Why Bicyclo[1.1.1]pentane (BCP) Has Become an Important Medicinal Chemistry Building Block: Phenyl Ring Replacement, Strain Release, and Functional-Group Interfaces

Introduction

The benzene ring can restrict molecular conformation, control substituent orientation, and participate in target binding through hydrophobic interactions, π–π interactions, or cation–π interactions. It is therefore widely present in drug molecules. However, in some lead compounds, aromatic rings may also be associated with excessive lipophilicity, tight crystal packing, insufficient solubility, or oxidative metabolism.

Bicyclo[1.1.1]pentane (BCP) is a rigid, cage-like carbon framework. A 1,3-disubstituted BCP can maintain substituent exit vectors similar to those of a para-disubstituted benzene ring, while removing the aromatic π system and altering molecular length, shape, surface properties, and metabolic exposure.

The emergence of BCP as an important medicinal chemistry building block is also closely related to advances in its synthetic methods. The unusual central bonding of [1.1.1]propellane enables radicals, nucleophiles, and organometallic reagents to rapidly construct 1,3-disubstituted BCPs. Functional groups such as carboxylic acids, amines, halogens, and organoboron groups allow BCP units to be incorporated into commonly used synthetic routes, including amidation, cross-coupling, and radical reactions.

The medicinal chemistry value of BCP mainly arises from three aspects:

① It can, to a certain extent, replace the spatial-support function of a para-disubstituted benzene ring;

② [1.1.1]Propellane can rapidly form the BCP framework through strain-release reactions;

③ Bridgehead functional groups provide reactive sites for subsequent molecular connection and structural expansion.

1 Structural Basis for Replacing a Para-Substituted Benzene Ring with BCP

1.1 Principal Roles of a Para-Disubstituted Benzene Ring in Drug Molecules

The two substituent bonds of a para-disubstituted benzene ring extend in approximately opposite directions, allowing two molecular fragments to be maintained in relatively fixed positions. In many molecules, the benzene ring primarily serves as a rigid linker and does not necessarily participate directly in aromatic interactions.

Its principal functions include:

① Controlling the relative orientations of two pharmacophores;

② Maintaining the distance between two connected fragments;

③ Reducing the number of conformations available to a flexible linker;

④ Occupying hydrophobic space within a binding site;

⑤ Modulating the electronic properties of substituents through aromatic conjugation.

When the benzene ring mainly provides directional control and rigid spacing, a 1,3-disubstituted BCP has clear value as a potential replacement.

1.2 What BCP Preserves and What It Changes

In a 1,3-disubstituted BCP, the two substituents are attached to the two bridgehead carbons. Their exit vectors are arranged at an angle of approximately 180°, similar to the substituent orientations of a para-disubstituted benzene ring.

However, the two frameworks are not identical in size. The distance between bridgehead carbons C1 and C3 in BCP is approximately 1.85 Å, whereas the distance between the para carbons C1 and C4 in benzene is approximately 2.8 Å. These values represent distances between framework atoms; in an actual molecule, the distance between the two terminal groups also depends on the connecting bonds and substituent structures.[3]

Comparison factor	Para-disubstituted benzene ring	1,3-Disubstituted BCP	Possible effect
Substituent exit vectors	Approximately 180°	Approximately 180°	May preserve the overall linear connection direction
Framework connection distance	Relatively long	Relatively short	The pharmacophores at the two ends may be brought closer together
Framework shape	Planar	Rigid and cage-like	Alters binding conformation and crystal packing
Carbon hybridization	Predominantly sp² carbon	Predominantly sp³ carbon	Increases local three-dimensional character
Aromatic π system	Present	Absent	Cannot preserve π–π or cation–π interactions
Transmission of electronic effects	Can occur through aromatic conjugation	Does not possess the same conjugative mode	May alter the electronic properties of adjacent functional groups
Molecular surface	Relatively flat and highly polarizable	Compact and nonplanar	May alter solvation, lipophilicity, and nonspecific binding

1.3 Structures for Which BCP Replacement Is More Suitable

BCP is more suitable for molecules in which the original benzene ring primarily performs the following roles:

① Serving as a rigid spacer between two pharmacophores;

② Maintaining two substituents in approximately opposite directions;

③ Providing hydrophobic volume without forming critical aromatic interactions;

④ Constituting a structural region potentially susceptible to aromatic or benzylic oxidation;

⑤ Contributing to strong crystal packing or high local planarity.

When the benzene ring directly participates in π–π stacking, cation–π interactions, electronic conjugation, or strict spatial positioning, replacement with BCP may reduce activity. The shorter connection distance introduced by BCP may also cause the pharmacophores at the two ends to deviate from their original binding positions. Evaluation should therefore combine target-structure analysis, conformational analysis, and matched molecular pair studies.

2 How Medicinal Chemistry Studies Validate the Replacement Value of BCP

2.1 Early Studies of BCP as a Bioisostere

In 1996, Pellicciari and co-workers reported (S)-(+)-2-(3′-carboxybicyclo[1.1.1]pentyl)glycine. This compound is a BCP analogue of (S)-(4-carboxyphenyl)glycine and exhibited antagonistic activity against metabotropic glutamate receptor 1 (mGluR1).[1]

This study demonstrated that when the original benzene ring primarily functions as a rigid linker and spatial-positioning element, BCP can position two bridgehead substituents in locations recognizable by the target.

2.2 BCP Replacement in a γ-Secretase Inhibitor

In 2012, Stepan and co-workers replaced the central para-disubstituted fluorobenzene ring in the γ-secretase inhibitor BMS-708,163 with a 1,3-disubstituted BCP.

The resulting BCP analogue displayed the following characteristics:

① Enzyme inhibitory activity similar to that of the original benzene-containing compound;

② Markedly improved aqueous solubility;

③ Increased passive membrane permeability;

④ Approximately fourfold increases in the maximum plasma concentration and the area under the plasma concentration–time curve in mice.[2]

These results indicate that, in this molecule, the aromatic π system of the central benzene ring was not the principal factor required to maintain activity, and that BCP could still hold the structural fragments on both sides in positions accommodated by the target. The framework replacement also altered solvation, lipophilicity, solid-state properties, and membrane-permeation behavior.

Different compounds do not respond identically to BCP replacement. Aqueous solubility depends on hydration capacity, ionization state, and lattice energy; membrane permeability is influenced by lipophilicity, polar surface area, hydrogen bonding, and conformation; metabolic stability also depends on how the entire molecule binds within metabolic enzymes.

2.3 BCP Replacement Requires Simultaneous Evaluation of Activity and Properties

BCP analogues should not be evaluated solely on the basis of enzyme activity or a single solubility measurement. A relatively comprehensive matched molecular pair study generally includes the following parameters:

Evaluation level	Principal parameters
Target activity	Biochemical activity, cellular activity, selectivity
Solution properties	Aqueous solubility, octanol–water partition coefficient, acid dissociation constant
Membrane transport	Passive permeability, efflux ratio
Metabolic behavior	Liver microsomal stability, metabolites, and major metabolic sites
In vivo exposure	Clearance, half-life, bioavailability
Solid-state properties	Melting point, crystal form, and lattice stability

BCP replacement alters multiple interrelated molecular parameters, and its advantages must be confirmed through experimental results obtained for the specific compound.

3 Why [1.1.1]Propellane Can Rapidly Form BCP

3.1 Central Bonding Characteristics of [1.1.1]Propellane

[1.1.1]Propellane and BCP possess the same five-carbon core framework, but [1.1.1]propellane also contains unusual transannular central bonding between its two bridgehead carbons.

A conventional Lewis structure represents this feature as a bridgehead carbon–bridgehead carbon bond. Theoretical calculations indicate that the central region exhibits pronounced σ–π electron delocalization, and its bonding characteristics differ from those of a conventional carbon–carbon σ bond in an alkane.[4]

The reactivity of [1.1.1]propellane is associated with the following factors:

① The three small rings produce substantial framework strain;

② The central region possesses unusual electron delocalization;

③ Addition can form relatively stable outward-directed carbon–carbon or carbon–heteroatom bonds;

④ The resulting BCP radical or organometallic intermediate can continue to react with a second reaction component.

General Process for the Difunctionalization of [1.1.1]Propellane

Reaction stage	Structural change
Step 1	The first reaction component adds to the central region of [1.1.1]propellane
Step 2	The central bonding is redistributed, generating a bridgehead-substituted reactive BCP intermediate
Step 3	The reactive intermediate is trapped by a second reaction component
Result	Formation of a 1,3-difunctionalized BCP

3.2 Radical Strain-Release Reactions

Radical addition is an important method for preparing 1,3-difunctionalized BCPs. Addition of the first radical to [1.1.1]propellane generates a bridgehead-substituted BCP radical, which subsequently forms the second bridgehead bond through atom transfer, hydrogen-atom abstraction, radical coupling, or capture by a transition metal.[3,5]

In the following equations, BCP represents the bicyclo[1.1.1]pentane framework.

General Radical-Addition Process

Step	Reaction equation
Radical addition	R· + [1.1.1]propellane → R—BCP·
Capture by the second component	R—BCP· + X donor or coupling component → R—BCP—X

R· may be generated from the following reagents or precursors:

① Alkyl halides;

② Carboxylic acids and their redox-active derivatives;

③ Nitrogen-containing radical precursors;

④ Sulfur- or fluorine-containing radical precursors;

⑤ Organoboron compounds;

⑥ Other substrates capable of generating carbon-centered radicals.

If the BCP radical is not captured promptly, it may add to another molecule of [1.1.1]propellane, forming oligomers containing multiple BCP units. Radical concentration, substrate ratio, trapping-reagent reactivity, and reactant polarity can all affect product selectivity.

3.3 Addition of Organometallic Reagents

Grignard reagents can undergo nucleophilic-type addition to [1.1.1]propellane, forming an organomagnesium BCP intermediate. This intermediate can react directly with an electrophile or participate in cross-coupling after transmetalation.[3]

BCP Difunctionalization Involving a Grignard Reagent

Step	Reaction equation
Nucleophilic addition	R—MgX + [1.1.1]propellane → R—BCP—MgX
Electrophilic trapping	R—BCP—MgX + E⁺ → R—BCP—E

Organomagnesium BCP intermediates can be connected to the following reaction components:

① Carbon dioxide;

② Aldehydes and ketones;

③ Halogenating reagents;

④ Boron reagents;

⑤ Allylic electrophiles;

⑥ Aryl or heteroaryl coupling partners.

This route is suitable for constructing carbon–carbon bonds between BCP and alkyl, aryl, or heteroaryl groups. Organometallic reagents are sensitive to water, oxygen, and certain electrophilic functional groups, and their specific application must therefore be selected according to the substrate structure and reaction conditions.

4 How Functional-Group Interfaces Determine the Uses of BCP Building Blocks

The BCP framework controls molecular shape and connection direction, whereas the bridgehead functional groups determine which subsequent reactions the building block can undergo.

BCP building blocks with practical synthetic value generally possess the following characteristics:

① Clearly defined reactive sites;

② The ability to be stored and used as solids or as solutions with controllable concentrations;

③ Compatibility with established bond-forming reactions;

④ The ability to modify the two ends selectively or sequentially;

⑤ Synthetic routes that support preparation on the gram scale or larger.

4.1 Carboxylic Acid, Ester, and Amine Interfaces

Carboxylic acids and amines are widely used connecting functional groups in medicinal chemistry and can be used to form amides, ureas, sulfonamides, and other nitrogen-containing structures.

BCP interface	Principal reactions	Principal uses
Carboxylic acid	Amidation, esterification, decarboxylative coupling	Connecting nitrogen- or oxygen-containing fragments
Ester	Hydrolysis, reduction, selective functional-group conversion	Serving as a protected carboxylic acid and synthetic intermediate
Amine	Amidation, urea formation, sulfonylation, reductive amination	Constructing nitrogen-containing termini or linking units
Amino acid or amino ester	Sequential coupling at the amine and carboxylic acid ends	Incorporating BCP into the molecular backbone
Monoprotected diamine	Stepwise acylation, alkylation, or urea formation	Constructing unsymmetrical difunctionalized molecules

BCP-1,3-dicarboxylic acid is an important general intermediate for the preparation of multiple classes of BCP building blocks.

In 2021, Ripenko and co-workers used a flow photochemical reaction between [1.1.1]propellane and 2,3-butanedione to prepare approximately 1 kg of 1,3-diacetyl-BCP within one day. A haloform reaction was then used to obtain BCP-1,3-dicarboxylic acid on a multigram scale. This diacid was further converted into building blocks containing alcohol, amine, amino acid, and trifluoroborate functionalities.[6]

Preparation Route for BCP-1,3-Dicarboxylic Acid

Reactants	Reaction conditions	Product
[1.1.1]Propellane + 2,3-butanedione	Irradiation, continuous flow	1,3-Diacetyl-BCP
1,3-Diacetyl-BCP	Haloform reaction	BCP-1,3-dicarboxylic acid

This route first forms a diketone that can be prepared on a relatively large scale and then uses the carboxylic acid functionalities for multidirectional transformations, providing a common source of BCP acid, amine, and amino acid building blocks.

4.2 Halogen and Organoboron Interfaces

Bridgehead-halogenated BCPs can participate in radical reactions, metal–halogen exchange, and transition-metal-catalyzed coupling. Organoboron BCPs can participate in Suzuki–Miyaura coupling, oxidation, amination, and radical coupling.

BCP interface	Principal transformations
Iodine or bromine	Radical generation, metalation, carboxylation, borylation, and cross-coupling
Boronic ester	Arylation, heteroarylation, oxidation to alcohols, and amination
Trifluoroborate	Photoredox coupling and transition-metal-catalyzed coupling
Redox-active ester	Decarboxylative radical reactions

In 2018, Caputo and co-workers reported the synthesis of 1-halo-3-substituted BCPs. This method employed triethylborane to initiate the atom-transfer radical addition of alkyl halides to [1.1.1]propellane, providing BCP products containing both a carbon substituent and a halogen interface under relatively mild conditions.[5]

In 2024, Ripenko and co-workers further reported a flow photochemical reaction between alkyl iodides and [1.1.1]propellane. Under the reported conditions, the reaction required no additional catalyst, initiator, or additive and could be conducted on milligram, gram, and kilogram scales.[8]

Flow Photochemical Preparation of BCP Iodides

Reactants	Reaction conditions	Product
R—I + [1.1.1]propellane	Irradiation, continuous flow; R is an alkyl group	R—BCP—I

The R group in the product constitutes one predefined bridgehead substituent, while the iodine atom remains as a second reactive site available for further transformation. The resulting BCP iodides can undergo:

① Metal–halogen exchange followed by reaction with an electrophile;

② Photoredox radical reactions;

③ Copper-, nickel-, or other transition-metal-catalyzed coupling;

④ Carboxylation, borylation, reduction, or deuteration.

These halogenated BCPs are suitable for preparing a series of 1,3-disubstituted derivatives from a common intermediate.

4.3 Heterobifunctional and Differentiated Reactive Interfaces

BCPs bearing identical functional groups at both bridgeheads generally require selective monoreaction or protecting-group control. BCPs bearing two different functional groups can connect different molecular fragments in a predetermined sequence.

Common combinations include:

① An amine at one end and a carboxylic acid or ester at the other;

② A halogen at one end and a carboxylic acid or ester at the other;

③ A protected amine at one end and a free amine at the other;

④ An organoboron group at one end and a halogen at the other;

⑤ Leaving groups with different reactivities at the two positions.

An iodinated BCP methyl thianthrenium reagent reported in 2025 contains both a thianthrenium-methyl reactive site and a BCP bridgehead iodine. The two functional groups exhibit different modes of reactivity, allowing nucleophilic substitution to be conducted first at the thianthrenium-methyl position, followed by metalation, radical reaction, or cross-coupling at the bridgehead iodine.[9]

This reagent expands the range of BCP structures containing a methylene linking unit. It is not equivalent to a conventional heterobifunctional 1,3-bridgehead-substituted BCP, but it likewise illustrates the design principle of stepwise modification through differentiated interfaces.

5 How the Method of Preparation Affects the Practical Application of BCP

5.1 Operational Characteristics of [1.1.1]Propellane

[1.1.1]Propellane is highly volatile and is generally stored and used as a solution in an ethereal solvent. Under suitable conditions, its solutions can be stored for a certain period, but larger-scale use still requires attention to the following issues:

① Concentration changes caused by solvent evaporation;

② Transfer and measurement of the reactive intermediate;

③ Control of water and oxygen during low-temperature preparation;

④ Effects of storage time on reaction reproducibility;

⑤ Heat transfer, mixing, and safety management during scale-up.[3]

5.2 On-Demand Generation in Continuous Flow

In 2021, Donnelly and Baumann established a continuous-flow method for preparing [1.1.1]propellane. The resulting solution could be introduced directly into subsequent BCP functionalization reactions without isolation, with a maximum throughput of 8.5 mmol/h, and was used to prepare BCP building blocks on the gram scale.[7]

The principal functions of the continuous-flow approach include:

① Reducing the instantaneous inventory of the volatile intermediate;

② Shortening the interval between generation, transfer, and use;

③ Improving mixing and heat transfer;

④ Improving irradiation uniformity when integrated with subsequent photochemical reactions;

⑤ Facilitating the integration of propellane generation with downstream reactions.

The development of BCP diacids, halogenated derivatives, and reagents with differentiated dual interfaces allows researchers to conduct late-stage structural expansion from common intermediates, eliminating the need to establish a complete framework-synthesis route for every individual BCP analogue.

6 How to Determine Whether BCP Is Suitable for a Specific Drug-Design Project

BCP replacement requires simultaneous consideration of the role of the original benzene ring, changes in framework dimensions, and the available synthetic routes.

Question to be considered	Influence on BCP selection
Does the benzene ring form critical π interactions?	If these interactions are significant, replacement may reduce binding ability
Does the benzene ring mainly serve as a rigid spacer?	If so, BCP has substantial value for comparative evaluation
Can the pharmacophores at the two ends tolerate a shorter distance?	Determines whether spatial positioning can be maintained after replacement
Which property does the project aim to improve?	Determines whether the replacement has a clearly defined experimental objective
Which type of chemical bond is planned?	Determines whether a carboxylic acid, amine, halogen, or organoboron interface should be selected
Is sequential modification of both ends required?	Determines whether a heterobifunctional or protected building block should be selected

When a target–ligand complex structure is unavailable, matched molecular pairs consisting of the benzene-containing compound and its BCP analogue can first be prepared, and the following parameters can be compared simultaneously:

① Biochemical and cellular activity;

② Aqueous solubility and octanol–water partition coefficient;

③ Passive permeability and efflux ratio;

④ Liver microsomal stability and metabolic sites;

⑤ Plasma protein binding and in vivo pharmacokinetics;

⑥ Melting point, crystal form, and other solid-state properties.

6.1 BCP Synthesis Is Expanding Toward Bridge-Substituted Structures

Addition reactions of [1.1.1]propellane are suitable for preparing monosubstituted and 1,3-disubstituted BCPs, but the synthesis of bridge-substituted and highly substituted BCPs is more complex.

Recently developed propellane-free routes can be categorized according to their principal bond-forming mechanisms as follows:

① Intramolecular diradical coupling;

② Carbene-mediated ring expansion;

③ Bond cleavage and ring closure based on two-electron polar processes.

These methods are primarily used to construct 1,2-disubstituted, bridge-substituted, and polysubstituted BCPs that are difficult to access through conventional propellane addition. Nevertheless, 1,3-disubstituted BCPs remain a relatively mature structural class in medicinal chemistry applications and the commercial development of building blocks.[10]

7 Classification and Research Applications of Representative Chemicals Related to BCP Phenyl Ring Replacement, Strain-Release Functionalization, and Synthetic Interfaces

Table 1. Core Precursors, Key Intermediates, and Functionalization Reagents

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Radical reaction initiator	97-94-9	T434625	Triethylborane solution	2.0 M in diethyl ether	Used for alkyl halide radical generation, atom-transfer radical addition, and strain-release functionalization studies of [1.1.1]propellane.
Organoboron interface-forming reagent	73183-34-3	B396365	Bis(pinacolato)diboron	≥99%	Used for the borylation of halogenated or organometallic bicyclo[1.1.1]pentane intermediates, construction of organoboron interfaces, and studies of subsequent carbon–carbon bond formation.
Photochemical addition starting material	431-03-8	B104601	2,3-Butanedione	≥98%	Used in photochemical addition with [1.1.1]propellane to prepare 1,3-diacetylbicyclo[1.1.1]pentane and to study routes to the corresponding dicarboxylic acid.
[1.1.1]Propellane synthetic precursor	98577-44-7	D178603	1,1-Dibromo-2,2-bis(chloromethyl)cyclopropane	≥97%	Used for the preparation and in-line generation of [1.1.1]propellane and for studies of synthetic routes to the bicyclo[1.1.1]pentane framework.
Core strain-release precursor	35634-10-7	O1297208	(1.1.1)Propellane	≥95%	A core precursor for bicyclo[1.1.1]pentane building blocks; used in radical addition, nucleophilic addition, organometallic addition, and 1,3-difunctionalization studies.
Key intermediate in the dicarboxylic acid route	115913-30-9	B166148	1,1′-(Bicyclo[1.1.1]pentane-1,3-diyl)bis(ethanone)	—	Used in haloform oxidation to prepare bicyclo[1.1.1]pentane-1,3-dicarboxylic acid and as a synthetic intermediate for acid, ester, amine, and other difunctionalized building blocks.

Table 2. Carboxylic Acid, Ester, Halogen, Organoboron, and Fluorinated Bicyclo[1.1.1]pentane Building Blocks

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Symmetrical diester building block	115913-32-1	D172214	Dimethyl bicyclo[1.1.1]pentane-1,3-dicarboxylate	≥98%	Used as a protected form of the dicarboxylic acid for selective hydrolysis, preparation of monoacid monoesters, and studies of differentiated functional-group transformations at the two termini.
Boronic ester building block	2152645-07-1	B588131	2-(Bicyclo[1.1.1]pentan-1-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane	≥98%	Used in studies of transition-metal-catalyzed carbon–carbon bond formation, oxidation to alcohols, amination, and radical functionalization.
Trifluoromethyl carboxylic acid building block	224584-18-3	B175798	3-(Trifluoromethyl)bicyclo[1.1.1]pentane-1-carboxylic acid	≥97%	Used in the preparation and amidation of trifluoromethyl-containing bicyclo[1.1.1]pentane derivatives and in studies of lipophilicity, electronic effects, and metabolic behavior.
Heterobifunctional acid–ester building block	83249-10-9	M177545	3-(Methoxycarbonyl)bicyclo[1.1.1]pentane-1-carboxylic acid	≥97%	Contains both a free carboxylic acid and a methyl ester interface; used for stepwise amidation, hydrolysis, and construction of unsymmetrical 1,3-disubstituted building blocks.
Brominated carboxylic acid building block	156329-70-3	B683336	3-Bromobicyclo[1.1.1]pentane-1-carboxylic acid	≥97%	Used in studies of bridgehead-bromine radical transformations, cross-coupling, and amidation at the carboxylic acid terminus.
Iodinated carboxylic acid building block	224584-16-1	I682663	3-Iodobicyclo[1.1.1]pentane-1-carboxylic acid	≥97%	Used in studies of radical generation from bridgehead iodine, borylation, cross-coupling, and connection reactions at the carboxylic acid terminus.
Fluorinated carboxylic acid building block	146038-53-1	F174291	3-Fluorobicyclo[1.1.1]pentane-1-carboxylic acid	≥97%	Used in the preparation of fluorinated bicyclo[1.1.1]pentane amide and ester derivatives and in studies of the effects of fluorine substitution on electronic and physicochemical properties.
Brominated ester building block	83249-14-3	M634622	Methyl 3-bromobicyclo[1.1.1]pentane-1-carboxylate	≥97%	Contains both a bridgehead bromine and a methyl ester interface; used in studies of radical reactions, metalation, cross-coupling, hydrolysis, and reductive transformations.
Monocarboxylate ester building block	106813-54-1	M626745	Methyl bicyclo[1.1.1]pentane-1-carboxylate	≥97%	Used in studies of monosubstituted bicyclo[1.1.1]pentane structures and can be converted through hydrolysis, reduction, and nucleophilic transformations into carboxylic acids, alcohols, and related derivatives.
Trifluoroborate building block	2410559-72-5	P633013	Potassium 1-bicyclo[1.1.1]pentanyl(trifluoro)boranuide	≥97%	Used in studies of photoinduced radical coupling, transition-metal-catalyzed carbon–carbon bond formation, and introduction of bicyclo[1.1.1]pentane fragments.
Hydroxymethyl building block	22287-32-7	B175784	Bicyclo[1.1.1]pentan-1-ylmethanol	≥97%	Used for oxidation to aldehydes or carboxylic acids, etherification, esterification, halogenation, and studies of hydroxymethyl-containing bicyclo[1.1.1]pentane derivatives.
Symmetrical dicarboxylic acid building block	56842-95-6	B176816	Bicyclo[1.1.1]pentane-1,3-dicarboxylic acid	≥97%	A building block with carboxylic acid connection sites at both termini; used in the preparation of diamides, diesters, monoacid monoesters, amino acids, and other 1,3-difunctionalized derivatives.
Monocarboxylic acid building block	22287-28-1	B175783	Bicyclo[1.1.1]pentane-1-carboxylic acid	≥97%	Used in amidation, esterification, decarboxylative radical reactions, and studies of monosubstituted bicyclo[1.1.1]pentane bioisosteres.

Table 3. Amine, Amino Acid, and Protected Bicyclo[1.1.1]pentane Building Blocks

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Bridgehead monoamine building block	22287-35-0	B168724	Bicyclo[1.1.1]pentan-1-amine hydrochloride	≥98%	Used in amidation, sulfonylation, urea formation, reductive amination, and the preparation of bridgehead nitrogen-containing bicyclo[1.1.1]pentane derivatives.
Protected amino acid building block	303752-38-7	T176057	3-[(tert-Butoxycarbonyl)amino]bicyclo[1.1.1]pentane-1-carboxylic acid	≥97%	Contains both a protected amine and a carboxylic acid interface; used for stepwise amidation, deprotection, and construction of unsymmetrical structures connected at both termini.
Protected aminomethyl ester building block	676371-64-5	M177149	Methyl 3-[((tert-butoxycarbonyl)amino)methyl]bicyclo[1.1.1]pentane-1-carboxylate	≥97%	Contains both a protected aminomethyl group and a methyl ester interface; used for side-chain amine connection, ester hydrolysis, and sequential functionalization of the two termini.
Protected bridgehead amine building block	1886967-60-7	T678982	tert-Butyl N-(1-bicyclo[1.1.1]pentanyl)carbamate	≥97%	Used for protection of bridgehead amines, followed by deprotection and amidation, urea formation, sulfonylation, and construction of nitrogen-containing terminal fragments.
Amino-acid-type heterobifunctional building block	406947-32-8	A731534	3-Aminobicyclo[1.1.1]pentane-1-carboxylic acid	≥95%	Contains both amine and carboxylic acid interfaces; used for stepwise construction of amide bonds, peptide-like linkage, and incorporation of bicyclo[1.1.1]pentane into molecular backbones.

Note: The products listed above are representative Aladdin products related to medicinal chemistry and organic synthesis research. Additional product specifications, grades, and COA information can be searched on the Aladdin website using the product name, CAS number, or catalog number.

References

[1] Pellicciari R, Raimondo M, Marinozzi M, Natalini B, Costantino G, Thomsen C. (S)-(+)-2-(3′-Carboxybicyclo[1.1.1]pentyl)-glycine, a structurally new group I metabotropic glutamate receptor antagonist. Journal of Medicinal Chemistry, 1996, 39(15): 2874–2876. DOI: 10.1021/jm960254o.

[2] Stepan A F, Subramanyam C, Efremov I V, et al. Application of the bicyclo[1.1.1]pentane motif as a nonclassical phenyl ring bioisostere in the design of a potent and orally active γ-secretase inhibitor. Journal of Medicinal Chemistry, 2012, 55(7): 3414–3424. DOI: 10.1021/jm300094u.

[3] Shire B R, Anderson E A. Conquering the synthesis and functionalization of bicyclo[1.1.1]pentanes. JACS Au, 2023, 3(6): 1539–1553. DOI: 10.1021/jacsau.3c00014.

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[5] Caputo D F J, Arroniz C, Dürr A B, et al. Synthesis and applications of highly functionalized 1-halo-3-substituted bicyclo[1.1.1]pentanes. Chemical Science, 2018, 9(23): 5295–5300. DOI: 10.1039/C8SC01355A.

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