PhD Thesis

This thesis has been submitted by me to the Department of Chemistry, Indian Institute of Technology Guwahati for the award of the degree of Doctor of Philosophy. I would also like to thank all my past and present laboratory members, including Dr. Narsimha Mamidi, Dr. Sukhamoy Gorai, Dr. Rituparna Borah, Dr. Sreeparna Das, Mr.

Introduction & Literature Review of Indoleamine 2,3-

Transition Metal, Azide, and Oxidant-Free Homo-and Heterocoupling of Ambiphilic Tosylhydrazones to the

4 Development of 2H-Triazoles Scaffold-Based Inhibitors for

Ring-opening of Indoles: An Unconventional Route for the

Overall study revealed some pyran compounds that have high potency for the inhibition of targeted IDO1 enzyme. The optimized conditions were used for the synthesis of several 4,5-diaryl-2H-1,2,3-triazole derivatives in moderate to excellent yields.

Chapter 4 describes the optimization of 2H-1,2,3-triazoles as potent IDO1 inhibitors

• Enzymes: A Regulator of Biochemical Reactions
• Oxygenases
• Tryptophan Catabolism
• Serotonin Pathway
• Kynurenine Pathway
• Biological role of Indoleamine 2,3-Dioxygenase 1 Enzyme (IDO1)
• Regulation of Indoleamine 2,3-Dioxygenase 1 Activity
• Designing of IDO1 Inhibitors
• Reported IDO1 Inhibitors
• Indole Derivatives
• Natural Products as IDO1 Inhibitors
• Quinone / Iminoquinone Scaffold
• Imidazole Scaffold
• Triazoles Derivatives
• Inhibitors with N-Hydroxyamidines Motif
• Inhibitors with Other Functional Moiety
• Inhibitors under Clinical Studies
• Objective of Research Work
• References
• Introduction
• Synthesis of 4-Phenyl-4H-Pyran Derivatives

All three of these enzymes (IDO1, IDO2, and TDO) catalyze the rate-determining initial step of L-Trp metabolism via the kynurenine pathway (Figure 1.1).5 However, the TDO enzyme is structurally quite different from the IDO1 and IDO2 enzymes. .6 The IDO1 enzyme is generally expressed in various tissues, including the spleen, lung, placenta, small intestine, central nervous system, and epididymis.7 However, during oxidative stress in the cell, the expression level of the IDO1 enzyme increases in various extrahepatic tissues. 8 Proinflammatory cytokines and other molecules activate this extrahepatic enzyme. 9 IDO1 is primarily involved in the catabolism of both D- and L-Trp. Gibellini, F.; Njuguna, N.; Lee, E.; Stennett, L., The lymph node microenvironment promotes B-cell receptor signaling, NF-κB activation and tumor proliferation in chronic lymphocytic leukemia.

Synthesis of 4H-pyran derivatives from aromatic aldehyde

• Results and Discussion
• Inhibitory Activities of Fused-Pyran Derivatives against Purified hIDO1 Enzyme
• Spectroscopy based Analysis of the Interaction between Selected Pyran Derivatives and IDO1 Enzyme
• The Cellular IDO1 Inhibitory Activity of Fused-Pyran Derivatives
• Cell Viability of Potent Pyran Compounds
• Determining the Mode of IDO1 Inhibition by Pyran derivatives
• Molecular Docking Analysis of Pyran Derivatives in IDO1 Active Site
• Inhibitory Activity of Pyran Compounds against Purified TDO Enzyme
• Conclusion
• Experimental Section
• Instrumentation and Characterization
• Procedure of Synthesized Compounds
• Purification of the Compounds by HPLC Analysis
• IDO1 and TDO Inhibition Assay by Spectrophotometric Method
• IDO1 and TDO Inhibition Assay by HPLC Method
• Spectroscopic Measurements
• Cellular Activity Assay
• Cell Viability Assay
• Determining the Mode of IDO1 Inhibition
• Molecular Docking Analysis
• Characterization of Synthesized Compounds
• References
• NMR Spectra of few Pyran Compounds
• Introduction
• Reported Synthetic Strategies of 1,2,3-Triazoles

The mentioned concentrations of the pyran compounds are the 2 × IC50 values ​​of the compounds obtained by the activity assay against the purified enzyme (round shape = dead cell, elliptical = live cell). The electronic properties of the fused heterocyclic ring and the halogen substitution on the aryl ring also play a key role in the binding of compounds to the active site of the IDO1 enzyme. The IC50 values ​​of the selected compounds against the purified TDO enzyme range from 3 to 79 μM (Table 2.5).

The IC50 values ​​of the compounds were analyzed using the GraphPadPrism Software (GraphPadPrism. Version 5.01 program). The cells were then treated with appropriate concentrations of the compounds (20 nM to 2 μM) for a period of 4 h.

Classic Huisgen 1,3-dipolar cycloaddition reaction for triazole synthesis

Copper catalyzed azide−alkyne cycloaddition reaction for triazole synthesis

• Concept for the Synthesis of 2H-1,2,3-Triazoles

However, the use of toxic heavy transition metal catalysts hinders the extension of these methods to triazole synthesis. Recently, several transition metal-free strategies have been developed.5 But most of them describe the use of dangerous and explosive azides. Furthermore, the limited accessibility of terminal alkynes has also limited the use of this approach.

Furthermore, very few literatures described the synthesis of 4,5-diaryl-2H-1,2,3-triazoles, although it exhibits various significant biological activities.6 To continue our findings for the development of potent IDO1 inhibitors, we are also very interested in the development of simple synthetic procedures for a rapid and contaminant-free compound library. Path a defined the usual cycloaddition reaction between azide and alkyne.7 Whereas path b described the reaction between hydrazone and imine for the synthesis of the desired triazole.

Retro-synthetic route of 1,2,3-triazole

We hypothesized that tosylhydrazone may be the correct precursor for these two intermediates (bold). In this regard, we wondered whether the ambiphilic nature of the tosylhydrazone anion (azaenolate, I) or corresponding diazome intermediate (II) could participate in the reaction with the electrophilic parent hydrazone (Scheme 3.5).11 We hypothesized that the pericyclic [3 + 2] cyclization of diazo or aza-enolate would form either 1,2,3- or 1,2,4-triazole after tosylamine elimination, but a polar mechanism can lead to both triazoles as well as six-membered tetrazoles.12 The regio- and chemoselectivity can depend on many factors such as the intermediality and reactivity of ambiphilic species, as well as the nature of a hydrazone electrophile. Researchers have often used alkyl, aryl, and acyl hydrazones as electrophiles, but to our knowledge, no literature has reported tosylhydrazones as electrophiles or sole reagent(s) for the regioselective construction of triazoles.

Tosylhydrazone as ambiphilic reagent for heteroaromatics

• Results and Discussion
• Optimization of Reaction Conditions
• Substrate Scope of 2H-Triazoles

A higher temperature and the use of 3 equiv. base (entry 4) gave the target product a better yield. We have also tested the targeted reaction in different solvents such as MeOH, toluene, water, acetonitrile, DCM, but DMF remained the best solvent with DMSO as the only other solvent for successful triazole formation. The electronic nature of the substituted aryl group influences the result, as both the starting hydrazone electrophile and the base-transformed nucleophile are in conjugation with the aryl substituent (Scheme 3.5).

For example, an electroneutral phenyl or electron-rich aryl makes the hydrazone less electrophilic and likely its deprotonation to I or II less favorable, leading to a longer reaction time and moderate yields (Scheme 3.6, 3ca−3cc). The one-pot reaction of 4-chlorobenzaldehyde and tosylhydrazide under these optimized reaction conditions also afforded 3cl (yield, 70%).

Substrate scope for the synthesis of 2H-triazole

• Plausible Mechanism of the Reaction

Mechanistically, the requirement for a base indicates deprotonation of the N−H of the hydrazone to generate the dipolar aza-enolate (I) (Scheme 3.7, path a). Reactions with a polar electrophile are known to proceed via this aza-enolate, but favor initial nucleophilic addition via the N center.10a, 10b, 15 The anionic intermediate is also known to lose Ts− (anion) to form diazoalkanes II (Scheme 3.7, path b ), which reacts with opposite regioselectivity.16 Although the hydrazone is a polar electrophile, we exclusively obtained a C-coupled product. This unusual reactivity may be an indication for diazoalkane intermediation or the unusual electrophilicity of the tosylhydrazone.

Formation of acylated triazole from acylhydrazone under the optimized reaction conditions also supports the dipolar mechanism, as diazo or azine intermediates are unlikely with the acyl group (Figure 3.3).15a, 17.

Proposed mode of reaction under the optimized reaction conditions

• Experimental Support of the Proposed Mechanism
• Heterocoupling of Tosylhydrazones with Various Electrophiles

Transition-metal, azide- and oxidant-free homo- and heterocompaction of ambiphilic tosylhydrazones into regioselective triazoles and pyrazoles. The presence of 1H NMR peak 'A' even after 4 hours of reaction indicates the presence of tosylhydrazone in the reaction medium, which may participate in the condensation with the aza-enolate. Since the electrophilic hydrazone can be considered as a functional group equivalent without a trace of the nitrile after elimination of the tosylamine to form the triazole, we therefore first attempted to use the nitrile itself as an electrophilic coupling partner.

However, the nitrile turns out to be a similar and/or weaker electrophile compared to the tosylhydrazone, leading to the formation of homocoupling products along with heterocoupling with nitriles (Scheme 3.8, compounds 3da, 3db; Table 3.2).

Reaction of tosylhydrazone with aromatic nitrile

However, the nitrile proved to be a similar and/or weaker electrophile compared to the tosylhydrazone, leading to the formation of homocoupling products along with heterocoupling with nitriles (Scheme 3.8, compounds 3da, 3db; Table To our great surprise, the regiospecific cross-coupling product, N-1 substituted triazole, was isolated (3ea, 3eb) in good yields. We believe that the presence of aerobic oxygen under basic conditions led to oxidative aromatization of the initially formed triazoline or isomerized dihydrotriazole. 2H-Triazole was also functionalized at the 2-position ( Scheme 3.10, compound 3ja); therefore, regioselective 1- and 2-substituted triazoles were obtained either by homo- and heterocoupling of tosylhydrazones or by coupling of tosylhydrazone with an imine.

Reaction of tosylhydrazone with aromatic imine

Synthesis of compound 3ja

Reactions of tosylhydrazone with alkenes and alkynes

• Inhibitory Activities of 2H-Triazoles against Purified hIDO1 Enzyme
• Conclusion
• Experimental Section
• Instrumentation and Characterization
• Procedure of Synthesized Compounds
• General procedure for the synthesis of 2H-triazoles (3ca-3cy)
• IDO1 Inhibition Assay by Spectrophotometric Method
• Characterization of Synthesized Compounds
• References
• NMR Spectra of few 2H-1,2,3-Triazole Compounds
• Introduction
• Design and Synthesis of 2H-Triazole Compounds
• Results and Discussion
• Conclusion
• Experimental Section
• Characterization of Synthesized Compounds
• References
• NMR Spectra of few 2H-Triazole Compounds
• Introduction

The halogen-substituted aryl ring may be involved in interactions with hydrophobic residues present in the "A pocket" of the IDO1 enzyme. To increase the IDO1 inhibition efficiency of 2H-triazoles, we extended our activity studies with N-substituted 4-carboxamide-5-aryl-2H-1,2,3-triazoles (Table 4.2). 10b, 13 The IC50 values ​​of the compounds were directly calculated from the amount of kynurenine produced from L-Trp in the presence of the IDO1 enzyme.

The results revealed that the IDO1 inhibitory activities of the selected 2H-triazoles are within the range of 0.06–. For deoxy-ferro-IDO1 enzyme, the Q band (at 551 nm for enzyme only) was shifted by 5-8 nm in the presence of the compounds. The IDO1 inhibition and binding studies described that the selected 2H-triazoles bind to the active site of the IDO1 enzyme and actively inhibit its L-Trp catabolic activity.

Probable mode of interaction of compounds 4ai (A) and 4ha (B) with the active site of the enzyme IDO1 (4PK5).

General reaction of Knorr pyrazole synthesis

In the year 1883, the German chemist Ludwig Knorr first synthesized the 1H-pyrazole via the condensation reaction between 1,3-diketone with hydrazine (Scheme 5.1).3 Later, several literatures described the synthesis of 1H-pyrazoles based on the concept of Knorr synthesis.4 In these methods, both the starting materials 1,3-diketone and hydrazine were widely available and led to the good substrate possibilities for 1H-pyrazole.

Copper catalyzed intramolecular cyclization reaction for the synthesis of 1H- pyrazole

Ma and co-workers have disclosed the silver-mediated cycloaddition reaction between terminal alkynes and 2-diazo-1,1,1-trifluoroethane for the construction of highly regioselective 3-trifluoromethyl-1H-pyrazoles (Scheme 5.3).6p.

Silver mediated cycloaddition reaction between alkyne and diazo compound

• Concept for the Synthesis of 1H-Pyrazole

Recently, unconventional opening of C2-N1 bond of indoles has been described (Scheme 5.4).11 On the other hand, the use of C2=C3 bond as electrophile for the in situ cyclization reaction followed by ring opening of indole is unknown. It is well known that the tosylhydrazone is an effective coupling partner with alkenes or alkynes. In chapter 3, section 3.2.4; we described the regioselective synthesis of 1H-pyrazoles through the heterocoupling reaction between the ambiphilic tosylhydrazones and alkenes or alkynes.12 We and other research groups have demonstrated the tremendous application of tosylhydrazones in synthetic organic chemistry, especially in cyclization reactions, but the reactivity towards indole is still not much investigated.13 Recent studies have described that the reaction of tosylhydrazone with indole favors the N-alkylation or N-vinylation of indole, rather than cyclization reactions with C2=C3 bond (Scheme 5.5).14.

Herein, we hypothesized that C2-N1 bond opening may be one of the driving forces for C2=C3 bond activation. It is well documented that carbonyl/sulfonyl groups form stable adducts with Lewis acids.15 Therefore, we judiciously introduced the carbonyl- or sulfonyl-containing directing groups such as acyl, benzoyl and tosyl at the N1 position of indoles, assuming that the complexation between Lewis acid and these directing groups will induce the C2-N1 bond opening (Scheme 5.6).11c, 16 Therefore, decrease in electron density of the C2=C3 bond will lead to the cyclization reaction with the tosylhydrazone leading to the formation of pyrazole.

Our hypothesis for C2-N1 bond opening in indole

• Results and Discussion
• Optimization of Reaction Conditions
• Substrate Scope of 1H-Pyrazoles

Our initial investigations started with the aim of improving the electrophilicity of C2=C3 bond of the indole unit, so that the C2-N1 bond opening would become much easier leading to annulation reaction with tosylhydrazone. To our delight, the tosylhydrazone (5bg) in the presence of Lewis acid BF3•OEt2 (catalytic amount) afforded the regioisomeric 1H-pyrazole (5cg) under ambient temperature (Table 5.1, entry 1). As expected, the model reaction in the absence of BF3•OEt2 failed to provide the desired product even at higher temperature (Table 5.1, entry 2).

Reaction at 50 °C with 0.3 equiv of BF3•OEt2 gave the target product in best yield (Table 5.1, entry 4). We hypothesized that the transformation of indole to pyrazole strongly depends on the C2-N1 bond opening ability of indoles.