Abstract

Indazole is a nitrogen-containing aromatic heterocycle formed by the fusion of a benzene ring with a pyrazole ring. Its value in medicinal chemistry does not lie merely in the fact that it “contains two nitrogen atoms” or that “a variety of biological activities have been reported.” Rather, its value arises from an interdependent set of structural features created by the adjacent nitrogen atoms, the rigid fused-ring framework, and multiple modifiable positions.

The adjacent nitrogen atoms determine the tautomerism of indazole and the arrangement of its hydrogen-bond donors and acceptors. The rigid scaffold fixes the spatial orientation of these interaction sites. Substitution at N1, N2, C3, and positions on the benzene ring determines the direction in which side chains extend and influences molecular properties. The desired substitution pattern, in turn, directly affects the choice of ring-forming method, N1/N2 regioselectivity, and late-stage functionalization strategy. Ultimately, whether a highly active indazole-containing molecule can be advanced further depends on target selectivity, cellular potency, solubility, membrane permeability, metabolic stability, and the scalability of its synthesis.

This article examines the continuous relationship among “structure–recognition–substitution–synthesis–developability” and explains the practical role, applicable conditions, and major limitations of the indazole scaffold in drug-molecule design.

1. The Indazole Structure Determines the Chemical Properties of Its Nitrogen Atoms

1.1 Indazole Is Not Simply a “Fused Aromatic Ring Containing Two Nitrogen Atoms”

Indazole consists of a benzene ring fused to a five-membered ring containing two adjacent nitrogen atoms. Compared with indole, indazole retains a similar scaffold size and rigidity while replacing one carbon atom in the five-membered ring of indole with a nitrogen atom.

This change simultaneously affects:

① the number of hydrogen-bond donors and acceptors;

② intraring electron delocalization;

③ the direction of the molecular dipole;

④ the reactivity of the N1 and N2 positions;

⑤ molecular interactions with water, proteins, and the crystal environment.

Therefore, indazole can serve as a bioisosteric replacement for indole in certain molecular designs, but the two scaffolds are not simply interchangeable. Replacing indole with indazole may substantially alter both the protein-binding mode and physicochemical properties, and these changes must be reassessed experimentally.

1.2 1H/2H Tautomerism Alters the Arrangement of Hydrogen-Bond Donors and Acceptors

Indazoles that retain an N–H group mainly exist as two cyclic tautomers, the 1H and 2H forms. Formally, the two structures differ in the redistribution of a proton between the adjacent nitrogen sites and can interconvert.

Simplified representation of hydrogen-bond donors and acceptors in neutral 1H/2H tautomers

1H-Indazole                              2H-Indazole

N1–H          N2                        N1          H–N2

Donor         Acceptor        ⇌         Acceptor    Donor

In the more commonly encountered 1H form, N1–H generally acts as a hydrogen-bond donor, whereas N2 generally acts as a hydrogen-bond acceptor. In the 2H form, the positions of the hydrogen-bond donor and acceptor are correspondingly reversed.

Quantum-chemical calculations on unsubstituted indazole in the gas phase showed that, at the level of theory used in the study, the electronic energy of the 1H form was approximately 3.6 kcal·mol⁻¹ lower than that of the 2H form. After thermodynamic corrections, the free-energy difference for the 1H→2H conversion at 298 K was approximately 4.1 kcal·mol⁻¹. Gas-phase spectroscopic data likewise showed that the 1H form was strongly favored.

However, the conclusion that “the 1H form is more stable” cannot be mechanically extended to all derivatives. Substituents, solvents, crystal packing, and intramolecular or intermolecular hydrogen bonding can all alter the tautomeric equilibrium. Studies have shown that the 2H forms of certain 3-substituted indazoles can be significantly stabilized by hydrogen bonding and may persist for extended periods in particular aprotic solvents or crystalline states.

1.3 Tautomerism Must Be Strictly Distinguished from N1/N2 Regioisomerism

The species that moves between the 1H and 2H forms is a proton; therefore, these forms are tautomers. Once the hydrogen atom on N1 or N2 is replaced by an alkyl, aryl, or other substituent, the substituent cannot freely migrate between the two nitrogen atoms. The resulting products are two distinct compounds:

① an N1-substituted indazole;

② an N2-substituted indazole.

N1- and N2-substituted compounds may differ in:

the spatial orientation of the side chain;

hydrogen-bond-acceptor properties;

electron distribution and dipole moment;

protein-binding mode;

solubility and membrane permeability;

metabolic stability;

crystal structure and melting point.

This distinction directly determines subsequent molecular-design and synthetic strategies. If the target structure requires a fixed N1- or N2-substituted form, the two regioisomers cannot be treated as interchangeable versions of the same compound.

2. How the Adjacent Nitrogen Atoms and Rigid Scaffold Create Molecular Recognition

2.1 Adjacent Nitrogen Atoms Provide a Compact Hydrogen-Bond Donor–Acceptor Arrangement

In 1H-indazole, N1–H and N2 can provide a hydrogen-bond donor and acceptor, respectively, within a compact structural region. Because both sites are fixed in a rigid five-membered ring, their distance and orientation do not vary as substantially as functional groups positioned along a flexible chain.

This arrangement enables indazole to provide the following features simultaneously:

However, the number of hydrogen bonds does not directly correspond to binding strength. Whether a hydrogen bond improves affinity also depends on distance, angle, the desolvation penalties of the ligand and protein, water molecules in the binding site, and whether the ligand experiences conformational strain.

2.2 Rigidity Reduces Conformational Freedom but Also Lowers Spatial Tolerance

The fused bicyclic structure of indazole restricts rotational freedom and maintains relatively fixed spatial relationships among N1, N2, C3, and substituents on the benzene ring. When these orientations match the protein pocket, rigidity can help maintain a stable binding conformation. When the orientations do not match, however, the molecule cannot readily bend to avoid steric repulsion.

This explains several phenomena commonly observed in the structure–activity relationships of indazole derivatives:

① the same group may produce substantially different activity when attached at different positions;

② even a small substituent may markedly reduce activity;

③ increasing linker length may allow access to a new interaction site, but may also increase the conformational entropy penalty upon binding;

④ rigidifying a flexible fragment may improve binding efficiency, but may also reduce adaptability to the binding pocket.

Therefore, the practical value of indazole rigidity depends on whether the fixed spatial orientations match the target protein.

2.3 The PLK4 Cocrystal Structure Reveals the Hinge-Binding Mode of Indazole

In certain kinase inhibitors, indazole can serve as a hinge-binding group in the adenosine triphosphate (ATP)-binding pocket.

The cocrystal structure of human Polo-like kinase 4 (PLK4) with inhibitor 400631 (PDB: 4JXF) shows that the two-nitrogen unit of indazole forms complementary hydrogen bonds with the backbone N–H of Glu90 and the backbone carbonyl of Cys92, thereby anchoring the ligand in the kinase hinge region. Because 4JXF is an X-ray crystal structure with a resolution of 2.40 Å, it cannot directly and reliably locate the indazole N–H proton. It is therefore not advisable to assign a particular hydrogen bond specifically to N1 or N2 solely on the basis of this structure.

This structure demonstrates that indazole can provide kinase inhibitors with a stable binding orientation, but it cannot by itself determine the activity or selectivity of the entire molecule. Peripheral substituents extending from C3 and the benzene-ring positions must also occupy adjacent subpockets, contact nonconserved residues, and modulate solubility, permeability, and metabolic stability.

3. How Molecular Recognition Determines Substitution Positions

3.1 N1 and N2: First Determine Whether the N–H Group Must Be Retained

When designing indazole derivatives, the first question is whether N1–H participates in a critical hydrogen bond with the protein.

If N1–H directly contributes to an important binding interaction, N-substitution may reduce affinity, and the 1H-indazole form should generally be retained.

If the N–H group does not participate in a critical interaction, N-substitution may be used to:

① fix the N1- or N2-substituted regioisomeric form;

② alter the direction in which a side chain extends;

③ remove one hydrogen-bond donor;

④ introduce an ionizable side chain;

⑤ alter crystal packing;

⑥ block certain N–H-related metabolic reactions.

3.2 C3: Extending the Molecule While Retaining the Two-Nitrogen Recognition Unit

The C3 position is located at the outer end of the five-membered ring and is one of the most important positions for structural extension of the indazole scaffold. The following groups may be attached through C3:

① aryl or heteroaryl groups;

② alkenyl or alkynyl groups;

③ carbonyl, amide, and carboxylic-acid derivatives;

④ flexible linkers;

⑤ a second protein-recognition group.

Three factors must be considered simultaneously when designing C3 substitution.

Length: Can the side chain reach the intended region?

Direction: Does the side chain enter a hydrophobic pocket or extend toward the solvent?

Rigidity and flexibility: Must the side chain maintain a fixed orientation, or does it require a degree of conformational adaptability?

There is no universally applicable function associated with the C3 direction. In one protein, a C3 substituent may project into a deep pocket; in another, it may point toward the solvent. C3 design must therefore be based on the actual binding pose rather than on indazole numbering alone.

3.3 C4–C7: Primarily Used to Optimize Selectivity and Properties

The C4–C7 positions on the benzene ring provide four distinct substitution vectors and are commonly used to access regions beyond the core indazole-binding site.

These positions may be used to:

① contact nonconserved amino acids and improve target selectivity;

② fill local hydrophobic cavities;

③ introduce polar or ionizable groups;

④ modulate lipophilicity and aqueous solubility;

⑤ install halogens, boronate esters, amino groups, or other sites for subsequent reactions;

⑥ modulate metabolic susceptibility.

In studies of PLK4 inhibitors, aryl and heterocyclic substituents at different positions had different effects on enzyme inhibition and cellular activity. Introduction of a methyl group at the 5-position of indazole markedly reduced activity, suggesting limited steric tolerance at this position, although the specific cause requires further structural evidence.

3.4 Functional Roles of the Different Positions

4. How the Desired Substitution Pattern Determines the Synthetic Route

The synthesis of an indazole derivative should be planned by working backward from the target structure.

The following questions should first be clarified:

1. Should the N–H group be retained?

2. Is N1 or N2 substitution required?

3. Should the C3 substituent be introduced before or after ring formation?

4. Should a cross-coupling handle be retained on the benzene ring?

5. Is the objective to prepare a single target or to rapidly expand a structure–activity relationship series?

6. Must the route be scalable beyond the gram scale?

4.1 Retaining N–H: Prefer Direct Construction of 1H-Indazole

When N1–H is a critical binding site, ring-forming methods that directly generate 1H-indazole should be prioritized because they can reduce the need for N-protection and deprotection.

Counceller and co-workers synthesized 1H-indazoles from ortho-aminobenzaldoximes using methanesulfonyl chloride and triethylamine at 0–23 °C. This transition-metal-free method is compatible with a variety of substituted substrates and has also been demonstrated in scale-up experiments.

Construction of 1H-indazole from an ortho-aminobenzaldoxime

ortho-Aminobenzaldoxime

          │  MsCl, Et3N, 0–23 °C

          ↓

      1H-Indazole

MsCl denotes methanesulfonyl chloride, and Et₃N denotes triethylamine.

The proposed mechanism in the original study was:

Mesylation of the oxime hydroxyl group

→ formation of an activated oxime

→ attack of the ortho-amino group on the sp² nitrogen of the oxime

→ N–N bond formation and cyclization

→ formation of 1H-indazole

4.2 Targeting an N2-Substituted Product: Encode the Substituent During Ring Formation

If the target is a defined N2-substituted indazole, incorporating the substituent into the nitrogen-containing reactant before ring formation is generally more reliable than first preparing 1H-indazole and then performing a nonselective alkylation.

Kumar and co-workers reported a copper-catalyzed three-component reaction in which 2-bromobenzaldehyde, a primary amine, and sodium azide undergo condensation followed by consecutive C–N and N–N bond formation to directly generate a 2-substituted-2H-indazole.

Three-component synthesis of an N2-substituted indazole

2-Bromobenzaldehyde + primary amine + NaN3

                         │ copper catalysis

                         ↓

               2-Substituted-2H-indazole

The advantage of this route is that the N2 substituent is introduced directly from the primary amine, and its attachment position is determined during ring formation. In practical applications, the safety of azides, reaction temperature, copper residues, and substrate scope must still be evaluated.

4.3 Direct N-Alkylation: Regioselectivity Is the Primary Evaluation Criterion

After treatment with a base, 1H-indazole forms an anion with two potential nucleophilic sites. Both N1 and N2 may react with an electrophile. Conventional alkylation may therefore generate two regioisomers.

1H-Indazole + R–X + base

             ↓

     N1–R product + N2–R product

In a 2024 process study, a specific indazole was reacted with isobutyl bromide in the presence of potassium carbonate in N,N-dimethylformamide, producing an approximately 58:42 mixture of the N1 and N2 regioisomers. Chromatographic separation was required.

The researchers subsequently changed the bond-forming strategy. Indazole was first reacted with isobutyraldehyde to form a reversible N1-enamine intermediate, followed by platinum-on-carbon-catalyzed hydrogenation. This two-step route afforded the N1-isobutyl product in 76% isolated yield. No N2 regioisomer was detected, and the process was demonstrated on a 100 g scale.

Indazole + isobutyraldehyde

          │ reversible condensation, water removal

          ↓

      N1-enamine intermediate

          │ H2, Pt/C

          ↓

      N1-isobutylindazole

This example shows that when a conventional bimolecular nucleophilic substitution cannot provide sufficient regioselectivity, changing the bond-forming mechanism may be more effective than continuing to screen bases and solvents.

For N2 selectivity, trifluoromethanesulfonic acid (TfOH) can catalyze reactions between indazoles and diazo compounds, providing products with N2/N1 ratios of up to 100:0. The reaction has also been demonstrated on the gram scale. This method is limited to substituents that can be introduced using the corresponding diazo reagents and should not be regarded as a general N2-alkylation method equivalent to conventional alkyl-halide conditions.

4.4 Systematic Structure–Activity Relationship Studies: Use Intermediates Suitable for Late-Stage Functionalization

Structure–activity relationship (SAR) studies commonly require independent variation of substituents at C3 or on the benzene ring while the indazole core is retained.

Intermediates suitable for late-stage diversification include:

① bromo- or iodo-substituted indazoles;

② indazole boronic acids or boronate esters;

③ aminoindazoles;

④ indazole carboxylic acids or esters;

⑤ C3-substituted intermediates suitable for further coupling.

A PLK4 inhibitor series used 6-bromo-1H-indazole as the starting material. Borylation, Suzuki–Miyaura coupling, C3 iodination, and a second coupling reaction were used to independently vary substituents extending from the benzene ring and C3 positions.

Such routes are suitable for rapidly preparing related compound series, but they require simultaneous control of the effect of N–H on catalysis, chemoselectivity among multiple reactive sites, precious-metal residues, overall yield, and purification methods.

4.5 Synthetic Routes Should Be Evaluated on the Basis of Overall Quality

In drug discovery, the highest single-step yield does not necessarily indicate the optimal route. High regioselectivity, fewer purification operations, and reliable support for series expansion often have greater practical value.

5. How the Indazole Structure Affects Developability

5.1 Enzyme Activity, Cellular Activity, and In Vivo Effects Must Not Be Conflated

A low half-maximal inhibitory concentration (IC₅₀) only indicates that a compound strongly inhibits an enzyme under a particular set of assay conditions.

To exert an effect in cells, a compound must also:

① remain sufficiently soluble in the culture system;

② cross the cell membrane;

③ avoid rapid efflux or degradation;

④ reach a sufficient intracellular unbound concentration;

⑤ bind the intended target and alter the corresponding signaling process.

The PLK4 indazole series was evaluated in terms of enzyme inhibition, tumor-cell proliferation, preliminary physicochemical properties, and animal models. In that study, the proposed binding mode of compound 14i was derived from molecular docking, whereas its PLK4 inhibition, cellular potency, and antitumor efficacy in animals were supported by separate experiments.

This distinction illustrates that structural models, biochemical activity, cellular phenotypes, and in vivo efficacy represent different levels of evidence and cannot be substituted for one another.

5.2 N–H Simultaneously Influences Binding, Solvation, and Solid-State Properties

The indazole N–H group may be an essential hydrogen-bond donor for protein binding, but it can also participate in hydrogen bonding with water molecules and with other molecules in the crystal lattice. N-substitution may therefore produce opposing effects.

Whether N-substitution should be introduced cannot be determined solely from calculated polarity or the number of hydrogen-bonding groups. A more appropriate approach is to compare the binding activity, solubility, permeability, and metabolic stability of the N–H, N1-substituted, and N2-substituted analogues in parallel.

5.3 Planar Aromatic Structures May Increase Lattice Constraints

Indazole itself is an approximately planar fused aromatic ring system. When additional conjugated aryl groups are attached through C3 or the benzene-ring positions, the molecule may form a larger planar system that favors regular intermolecular packing.

Low solubility, however, cannot be attributed solely to planarity. Actual solubility is jointly influenced by: lattice stability; molecular ionization state; lipophilicity; hydrogen-bonding capacity; solid-state polymorphism and solvation capacity.

In a study of ciprofloxacin derivatives, increasing molecular flexibility, disrupting crystal-lattice packing, and simultaneously adjusting lipophilicity improved solubility and membrane permeability within that series. This result represents a general medicinal-chemistry strategy rather than direct evidence for the indazole scaffold.

Therefore, when solubility problems arise in an indazole series, melting point, polymorphism, acid dissociation constant, and lipophilicity should be considered together, rather than simply adding a polar group at an arbitrary position.

For coplanar aromatic structures, possible strategies include:

① reducing the number of continuously conjugated aromatic rings;

② introducing noncoplanarity between adjacent aromatic rings;

③ introducing an ionizable group at a solvent-facing position;

④ replacing part of the aromatic system with a saturated ring or another nonplanar fragment;

⑤ disrupting molecular symmetry and regular crystal packing.

These modifications must be made without disrupting critical protein interactions.

5.4 Hinge Binding Provides Basic Positioning but Cannot Alone Determine Kinase Selectivity

In certain adenosine triphosphate (ATP)-competitive kinase inhibitors, indazole can serve as a hinge-binding group. Its adjacent ring nitrogen atoms can form hydrogen bonds with the kinase hinge backbone, thereby defining the basic position of the ligand in the ATP-binding pocket and providing a foundation for target affinity.

Protein kinases, however, have similar catalytic architectures, and the backbone geometries of the hinge regions of many kinases are also similar. The same indazole hydrogen-bonding unit may therefore form comparable interactions in multiple kinases. The ability to bind the hinge region alone does not demonstrate that a compound is highly selective.

Kinase selectivity mainly depends on whether the complete molecule can exploit structural differences outside the hinge region, for example by:

① contacting nonconserved amino acids in the target kinase;

② matching the size and shape of an adjacent hydrophobic pocket;

③ stabilizing a specific conformation that is more readily adopted by the target kinase;

④ creating unfavorable steric interactions in off-target kinases.

In indazole-containing kinase inhibitors, indazole is commonly used to establish the basic binding orientation, whereas substituents at C3 and C4–C7 can extend into adjacent regions and form additional interactions. The specific function of each position is not fixed and must be determined from cocrystal structures, reliable molecular models, and SAR data.

6. The Iterative Process from the Indazole Core to a Candidate Molecule

The design of indazole-containing drug molecules follows a logical sequence, but in practice it is not a one-directional process completed in a single cycle. Instead, it requires repeated refinement among structural design, chemical synthesis, and property evaluation.

When experimental results are inconsistent with the original hypothesis, earlier stages should be revisited. For example:

① if N-substitution improves permeability but reduces activity, determine whether N–H participates in binding;

② if enzyme activity improves but cellular activity remains unchanged, examine solubility, permeability, and efflux;

③ if the desired N1 product is difficult to prepare, revise the ring-forming strategy or bond-forming mechanism;

④ if broad inhibition of multiple kinases is observed, redesign substituent vectors outside the hinge region;

⑤ if solid-state solubility is insufficient, adjust planarity, ionization state, and crystal packing.

7. From Structure to Developability: Representative Chemicals for Indazole-Scaffold Research

Table 1. Core Scaffold and N-Regioisomer Reference Compounds

Table 2. Halogenated and Borylated Indazole Building Blocks for Coupling Reactions

Table 3. Amino-, Carboxy-, and Formyl-Functionalized Indazole Building Blocks

Table 4. Supporting Reagents for Indazole Ring Formation and N-Regioselective Functionalization

Table 5. Catalytic Systems for Cross-Coupling and Borylation Reactions

Note: The products listed above are representative Aladdin products intended for scientific and formulation research. Additional product specifications, grades, and certificates of analysis may be retrieved from the Aladdin website by searching by product name, CAS number, or item number.

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