ B₃	A₂ B₃	A₃ B₃	A₄ B₃
B₄	A₁ B₄	A₂ B₄	A₃ B₄	A₄ B₄

Here in the above table the reagents A₁to A₄reacts with substrates from B₁to B₄ to form products A_1-4B_1-4at the same time as that of the time taken to produce a single compound. The number of molecular entities (N) generated by an ideal combinatorial synthesis in parallel synthesis given by

N = (a x b)^x

Where a is the number of building blocks of A type and b is the number of B type available and x is the number of permutations of covalent links between the entities whichfunctional group dependent.

Parallel reactors for solution and solid phase chemistries using electronic control of temperature, pressure and vacuum. Generally apparatus contain the reactor tubes in multiple of 8 and the following apparatus contain 96 (12*8) reactor tubes.

Fig: 10 Equipment used for Parallel synthesis

Split-Mix Synthesis^{1, 3}

The Synthesis can be performed by repeating the following the three simple operation that form a circle

1 Dividing the solid support into equal portions.

2 Reacting the each portion with only one of the reagent.

3 Mixing and homogenizing the portions.

Generally this mix-split method is used for peptide synthesis

Fig: 11 Schematic synthesis of tripeptides by Mix and Split Method

If same building block is used at each step the maximum possible numbers of compounds that can be synthesized for the given number of different building blocks (b) are given by

Number of Compounds = b^x

Unlike in parallel synthesis, the history of the bead cannot be traced from the grid reference; it is traced by using suitable encoding method or deconvolution

SOLUTION PHASE COMBINATORIAL CHEMISTRY

Most ordinary synthetic chemistry takes place in solution phase. In this method synthesis of compounds takes place in solution form without the aid of solid support.

Reagent Filter

Substrate Product + Excess Reagent Product

Time consuming

In solution phase synthesis we use soluble polymer as support for the product. PEG is a common vehicle which is used in solution phase synthesis it can be liquid or solid at room temperature and show varying degrees of solubility in aqueous and organic solvent. By converting one OH group of PEG to methyl ether (MeO-PEG-OH) it is possible to attached a carboxylic acid to the free OH and use in solution phase combinatorial synthesis. Another common support which is used in solution phase synthesis is liquid Teflon consisting mainly of long chain of (-CF2-) groups attached to a silicon atom. When these phases are used as a soluble support for synthesis the resulting product can be easily separated from any organic solvent.

Examples for solution-phase synthesis:

1. Synthesis of Polymer by Solution Phase Combinatorial Chemistry^{20-22, 24}

Tartar and co-workers reported the synthesis of polymer supported 1-hydroxybenzotriazole. Reaction of the reagent with a carboxylic acid in the presence of an activating agent afforded the polymer bound activated ester which was reacted with amines to liberate the amide in solution. Supported electrophilic, nucleophilic or ionic reagents used to remove impurities from solution have been termed scavenger reagents; polymer supported quenching reagents (PSQ) or complementary molecular reactivity/ molecular recognition polymer (CMR/R polymer). Use of such reagents provides a versatile counterpart to the approach. Booth and Hodges utilised a high loading amine resin derived from chloromethylpolystyrene and tris (2- aminoethyl) amine in the preparation of ureas, thioureas, sulphonamides and amides.

Fig no: 12

2. Synthesis of Thiohydantoins^20,
23

Sim and Ganesan developed a one-pot three component synthesis of thiohydantoins using the reductive amination of amino esters with aromatic aldehydes and sodium triacetoxyborohydride followed by the reaction with an isocyanate in the presence of triethylamine. The thiohydantions were isolated by an aqueous work-up protocol which incorporated the addition of glycine to convert unreacted reagents into water soluble materials. The methodology was used in the preparation of an array of 600 discrete compounds.

Fig no: 13

3. Solution Phase synthesis of Biologically important oligosaccharides

The use of solution phase techniques has been explored as an alternative to solid-phase chemistry approaches for the preparation of arrays of compounds in the drug discovery process. Solution-phase work is free from some of the constraints of solid-phase approaches but has disadvantages with respect to purification.

The purification process can be made somewhat easy by using the scavenger resins. Scavenger resins react with the reagents so that products can be obtained by simple filtration as shown below. Here scavenger resins are chosen according to the nature reagents employed in the reaction.

1. If the reagent is nucleophillic we will use electrophillic scavengers.

Fig no: 14

Example: -

Fig no: 15

2. If the reagent is electrophillic we will use nucleophillic scavengers.

Example: -

Fig no: 16

Example: -

Fig no: 17

Pooling Strategies

Generally pooling strategies are used to make easy the screening process. Although some solid-phase combinatorial chemistry is conducted by use of the one-bead one-compound strategy, chemists have devised numerous other approaches to pooling reactants and intermediates to generate libraries. The goal is generally to achieve a balance between the simplicity of mixing everything together in one step but then having to “Deconvolute” the resulting mixture and working with more, but smaller, mixtures. It has been linked to someone giving you a rake and a magnet and telling you to go find and describe a needle in a field of hay. you can make one big haystack you know contains the needle, then have to deal with ever-smaller “Sub haystacks,” or you can use more clever approaches, such as dividing the field into regions, using overlapping regions, etc,. The major approaches that have been used include the following: ²⁵

1. One bead one compound strategy (Spatially addressable synthesis)

2. Iterative deconvulution.

3. Subtractive deconvulution.

4. Bogus-coin detection.

5. Orthogonal Pooling,

6. Positional scanning.

One bead one compound strategy (Spatially addressable synthesis) ²⁶

With this strategy, a specific quantity of beads is allocated for each possible structure in the library; those beads contain only molecules of the given library member. The beads may be tagged in various ways to help identify the synthetic compound. The advantage of the one compound strategy is the simplicity of analysis and screening. The disadvantage is keeping the beads searate and having to deal with a large number of synthesis in parallel It is otherwise called as Split and Mix technique.

Iterative deconvolution ²⁷

This is the strategy first described 20 yrs ago when combinatorial chemistry was started. Each group has beads bearing a variety of compounds, but a given structure only appears in one of the groups. Suppose the active structure is ABC in the 3rd group. Since it is in the 3rd group, we know a C in position 3 is needed for activity. We synthesize a smaller library of the structures, in 3 groups.

(AAC+BAC+CAC, ABC+BBC+CBC, and ACC+BCC+CCC)

Now when we screen those mixtures, we find activity in the middle group of beads. This tells us that a B in position 2 is required for activity. The final step is to synthesize ABC, BBC, and CBC, keeping them separate, and screen each to find ABC as the active structure.

Subtractive deconvolution ²⁸

This is the strategy similar to iterative deconvulution but uses negative logic, namely, leave out a functional group, and if activity is absent, the functional group that is missing must be needed for activity. This is particularly useful for QSAR-type studies in which, say, a cl group is placed at several positions on a phenyl ring. The entire library is screened as a mixture to get the baseline activity level. If activity is detected, a set of sub libraries is prepared; with each missing one building block (subtraction of functional groups from the active compounds) will be less active than the parent library. The Least active sub libraries identify the most important functional groups. A reduced library containing only these functional groups are then prepared, and the most active compounds are identified by either one compound synthesis or iterative deconvulution.

Bogus-coin detection ²⁹

This begins with generating and screening the entire library as a single mixture. If activity is detected, the building blocks are divided into 3 groups (alpha, beta, and gamma) and additional sub libraries are prepared. In these subsets, the number of building blocks from the alpha group is decreased, the number from the beta group is increased, and the number from the gamma group is unchanged. The resulting effect on activity (up and down, unchanged) suggests which group of building blocks was contributing most to activity. This approach is applied iteratively to zoom in one of the groups that are most active.

Orthogonal Pooling ³⁰

The orthogonal pooling means perpendicular or uncorrelated. In this type of pooling, we distribute the functional groups to be considered into sets of libraries A, B, C etc., which can contain mixtures of the same compounds. However, the functional groups are distributed such that any subset in A,B shares only one functional group, For example, if we have a very small library of structures aa, ab and ac .We might put aa and ab into group A, aa and ac into group B, ab and ac into group C. If ab is the active structure, screening A, B, C would show activity in A and C, but not in B, telling us that ab is the active one.

Positional Scanning ³¹

This is a noninterative deconvulution screening strategy in which a subset library is created with a single building block fixed at one position and all building blocks in the other positions. In principle, by selecting the functional group from the most active subset at each position, the most active compound overall is discovered. This ignores interactive between building blocks, which may complicate the results.

Methods of detection, purification and analysis³²

Detection analysis and purification of combinatorial libraries places high demands on existing analytical techniques because (a) the quantities to be analyzed are very small, sometimes pico moles of material, (b) the analysis should be nondestructive, to allow recovery of the compound if possible, and (c) The methods must be suitable for rapid, parallel analysis-analysis cannot be the rate limiting step in the procedure. No single analytical technique can fit all the requirements, so usually some “Hyphenated” analytical techniques are used such as HPLC-MS.

Chromatography is usually the first step in the analysis of a combinatorial mixture. If the sample solution contains a single compound we can directly analyze by using spectroscopic methods such as (UV/IR/NMR) and if the sample contains a mixture of compounds we must separate them before determining their structures. HPLC is most commonly used chromatographic technique for separation of combinatorial libraries. The main principle involved in HPLC is a sample of mixture is injected into the flow of solvent entering a chromatographic column. The components in the mixture travel down the column at different rates, depending on their affinity for the stationary phase in the column, and they exit or elute form the column at different times they are detected by some optical methods.

IR spectroscopy is also used, Since IR light can be reflected from materials, and one can analyze the resin beads directly, without cleaving the products from them. Since the loading of products on any given bead is very small, usually computer-enhanced methods like FTIR (Fourier Transform IR) is employed

NMR spectroscopy is used for analyzing the structural elucidation of compounds. Now a-days NMR is using in combination with separation techniques such as HPLC-NMR and CE-NMR (CE-Capillary Electrophoresis) NMR is even used to detect the binding of drug to receptors to identify the activity. A type of NMR called magic angle spinning NMR (MAS-NMR), in which the sample is inserted into the magnetic field at an angle of about 54.7⁰, reduces the peak broadening and has been used to analyze swollen polymer beads directly.

Fig no: 18

Mass spectroscopy is most widely used in the quality control of combinatorial libraries. The measurements can be made directly using resin beads. A wide range of compounds can be analyzed by using MS

Methods of screening ³³

Without the ability to screen libraries rapidly for activity, there would be no combinatorial chemistry. So, screening is an important step

1. High throughput screening (HTS) 2. Virtual Screening (In silico screening)

High throughput screening ³⁴

It is a broad topic, encompassing enzymes, organelles, cells, various tissues, and whole animal testing via cassette dosing. Successful HTS, programs integrate several activities,

Include target identification (genomics and molecular biology groups). Reagent preparation (protein expression and purification groups), compound management (information management group), assay development (biologist and pharmacologist), and high throughput library screening (biologists and chemists this increase efficiency of the screening process. Another route to increase efficiency is a move to higher density screening platforms. The methods for detection in HTS fall into the categories of non radiometric and radiometric. Non radiometric includes absorbance, fluorescence and luminescence spectroscopy. Enzyme assays are of a common example. Radiometric includes filtration and scintillation proximity assay (SPA).Assays use radioisotopes, so safe storage and handling are of concern.

Fig: 19 schematic of an high throughput screening process

Advantages:

Over filtration method SPA is that no filtering of the solution is needed, so beads can be added directly to the assay mixture in wells or test tubes. In HTS assay use of microorganisms such as bacteria, yeast, the cloning and expression of mammalian receptors in microorganisms, probing protein-protein interactions, and very importantly, DNA and Protein arrays.

Virtual Screening

It refers to the use of computers to predict whether a compound will show desired properties or activity on the basis of its (2D) or (3D) chemical structure or its physicochemical properties. The motivation for using virtual screening arises from the flood of new structures coming from combinatorial chemistry, the expense and time required to run conventional HTS, is less in the virtual screening programs in combinatorial chemistry. Because, virtual screening method of synthesizing compounds is mainly done on the basis of 3D and 2D.But, it has not been done in HTS method. This is the further advanced program as the method of synthesizing compounds.

Applications of Combinatrial chemistry²⁴

The following are various applications of combinatorial chemistry

1. Mainly it is applied in the discovery of drugs.

Eg:- Raloxifen (Fig no: 20)

Fig no: 20

2. Combinatorial chemistry is also applied for synthesizing analogues of existing lead structure to elucidate the Structure Activity Relationships (SAR).

3. Synthesis of molecular libraries

Combinatorial chemistry beagn with the synthesis of large libraries like peptides and oligo nucleotides. In the recent years combinatorial research has shifted to synthesis of small libraries having the molecular weight of 500 daltons or less due to poor oral bioavailability

Example synthesis of 1,4-Benzodiazepine library

4. Applications of antibody libraries obtained by combinatorial chemistry

Antibody combinatorial libraries have been developed to treat a series of human viral disease such as HIV – 1, RSV and Herpes Simplex viruses 1and 2 infections

5. Development of enzyme inhibitors by combinatorial chemistry

various enzyme inhibitors like acetylcholinesterase enzyme inhibitors by combinatorial library (carbamates, tetra hydroacridines) were developed by combinatorial methods

6. Combinatorial chemistry has been developed as a tool for lead optimization

Lead molecules were optimized by combinatorial chemistry Eg: - Optimization of leukotriene D4 antagonist

7. Qualitative and Quantitative characterization of drug database through combinatorial libraries.

Pharmacologically active compounds: ²⁵

examples of combinatorial chemicals synthesised by ‘combinatorial chemistry’

Fig no: 21

BOC-Val-MBHA-Resin(Peptides)

Fig no: 22

CONCLUSION:

Combinatorial chemistry continues to provide an important technique particularly to the medicinal chemist engaged in lead optimization work. Combinatorial chemistry and parallel synthesis can greatly benefit by their unique features offered by new synthetic technology. These include the possibilities of high-speed parallel processing of chemical transformations in the context of library production, and the rapid optimization of reaction conditions. Among the solid and solution phase synthesis Solid-phase organic synthesis (SPOS) is the most important method for the production of combinatorial libraries because all the synthetic transformations successfully applied to solid phase and with the development of high-throughput screening, libraries are widespread in pharmaceutical fields by doing the synthesis of biologically, pharmacologically and physicochemically active compounds and it has been correlated with the QSAR (Quantitative Structural Activity Relationship) parameters.

The last five years has seen an explosion in the exploration and adoption of combinatorial techniques. Indeed, it is difficult to identify any other topic in chemistry that has ever caught the imagination of chemists with such passion. For pharmaceutical chemists at least the reason for this change is not hard to fathom. 15 years ago the market for pharmaceuticals was growing at around 10% per annum but more recently the rate of the market growth as decline. At the same time, cost constraints on pharmaceutical research have forced the investigation of methods that offer higher productivity at lower expenses. The belief that combinatorial chemistry will allow the productive and cost-efficient generation of both compounds and drug molecules has fuelled enormous investment in this area. However, much work remains to be done in this area and this is clearly an area of massive growth for the future.

REFERENCES:

1) K. Ilango, P. Valentina Text book of Medicinal Chemistry Volume-II Pg no :- 447-448

2) G. Devala Rao A Text book of Synthetic and Medicinal Chemistry Pg no:-306-316

3) Arpad Furka Combinatorial Chemistry Principles and techniques

4) J. H. Block, J. M. Beale, Ed. 11^th, Lippincott Williams and Wilkins, pp. 43-49 (A Wolters Kluwer company: Philadelphia)

5) J. Giraldes, a dissertation for the degree of Doctor of philosophy in The Department of Chemistry 2003, 16-21.

6) S. Borman, combinatorial chemistry 2002, 80 (45), 43-57.

7) Z. Yu, M. Bradely, current opinion in chemical biology 2002, 6, 347-352.

8) S. Rana, P. White, M. Bradely, J Comb chem. 2001, 3, 9-13.

9) M. Kemp, G. Barany, J Am chem soc 1996, 118, 7083-7093.

10) P. F. Forns, a Practical Guide 2000, (New York).

11) C. P. Holmes, J. Org. chem 1997, 62, 2370-2380.

12) R. Reents, D. A. Jeyaraj, H. Waldman, Drug Discovery Today 2002, 7, 71-75.

13) K. Nakamura, Y. Ohnishi, Tetrahedron Letters 2003, 44, 5445-5448.

14) S. Punna, M. G. Finn, Tetrahedron Letters 2005, 46, 1181-1184.

15) F. Albericio, Tetrahedron Letters 2003, 44, 5319-5321.

16) R. Reents, H. Waldman, Drug Discovery Today, 2002, 7, 1-10.

17) K. Fukase, K. Egasa, Y. Nakai, S. Kusumoto, K. Fukase, Molecular diversity 1996, 2, 18-138.

18) K. Fukase, H. Tanaka, S. Torii, S. Kusumoto, Tetrahedron Letters. 1990, 31, 389-392.

19) A. Burgers, a Wiley; Inter Science Publication, (Ed. Abrahmam DJ), Vol II Ed. 6^t1-36 (Virginia: Midlothian pp).

20) M. C. Diane, R. Storerb, Molecular Diversity 1999, 4: 31–38.

21) E. Pop, B. P. Deprez, A. L. Tartar, J. Org. Chem. 1997, 62, 2594–2603.

22) R. J. Booth, J. C. Hodges, J. Am. Chem. Soc. 1997, 119 4882–4886.

23) M. M. Sim, A. Ganesan, J. Org. Chem., 62, 3230–3235.

24) Rama Rao Nadendla, Medicinal chemistry Revised edition Pg no 60-61.

25) http.www.google.com/.

26) Wilson and Gisvold Textbook of Pharmaceutical and medicinal chemistry VI^th edition .2007

27) R. W. Schafer, R. M. Mersereau, and M. A. Richards, “Constrained Iterative deconvolution algorithms,” Proc. IEEE., 1981,69, 432–450.

28) www.mprg.org/people/buehrer/ultra/pdfs/UWBDeconvolution.pdf.pubs.acs.org/cgibin/jtext?jmcmar/40/i26/abs/jm970503o.

29) C. J. Harris, X. Hong, and Q. Gan, Adaptive Modeling, Estimation and Fusion from Data: A Neurofuzzy Approach. New York: Springer-Verlag. 2002.

30) Pinilla C., Martin R., Gran B., Appel J. R., Boggiano C., Wilson D. B.,Houghten R. A. Exploring immunological specificity using synthetic peptide combinatorial libraries. Curr. Opin. Immunol. 1999, 11: 193-202.

31) Sasa.M, Dimitrijevi.c , Ursula Humer , Mayadah Shehadeh, W.Jonathan Ryves, Nahed M. Hassan and Fred J. Evans.Journal of Pharmaceutical and Biomedical Analysis., 1996,15(3),393-401.

32) W. F. Maier and P. von R. Schleyer, J. Am. Chem. Soc., 1981, 103, 1891.

33) Teodoro ML, Kavraki LE. Curr Pharm Des., 2003, 9, 1635. Dearden JC. J Comput Aided Mol Des. 2003,17,119

Received on 25.02.2012 Modified on 15.03.2012

Research J. Pharm. and Tech. 5(5): May2012; Page 570-579