Why Are Adamantane Compounds Special? From Rigid Cage Structures and Selective Functionalization to Medicinal Chemistry and Functional Materials Applications

Introduction

Adamantane has the molecular formula C₁₀H₁₆ and consists solely of carbon and hydrogen, without aromatic rings, heteroatoms, or unsaturated bonds. In terms of elemental composition, it is simply a saturated hydrocarbon; nevertheless, adamantane and its derivatives have established applications in medicinal chemistry, photoresist materials, low-dielectric polymers, porous materials, and supramolecular hydrogels.[3,8–12]

Adamantane has attracted considerable attention because its molecular structure combines several interrelated features: a rigid three-dimensional cage framework, low conformational freedom, a concentrated hydrophobic volume, and carbon sites that can be functionalized in different spatial directions.[2,3]

The adamantane parent structure lacks strongly polar or highly reactive functional groups. Through halogenation, oxidation, amination, carboxylation, or modification with polymerizable groups, it can be converted into an organic synthesis intermediate, a structural fragment in drug molecules, a polymerizable monomer, or a three-dimensional crosslinking node.

Keywords: adamantane; adamantane compounds; diamondoids; three-dimensional cage structure; bridgehead position; bridge position; selective functionalization; conformational restriction; hydrophobic volume; medicinal chemistry; photoresist materials; low-dielectric polymers; host–guest recognition; porous organic materials.

1 Why Is the Cage Structure of Adamantane Special?

1.1 What Is Adamantane?

Adamantane, systematically named tricyclo[3.3.1.1³,⁷]decane, is a saturated bridged hydrocarbon composed of ten sp³-hybridized carbon atoms. Its carbon connectivity corresponds to a finite fragment of the diamond lattice, and adamantane is therefore classified as a diamondoid.[1]

The term “diamond” in the Chinese name of adamantane describes the spatial connectivity of its carbon atoms; it does not imply that adamantane crystals possess the macroscopic hardness of diamond. In diamond, carbon atoms form a continuous three-dimensional covalent network, whereas solid adamantane consists of discrete molecules held together primarily by intermolecular forces.

The ten carbon atoms in adamantane can be divided into two categories:

Carbon site	Number	Numbering	Structural characteristics
Bridgehead carbon	4	Positions 1, 3, 5, and 7	Tertiary carbon; each carbon is bonded to one hydrogen atom
Bridging methylene carbon	6	Positions 2, 4, 6, 8, 9, and 10	Secondary carbon present as a —CH₂— group

In unsubstituted adamantane, the four bridgehead carbons are equivalent to one another, and the six bridging carbons are likewise equivalent. The bridgehead positions are conventionally numbered 1, 3, 5, and 7 and are oriented approximately toward the four vertices of a tetrahedron. Multifunctionalized adamantanes can use this directional arrangement to construct three-dimensional molecules or polymer networks.[1,2]

1.2 Rigidity Arises from Interlocked Ring Structures

Adamantane can be understood as a cage structure formed by several fused and interlocked six-membered ring units that closely resemble chair conformations. Ordinary cyclohexane can undergo chair–chair conformational inversion, whereas the individual rings in adamantane cannot invert independently. Consequently, the entire carbon framework can undergo only small-amplitude conformational changes.[2]

The rigidity of adamantane is mainly reflected in the following features:

① The carbon framework does not readily undergo large conformational changes.

② The three-dimensional molecular shape is relatively stable.

③ Substituents attached to adamantane have relatively well-defined spatial orientations.

④ Incorporation of adamantane into a polymer can restrict the motion of adjacent chain segments.

Here, “rigidity” refers to restriction of molecular conformation and is not equivalent to the macroscopic hardness of a material. The hardness, toughness, and processability of adamantane-containing materials also depend on molecular weight, linking groups, degree of crosslinking, and other structural units.

1.3 A Cage Structure Does Not Necessarily Mean High Ring Strain

Some cage hydrocarbons contain small rings with bond angles that deviate substantially from ideal values and therefore possess high ring strain. In adamantane, the carbon atoms closely approach the tetrahedral geometry of sp³ carbon, and the six-membered ring units resemble chair conformations. As a result, the overall ring strain is relatively low.[2]

Two concepts should be distinguished:

① Rigidity describes how difficult it is for a molecule to undergo conformational changes.

② Ring strain describes the energy accumulated when chemical bonds deviate from ideal bond angles and torsional conformations.

Adamantane is a cage hydrocarbon with high rigidity but low ring strain. This structural combination allows it to maintain a stable three-dimensional shape without readily releasing strain energy through skeletal bond cleavage, as may occur in some highly strained cage hydrocarbons.[2]

1.4 Hydrophobicity Is Concentrated within a Fixed Three-Dimensional Volume

Adamantane contains only carbon and hydrogen and lacks functional groups capable of forming strong hydrogen bonds with water. The parent molecule is therefore distinctly hydrophobic. Compared with a flexible linear alkyl group, the hydrophobic surface of an adamantyl group is concentrated within a relatively fixed, approximately spherical three-dimensional volume.[3]

This structural feature may influence:

① Molecular partitioning between aqueous and lipid phases.

② Size matching and spatial occupancy between the molecule and a hydrophobic protein-binding pocket or host-molecule cavity.

③ The spatial orientations and distances between adjacent functional groups.

④ Molecular packing in crystals, thin films, or polymers.

⑤ The packing density and free volume of polymer chain segments.

An adamantyl group should therefore not be regarded merely as an alkyl group that increases lipophilicity. It can also alter the overall molecular shape, steric environment, substituent orientation, and mode of molecular packing.[3]

2 How Is Adamantane Converted into Functionalized Derivatives?

The parent adamantane structure lacks functional groups that can directly undergo salt formation, amidation, crosslinking, or polymerization. To convert it into a drug or materials-related structure, halogen, hydroxy, carbonyl, carboxyl, amino, or polymerizable groups are generally first introduced at bridgehead or bridging positions.

In the reaction equations below, Ad denotes an adamantyl group. When substitution at positions 1 and 2 must be distinguished, the position is stated explicitly.

2.1 Bridgehead and Bridging Positions Have Different Reactivities

Bridgehead carbons are tertiary carbons, whereas bridging carbons are secondary carbons. The two types of carbon–hydrogen bonds differ in their spatial environments, the properties of the corresponding radical intermediates, and their behavior in ionic reactions.

Many radical substitution and oxidation reactions generate bridgehead-substituted products, but bridgehead selectivity is not a universal rule for all adamantane reactions. The actual product distribution is also affected by:

① The electrophilic or nucleophilic character of the hydrogen-atom-transfer reagent.

② The steric size of the catalyst and radical reagent.

③ The inductive effects and steric hindrance produced by existing substituents.

④ Solvent, temperature, and reaction concentration.

⑤ Whether the reaction proceeds through a radical, carbocationic, or organometallic intermediate.

Many recently developed methods for direct carbon–hydrogen bond functionalization first generate an adamantyl radical through hydrogen-atom transfer and then use this radical to construct a new carbon–carbon bond.[2]

General hydrogen-atom-transfer process:

Ad—H + X· → Ad· + X—H

Here, X· represents a radical capable of abstracting a hydrogen atom from an adamantane carbon–hydrogen bond.

Addition of an adamantyl radical to an alkene:

Ad· + CH₂=CH—Z → Ad—CH₂—CH·—Z

The resulting carbon-centered radical must subsequently undergo hydrogen abstraction, atom transfer, single-electron transfer, or another termination process to form a stable product. The subsequent steps vary with the specific reaction system.[2]

2.2 Halogenated Adamantanes Provide Interfaces for Substitution Reactions

1-Bromoadamantane and 1-chloroadamantane are common bridgehead intermediates. In suitable polar protic solvents or under other conditions that promote ionization, 1-haloadamantanes can undergo solvolysis or nucleophilic substitution reactions with pronounced carbocationic character.

Using hydrolysis as an example, the overall reaction can be summarized as:

1-Bromoadamantane + H₂O → 1-Adamantanol + HBr

This equation expresses the stoichiometric relationship between the reactants and major products. It does not indicate that the reaction proceeds in a single step through a free 1-adamantyl carbocation. The actual process may involve ion pairs, solvent-stabilized intermediates, and multiple proton-transfer steps.[2]

Under radical conditions, halogenated adamantanes can also serve as precursors to adamantyl radicals for the formation of carbon–carbon or carbon–heteroatom bonds.

2.3 Alcohols and Ketones Provide Access to Esterification, Oxidation, and Carbonyl Reactions

Common oxygen-containing adamantane derivatives include 1-adamantanol, 2-adamantanol, and 2-adamantanone. Although all contain oxygen, the hydroxy or carbonyl groups occupy different positions, resulting in different reactivities.

2.3.1 1-Adamantanol

1-Adamantanol is a tertiary alcohol that can undergo esterification and etherification and can also be converted into a leaving group.

An acyl chloride esterification using triethylamine as an acid scavenger can be represented as:

1-Ad—OH + RCOCl + Et₃N → RCOO—Ad + Et₃NHCl

Here, Et₃N is triethylamine, and RCOO—Ad is an adamantyl ester.

Because the carbon bearing the hydroxy group in 1-adamantanol has no attached hydrogen atom, it cannot be directly converted into 2-adamantanone through the conventional dehydrogenative oxidation pathway of a secondary alcohol. Other reactions may occur under strongly oxidizing conditions, but these should not be generalized as conventional “oxidation of an alcohol to a ketone.”

2.3.2 2-Adamantanol and 2-Adamantanone

2-Adamantanol is a secondary alcohol and can be oxidized to 2-adamantanone:

2-Adamantanol + [O] → 2-Adamantanone + H₂O

Here, [O] represents an oxidant that provides one oxidation equivalent and does not refer to any specific reagent.

2-Adamantanone exhibits typical carbonyl reactivity and can undergo:

① Reduction to form 2-adamantanol.

② Reaction with hydroxylamine to form an oxime.

③ Nucleophilic addition of an organometallic reagent to the carbonyl group.

④ Reductive amination with a primary or secondary amine.

Reductive amination generally proceeds through initial formation of an imine or iminium intermediate, followed by reduction. The form of the intermediate and the reduction pathway depend on the type of amine, the acid–base conditions of the solution, and the reducing agent.

2.4 Carboxylic Acids and Amines Provide Interfaces for Amidation and Molecular Conjugation

Adamantanecarboxylic acids can be used to prepare esters, amides, and other carboxylic acid derivatives. Direct mixing of a carboxylic acid and an amine generally first produces an acid–base salt. Efficient amide formation usually requires activation of the carboxyl group or conversion of the carboxylic acid into an acid chloride, active ester, or related derivative.

Using adamantanecarbonyl chloride as an example:

Ad—COCl + H₂N—R + Et₃N → Ad—CONH—R + Et₃NHCl

Here, Ad—COCl represents adamantanecarbonyl chloride, and Ad—CONH—R represents the corresponding amide.

When a carbodiimide, uronium salt, or another coupling reagent is used, the stepwise process can be represented as:

Ad—COOH → Activated carboxylic acid intermediate

Activated carboxylic acid intermediate + H₂N—R → Ad—CONH—R

The amino group in an adamantylamine can also:

① Form a salt with an acid.

② Form an amide with an activated carboxylic acid derivative.

③ React with an isocyanate to form a urea.

④ Participate in alkylation or reductive amination.

The adamantane cage provides hydrophobic volume and spatial shape, whereas the amino group provides basicity, ionizability, and further reactivity. The properties of nitrogen-containing adamantanes arise from the combined contribution of both components.

2.5 Polymerizable Derivatives Introduce Adamantane into Polymers

Adamantyl acrylates and adamantyl methacrylates can undergo radical addition polymerization to introduce adamantane into polymer side chains.

Using adamantyl methacrylate as an example:

n CH₂=C(CH₃)—COOAd → [—CH₂—C(CH₃)(COOAd)—]ₙ

During polymerization, the carbon–carbon double bond of the monomer is converted into carbon–carbon single bonds in the polymer backbone, while the adamantyl group remains in the side chain. The polymer backbone provides the fundamental film-forming and mechanical structure, whereas the adamantyl side group contributes to the regulation of chain-segment motion, hydrophobicity, and molecular packing.

2.6 The Number of Functional Groups Determines the Structural Role of Adamantane

Form of functionalization	Primary structural role	Representative uses
Monofunctionalization	Serves as an end group, drug fragment, or single recognition group	Drug molecules, surface modification, host–guest recognition
Difunctionalization	Serves as a linker, chain extender, or crosslinker	Linear polymers, crosslinked networks
Multifunctionalization	Serves as a three-dimensional network node	Porous materials, highly crosslinked resins, multivalent assembly

When reactive groups are attached at positions 1, 3, 5, and 7, adamantane can construct a three-dimensional network along approximately tetrahedral directions. In this case, the principal role of adamantane changes from terminal molecular modification to control of network connection directions and topology.

3 How Does Functionalized Adamantane Alter Molecular Properties?

Functional groups determine which chemical reactions a molecule can undergo, whereas the adamantane cage determines the three-dimensional environment in which those functional groups are positioned. Together, these two components control the final properties.

3.1 Conformational Restriction Improves the Predictability of Three-Dimensional Shape

Flexible molecules generally exist as multiple interconverting conformations. When binding to a protein or entering an ordered material structure, only some conformations may possess the appropriate spatial arrangement.

After introduction of adamantane, the number of rotatable degrees of freedom in a molecule may decrease, and the relative orientations of substituents may become constrained. If the preorganized conformation resembles the target binding conformation, the loss of conformational freedom upon binding may be reduced. If the fixed orientation does not match the binding site, however, the rigid structure may hinder effective binding.[3]

The structural role of adamantane is therefore to restrict conformation and improve the predictability of three-dimensional shape, rather than to universally enhance molecular binding to a target.

3.2 Hydrophobic Volume Alters Solubility and Partitioning Behavior

Introduction of an adamantyl group generally increases the hydrophobic surface area and tendency of a molecule to partition into lipid phases, although the actual effect also depends on other polar and ionizable groups in the molecule.[3,4]

An adamantyl group may:

① Occupy a hydrophobic binding pocket in a protein.

② Alter molecular partitioning among aqueous phases, lipid membranes, and hydrophobic materials.

③ Change the extent to which adjacent functional groups are exposed to water or reactive reagents.

④ Alter the exposure of metabolically labile sites and the tendency toward nonspecific binding.

At the same time, an adamantyl group may also introduce the following limitations:

① Reduced aqueous solubility.

② Increased tendency toward molecular aggregation.

③ Increased nonspecific protein binding.

④ Greater difficulty in formulation development.

⑤ Altered in vivo distribution and clearance behavior.

Adamantyl groups in drug molecules are therefore often combined with amines, carboxylic acids, hydroxy groups, or other polar structures. The polar component provides ionization, hydrogen bonding, or specific recognition, whereas the adamantane component provides hydrophobic volume and three-dimensional support.[3,4]

3.3 Steric Hindrance Alters the Accessibility of Reaction Centers

Adamantyl groups are bulky and cannot substantially change shape through rotation in the manner of flexible alkyl groups. They can therefore partially shield adjacent reaction centers and alter the ease with which reagents, enzymes, or other molecules approach those sites.

This steric effect may be used to:

① Regulate the regioselectivity or stereoselectivity of chemical reactions.

② Reduce excessive proximity and aggregation between chromophores.

③ Alter contacts between polymer chain segments.

④ Regulate the exposure of active sites or metabolically labile chemical bonds.

Steric hindrance may also reduce the rate of polymerization, coupling, or crosslinking. The length of the linker between adamantane and the reaction center, the substitution position, and the functional-group density must therefore be selected according to the specific system.

3.4 Rigidity and Volume Jointly Affect Polymer Chain-Segment Motion

Introduction of adamantane into a polymer generally produces two interrelated effects that may act in different directions.

First, the rigid cage structure can restrict chain-segment motion and may increase the glass transition temperature (Tg) and high-temperature dimensional stability.

Second, the bulky, nonplanar structure may prevent tight packing of polymer chains and increase free volume, thereby affecting density, water absorption, gas transport, and dielectric properties.

Structural change	Possible effect
Restricted chain-segment motion	Increased Tg and reduced deformation at elevated temperatures
Disruption of tight interchain packing	Changes in free volume and density
Multisite crosslinking	Increased solvent resistance and dimensional stability
Excessive adamantane content or crosslink density	Reduced flexibility, solubility, and processability

These properties cannot be determined by adamantane alone. For example, in polymers containing benzocyclobutene (BCB) groups, adamantane provides rigidity and spatial volume, BCB groups provide thermal crosslinking, and fluorinated aromatic structures also influence polarizability and water absorption.[10]

3.5 Host–Guest Recognition Provides Dynamic Reversibility

The size and hydrophobic surface of an adamantyl group are well matched to the hydrophobic cavity of β-cyclodextrin (β-CD). When an adamantane guest enters the β-CD cavity, exposure of the hydrophobic surface to water is reduced, while van der Waals contacts and hydrophobic interactions are established.[11]

The inclusion equilibrium can be represented as:

β-CD + Ad—R ⇌ β-CD⊃Ad—R

Here, β-CD is the host, Ad—R is the adamantane guest, and the symbol “⊃” indicates that the guest is included within the host cavity. This equation represents a noncovalent binding equilibrium rather than a chemical reaction that forms a new covalent bond.

If one polymer is functionalized with adamantyl groups and another with β-CD, mixing the two can produce a physically crosslinked network through multiple host–guest complexation sites. Under an external force, some complexation sites dissociate; when the force is reduced, the host–guest structures can re-form.[11]

4 How Is the Adamantane Structure Translated into Representative Functions?

4.1 Medicinal Chemistry: Adamantane Is a Three-Dimensional Structural Fragment, Not a Fixed Pharmacophore

Adamantane-containing drugs can act on different targets, demonstrating that adamantane itself does not correspond to a single pharmacological mechanism. It primarily provides hydrophobic volume, spatial occupancy, and conformational restriction. The complete pharmacological effect is jointly determined by adamantane, polar functional groups, aromatic structures, and other pharmacophoric components.[3,4]

4.1.1 Amantadine

Amantadine consists of an amino group attached to a bridgehead position of adamantane. The amino group provides basicity and salt-forming ability, whereas the adamantane cage provides hydrophobic volume.

Amantadine is used for symptoms associated with Parkinson’s disease and for certain drug-induced extrapyramidal reactions. It can also inhibit the M2 ion channel of influenza A viruses. However, because circulating seasonal influenza A viruses show widespread resistance to this class of drugs, the U.S. Centers for Disease Control and Prevention currently does not recommend amantadine or rimantadine for the treatment or prevention of seasonal influenza.[4,7]

4.1.2 Memantine

Memantine is 1-amino-3,5-dimethyladamantane. It is a low- to moderate-affinity, open-channel antagonist of the N-methyl-D-aspartate (NMDA) receptor and is used to treat moderate-to-severe dementia of the Alzheimer’s type.[5]

The adamantane cage and methyl groups of memantine together form a hydrophobic three-dimensional framework, while the amino group can be protonated under physiological conditions. Its activity arises from the charge state, spatial volume, and channel-binding kinetics of the complete molecule and cannot be attributed solely to the adamantane structure.

4.1.3 Adapalene

Adapalene is a topical retinoid containing an adamantyl group and is used for the topical treatment of acne vulgaris.[6]

Pharmacological studies indicate that adapalene can regulate processes associated with cellular differentiation, keratinization, and inflammation, although the precise relationship between these findings and its mechanism of action in acne treatment has not been fully established. The adamantyl group forms part of its rigid, hydrophobic structure, but the pharmacological activity of adapalene should not be attributed to the adamantyl group alone.[6]

4.2 Photoresist Materials: Using Alicyclic Structures, Rigidity, and Solubility Changes

Deep-ultraviolet lithography (DUVL), represented by exposure at 193 nm, requires photoresists to combine transparency at the exposure wavelength, film-forming ability, development contrast, and resistance to dry etching.

Aromatic resins may exhibit strong absorption near 193 nm. Early studies of 193 nm photoresists therefore employed acrylic or methacrylic backbones and introduced alicyclic structures such as adamantane to balance optical transparency, thermal properties, and etch resistance.[8]

In chemically amplified positive-tone photoresists, exposure generates an acid, which catalyzes deprotection of protecting groups and increases the polarity and alkaline solubility of the exposed regions. Adamantane or methyladamantane structures may simultaneously serve the following functions:

① Form part of an acid-labile ester structure and participate in the change in solubility before and after exposure.

② Provide a rigid alicyclic structure that regulates Tg and pattern stability.

③ Increase the cyclic carbon content of the resin and contribute to regulation of dry-etch resistance.

④ Alter resin hydrophobicity, film-forming properties, and compatibility with other photoresist components.

Early studies showed that 2-methyl-2-adamantyl esters could serve as acid-labile protecting structures in chemically amplified 193 nm photoresists.[8] A 2024 study incorporated adamantane, methyladamantane, hydroxy groups, and a small proportion of crosslinking units into acrylic resins for deep-ultraviolet lithography and thermal nanoimprint lithography.[9]

The performance of these materials arises from the combined effects of multiple structural units. Excessive adamantane content may also lead to poor resin solubility, film embrittlement, altered development behavior, or reduced compatibility with other components.

4.3 Low-Dielectric Polymers: Combining Low Polarity, Free Volume, and Crosslinking

High-frequency electronic materials require control over molecular polarization and dielectric energy loss under an electric field. Adamantane consists of a low-polarity hydrocarbon framework, and its bulky, nonplanar volume may also hinder tight packing of polymer chains.

In the design of low-dielectric polymers, adamantane may serve the following functions:

① Its low-polarity hydrocarbon framework reduces some sources of dipolar polarization.

② Its bulky, nonplanar structure regulates interchain packing and free volume.

③ Its rigid structure restricts chain-segment motion.

④ When combined with thermally crosslinkable groups, it improves the high-temperature dimensional stability of the network.

A polymer reported in 2024 that contained adamantane linkers and BCB crosslinking groups exhibited low dielectric properties at 10 GHz together with high thermal stability.[10]

However, dielectric performance is also affected by the polarizability of linking groups, fluorinated structures, residual polar groups, water absorption, crosslinking uniformity, and pore structure. The presence of adamantane alone therefore does not establish that a material will have low dielectric properties.

4.4 Supramolecular Hydrogels: Using Reversible Host–Guest Crosslinking

In supramolecular hydrogels, adamantane commonly acts as the guest and β-CD as the host. When the two are separately attached to polymers such as hyaluronic acid, mixing the polymers allows a physical network to form through host–guest inclusion.[11]

When the network is subjected to shear, some host–guest junctions dissociate and the material viscosity decreases. When the shear force is reduced, adamantane re-enters the β-CD cavity and the network gradually recovers. This process provides the molecular basis for shear-thinning and self-healing behavior.[11]

Such systems can be used for:

① Injectable materials.

② Local delivery of drugs or cells.

③ Bioinks for three-dimensional printing.

④ Dynamic cell-culture matrices.

In these systems, adamantane does not directly provide biological activity. Instead, it provides a molecular recognition site capable of repeated association and dissociation. The strength, degradation rate, and biocompatibility of the material remain dependent on the polymer backbone, degree of substitution, polymer concentration, and additional crosslinking methods.

4.5 Porous Organic Materials: Constructing Pore Structures through Multidirectional Connectivity

Multifunctionalized adamantanes can serve as crosslinking nodes in covalent networks. Their rigid, nonplanar geometry can restrict excessive rearrangement of network segments and help form or preserve microporous and nanoporous structures.

A 2024 study used a Friedel–Crafts reaction between 1,3-dibromoadamantane and hexaphenylbenzene to prepare a nanoporous organic polymer and evaluated its adsorption and separation performance toward benzene and cyclohexane.[12]

In this system, adamantane primarily serves the following functions:

① Providing a rigid alicyclic crosslinking unit.

② Fixing the spatial orientations of the linking groups.

③ Regulating the aliphatic chemical environment within the pores.

④ Restricting excessively dense packing of the network.

The adsorption capacity and selectivity of the material also depend on pore size, specific surface area, aromaticity of the pore walls, crosslink density, and the size and interaction characteristics of the target molecules. The presence of adamantane nodes does not inherently confer a particular adsorption selectivity.

5 Design Logic and Research Priorities of Adamantane Compounds

5.1 Selecting the Form of Functionalization According to the Target Function

Research objective	Adamantane derivatives of interest	Basis for selection
Bridgehead substitution and fundamental reaction studies	Adamantane and 1-haloadamantanes	Applicable to studies of radical reactions, ionic substitution, and carbon–carbon bond formation
Synthesis of carbonyl and nitrogen-containing derivatives	2-Adamantanol and 2-adamantanone	Can undergo oxidation, reduction, nucleophilic addition, and reductive amination
Ester and amide conjugation	Adamantanols, adamantanecarboxylic acids, and adamantylamines	Facilitate conjugation to pharmacophores, polymers, or surface groups
Side-chain polymers	Adamantyl acrylates and adamantyl methacrylates	Enable the introduction of rigid side groups through radical polymerization
Covalent crosslinking and porous networks	Difunctional and multifunctional adamantanes	Can form permanent multidirectional connections
Dynamic supramolecular networks	Monofunctional adamantane guests	Can form reversible inclusion complexes with cyclodextrins

When selecting an adamantane derivative, the role of adamantane in the target structure should first be defined: whether it is intended to provide hydrophobic occupancy, fix the orientation of functional groups, provide a reactive interface, or construct a multidirectional network. Different roles require different substitution positions, numbers of functional groups, and modes of connection.

5.2 Structural Functions and Performance Limitations Should Be Evaluated Together

Structural feature	Potential contribution	Potential limitation
Rigid carbon cage	Restricts conformation and improves dimensional stability	Reduced flexibility and conformational adaptability
Concentrated hydrophobic volume	Enhances hydrophobic interactions and partitioning into lipid phases	Reduced aqueous solubility and increased aggregation or nonspecific binding
Substantial steric bulk	Regulates exposure of reaction centers and molecular packing	Reduced reaction rates, crosslinking efficiency, or access to biological targets
Multidirectional functionalization	Constructs crosslinked networks and pore structures	Greater difficulty in controlling synthetic selectivity and network uniformity
Host–guest recognition	Forms dynamically reversible networks	Mechanical strength and stability are affected by the binding equilibrium

5.3 Core Issues in Current Research

In recent years, research on adamantane compounds has mainly focused on the following areas:

① Improving the positional selectivity of carbon–hydrogen bond functionalization.

Radical, photochemical, or catalytic methods are used to reduce the need for preformed halogenated and oxygen-containing intermediates and to control the proportions of bridgehead, bridging, monosubstituted, and multisubstituted products.[2]

② Transforming adamantane from a terminal hydrophobic group into a multisubstituted three-dimensional drug scaffold.

Asymmetric substitution and multidirectional connection are used to regulate the spatial relationships among multiple pharmacophoric groups.[3]

③ Balancing transparency, solubility, and etch resistance in photoresist materials.

Adamantane must be designed in combination with acid-labile groups, polar monomers, and crosslinking units rather than simply being incorporated at a higher content.[8,9]

④ Balancing free volume, crosslinking, and water absorption in low-dielectric materials.

A larger free volume can help reduce the density of polarizable groups per unit volume, whereas excessive porosity or residual polar groups may increase water absorption and dielectric loss.[10]

⑤ Distinguishing permanent covalent networks from dynamic supramolecular networks.

Multifunctional adamantanes can serve as connection nodes in permanently crosslinked and porous networks. A single adamantane guest group can form a dynamically reversible inclusion complex with cyclodextrin, whereas macroscopic physical networks generally depend on multivalent host–guest interactions along polymer chains.[11,12]

The common issue underlying these research directions is not whether adamantane can impart a particular function, but how its structural effects can be controlled within a range appropriate for a specific application through the substitution position, number of functional groups, linking groups, and overall molecular composition.

6 Classification and Research Applications of Representative Chemicals Related to Adamantane Structures, Functionalization, and Drug and Materials Research

Note: The products listed below are primarily used in studies of adamantane structures, functionalization reactions, pharmaceutical analysis, polymerization and crosslinking, host–guest recognition, and material properties. Some applications are summarized on the basis of product structures and related research routes and do not indicate that every product has been directly validated in the specific drug or materials systems discussed in this article.

Table 1. Adamantane Parent Structures, Halogenated Derivatives, and Oxygen-Containing Functionalization Intermediates

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or purity	Product features and applications
Basic adamantane parent structure	281-23-2	A105219	Adamantane	≥99%	Used in studies of rigid three-dimensional cage frameworks, bridgehead and bridging-position reactivity, carbon–hydrogen bond functionalization, and structure–property relationships.
Higher diamondoid	2292-79-7	D155205	Diamantane	≥97%	Used in studies of higher diamondoid frameworks, cage-size effects, carbon–hydrogen bond selectivity, and nanocarbon molecules.
Bridgehead chlorinated intermediate	935-56-8	C153451	1-Chloroadamantane	≥98% (GC)	Used in bridgehead nucleophilic substitution, solvolysis, carbocation reactions, and the synthesis of oxygen- and nitrogen-containing derivatives.
Bridgehead brominated intermediate	768-90-1	B120308	1-Bromoadamantane	≥97% (GC)	Used in bridgehead substitution, radical generation, and the construction of carbon–carbon and carbon–heteroatom bonds.
Bridgehead iodinated intermediate	768-93-4	I472626	1-Iodoadamantane	≥95%	Used as a precursor to adamantyl radicals and in studies comparing halogen leaving ability and bridgehead functional-group interconversion.
Two-site halogenated intermediate	876-53-9	D155459	1,3-Dibromoadamantane	≥97% (GC)	Used in 1,3-disubstitution, the preparation of difunctional adamantanes, crosslinking units, and porous organic networks.
Four-site halogenated network precursor	7314-86-5	T304500	1,3,5,7-Tetrabromoadamantane	≥95%	Used in functionalization of all four bridgehead positions and in studies of tetrahedral connection nodes, three-dimensional crosslinked networks, and porous materials.
Bridgehead tertiary alcohol	768-95-6	A118555	1-Adamantanol	≥99%	Used in the synthesis of adamantyl esters, ethers, and acid-labile derivatives and in studies of bridgehead tertiary-alcohol reactivity.
Hydroxymethyl linking intermediate	770-71-8	A118554	1-Adamantanemethanol	≥99%	Used to introduce hydroxymethyl linkers and in esterification, etherification, oxidation, and polymer side-group modification.
Bridging-position secondary alcohol	700-57-2	A118553	2-Adamantanol	≥98%	Used in bridging-position functionalization, secondary-alcohol oxidation, preparation of 2-adamantanone, and studies of 2-substituted structures.
2-Methyl-substituted tertiary alcohol	702-98-7	M121786	2-Methyl-2-adamantanol	≥99%	Used in the synthesis of acid-labile 2-methyl-2-adamantyl esters, chemically amplified photoresist monomers, and deprotection studies.
2-Ethyl-substituted tertiary alcohol	14648-57-8	E121785	2-Ethyl-2-adamantanol	≥98%	Used in the synthesis of acid-labile 2-ethyl-2-adamantyl esters, photoresist-resin monomers, and hydrophobicity-regulating structures.
Bridging-position ketone	700-58-3	A107048	2-Adamantanone	≥98%	Used in carbonyl nucleophilic addition, reduction, oxime formation, reductive amination, and preparation of nitrogen-containing derivatives substituted at the 2-position.
Difunctional diol	5001-18-3	A121783	1,3-Adamantanediol	≥99%	Used in the synthesis of difunctional esters, ethers, acrylates, and methacrylates and in the construction of crosslinked networks.

Table 2. Adamantanecarboxylic Acids and Nitrogen- and Sulfur-Containing Linking Reagents

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or purity	Product features and applications
Carboxylic-acid linking intermediate	828-51-3	A107246	1-Adamantanecarboxylic acid	≥98%	Used in the synthesis of adamantyl esters, amides, active esters, and linking structures for drugs and materials.
Carboxylic acid with an extended linker	4942-47-6	A111906	1-Adamantaneacetic acid	≥98%	Used to introduce a methylene carboxylic-acid linker and in amide coupling, esterification, and regulation of intermolecular spacing.
Difunctional dicarboxylic acid	39269-10-8	A151180	Adamantane-1,3-dicarboxylic acid	≥97% (GC)	Used in bidirectional amidation, construction of polyesters and polyamides, coordination linking, and crosslinked-material research.
Tetrafunctional carboxylic-acid network node	100884-80-8	T302055	Adamantane-1,3,5,7-tetracarboxylic acid	≥98%	Used in four-directional carboxylate coordination and in the construction of three-dimensional connection nodes, coordination networks, and porous materials.
Activated carboxylic-acid intermediate	2094-72-6	A117233	1-Adamantanecarbonyl chloride	≥97%	Used in adamantanecarboxylic-acid activation, amide and ester formation, and connection of drug fragments to polymer side groups.
Basic aminoadamantane	768-94-5	A136871	1-Adamantylamine	Moligand™, ≥98%	Used in studies of nitrogen-containing adamantane derivatives, amides, ureas, salt forms, and medicinal chemistry.
Bifunctional amino alcohol intermediate	702-82-9	A121784	3-Amino-1-adamantanol	≥98%	Used in selective modification of amino and hydroxy groups, adamantane-containing drug intermediates, and construction of bifunctional molecules.
Isocyanate linking reagent	4411-25-0	A151185	1-Adamantyl isocyanate	≥98%	Used in the synthesis of adamantane-containing ureas, carbamates, polyurethane side groups, and biomolecular conjugates.
Isothiocyanate linking reagent	4411-26-1	A170371	1-Adamantyl isothiocyanate	≥98%	Used in the synthesis of adamantane-containing thioureas, amine labeling, molecular conjugation, and biologically active derivatives.
Sulfur-containing surface-modification reagent	34301-54-7	A169723	1-Adamantanethiol	≥95%	Used in thioether construction, thiol addition, gold-surface modification, and adamantane-based host–guest interfacial assembly.

Table 3. Adamantane Monomers for Polymerization, Crosslinking, and Photoresist Materials

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or purity	Product features and applications
Monofunctional acrylate monomer	121601-93-2	A302637	Adamantan-1-yl acrylate (stabilized with BHT)	≥99%	Used in adamantane-containing acrylic polymers and in studies of rigid side chains, film-forming properties, and chain-segment motion.
Hydroxy-containing acrylate monomer	216581-76-9	A151188	1-Acryloyloxy-3-hydroxyadamantane	≥98% (GC)	Used in hydroxy-functional adamantane-containing polymers, polarity regulation, crosslinking reactions, and photoresist-resin research.
Monofunctional methacrylate monomer	16887-36-8	A404639	Adamantan-1-yl methacrylate (stabilized with MEHQ)	≥98%	Used in methacrylic polymers with rigid side chains and in studies of glass transition, film formation, and low-polarity materials.
Hydroxy-containing methacrylate monomer	115372-36-6	H157098	3-Hydroxy-1-methacryloyloxyadamantane	≥98% (GC)	Used in hydroxy-functional methacrylic resins and in studies of polarity and adhesion regulation, crosslinking, and photoresist materials.
2-Ethyl acid-labile photoresist monomer	209982-56-9	E156351	2-Ethyl-2-adamantyl methacrylate	≥97%, stabilized	Used in chemically amplified positive-tone photoresists and in studies of acid-catalyzed deprotection and changes in polarity and alkaline solubility in exposed regions.
2-Methyl acid-labile photoresist monomer	177080-67-0	M158412	2-Methyl-2-adamantyl methacrylate	≥97% (GC), stabilized with MEHQ	Used in chemically amplified 193 nm photoresists and in studies of acid-labile protecting units and changes in solubility during development.
Difunctional acrylate crosslinking monomer	81665-82-9	P953907	1,3-Adamantanediol diacrylate	≥95%	Used in radical crosslinking, ultraviolet curing, highly crosslinked resins, and studies of network rigidity and solvent resistance.
Difunctional methacrylate crosslinking monomer	122066-43-7	A735920	Adamantane-1,3-diyl bis(2-methylacrylate)	≥95%	Used in methacrylic crosslinked networks, lightly crosslinked resins, dimensional-stability studies, and functional coatings.

Table 4. Adamantane-Containing Active Pharmaceutical Molecules and Host–Guest Research Materials

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or purity	Product features and applications
Adamantane-containing antidiabetic active pharmaceutical molecule	274901-16-5	V125454	Vildagliptin (LAF-237)	Moligand™, ≥99%	Used in studies of adamantane-containing dipeptidyl peptidase-4 inhibitor structures, enzyme-inhibition mechanisms, and antidiabetic-drug analysis.
Amantadine pharmaceutical salt	665-66-7	A109736	Amantadine hydrochloride	≥99% (T)	Used in studies of amantadine salt forms, neuropharmacology, M2 ion-channel inhibition, and pharmaceutical quality analysis.
Adamantane-containing retinoid active pharmaceutical molecule	106685-40-9	A120006	Adapalene	Moligand™, ≥98%	Used in studies of adamantane-containing retinoid structures, retinoic-acid-receptor-related effects, regulation of keratinization, and pharmaceutical analysis.
Rimantadine-type pharmaceutical salt	1501-84-4	A118559	1-(1-Adamantyl)ethylamine hydrochloride	≥99%	Used in studies of rimantadine structures, M2 ion-channel inhibitors, chiral adamantane-containing amines, and antiviral-drug analysis.
Memantine-type pharmaceutical salt	41100-52-1	M107928	Memantine hydrochloride	≥98%	Used in studies of amino-substituted adamantane structures, N-methyl-D-aspartate receptor-channel modulation, and pharmaceutical analysis.
Tromantadine-type pharmaceutical salt	41544-24-5	T648757	Tromantadine hydrochloride	—	Used in studies of adamantane-containing antiviral amide structures, herpes simplex virus entry, and pharmaceutical analysis.
Cyclodextrin host molecule for host–guest studies	7585-39-9	C104384	β-Cyclodextrin	≥98%	Used in adamantane-guest inclusion, binding-equilibrium studies, dynamic crosslinking, supramolecular hydrogels, and delivery systems.
Cucurbituril host molecule for host–guest studies	259886-50-5	C395771	Cucurbit[7]uril hydrate	≥95%	Used in host–guest recognition of adamantane and its cationic derivatives, binding-affinity comparisons, competitive binding, supramolecular assembly, and reversible functional materials.

Note: The products listed above are representative Aladdin research products. Additional product specifications, grades, and certificate of analysis information can be retrieved from the Aladdin website using the product name, CAS number, or catalog number.

References

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[9] Yang Z. H., Zhao J., Cai Y. J., et al. Synthesis of micro-crosslinked adamantane-containing matrix resins designed for deep-UV lithography resists and their application in nanoimprint lithography. Nanoscale, 2024, 16(24): 11651–11662. DOI: 10.1039/D4NR00844H.

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