Heterobicyclo[n.1.1]alkanes: Structural Features and Synthesis, Medicinal Chemistry Value, and Representative Building Blocks

1. Research Background

In their review titled “Synthesis of Heterobicyclo[n.1.1]alkanes,” published in Chemical Society Reviews, Revie, Ragus, and Anderson systematically summarized advances in the synthesis of heterobicyclo[n.1.1]alkanes, with a particular focus on heterobicyclo[2.1.1]hexanes and heterobicyclo[3.1.1]heptanes.

These scaffolds have attracted attention because they combine three features of relevance to medicinal chemistry:

① Rigid three-dimensional scaffolds: They can restrict conformational freedom and orient substituents along relatively well-defined spatial vectors;

② Potential as aromatic-ring replacements: Specific substitution patterns may serve as saturated bioisosteres of certain ortho- or meta-disubstituted aromatic rings, as well as selected heteroaromatic rings;

③ Heteroatom-enabled property modulation: Oxygen, nitrogen, sulfur, and other heteroatoms can be used to alter polarity, hydrogen-bonding capacity, pKa, solubility, and options for late-stage derivatization.

The core value of heterobicyclo[n.1.1]alkanes lies in their ability to combine a saturated three-dimensional scaffold with heteroatom-enabled property modulation. As a result, aromatic-ring replacement is no longer limited to spatial mimicry, but may also provide opportunities to modulate physicochemical properties and binding interactions.

2. Structural Fundamentals

2.1 Comparison of Core Scaffolds

Scaffold type	English name and abbreviation	Typical parent molecular formula	Main medicinal chemistry significance
BCP	bicyclo[1.1.1]pentane, BCP	C₅H₈	Commonly used as a three-dimensional replacement for para-disubstituted benzene
BCHex	bicyclo[2.1.1]hexane, BCHex	C₆H₁₀	Can be explored as a replacement for ortho- or meta-disubstituted benzene rings
BCHep	bicyclo[3.1.1]heptane, BCHep	C₇H₁₂	Commonly explored as a replacement for meta-disubstituted benzene rings
HBCHex	heterobicyclo[2.1.1]hexane, HBCHex	e.g., C₅H₈O, C₅H₉N, C₅H₈S	Possesses a BCHex-type bicyclic framework containing heteroatoms such as O, N, or S within the ring skeleton
HBCHep	heterobicyclo[3.1.1]heptane, HBCHep	e.g., C₆H₁₀O, C₆H₁₁N, C₆H₁₀S	Possesses a BCHep-type bicyclic framework containing heteroatoms such as O, N, or S within the ring skeleton

Note: The molecular formulas shown for HBCHex and HBCHep are examples of unsubstituted parent scaffolds or parent scaffolds containing an N–H group. The actual molecular formula varies depending on substitution at the bridgehead positions, bridge positions, and heteroatoms.

2.2 Simplified Structural Representation

Scaffold	Simplified structural description
bicyclo[n.1.1]alkane	Two bridgehead atoms are connected by three bridges: one bridge contains n atoms, while each of the other two bridges contains one atom
HBCHex	Has a [2.1.1] bicyclic connectivity pattern and contains at least one heteroatom, such as O, N, or S, within the ring skeleton
HBCHep	Has a [3.1.1] bicyclic connectivity pattern and contains at least one heteroatom, such as O, N, or S, within the ring skeleton

3. Why Are These Scaffolds Relevant to Medicinal Chemistry?

3.1 Benzene Is Not Always Optimal

The benzene ring is one of the most common structural units in drug molecules. It is stable, readily functionalized, and supported by mature synthetic chemistry. It can contribute to hydrophobic occupancy, π–π interactions, and the connection of substituents. However, benzene rings may also introduce certain limitations:

Characteristic of benzene	Potential medicinal chemistry issue
Strong planarity	Limited molecular three-dimensionality and restricted exploration of three-dimensional conformational space
Aromatic character	May become a site of aromatic oxidative metabolism in some cases
Relatively high hydrophobicity	May adversely affect solubility, nonspecific binding, and clearance
Substituent vectors confined to a plane	May not be well suited to three-dimensional binding pockets

The purpose of using benzene bioisosteres is not simply to “replace the benzene ring.” Rather, the aim is to preserve the spatial relationship between key substituents while improving molecular shape, polarity, metabolic stability, or selectivity.

3.2 Saturated Small Bridged Rings Address the “Spatial Problem”

Saturated small bridged rings such as BCP, BCHex, and BCHep have attracted attention primarily because they can replace planar aromatic rings with rigid three-dimensional scaffolds. Compared with benzene, these structures generally have greater sp³ character and can direct substituents along vectors that differ from those defined by an aromatic plane.

The central rationale behind this type of replacement is:

To replace a planar aromatic ring with a rigid three-dimensional scaffold in an attempt to improve conformational behavior, metabolic stability, and physicochemical properties.

However, all-carbon bridged rings also have limitations. They mainly alter molecular shape and offer relatively limited control over polarity, hydrogen-bonding capacity, pKa, and local electronic properties.

3.3 Heterobicyclo[n.1.1]alkanes Further Address the “Property Problem”

The incorporation of O, N, S, and other heteroatoms into all-carbon bridged-ring frameworks means that heterobicyclo[n.1.1]alkanes are not merely spatial replacements. They also provide a broader range of opportunities for property modulation. Their medicinal chemistry significance can be summarized as follows:

Design objective	What all-carbon BCH/BCP scaffolds can provide	What heterobicyclo[n.1.1]alkanes may additionally provide
Alter conformation	Yes	Yes
Increase three-dimensionality	Yes	Yes
Change substituent orientation	Yes	Yes
Alter polarity	Limited	More direct modulation
Introduce hydrogen-bonding capacity	Limited	Modulation through O, N, or S
Modulate pKa	Limited	Greater potential with nitrogen-containing scaffolds
Enable late-stage functionalization	Depends on the substituents	Certain heteroatoms, particularly unprotected ring nitrogen atoms, may provide additional sites for modification

4. The Value of Heteroatoms

4.1 Modulation of Polarity and Hydrogen-Bonding Capacity

The incorporation of oxygen, nitrogen, or sulfur atoms into a bridged-ring framework changes the local polarity and hydrogen-bonding pattern of the molecule. Ether oxygen can generally act as a hydrogen-bond acceptor, whereas the hydrogen-bonding and acid–base properties of sulfur and nitrogen depend on their oxidation state, substitution pattern, and protonation state. Nitrogen-containing scaffolds may also affect basicity, protonation state, and salt-forming ability.

This means that heterobicyclo[n.1.1]alkanes can retain a rigid scaffold while influencing the following properties:

Aqueous solubility; LogD; The distribution of hydrogen-bond acceptors and donors; Local dipole moment; pKa; Polar interactions within protein-binding pockets.

4.2 Modulation of the Bound Conformation

The position of a heteroatom can affect how the entire scaffold fits within the target-binding pocket. Even when two compounds have similar distances between their substituents, differences in heteroatom position may lead to substantially different interactions with water molecules, polar amino acid residues, or hydrophobic regions.

4.3 Opportunities for Late-Stage Modification

Medicinal chemistry optimization relies on exploration of structure–activity relationships, or SAR. A new scaffold has limited value if only a single compound can be prepared. Its value is considerably greater when substituents can be systematically varied at multiple positions, allowing the scaffold to enter a structured optimization program.

The incorporation of heteroatoms can increase the electronic and structural diversity of a scaffold. Derivatizable nitrogen or sulfur atoms may also provide opportunities for late-stage modification, including:

① Derivatizable heteroatoms can create additional entry points for functional-group transformations;

② Certain heteroatoms can provide attachment points for further derivatization;

③ Polarity and spatial occupancy can be adjusted within the same scaffold;

④ A more complete SAR series can be established.

The primary role of an endocyclic oxygen atom is generally to modulate polarity, conformation, and local electronic properties. It does not necessarily serve directly as a late-stage attachment point.

5. HBCHex and HBCHep Have Different Medicinal Chemistry Roles

5.1 HBCHex: A More Compact Candidate for Ortho/Meta Aromatic-Ring Replacement

HBCHex, or heterobicyclo[2.1.1]hexane, generally has a relatively compact framework. It may therefore be prioritized when exploring aromatic-ring replacements with shorter distances between substituents or in more spatially constrained environments. However, its suitability must still be assessed according to the specific substitution pattern and exit vectors.

It is particularly relevant when addressing the following medicinal chemistry questions:

① Is the original aromatic ring excessively planar?

② Do the key substituents require more clearly defined three-dimensional orientations?

③ Is conformational freedom required to be restricted within a small binding pocket?

④ Is the introduction of a polar atom into a compact scaffold desirable?

5.2 HBCHep: A More Extended Candidate for Meta Aromatic-Ring Replacement

HBCHep, or heterobicyclo[3.1.1]heptane, generally has a larger framework than HBCHex. In certain common substitution patterns, its substituents may span a greater spatial distance. It is therefore more suitable for mimicking certain meta-disubstituted aromatic rings or replacing more extended hydrophobic aromatic fragments.

It is particularly relevant when addressing the following medicinal chemistry questions:

① Does the original meta-disubstituted aromatic ring introduce metabolic or hydrophobicity-related liabilities?

② Can the binding pocket accommodate a larger three-dimensional scaffold?

③ Is it desirable to increase polarity while maintaining the required spatial distance?

④ Is it necessary to expand SAR through modification at the bridgehead or bridge positions?

5.3 The Two Scaffold Classes Should Not Be Used Interchangeably

The difference between HBCHex and HBCHep is not simply one of size. They differ in substituent orientation, spatial reach, and scaffold volume, and are therefore suited to different design problems.

Question	HBCHex is more suitable	HBCHep is more suitable
Small binding pockets	May be prioritized for exploration	Requires case-specific evaluation
More compact aromatic-ring replacement	Generally more favorable	Generally less favorable
Meta aromatic-ring replacement	Can be explored	More commonly used
Greater spatial reach	Depends on the substitution pattern	Certain substitution patterns may be more favorable
Introduction of polar heteroatoms	Yes	Yes
Expansion of substituent SAR	Depends on the synthetic route	Depends on the synthetic route

Note: The comparison above reflects only general structural trends. Actual suitability depends on the specific substitution pattern, relative configuration, distance between substituents, and exit vectors.

6. Why Are Synthetic Methods Central to the Discussion?

6.1 Synthetic Accessibility Determines Medicinal Chemistry Utility

In medicinal chemistry, structural potential does not necessarily translate into practical utility. A genuinely useful scaffold must satisfy several requirements:

Medicinal chemistry requirement	Requirement for the synthetic route
Rapid validation of a design hypothesis	Short route and readily available starting materials
Establishment of SAR	Substituents can be varied systematically
ADMET evaluation	Sufficient quantities of material can be prepared
Entry into lead optimization	Good functional-group compatibility
Advancement of candidate compounds	Scalable, reproducible, and amenable to purification

ADMET refers to absorption, distribution, metabolism, excretion, and toxicity.

6.2 Major Current Synthetic Strategies

Synthetic strategy	Representative feature	Relevance to medicinal chemistry
BCB strain release	Exploits the highly strained central bond of bicyclo[1.1.0]butane	Can be used to construct a range of HBCHex or HBCHep scaffolds
Radical strategies	Generate radical or diradical intermediates through single-electron transfer or energy transfer	Suitable for rapid fragment assembly and expansion of structural diversity
Polar cyclization	Cyclization promoted by Lewis acids or ionic processes	May improve regioselectivity and functional-group compatibility
Intramolecular cycloaddition	Uses preorganized substrates to construct rigid bicyclic frameworks	Suitable for preparing specific scaffolds and stereochemical arrangements
Ring closure/rearrangement/ring opening	Accesses the target scaffold from preformed precursors	Provides access to different heteroatom positions and substitution patterns
Asymmetric synthesis	Uses chiral catalysts or chiral substrates to control configuration	An important route to enantiomerically defined medicinal chemistry building blocks

7. Current Limitations: Why Have These Scaffolds Not Yet Become Mature Medicinal Chemistry Building Blocks?

7.1 Substituent Control Remains Inadequate

Medicinal chemistry optimization requires the systematic variation of substituents at multiple positions. If only a limited number of substitution patterns can be prepared on a given scaffold, it is difficult to support a comprehensive SAR program.

Many current synthetic routes to heterobicyclo[n.1.1]alkanes still depend on specific substrates, activation modes, or substituent classes. For example, some reactions require an aryl group or an electron-withdrawing group at the bridgehead position of a BCB, whereas certain routes introduce fixed functional groups that may not align with medicinal chemistry requirements. These limitations reduce the efficiency with which the scaffolds can be used in real drug discovery programs.

7.2 Regioselectivity and Stereoselectivity Still Require Improvement

Small bridged-ring systems are compact and highly strained, making control of regioselectivity and stereoselectivity challenging. For medicinal chemistry applications, access to a single, well-defined configuration through a reproducible synthetic route is highly important. If a route provides only racemic products, or achieves high enantioselectivity for only a narrow range of substrates, its ability to support practical drug discovery programs remains limited.

7.3 Functional-Group Compatibility and Scalability Remain Practical Challenges

Real drug-like molecules commonly contain multiple functional groups, including amides, heteroarenes, amines, ethers, halogens, and carboxylate esters. If the reaction conditions are excessively harsh, or if a method is applicable only to simple model substrates, it will be difficult to use for the late-stage modification of complex molecules.

Furthermore, successful synthesis on the milligram scale does not necessarily mean that a method can support a medicinal chemistry program. Medicinal chemistry evaluation generally requires sufficient material for studies of potency, selectivity, solubility, metabolic stability, and in vivo exposure. Scalability is therefore an important criterion when assessing the maturity of these scaffolds.

7.4 Publicly Available Medicinal Chemistry Validation Remains Limited

Published research on heterobicyclo[n.1.1]alkanes currently focuses mainly on synthetic methods and scaffold construction. Although analogues of drugs and agrochemicals have been used to demonstrate the replacement potential of these structures, publicly available information still lacks widely recognized examples of clinical candidates or marketed drugs incorporating these scaffolds, particularly when compared with more established frameworks such as BCP.

8. When Should Heterobicyclo[n.1.1]alkanes Be Considered?

8.1 Situations in Which They May Be Worth Exploring

Project issue	Potential value of introducing a heterobicyclo[n.1.1]alkane
An aromatic ring may be a metabolic soft spot	Attempt to reduce the original aromatic oxidation pathway while assessing whether new aliphatic oxidation sites are introduced
The molecule is excessively planar	Increase three-dimensionality and alter the conformational distribution
Solubility is inadequate	Use heteroatoms to explore modulation of polarity, hydrogen-bonding capacity, and ionization state
Selectivity is inadequate	Change the spatial orientation of substituents and attempt to modify target-binding interactions
Conformational restriction is required	Use a rigid bridged ring to reduce unproductive conformations

8.2 Situations in Which Blind Replacement Is Not Appropriate

If the original benzene ring participates in a defined π–π interaction or if a strictly planar conformation must be maintained, direct replacement with a heterobicyclo[n.1.1]alkane may reduce activity. If the target region serves only as a general hydrophobic contact and there are no issues related to solubility, metabolism, or selectivity, the introduction of a complex bridged-ring scaffold may also be unnecessary.

Before using these scaffolds, three questions should therefore be addressed:

① What is the specific problem associated with the original aromatic ring?

② Is the objective to change spatial orientation, polarity, metabolism, or the available intellectual-property space?

③ Can the target scaffold be synthesized rapidly and support systematic SAR exploration?

Only after these questions have been clearly answered can a heterobicyclo[n.1.1]alkane become an effective molecular-design tool.

9. Representative Heterobicyclo[n.1.1]alkanes and Related Three-Dimensional Aromatic-Ring Bioisosteres

Table 1. Core Heterobicyclo[2.1.1]hexane Building Blocks

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Oxabicyclo[2.1.1]hexane carboxylic acid building block	2228699-81-6	O632279	2-Oxabicyclo[2.1.1]hexane-1-carboxylic acid	≥97%	Contains a rigid, three-dimensional oxa-bridged scaffold and a carboxylic acid at C1; can be used for amidation, esterification, and the construction of aromatic-ring bioisosteric fragments
Oxabicyclo[2.1.1]hexane carboxylic acid building block	1784145-36-3	O630423	2-Oxabicyclo[2.1.1]hexane-4-carboxylic acid	≥97%	Contains an oxa-bridged ring and a carboxylic acid at C4; can be used to construct amides, esters, and polar three-dimensional fragments with different exit-vector orientations
Bifunctional oxabicyclo[2.1.1]hexane acid–ester building block	2385186-83-2	M682136	1-Methoxycarbonyl-2-oxabicyclo[2.1.1]hexane-4-carboxylic acid	≥97%	Contains both a carboxylic acid and a methyl ester; can be used for selective amidation, ester hydrolysis, and the design of fragments with two points of attachment
Oxabicyclo[2.1.1]hexane hydroxymethyl building block	2060007-65-8	O631455	2-Oxabicyclo[2.1.1]hexan-1-ylmethanol	≥97%	Contains an oxa-bridged ring and a hydroxymethyl group at C1; can be used for etherification, esterification, oxidation, and linker modification
Oxabicyclo[2.1.1]hexane hydroxymethyl building block	2138162-49-7	M631763	(1-Methyl-2-oxabicyclo[2.1.1]hexan-4-yl)methanol	≥97%	Contains a methyl-substituted oxa-bridged ring and a hydroxymethyl group at C4; can be used to modulate steric occupancy and prepare ether and ester derivatives
Oxabicyclo[2.1.1]hexane aldehyde building block	2137762-49-1	M682131	1-Methyl-2-oxabicyclo[2.1.1]hexane-4-carbaldehyde	≥97%	Contains a reactive aldehyde group; can be used for reductive amination, oxime formation, hydrazone formation, and the introduction of nitrogen-containing side chains
Oxabicyclo[2.1.1]hexane iodomethyl building block	1935986-52-9	O631208	1-(Iodomethyl)-2-oxabicyclo[2.1.1]hexane	≥97%	Contains a reactive iodomethyl group; can be used for nucleophilic substitution, alkylation, and late-stage functional-group introduction
Oxabicyclo[2.1.1]hexane aminonitrile salt building block	2170372-36-6	A631952	4-Amino-2-oxabicyclo[2.1.1]hexane-1-carbonitrile hydrochloride	≥97%	Contains both amino and nitrile groups; can be used to construct amides, ureas, nitrile-derived products, and multifunctional three-dimensional fragments
Protected amino alcohol oxabicyclo[2.1.1]hexane building block	2170372-32-2	T631951	tert-Butyl N-[1-(hydroxymethyl)-2-oxabicyclo[2.1.1]hexan-4-yl]carbamate	≥97%	Contains a protected amino group and a hydroxymethyl group; can be used for stepwise functionalization, amino alcohol fragment construction, and modification of polar three-dimensional fragments
Azabicyclo[2.1.1]hexane parent amine salt building block	871658-02-5	A177805	2-Azabicyclo[2.1.1]hexane hydrochloride	≥97%	Contains a secondary amine hydrochloride and a rigid small bridged ring; can be used in the design of basic fragments, acylation, alkylation, and conformational-restriction studies
Azabicyclo[2.1.1]hexane carboxylic acid salt building block	116129-07-8	A627068	2-Azabicyclo[2.1.1]hexane-1-carboxylic acid hydrochloride	≥97%	Contains an aza-bridged ring, a carboxylic acid, and an amine salt; can be used to construct rigid amino acid analogues, peptidomimetics, and amide fragments

Table 2. Heterobicyclo[3.1.1]heptane and Related Extended Oxa-Bridged Building Blocks

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Oxabicyclo[3.1.1]heptane carboxylic acid building block	2305253-19-2	M682169	1-Methyl-2-oxabicyclo[3.1.1]heptane-5-carboxylic acid	≥97%	Contains an extended oxa-bridged ring and a carboxylic acid; can be used in studies of meta-aromatic-ring replacement, amide-fragment construction, and polarity modulation
Azabicyclo[3.1.1]heptane parent amine salt building block	2171280-54-7	A631958	2-Azabicyclo[3.1.1]heptane hydrochloride	≥97%	Contains a 2-aza-bridged ring and an amine salt; can be used to construct basic three-dimensional fragments and in acylation, alkylation, and saturated-heterocycle replacement studies
Azabicyclo[3.1.1]heptane parent amine salt building block	1427380-44-6	A629128	3-Azabicyclo[3.1.1]heptane hydrochloride	≥97%	Contains a 3-aza-bridged ring and an amine salt; can be used to construct aza-containing three-dimensional fragments, piperidine analogues, and polar fragments
Protected amino alcohol azabicyclo[3.1.1]heptane building block	1357353-36-6	A173543	tert-Butyl 6-hydroxy-3-azabicyclo[3.1.1]heptane-3-carboxylate	≥97%	Contains a protected nitrogen atom and a hydroxyl group; can be used for stepwise derivatization, etherification, esterification, and the modification of nitrogen-containing three-dimensional fragments
Oxa-azabicyclo[3.1.1]heptane dual-heteroatom building block	1414958-33-0	O628988	6-Oxa-3-azabicyclo[3.1.1]heptane hydrochloride	≥97%	Contains both oxygen and nitrogen heteroatoms together with an amine salt; can be used to construct highly polar saturated heterocyclic fragments and in studies of aqueous-solubility modulation and conformational restriction
Extended oxabicyclo[3.2.1]octanone building block	77745-32-5	O177428	8-Oxabicyclo[3.2.1]octan-3-one	≥97%	Contains an oxa-bridged ring and a ketone group; can be used for reductive amination, oxime formation, carbonyl reduction, and the construction of oxa-containing three-dimensional fragments

Table 3. All-Carbon Bicyclo[2.1.1]hexane, Bicyclo[3.1.1]heptane, and Bicyclo[2.2.2]octane Reference Building Blocks

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Bicyclo[2.1.1]hexane dicarboxylic acid building block	85407-65-4	B634691	Bicyclo[2.1.1]hexane-1,4-dicarboxylic acid	≥97%	A rigid dicarboxylic acid scaffold; can be used to construct diamides, diesters, fragments with two points of attachment, and all-carbon scaffold controls
Bicyclo[2.1.1]hexane diester building block	42145-38-0	D633867	Dimethyl bicyclo[2.1.1]hexane-1,4-dicarboxylate	≥97%	A rigid dimethyl diester scaffold; can be used for hydrolysis, transamidation, bifunctional derivatization, and bridged-ring modification
Bifunctional bicyclo[2.1.1]hexane acid–ester building block	141046-52-8	M678795	4-Methoxycarbonylbicyclo[2.1.1]hexane-1-carboxylic acid	≥97%	Contains a carboxylic acid and a methyl ester; can be used for selective amidation, ester transformations, and comparative studies of ortho/meta aromatic-ring replacement
Protected amino acid bicyclo[2.1.1]hexane building block	1050886-56-0	B626668	4-(tert-Butoxycarbonylamino)bicyclo[2.1.1]hexane-1-carboxylic acid	≥97%	Contains a protected amino group and a carboxylic acid; can be used to construct rigid amino acid analogues and amide fragments and in conformational-restriction studies
Bicyclo[2.1.1]hexane amino ester salt building block	1638772-04-9	M630094	Methyl 4-aminobicyclo[2.1.1]hexane-1-carboxylate hydrochloride	≥97%	Contains an amine salt and a methyl ester; can be used to construct amides, ureas, sulfonamides, and amino acid analogues
Protected diamine bicyclo[2.1.1]hexane building block	1050890-47-5	T678664	tert-Butyl N-(4-amino-1-bicyclo[2.1.1]hexanyl)carbamate	≥97%	Contains a free amino group and a protected amino group; can be used for stepwise acylation, urea construction, and modification of diamino fragments
Bicyclo[3.1.1]heptane dicarboxylic acid building block	75328-55-1	B634485	Bicyclo[3.1.1]heptane-1,5-dicarboxylic acid	≥97%	An extended dicarboxylic acid bridged-ring scaffold; can be used in comparative studies of meta-aromatic-ring replacement and in the design of diamides and diesters
Bifunctional bicyclo[3.1.1]heptane acid–ester building block	110371-28-3	M626874	5-Methoxycarbonylnorpinane-1-carboxylic acid	≥97%	Contains a carboxylic acid and a methyl ester; can be used for selective functional-group transformations, amidation, and comparative studies of the bicyclo[3.1.1]heptane scaffold
Protected amino acid bicyclo[3.1.1]heptane building block	1035325-28-0	N626623	5-(tert-Butoxycarbonylamino)norpinane-1-carboxylic acid	≥97%	Contains a protected amino group and a carboxylic acid; can be used in comparative studies of meta-aromatic-ring replacement and to construct rigid amino acid analogues and amide series
Bicyclo[3.1.1]heptane amino ester building block	1824132-78-6	M630787	Methyl 5-aminonorpinane-1-carboxylate	≥97%	Contains an amino group and a methyl ester; can be used to construct amides, ureas, amino acid analogues, and extended three-dimensional fragments
Protected diamine bicyclo[3.1.1]heptane building block	1049607-15-9	T680230	tert-Butyl N-(5-aminonorpinan-1-yl)carbamate	≥97%	Contains a free amino group and a protected amino group; can be used to construct diamine bioisosteres and urea fragments and in studies of conformationally restricted structures
Extended protected amino acid bicyclo[2.2.2]octane building block	863304-76-1	T177717	4-[(tert-Butoxycarbonyl)amino]bicyclo[2.2.2]octane-1-carboxylic acid	≥97%	Contains a protected amino group and a carboxylic acid; can be used to construct extended saturated three-dimensional scaffolds and rigid amino acid analogues and in comparative studies of aromatic-ring replacement

Table 4. Bicyclo[1.1.1]pentane- and Propellane-Related Building Blocks

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Key bicyclo[1.1.1]pentane precursor	35634-10-7	O1297208	[1.1.1]Propellane	≥95%	A key precursor to the bicyclo[1.1.1]pentane scaffold; can be used for ring-opening addition, radical functionalization, and the synthesis of three-dimensional small bridged-ring derivatives
Bicyclo[1.1.1]pentane carboxylic acid building block	22287-28-1	B175783	Bicyclo[1.1.1]pentane-1-carboxylic acid	≥97%	A basic carboxylic acid-functionalized bicyclo[1.1.1]pentane building block; can be used for amidation, esterification, and studies of para-aromatic-ring bioisosteres
Bicyclo[1.1.1]pentane dicarboxylic acid building block	56842-95-6	B176816	Bicyclo[1.1.1]pentane-1,3-dicarboxylic acid	≥97%	A dicarboxylic acid scaffold with two points of attachment; can be used for para-benzene replacement and the design of diamide and diester structures
Bicyclo[1.1.1]pentane hydroxymethyl building block	22287-32-7	B175784	Bicyclo[1.1.1]pentan-1-ylmethanol	≥97%	Contains a hydroxymethyl-functionalized bicyclo[1.1.1]pentane scaffold; can be used for etherification, esterification, oxidation, and linker construction
Bicyclo[1.1.1]pentane diol building block	1312790-52-5	B628220	Bicyclo[1.1.1]pentane-1,3-diol	≥97%	A dihydroxy scaffold with two points of attachment; can be used for etherification, esterification, carbonate construction, and polar-fragment modification
Bicyclo[1.1.1]pentane bromomethyl building block	161043-38-5	B174609	1-(Bromomethyl)bicyclo[1.1.1]pentane	≥97%	Contains a reactive bromomethyl group; can be used for nucleophilic substitution, alkylation, and the introduction of bicyclo[1.1.1]pentane side chains
Bicyclo[1.1.1]pentane iodocarboxylic acid building block	224584-16-1	I682663	3-Iodobicyclo[1.1.1]pentane-1-carboxylic acid	≥97%	Contains an iodo substituent and a carboxylic acid; can be used for coupling reactions, amidation, and the construction of diverse bicyclo[1.1.1]pentane fragments
Bicyclo[1.1.1]pentane iodo ester building block	141046-59-5	M587168	Methyl 3-iodobicyclo[1.1.1]pentane-1-carboxylate	≥98%	Contains an iodo substituent and a methyl ester; can be used for coupling, ester transformations, and the construction of para-aromatic-ring bioisosteres
Bicyclo[1.1.1]pentane trifluoromethyl carboxylic acid building block	224584-18-3	B175798	3-(Trifluoromethyl)bicyclo[1.1.1]pentane-1-carboxylic acid	≥97%	Contains a trifluoromethyl group and a carboxylic acid; can be used in studies of lipophilicity modulation and metabolic stability and for the construction of amide fragments
Bicyclo[1.1.1]pentane tert-butyl carboxylic acid building block	132663-73-1	T628278	3-tert-Butylbicyclo[1.1.1]pentane-1-carboxylic acid	≥97%	Contains a hydrophobic tert-butyl substituent and a carboxylic acid; can be used in studies of steric occupancy, the construction of hydrophobic fragments, and the preparation of amide series
Bicyclo[1.1.1]pentane phenyl carboxylic acid building block	83249-04-1	P177543	3-Phenylbicyclo[1.1.1]pentane-1-carboxylic acid	≥97%	Contains a phenyl substituent and a carboxylic acid; can be used for hydrophobic-fragment extension, amidation, and comparative studies of aromatic-ring bioisosteres
Protected diamine bicyclo[1.1.1]pentane building block	1638767-25-5	T174821	tert-Butyl N-[3-aminobicyclo[1.1.1]pentan-1-yl]carbamate	≥97%	Contains a free amino group and a protected amino group; can be used for stepwise acylation and the construction of ureas, amides, and fragments with two points of attachment
Bicyclo[1.1.1]pentane dicarboxamide building block	1379318-53-2	B683067	Bicyclo[1.1.1]pentane-1,3-dicarboxamide	≥97%	A dicarboxamide-functionalized bicyclo[1.1.1]pentane scaffold; can be used in studies of para-disubstituted benzamide replacement and polar fragments
Bicyclo[1.1.1]pentane amidine salt building block	2231674-40-9	B632361	Bicyclo[1.1.1]pentane-1-carboxamidine hydrochloride	≥97%	Contains an amidine hydrochloride group; can be used to construct basic three-dimensional fragments and amidine-containing fragments and in studies of polar interactions

References

[1] Revie, R. I.; Ragus, J.; Anderson, E. A. Synthesis of heterobicyclo[n.1.1]alkanes. Chem. Soc. Rev. 2026, 55, 941–954. DOI: 10.1039/D5CS01128K.

[2] Tsien, J.; Hu, C.; Merchant, R. R.; Qin, T. Three-dimensional saturated C(sp³)-rich bioisosteres for benzene. Nat. Rev. Chem. 2024, 8, 605–627. DOI: 10.1038/s41570-024-00623-0.

[3] Subbaiah, M. A. M.; Meanwell, N. A. Bioisosteres of the phenyl ring: recent strategic applications in lead optimization and drug design. J. Med. Chem. 2021, 64, 14046–14128. DOI: 10.1021/acs.jmedchem.1c01215.

[4] Lovering, F.; Bikker, J.; Humblet, C. Escape from flatland: increasing saturation as an approach to improving clinical success. J. Med. Chem. 2009, 52, 6752–6756. DOI: 10.1021/jm901241e.

[5] Mykhailiuk, P. K. Saturated bioisosteres of benzene: where to go next? Org. Biomol. Chem. 2019, 17, 2839–2849. DOI: 10.1039/C8OB02812E.

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

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