Surgery - the cancerous tissue may be cut out by a surgeon. It is a difficult process to destroy the cancerous cell.
Radiotherapy - a beam of high-energy radiation destroys the cancer cells and can affect some normal cells as well, producing some side effects.
Chemotherapy - chemicals which has been used for to destroy cancer cells. This treatment may also have many other side effects and may affect other parts of the body.
Combination therapy - this may involve all, some or a combination of some of the above processes.
If cancer is detected and treated at an early stage there is a greater chance of a cure and possibly less extensive treatment.
N N O
O N
HN
F Cl O
To slow down or prevent the growth and spread of cancer cells in humans, researchers are now thinking for angiogenesis inhibitors. Two dozen angiogenesis inhibitors are currently being tested in cancer patients to answer this question. Depending on their mechanism of action the inhibitors being tested fall into several different categories.
Some inhibit endothelial cells directly, while others inhibit the angiogenesis signaling cascade or block the ability of endothelial cells to break down the extracellular matrix as shown in Figure 1.18. 33-36
2.11. Small molecules for Inhibtion of Angiogenesis process.
2.11.1 Gefitinib
Gefitinib, the trade name Iressa is a drug used in the treatment of cancer like non-small cell lung and breast cancer. Gefitinib is the first selective inhibitor of epidermal growth factor receptor's. Gefitinib inhibits EGFR tyrosine kinase by attaching to the adenosine triphosphate (ATP)-binding site of the enzyme.37-38
2.11.2. Erlotinib hydrochloride
N N
HN O
O O O
O N
O HN
NH O
NH Cl
F F F
NH F
O NH
O NH
N
Erlotinib hydrochloride is a drug used for treatment of non-small cell lung cancer, pancreatic cancer and several other types of cancer. It is a tyrosine kinase inhibitor, which acts on the epidermal growth factor receptor.39-40
2.11.3. Sunitinib
Sunitinib malate is a multitarget oral tyrosine kinase receptor inhibitor which was approved by FDA in for the treatment of renal cells carcinoma. Sunitinib inhibits cellular signaling by targeting multiple receptor tyrosine kinases (RTKs).41
2.11.4. Sorafenib
Sorafenib is a small molecule drug approved for the treatment of primary kidney cancer and advanced primary liver cancer. 42
N N
HN
Cl O
F O
HN O S
O
2.11.5. Lapatinib
Lapatinib used in the form of lapatinib ditosylate, is an orally active drug for breast cancer and different solid tumours. Lapatinib, the first of its type of dual inhibitor of epidermal growth factor receptorand human epidermal growth factor receptor 2 tyrosine kinases, was approved by the US Food and Drug Administration in 2007.43-44
2.12 .Protein Lysine Methyltransferase G9a Inhibitors
Methylation of Lysine 9 of histone H3 (H3K9) and lysine 373 (K373) of p53 has been catalyzed by protein lysine methyltransferase G9a, which is often overexpressed in human cancers. Dimethylation of p53 K373 results in the inactivation of p53 gene and also genetic reduced of G9a, inhibits cancer cell growth by the dimethylation mechanism.
Protein lysine methyltransferases (PKMTs) that catalyze mono, di, or trimethylation of lysine residues of various proteins including histones have received great attention because of the essential function of histone lysine methylation in many biological processes such as gene expression and transcriptional regulation, heterochromatin formation, and X chromosome inactivation as shown in Figure 2.7. The dimethylation of p53 K373 results in the inactivation of p53, which is concerned in over 50% of cancers.
These interpretation suggest that inhibition of G9a is a potential approach for cancer
N N HN O
O
N
N
N CH3
. XH2O . 3HCl
treatment. Till date the researcher has found a only one small molecule BIX01294, which is G9a inhibitor. Thus, small molecule PKMT inhibitors could play an important role in stem cell biology and regenerative medicine.
Figure 2.7. Histone K methylation, courtsey. Galinari et al, cell res, 2007, 17, 195.
In Figure 2.8 has shown a small molecule BIX 01294, which can inhibit G9a. 45-47
Figure 2.8. BIX 01294 is a selective histone methyl transferase inhibitor
2.13. Soluble Polymer Supported Organic Synthesis
Generally classical reaction has been carried out in solution phase involving the separation of the desired product from reagents and by-products after the reaction. This purification step can however turn out to be extremely time consuming and often meticulous. In 1963 Merrifield first introduced the solid-phase synthesis of peptide and oligosaccharide. This methodology, which was limited to the synthesis of peptides and oligosaccharides48 however remained predominantly limited to this field until the introduction of liquid phase techniques. To evade the drawbacks innate to solid-supported technologies such as, nonlinear kinetics, unequal distribution to the chemical reaction, solvation problems, alternative approaches using homogeneous ′beadless′ phase-tagged chemistry have been introduced to facilitate separation whilst retaining solution phase kinetics. Amongst all of these beadless approaches, soluble polymer phase attaching of substrates enable easy separation, monitoring, analysis and characterisation has become the method of choice.49 The polymers employed as soluble supports in liquid phase organic synthesis possess some qualities such as a) easy availability, b) good mechanical and chemical stabilities, c) homogeneous reaction condition, d) having appropriate functional groups for easy anchoring to organic moiety and more importantly showing high solubility to dissolve molecular entities etc.50 Moreover, it has been observed that polymer supports normally used in organic synthesis are macromolecules varying different sizes. These supports should with stand the reaction condition used in solution phase chemistry and consequently most polymer supports used in liquid phase synthesis possess alkyl ether backbone structures. By variation of functional groups of backbone structures, polymer properties are determined and may provide sites for attachment of
On
Oxide Polyvinyl alcohol Polyethylene imine Polyacrylic acid
Polyacrylamide Polystyrene PEG with 3,5-diisocyanatobenzyl
chloride Cellulose molecule organic synthesis. These includes polyethylene glycol (PEG), polystyrene, poly (propylene oxide), poly(vinyl alcohol), polyethylene imine, polyacrylic acid, polyacryl amide, PEG with 3,5-diisocyanatobenzyl chloride, and cellulose (Figure 1.21).51
Figure 2.9. Different Soluble Supports used in Small Molecule Organic Synthesis
2.13.1. Application and recent development of polyethylene glycol as soluble support in organic synthesis
Among all polymer support, polyethylene glycol has been frequently used in small molecule organic synthesis. Polyethylene glycol (PEG) (Figure 2.10) is a polyether compound which has many applications from industrial manufacturing to medicine.
Figure 2.10. Different PEG Soluble Supports.
PEGs are commercially available over a wide range of molecular weights from 200 g/mol to 10,000,000 g/mol and are prepared by polymerization of ethylene oxide. PEG has different physical properties (e.g., viscosity) due to chain length effects and their chemical properties are nearly identical and PEO with different molecular weights find use in different applications.52-54
2.13.2. PEG for Small molecule Synthesis
Polyethylene glycol has long been applied as soluble polymer supports for the synthesis of oligopeptide, oligosaccharide as well as small molecules. Normally for the supported synthesis PEG5000 and PEG4000, PEG6000 are used as soluble supports based on the loading capacity and hydroxyl functionalities contained. PEG5000 contains one hydroxyl group and its loading capacity is 0.2 mmol/g, whereas PEG4000 and PEG6000 consist of two hydroxyl groups and their loading capacities are 0.5 mmol/g and 0.33 mmol/g, respectively. Employed as a protecting group, this linear homopolymer exhibits solubility in a wide range of organic solvents and water. PEG5000 and PEG4000, PEG6000 in can be use as a support but some cases low moleucular weight PEG can be use a solvent. High molecular weight PEG is insoluble in hexane, diethyl ether and tert-butyl methyl ether, and these solvents have been used to induce PEG precipitation.55-56 Primarily, the uses of these polymers in synthesis have fallen into one of two areas: (A) the use of the polymer as a support for reactants or (B) the use of the polymer as a support for reagents and catalysts during a reaction. Both of these methods allow rapid product purification and the ability to drive a given reaction to completion through the use of an excess of reagents.
R + OSO4
Chiral ligand 1 or 2
HO ligand to carry out the asymmetric hydroxylation. They believe that MeO-PEG polymer will useful effecting the separation of catalyst from product in homogeneous industrial applications as shown in scheme 1.57
Scheme 2.0 Asymmetric hydroxylation using PEG supported catalyst
MeOPEG-5000 was coupled to a substituted styrene using a succinate linker to provide, MeO-PEG-supported Grubbs type catalyst, after treatment with a ruthenium alkylidene in figure 1.23. This catalyst was used for the ring-closing metathesis reaction of a number of dienes and demonstrated excellent conversions (>92%) for all studied examples and only a slight decrease in catalytic activity after repeated use.58
Scheme 2.1. Grubbs catalyst in PEG support
NH2
COOH HO
1. NaNO2, H3O+, ooC 2. KI, 90oC 3. EtOH, HCl, reflux
I
COOH HO
1. Cs2CO3, DMF, 70 oC 2. Ba(OH)2, 8H2O/MeOH
OMs
I
COOH O
Bu4N-Oxone, MeSO3H CH2Cl2
O I
O HO O
O
Recently, Janda and co-workers have reported a soluble polymer-supported version of IBX that has the advantage of being soluble in a greater range of solvents. This reagent was prepared by first synthesizing the appropriate m-hydroxyiodobenzoic acid in scheme 2. Loading of this compound onto NCPS, followed by ester hydrolysis and oxidation of the iodine from I(III) to I(V), led to NCPS-supported reagent. Using 2 equiv of NCPS supported IBX, it was demonstrated that the conversion of benzyl alcohol to benzaldehyde proceeded in quantitative yield after only 1 h in methylene chloride as in scheme 2.59
Scheme 2.2. PEG supported IBX reagent
Till date several research efforts has been published using PEG as soluble supports.
Janda et al. first reported the synthesis of sulfonamide libraries using the PEG as a support. This was the first report of the use of PEG in small organic molecule synthesis shown in Scheme 2.3.60
MeO-PEG-OH +O C N S Cl
Peptidomimetric azatide Tyr-Gly-Gly-Ph-Leu prepared by PEG support
Scheme 2.3. Synthesis of sulfonamide in PEG support.
Similar methodology was applied for the synthesis of a new class of peptidomimetics called azetides as shown in Scheme 2.4.61
Scheme 2.4. Synthesis of peptide by PEG support.
Subsequently, in the year 1999, first our lab has developed pharmacologically interesting guanidine and urea functional groups by combinatorial approach using soluble PEG support in Scheme 2.5.62
N
O O
HN NBoc NHBoc
R1R2NH THF, reflux
N
O O
HN NBoc NH
NR1R2 O
1% KCN/MeOH N
O O
HN NBoc NH
NR1R2 O
Scheme 2.5. Synthesis of guanidine and urea functional group
2.14. Pyrrolo[1,2-a]quinoxalines its importance and synthesis
The heterocyclic compounds with azole-fused quinoxaline rings such as imidazo[1,2-a]quinoxalines, imidazo[1,5-imidazo[1,2-a]quinoxalines, [1,2,4]triazolo[4,3-imidazo[1,2-a]quinoxalines, 1H-imidazo[4,5-b]quinoxalines, and pyrrolo[1,2-a]quinoxalines are known to demonstrate a ample range of pharmacological activities.63-67, It has been observed that many best selling drugs contain structurally complex biheterocyclic moiety as a key component.68 Our research group has been long involved in developing the novel therapeutics in quinoxaline analogues as quinoxaline skeleton fused with pyrrole is most representative structure with extensive chemical and biological profiles (Figure. 2.11 A).69 Recently, it was identified that the pyrrolo[1,2-a]quinoxaline derivatives are In binding studies shows several pyrroloquinoxaline compounds proved to be potent and selective 5-HT3 receptor ligands as well as potent antileshmanial agents (Figure. 2.11 B, C).70 Furthermore, the pyrrolo[1,2-a]quinoxaline derivatives anchored benzimidazole moiety are shown to
exhibit as an antiulcer agent (Figure. 2.11 D).71 It is noticed that the several bis pyrrolo[1,2-a]quinoxalines possesses in vitro anti-malerial activity.72
N
N N
N
N N
N N
N
N N Cl
S O
N
HN OMe
N N
A B C D
F F F
Figure 2.11. Representative examples of biologically active pyrrolo[1,2-a]quinoxalines.
2.15. Various method of preparation of Pyrrolo[1,2-a]quinoxalines derivatives
In 1997, for the first time Campiani et. al. had described the synthesis and the biological evaluation of a series of novel pyrroloquinoxaline derivatives. They used the o-fluoroaniline which underwent Clauson-Kaas reaction to give the corresponding arylpyrroles and subsequently transformed into 1-aryl-2-cyanopyrroles through a one-pot sequence involving formylation, oximation,and dehydration. The intermediates were progressively cyclized to the desired pyrrolo[1,2-a]quinoxalinones by basification in ethylene glycol at high temperature. Upon treatment of pyrrolo[1,2-a]quinoxalinones with POCl3 and Nucleophillic aromatic substitution to generate the target pyrrolo[1,2-a]quinoxalines as shown in Scheme 2.6.73
NH2
Scheme 2.6. Synthesis of pyrrolo[1,2-a]quinoxalines.
Simailarly, In 1999 the Campiani et. al. has modified the procedure for the synthesis of pyrroloquinoxaline moiety. Instead of o-fluoroaniline they took o-onitroaniline which underwent the same set of Clauson-Kaas reaction to give the corresponding arylpyrroles.
Subsequently reduction of nitro group ortho to the pyrrole moiety with SnCl2 and cyclisation with triphosgene generated the pyrrolo[1,2-a]quinoxalinones. The desired compounds were achived by the Reaction of POCl3 with pyrrolo [1,2-a]quinoxalinones as shown in the Scheme 2.7.74
Scheme 2.7. Synthesis of pyrrolo[1,2-a]quinoxalines by Campiani et. al.
N derivatives, sharing structural analogies with omeprazole, a eukaryotic efflux pump inhibitor (EPI) used as an antiulcer agent. But instead of cyclisating with triphosgene they used chloroacetyl chloride and POCl3 to obtain the target structures as drawn in Scheme 2.8.75
Scheme 2.8. Synthesis of pyrrolo[1,2-a]quinoxalines by Vidaillac et. al..
In 2009, For the first time, Patil et. al. has developed an efficient method for Markownikoff‟s hydroamination-hydroarylation of alkynols using PtBr2 as catalyst. The platinum-catalyzed reactions of alkynols with amino group containing aromatics were achieved in methanol over a reaction time of 6-24 h and temperature ranging from r.t to 80 oC in Scheme 2.9.76
Scheme 2.9. Synthesis of pyrrolo[1,2-a]quinoxalines by PtBr2 catalysedreaction by Patil et. al.
N H2N +
5 mol% AuCl DCE, rt-12 h
N
N Ph
O H
96 % yield
NH2 N
+ nHex
2 mol% Ph3PAuNTf2, toluene
100 oC N
NH Me
nHex
90 % yield
Subsequently in 2010, Patil et. al. has developed a gold(I)-catalyzed, coupling–
cyclization technique for the synthesis of isoquinoline- fused polycyclic compounds. The reaction composed of o-alkynylbenzaldehydes and aromatic amines having tethered nucleophiles in Scheme 2.10.77
Scheme 2.10. AuCl catalysed synthesis of pyrrolo[1,2-a]quinoxalines
In the same year, Patil et. al. has developed an efficient method for formal Markownikoff hydroamination/ hydroarylation and double hydroamination of terminal alkynes using 2–
5 mol-% of Ph3PAuNTf2 in toluene at 100 °C has been developed scheme 2.11. 78
Scheme 2.11. Synthesis of pyrrolo[1,2-a]quinoxalines by Au catalysed reaction
In 2011, Liu et. al. has developed an efficient tandem process of hydroamination and hydroarylation using a gold catalyst to enable and study the reactions between
pyrrole-N NH2 R3
R4
R1 R2 Au catalyst Toluene 80 oC, 1-6 hrs
R3
NH N
R1R2 R4
R1= aryl, alkyl R2=H, aryl, alkyl R3=H, Me, OMe, CN R4=H, Me, Et, OMe, CN, F
substituted anilines and alkynes. The gold (I)-catalyzed reactions were accomplished in toluene at 80 oC over a reaction time of 1-6 h in scheme 2.12. 79
Scheme 2.12. Synthesis of pyrrolo[1,2-a]quinoxalines by Au catalysed reaction
In 2010, Our lab has described the synthesis diverse indolo-fused pyrazino-/diazepinoquinoxalinones using amino acid and indoline-substituted dinitrobenzene on a soluble polymer support (PEG) and its further reductive double-ring closure to afford structurally diverse final compounds. To furnish these novel scaffolds traceless synthesis of quinoxalinones coupled with application of the Pictet-Spengler-type condensation reaction uder microwave irradiation has been developed in Scheme 2.13. 80
N
Scheme 2.13. Synthesis of indolo-fused pyrazino-/diazepinoquinoxalinones on PEG support
In 2011, Our lab has developed the diversity-oriented synthesis of novel benzimidazole linked indolo-benzodiazepine/quinoxaline ring systems using poly-(ethylene glycol) as soluble polymer support. To construct these types of privileged heterocyles commercially available 4-fluoro-3-nitrobenzoic acid and indoline were along with focused microwave irradiation in Scheme 2.14. 81
Scheme 2.14. Synthesis of benzimidazole linked indolo-benzodiazepine/quinoxaline on PEG support.
However, it has observed that for the construction of both tetrahydro-β-carbolines and tetrahydroisoquinolines the Pictet-Spengler reaction,82a which entails the cyclization of electron-rich aryl or heteroaryl groups onto imine or iminium ion electrophiles, has long been a standard method.82b For assemble diverse benzimidazole-pyrrolo[1,2-a]quinoxaline core small molecules, here for the first time we have described an application of SNAr reaction and subsequent Pictet-Spengler cyclization involving electron-rich heteroaryl pyrrole groups onto iminium ion electrophiles. To the best of our knowledge, no research group ever reported the nucleophilic aromatic substitution reaction of pyrrole ring to the aromatic substrate directly and subsequent Pictet-spengler reaction. Prompted by this observation, we undertook the present investigation and the results of our studies are reported herein.
2.16. Results and Discussions
The present approach initiated with the synthesis of polymer immobilized o-phenylene diamine 3 from 4-fluoro-3-nitrobenzoic acid 1 with built-in structural diversity (R1) through three step protocol as developed by our group previously.83
Scheme 2.15. PEG supported synthesis of o-phenylenediamine 4.
PEG 4000
N C N
The synthetic method depicted in Scheme 2.15 was utilized HO-PEG-OH (MW: 4000) as a soluble support was reacted with the commercially available 4-fluoro-3-nitrobenzoic acid 1 through the N,N'-dicyclohexylcarbodiimide (DCC) and catalytic amount of 4-dimethylaminopyridine (DMAP) activation to afford the polymer immobilized o-fluoronitrobenzene 2 as pale yellow compounds in quantitative yields. However, we have observed that completion of the reaction was achieved in 1 day at room temperature condition. But with the application of sealed vessel microwave irradiation (80 °C, 2 bar), the same reaction completed in 20 minutes. After completion of the reaction time, the insoluble DCU was filtered off and purified by precipitating out the reaction mixtures with cold ether. The mechanism of the formation of compound 2 was explained in Scheme 2.16.
Scheme 2.16. Mechanism of formation of compound 2 on soluble support
The first point of structural diversity was introduced by nucleophilic aromatic substitution (SnAr) of readily available primary amines with 1 via an ipso-fluoro
displacement to give polymer bound yellow nitroaniline compound 2. NMR analysis of 2 showed complete conversion to 3 after a reaction time of 12 h at room temperature. The reaction proceeded efficiently with various amines without cleavage of the ester bond at the polymer attached site. With the application of microwave irradiation (100 °C, 2 bar) reduced the reaction time to 10 minutes. Purification was achieved by the precipitation with cold ether. Reduction of the aryl nitro group in the resulting polymer immobilized nitro derivative 3 was successfully accomplished with a suspension of Zn/HCOONH4 in methanol to afford immobilized diamine 4 at room temperature for 20 minutes.
Formation of the amine conjugates 4 was confirmed from change of yellow to blue color upon spotting on the TLC plate. Upon completion of the reaction, reaction mixtures were filtered through fritted funnel to get rid of the Zn. The reaction mixtures were evaporated and dichloromethane was added to salt out the ammonium formate to obtain the colourless compound 4. The exact mechanism reharding the formation of compound 4 was observed in Scheme 2.17.
Scheme 2.17. Mechanism of formation of compound 4 on soluble support.
R N
OH e, H
R NH OH
e, H -H2O
e, H
R N O OH
R NO2 e, H
-H2O R NO e, H
R N
H e, H
R NH2
1eq. Zinc 1eq. Zinc
1eq. Zinc
4 3
To construct the benzimidazole ring in the present synthesis another molecule of 4-fluoro-3-nitrobenzoic acid 1 has been used in presence of DCC/DMAP coupling reagent to afford the polymer immobilized 5 as pale brownish compounds in quantitative yields in Scheme 2.18.
Scheme 2.18. PEG supported synthesis of o-nitrofluoro benzimidazol derivatives 6.
However, we have observed that completion of the reaction was achieved in 2 day at room temperature condition. But with the application of sealed vessel microwave irradiation (85 °C, 2 bar), the same reaction completed in 15 minutes. After completion of the reaction time, the insoluble DCU was filtered off and purified by precipitating out the reaction mixtures with cold ether. The mechanism of the formation of compound 5 was explained in Scheme 2.19.
NH
Scheme 2.19. Mechanism of formation of compound 5 on soluble support
The obtained anilide conjugates 5 were converted into benzimidazoles 6 by an intramolecular ring closure through the nucleophilic attack of the secondary amine on to the amide carbonyl which was induced by a mild acid (12 % TFA). Addition of anhydrous magnesium sulphate in this transformation reduced the reaction time, by facilitating the removal of water during this step, which needed 15 h under refluxing conditions in dichloroethane. The time for the formation of benzimidazole was reduced to 15 minutes by domestic MW reactor. However, the reaction time was reduced to 5 minutes in sealed vessel MW conditions (5 bar, 100 0C). Magnesium sulphate was filtered off and the polymer conjugate 6 was purified by precipitating out the reaction
The obtained anilide conjugates 5 were converted into benzimidazoles 6 by an intramolecular ring closure through the nucleophilic attack of the secondary amine on to the amide carbonyl which was induced by a mild acid (12 % TFA). Addition of anhydrous magnesium sulphate in this transformation reduced the reaction time, by facilitating the removal of water during this step, which needed 15 h under refluxing conditions in dichloroethane. The time for the formation of benzimidazole was reduced to 15 minutes by domestic MW reactor. However, the reaction time was reduced to 5 minutes in sealed vessel MW conditions (5 bar, 100 0C). Magnesium sulphate was filtered off and the polymer conjugate 6 was purified by precipitating out the reaction