Recent Advances in the Synthesis and Biological Activity of Pyrrolo[2,3-c ]pyridines

Pyrrolo[2,3-c ]pyridines (6-azaindoles) are the most promising nitrogen-containing heterocyclic compounds in the field of drug development. Exhibiting extraordinary versatility as pharmacophores, they are widely used in the development of kinase in - hibitors, antiproliferative agents, and as potential therapeutic agents for the treatment of various diseases, including cancer and Alzheimer’s disease. A large number of works focusing on new methods and approaches in the synthesis of 6-azaindoes, as well as on the study of their biological activity, have been published worldwide. In our review, we tried to classify all currently known strategies for the construction of the 6-azaindole core, which were published within the last 15 years, the chemical diversity of the derivatives obtained, and their therapeutic potential in the context of medicinal chemistry. We hope that this work will generalize and facilitate the understanding of the strategy for the synthesis of pyrrolo[2,3-c ]pyridines, as well as help scientists in their further research in the direction of 6-azaindoles.


■ Introduction
Nitrogen-containing heterocyclic compounds play one of the central roles in the realm of drug development, mainly thanks to their inherent molecular polarity, water solubility, and the ability to permeate cellular membranes.The analysis of FDA-approved drugs reveals that an astonishing 59 % of unique small-molecule drugs contain at least one nitrogen heterocycle, which demonstrates their importance in drug design and discovery [1].This predominance is attributed not only to the versatility of nitrogen heterocycles in mimicking the biological landscape, but also to their structural diversity, which offers myriad possibilities for the modulation of pharmacokinetic and pharmacodynamic properties.
Among all of the nitrogen-containing heterocycles, pyrrolo [2,3-c]pyridines stand out as a very promising scaffold due to its unique structural features and considerable biological activity.This class of compounds with the condensed pyrrole and pyridine ring has long attracted a widespread interest from the research community.This interest is demonstrated by the vast list of literature on synthetic methodologies, structural modifications, and the study of the medicinal and biological potential of the 6-azaindole core.The review by Popowycz et al. (2007) meticulously summarized this data, highlighting the versatility of 6-azaindoles in drug development and underscoring the synthesis of compounds via diverse strategies, including the Reissert, Batcho-Leimgruber, Hemetsberger-Knittel syntheses, and their functionalization in various positions to enhance the biological activity [2].It delves into the design of 6-azaindoles as biological targets and demonstrates their potential across a range of applications, from therapeutic agents to key synthetic intermediates.
However, since 2007, the synthesis and functionalization capabilities of pyrrolo [2,3-c]pyridines have significantly expanded due to advancements in synthetic chemistry, the availability of new reagents, increased technical capabilities, and so on.This synthetic versatility combined with the inherent biological relevance of the pyrrolo [2,3-c]pyridine core has led to the emergence of an immense amount of research, publications, and scientific works over the past 17 years.
Therefore, it seems to be very reasonable to complement the work of Popowycz, conduct a thorough analysis of all the new scientific achievements and provide a fresh thorough overview of the current state of research on pyrrolo [2,3-c]pyridines, encompassing their synthesis, structural modifications, and pharmacological potential.By studying the current scientific developments in this field and identifying promising areas for future research, we hope to contribute to the ongoing efforts to use pyrrolo [2,3-c]pyridines in the search for new therapeutic agents.

■ Results and discussion
Pyrrolo [2,3-c]pyridines and their annulated derivatives can be synthesized by various synthetic strategies.However, it makes sense to highlight three main principal approaches that stand out due to their efficiency and versatility: (1) the annulation of the pyrrole nucleus to the pyridine cycle; (2) the annulation of the pyridine nucleus to the pyrrole cycle; (3) the synchronous formation of the 6-azaindole system where both the pyrrole and pyridine rings are constructed in a single, concerted step.Each of these methods offers its own unique advantages in terms of reaction conditions, functional group tolerance, and overall yield.
Initially, we propose focusing on the first method, namely the annulation of the pyridine nucleus to the pyrrole cycle.

Annulation of the pyrrole nuleus to the pyridine cycle
The first and one of the most common methods for forming the pyrrolo [2,3-c]pyridine framework 3 involves the Bartoli reaction of 2-halogen-3-nitropyridines 1 with vinyl magnesium bromide 2 in the THF solution [3 -6] or using toluene as a solvent in the presence of a base [7] (Scheme 1).
The widespread application of this method can be attributed to its versatility, the high yields of targeted compounds it can achieve, and, of course, the possibility of using halogenated nitropyridines as precursors.The Bartoli reaction is a classic, described in an immense number of scientific studies and publications.
It is worth noting that this approach facilitates the incorporation of versatile alkyl groups in critical positions of the pyrrolopyridine core, allowing to fine-tune the molecule interaction with the H + /K + -ATPase enzyme.The subsequent functionalization of these derivatives has led to the identification of compounds exhibiting remarkable in vitro and in vivo inhibitory activities against gastric acid secretion, positioning them as promising leads for the development of new therapies for diseases associated with the increased stomach acid production.
Thus, the utility of the Bartoli reaction consists not only in constructing complex nitrogencontaining heterocycles, but also in enabling the targeted modification of these molecules to enhance their pharmacological profiles, thereby offering a valuable strategy for the discovery and optimization of novel P-CABs.
The next described method for constructing the 6-azaindole core is the Sonogashira reaction.The interaction of tert-butyl (4-iodopyridine-3-yl) carbamate 8 with a terminal alkyne 9 at room temperature gave the alkylation product 10; its heating at 80 °C provided a smooth cyclization to 2-benzyl-oxymethylpyrrolo[2,3-c]pyridine 11, from which a new tricyclic diamine 12 could be synthesized by further functionalization [15].In addition, to implement a tandem Sonogashira coupling/intramolecular cyclization reaction and obtain 6-azaindole 11 in one stage, the reaction mixture of iodopyridine 8 with a terminal alkyne 9 was heated (Scheme 4).
Undoubtedly, our review would be incomplete without mentioning the study from 2005 [16] where the authors described a one-step method for constructing a combinatorial library of 6-azaindole derivatives 15, it involves the direct dilithiation of unprotected 3-amino-4-picoline 13.The condensation of the dianion A obtained with carboxylic acid esters, thioester, or dihydrofuranone 14 led to a number of 2-substituted pyrrolo[2,3-c]pyridines with quite good and competitive yields (Scheme 5).Another convenient method for synthesizing 2-alkyl(aryl, heteroaryl)-substituted 6-azaindoles 19 is the palladium-catalyzed reaction of gem-dichloro olefins 17 and boronic acids 18, which includes a tandem intramolecular C-N coupling and the intermolecular Suzuki process (Scheme 6) [17].
The work documented in [21] outlines a one-pot method for synthesizing 3-substituted   diethyl malonates 35 were converted into ethyl esters of acetic acid 37 by the decarboxylation treated with LiCl in a water/DMSO mixture at reflux.The reductive cyclization of derivatives 37 with zinc in acetic acid produced 5-amino-and 7-amino-6-azaindoles 38 (Scheme 11) [26].On the example of the synthesis of ethyl 5-amino-2-hydroxy-1H-pyrrolo[2,3-c]pyridine-3-carboxylate 36, the authors of work [27] tried the heterocyclization in a Parr hydrogenator using a catalytic amount of Pd on carbon in ethanol and the treatment with the 18 % solution of hydrochloric acid, as well as treating diethyl malonate 35 with an excess of SnCl 2 •H 2 O in ethanol under ultrasound activation (Scheme 11) [27].
In 1970, the synthesis of 3-formyl-6-azaindole by the Vilsmeier-Haack formylation in 19 % yield was described for the first time.However, in 2024, this work was expanded and supplemented by a study concerning the scope and limitations of the synthesis of 3-formyl-6-azaindoles 56 via the Vilsmeier-Haack formylation of the corresponding 3-amino-4-methyl pyridines 55 (Scheme 16) [33].
This method was demonstrated to be very effective, scalable, and regioselective, requiring no catalysts and quite easy to perform.Also, the same year, the synthesis of 6-azaindoles via the formal electrophilic [4+1]-cyclization of 3-amino-4-methyl pyridines from the whole set of 3-amino-4-methylpyridine derivatives was described in detail (Scheme 17) [34].The essential difference compared to all similar reactions previously known is the absence of the activation of the methyl group by a strong base.It allows to provide the cyclization in mildly acidic conditions and significantly enlarges its scope.3-Methylamino-4-methylpyridine and 3-hydroxy-4-methylpyridine were preparatively entered into the reaction, giving the corresponding fused pyrrolo-/ furano-derivatives though in hydrated form.
While pyrrolo[2,3-c]pyridines themselves are of substantial interest mainly due to their potential as pharmacophores, the move towards synthesizing their annulated derivatives opens new possibilities in drug design.Approaches to them include strategies, such as intramolecular cyclization reactions, the use of transition metalcatalyzed cross-coupling reactions, and employing heteroatom insertions.Each method offers its own set of advantages in terms of selectivity, yield, and the types of annulated structures that can be achieved.A one-pot, two-step method for synthesizing highly functionalized derivatives of 6-azaindole 61 was developed based on the nucleophilic aromatic substitution reaction of perfluoropyridine 59 with heterocyclic ketene aminals 60 promoted by two bases, K 2 CO 3 and Cs 2 CO 3 (Scheme 18) [35].
A convenient route for obtaining condensed derivatives of 6-azaindoles 63 and 64 is based on a simple four-step cascade sequence; its key stages are Cu-catalyzed coupling of boronic acids 62 with di-tert-butyl diazodicarboxylate (DBAD) and the Fischer indolization (Scheme 19) [36].
The interaction of 3-hydrazinyl-2-methoxypyridine 29 with cyclohexanone under the Fischer cyclization conditions was also effective for the annulation of the tricyclic system 63 (Scheme 19) [22].

Annulation of the pyridine nucleus to the pyrrole cycle
The next strategy for the synthesis of the pyrrolopyridine core is through the annulation of the pyridine nucleus onto the pyrrole cycle.In work [42] a regioselective approach to the synthesis of pyrrolo [2,3-c] A method for constructing highly functionalized 6-azaindoles 87 involved the iodine-mediated electrophilic cyclization of 2-alkynyl-1-methylene azides 86 (Scheme 25) [44].
The Ir(III)-catalyzed reaction of pyrroloxime 92 and α-diazocarbonyl derivative 93 proved to be effective for the synthesis of N-oxide pyrrolo-[2,3-c]pyridine 94.It is worth noting that this represents a straightforward method for the synthesis of phosphorylated heterocycles, which are highly important in the organic synthesis and medicinal chemistry (Scheme 27) [46].
In terms of the synthesis of β-carboline derivatives through the annulation of the pyridine nucleus onto the pyrrole cycle, the Pictet-Spengler cyclization is one of the most common methods.The interaction of tryptamine or serotonin 95 with aldehydes in acetic acid led to the formation of tetrahydro-β-carbolines 96; its structural modification yielded derivatives 97 and 98 -potential phosphodiesterase-4 inhibitors (Scheme 28) [47].
A series of tetrahydro-β-carbolines and methyl tetrahydropyrido [3,4-    The authors of work [63] developed a one-step method for synthesizing β-carboline derivatives 124 from substituted methyl ester of tryptophan 123 and aldehydes in a methylene chloride solution in the presence of catalytic amounts of TFA at room temperature, followed by further treatment of the reaction mixture with trichloroisocyanuric acid (Scheme 35).
The biomimetic approach is a convenient alternative to the methods involving the stepwise synthesis of β-carbolines.Treating a mixture of substituted tryptophan 125 and amino acids with molecular iodine and trifluoroacetic acid successively undergoes decarboxylation, deamination, the Pictet-Spengler reaction, and oxidation, resulting in the formation of target β-carbolines 127 (Scheme 36).In contrast, the reaction of tryptophan hydrochloride 126 leads to the formation of methyl 9H-pyrido [3,4-b]indole-3-carboxylate 127.This indicates that the carboxylic group esterification in tryptophan blocks the decarboxylation, but does not impede other reactions in the process [64].
The conditions of the cascade reaction proved to be effective for the synthesis of ethyl 9Hpyrido [3,4-b]indole-3-carboxylate 143 as well.The required enone 141, which was generated by the Wittig olefination of aldehyde 139 with phosphorane 140 in the subsequent one-pot process with 2-bromo-1-phenylethanone 142, gave the target carboxylate 143 with the yield of 63 % (Scheme 40) [66].
2-(1H-Pyrrole-3-yl)anilines 146 have proven to be convenient substrates in the synthesis of pyrroloquinolines 149 through electrocyclization reactions.The interaction of the initial anilines with the pyrrol-3-yl fragment 146 with isocyanates in the DCM solution at room temperature yielded urea derivatives 147.The treatment of these compounds with CBr 4 , PPh 3 , and TEA led to the formation of carbodiimides 148.The subsequent deprotection of carbodiimides 148 with tetrabutylammonium fluoride (TBAF) was accompanied by the electrocyclization reaction and the in situ formation of the desired marinoquinolines 149 (Scheme 42) [70].
The authors of work [71] developed a Pd-catalyzed cyclization of imines 150 to create the 3Hpyrrolo[2,3-c]quinoline system 151.It is a part of the natural antimalarial marine products aplidopsamine A and marinoquinoline A. The baseinduced deprotection of the phenylsulfonyl fragment from pyrroloquinoline 151 led to the formation of marinoquinoline A 152 with the yield of 96 %.For the synthesis of aplidopsamine A 154, the benzoyl peroxide (BPO) catalyzed bromination of pyrroloquinoline 151 was carried out using NBS to obtain bromide 153.The reaction of the latter with 6-chloropurine in the DMF solution

The synthesis of the 6-azaindole system with a single-step formation of pyrrole and pyridine rings
A new variant of constructing the 6-azaindole core 159 has been developed based on the intramolecular Diels-Alder cycloaddition of oxazole 158 obtained from the reaction of oxazole 156 with diene 157 (Scheme 44) [72].
The reaction of alkyne-allene isomerization of esters 162 in situ proved to be convenient for

6-Azaindoles of high MedChem importance
Pyrrolo[2,3-c]pyridines represent a significant class of heterocyclic compounds that exhibit a wide range of biological activities.Due to their structural similarity to natural alkaloids and their ability to interact with various biological targets, these compounds have attracted considerable interest in medicinal chemistry and drug development.The key areas of the biological activity for pyrrolo [2,3-c]pyridines include the anticancer activity, antiviral properties, neuroprotective effects, anti-inflammatory and analgesic activities, antimalarial activity, modulation of ion channels and receptors activity, etc.
The 6-azaindole core is incorporated into the approved antiretroviral drug Fostemsavir 169 (Rukobia™) [76] and its prodrug Temsavir 170 (BMS-626529) [77] (Figure 1).They inhibit the attachment of the viral gp120 and prevent HIV entry.Both structures are widely used in the treatment of patients who have intolerance or resistance to other HIV/AIDS medications.
Tetrahydro-β-carbolines 169 and 170 demonstrated a good selectivity for inhibiting butyrylcholinesterase (BuChE), disaggregation of A β1-42 , and an excellent neuroprotective activity by alleviating damage induced by H 2 O 2 , okadaic acid, and A β1-42 , without cytotoxicity in SH-SY5Y cells.Thus, compounds 169 and 170 are potent multifunctional agents against Alzheimer's disease and can serve as promising lead candidates for further development [84] (Figure 7).
Compound 183 proved to be a potent, selective, and metabolically stable antagonist of the transient receptor potential melastatin 8 (TRPM8) ion channel (Figure 8).In vivo, 183 demonstrated a significant target coverage in murine models of icilin-induced wet dog shakes (WDS), cold allodynia induced by oxaliplatin, and thermal hyperalgesia induced by the chronic constriction injury (CCI).These results confirm the tryptophan moiety as a solid pharmacophore matrix for the development of high-potency modulators of the TRPM8-mediated activity [49].
A derivative of pyrido [4',3':4,5]pyrrolo[2,3-d]pyrimidine 184 was identified as an effective inhibitor of checkpoint kinases 1 and 2 (CHK1, CHK2) belonging to serine/threonine kinases and playing a central role in the mechanisms of the cellular regulation and DNA repair [41].It is noteworthy that among compounds of this class of heterocycles, a potent and orally bioavailable dual inhibitor 185 (AMG 925) of cyclin-dependent kinase (CDK4) and tyrosine kinase (FLT3) was found.The derivative 185 inhibits the proliferation of a range of human tumor cell lines, including Colo205 (Rb+) and U937 (FLT3WT), induces cell death in MOLM13 (FLT3ITD), and even in MOLM13 (FLT3ITD, D835Y), which shows resistance to several FLT3 inhibitors.In well-tolerated doses, compound 185 leads to the significant inhibition of the growth of MOLM13 xenografts in mice, and the activity correlates with the inhibition of STAT5 and Rb phosphorylation [41] (Figure 9).

■ Conclusions
Thus, the analysis of the literature sources for the last 15 years has shown that the construction of the 6-azaindole core and its structural modification remains a topical issue in the organic synthesis and medicinal chemistry.Biologically, pyrrolo [2,3-c]pyridines have emerged as a significant class of compounds with a potent activity across the spectrum of targets.The elucidation of their mechanisms of action and the optimization of their pharmacokinetic profiles are still crucial for drug development.The identification of derivatives with activity against challenging targets, such as protein kinases and viral proteins underscores the potential of pyrrolo [2,3-c]pyridines in addressing unmet medical needs.
In this sense, the future of pyrrolo[2,3-c]pyridine study is promising, and we anticipate discoveries that will further enrich our pharmacological arsenal and contribute to the advancement of medicinal chemistry.In particular, among the vast array of pharmacophores attached to the pyrrolo[2,3-c]pyridine framework, the promising trifluoromethyl group has been understudied, and we expect the results in the field will appear in the near future.

Scheme 20 .
Scheme 20.An effective method for the synthesis of β-carbolines