Introduction

2-Oxabicyclo[2.1.1]hexane (2-oxa-BCH) is a rigid, sp³-rich, oxygen-containing bicyclic scaffold. Based on crystallographic structures, a 2020 study proposed that 1,4-disubstituted 2-oxa-BCH could mimic meta-disubstituted benzene, although the corresponding biological activity had not yet been validated [2]. A 2023 study further showed that 1,5-disubstituted 2-oxa-BCH and ortho-disubstituted benzene have relatively similar exit-vector relationships and compared the physicochemical properties of the corresponding bioactive molecules; the 1,5-disubstituted analogs of Fluxapyroxad and Boscalid were also evaluated for antifungal activity [3]. A subsequent study published in 2024 expanded the scalable synthesis and functionalization scope of this scaffold. The meta-disubstituted benzene in Sonidegib and the ortho-disubstituted benzenes in seven other bioactive molecules were replaced to investigate the geometric parameters, physicochemical properties, and biological activities of 1,4- and 3,4-disubstituted 2-oxa-BCH scaffolds, respectively [1].

These studies show that geometric similarity alone is insufficient to establish a bioisosteric relationship. A saturated scaffold that is geometrically similar to benzene must also satisfy the relevant molecular-recognition requirements within a specific bioactive molecule and undergo biological activity validation before it can be regarded as a bioisostere of practical significance.

1. Functional Differences of the Saturated Sonidegib Analog

The 2024 study first replaced the meta-disubstituted benzene in Sonidegib with 1,4-disubstituted 2-oxa-BCH to obtain saturated analog 78. Sonidegib is a Smoothened (Smo) receptor antagonist that acts by inhibiting the Hedgehog signaling pathway. The researchers compared the pathway-inhibitory activities of the two compounds in a Gli-Luc reporter NIH3T3 cell system and simultaneously measured their bidirectional permeability across Caco-2 cell monolayers [1].

Sonidegib and analog 78 showed similar permeability in the Caco-2 assay, but their inhibitory potencies against Hedgehog signaling differed markedly. The half-maximal inhibitory concentration (IC₅₀) was 1.462 nM for Sonidegib and 179 nM for analog 78. Although analog 78 retained nanomolar activity, its potency decreased by approximately two orders of magnitude [1].

Figure 1. Structures and experimental results of Sonidegib and its saturated 2-oxa-BCH analog 78. The meta-disubstituted benzene replaced in Sonidegib is shown in blue, and the 1,4-disubstituted 2-oxa-BCH in analog 78 is shown in red. The two compounds exhibited similar bidirectional Caco-2 permeability, whereas the IC₅₀ for Hedgehog pathway inhibition increased from 1.462 nM to 179 nM. Caco-2 data reflect properties related to transport across cell monolayers and cannot, by themselves, exclude other factors affecting cellular activity. Reproduced from Figure 2 of Ref. [1] under CC BY 4.0.

These results distinguish three levels of assessment: whether the chemical replacement can be achieved, whether the two scaffolds possess a certain degree of geometric similarity, and whether the original biological function is retained at a comparable level. These are not equivalent conclusions.

The residual activity of analog 78 indicates that 1,4-disubstituted 2-oxa-BCH can connect the two flanking structural fragments in this molecule. However, the marked loss of potency suggests that the effective molecular-recognition or intracellular activity requirements of the complete molecule were not maintained to the same extent after replacement. Because the Gli-Luc IC₅₀ reflects cellular inhibition of the Hedgehog pathway rather than direct binding affinity for Smo, the observed potency difference cannot be attributed solely to the exit-vector angle.

2. Spatial Constraints of Benzene Substituents and Exit Vectors

A disubstituted benzene ring not only provides an aromatic π-electron system but also serves as a rigid spatial-positioning unit. Ortho, meta, and para substitution patterns define both the distance between the two attachment sites and the directions in which the substituent bonds extend from the aromatic ring. When an aromatic ring is replaced with another rigid scaffold, the positions of the terminal functional groups in three-dimensional space may change even if the overall connectivity of the molecule remains unchanged.

These spatial relationships are commonly described using exit-vector analysis. The bonds connecting the core scaffold to its external substituents are treated as directional vectors. Parameters such as the distance between attachment sites, the distance between substituents, the angle between the vectors, and the dihedral angle can be used to determine whether two different cores project their external functional groups into similar spatial positions [1,3]. This method has been used to compare 2-oxa-BCH scaffolds with different connectivity patterns. The 2023 study quantitatively compared the spatial relationship between 1,5-disubstituted structures and ortho-disubstituted benzenes [3], whereas the 2024 study compared the geometric parameters of 3,4-disubstituted structures and ortho-disubstituted benzenes [1].

For drug molecules, both distance and direction must remain within reasonable ranges. Even when two cores have similar distances between their substituents, markedly different exit directions can project pharmacophores extending along the substituent bonds into different spatial regions. The longer the substituents, the more likely an angular difference at the core is to produce a pronounced positional displacement of the terminal functional groups.

Crystallographic comparisons show that 2-oxa-BCH scaffolds with different substitution patterns exhibit distinctly different degrees of geometric matching with benzene.

Figure 2. Geometric comparison of 2-oxa-BCH with meta- and ortho-disubstituted benzenes. Left: the two exit directions of 1,4-disubstituted 2-oxa-BCH form an angle of approximately 150°, compared with approximately 120° for meta-disubstituted benzene; superimposition reveals a clear angular difference. Right: 3,4-disubstituted 2-oxa-BCH and ortho-disubstituted benzene have relatively similar principal attachment positions and overall spatial profiles, although 2-oxa-BCH retains a distinctly nonplanar three-dimensional structure. Source: Figures 1b and 1d of Ref. [1], CC BY 4.0.

3. The Intermediate meta–para Geometry of 1,4-Disubstituted 2-oxa-BCH

The left panel of Figure 2 explains why 1,4-disubstituted 2-oxa-BCH cannot simply be regarded as an exact spatial replica of meta-disubstituted benzene.

X-ray crystallographic data show that some distance parameters of the two types of cores are indeed similar. The substituent distance, d, is 4.92–4.95 Å for 2-oxa-BCH and 4.93–5.05 Å for the meta-disubstituted benzenes examined. Based on this distance alone, the two scaffolds show a high degree of similarity [1].

A more pronounced difference lies in the angle γ between the two exit vectors:

The 153° angle of 1,4-disubstituted 2-oxa-BCH is not a minor deviation from the approximately 120° angle of meta-disubstituted benzene; instead, it is substantially closer to that of para-disubstituted benzene. Accordingly, the original study described this structure as occupying chemical space between meta- and para-disubstituted benzenes and proposed that it could serve as an alternative scaffold between these two structural classes [1].

This finding demonstrates that the spatial mimicry of disubstituted benzenes by 2-oxa-BCH strongly depends on the substitution pattern. The corresponding aromatic substitution pattern cannot be assigned solely from the type of scaffold.

The activity results for Sonidegib and analog 78 are consistent with this structural difference. The original study proposed that their potency difference might be related to the different angular characteristics of meta-disubstituted benzene and 1,4-disubstituted 2-oxa-BCH [1].

Replacing the aromatic ring also removes the aromatic π-electron surface and changes the local electronic distribution, hydrophobic surface, solvation, and three-dimensional shape. The bridging oxygen in 2-oxa-BCH additionally introduces a new polar feature. If the original benzene ring directly participates in π–π interactions, π–amide stacking, or other aromatic interactions, molecular recognition may still be affected even when the external pharmacophores occupy similar positions [3].

4. Ortho Geometry and Functional Validation of 3,4-Disubstituted 2-oxa-BCH

The overall geometric match between 3,4-disubstituted 2-oxa-BCH and ortho-disubstituted benzene is clearly better than that observed in the meta system described above.

In the crystal structures, the substituent distance, d, is 3.11–3.17 Å for 3,4-disubstituted 2-oxa-BCH and 2.87–2.95 Å for ortho-disubstituted benzene. The corresponding attachment-site distances, r, are 1.55–1.56 Å and 1.40–1.41 Å, respectively. The in-plane angles φ₁ and φ₂ of the exit vectors are 61–65° and 57–59°, respectively, indicating relatively similar principal exit directions [1].

The two structures are not completely identical. Ortho-disubstituted benzene remains essentially planar, with a dihedral angle |θ| of only 0–6°, whereas |θ| reaches 54–56° for 3,4-disubstituted 2-oxa-BCH. Thus, this replacement introduces pronounced three-dimensionality while largely preserving the principal exit directions [1].

This structural relationship is more meaningful in medicinal chemistry than simply increasing the proportion of sp³ carbon. Three-dimensionality is not an independent objective. Introducing a new nonplanar core may alter molecular shape and physicochemical properties while preserving the original recognition mode only when the spatial relationship between key pharmacophores remains within an acceptable range.

Geometric matching still does not directly demonstrate retention of biological function. The researchers incorporated 3,4-disubstituted 2-oxa-BCH into structures corresponding to Fluxapyroxad, Bixafen, and Boscalid to obtain saturated analogs 83–85 and tested their antifungal activities against Fusarium culmorum and F. oxysporum. Analogs 83 and 85 showed no activity under the assay conditions, whereas Bixafen analog 84 retained high antifungal activity, although the parent Bixafen remained slightly more potent [1].

The 1,5-disubstituted analogs 29 and 31 reported in Ref. [3] corresponded to Fluxapyroxad and Boscalid, respectively, and retained activity in the corresponding antifungal assays. By contrast, compounds 83–85 in Ref. [1] employed a 3,4-disubstituted connectivity pattern. Because the two sets of compounds differ in their attachment positions and spatial orientations of the substituents, and because the fungal strains and specific experimental conditions were not identical, the results of the two studies cannot be compared directly on a one-to-one basis. Together, they demonstrate that the effectiveness of 2-oxa-BCH replacement depends on the specific connectivity pattern and complete molecular structure.

A good exit-vector match increases the likelihood that key pharmacophores will remain in reasonable spatial positions after aromatic-ring replacement, but it cannot guarantee that every complete molecule will retain its original function. Proteins recognize the overall three-dimensional, electrostatic, and solvation surfaces formed by a ligand rather than the exit vectors alone. Functional outcomes are also influenced by aromatic interactions, local polarity, conformation, cellular exposure, and the fit of the complete molecule within the binding site.

5. Changes in Physicochemical Properties and Their Relationship to Bioisosterism

Replacing benzene with 2-oxa-BCH also changes the physicochemical properties of a molecule. The 2024 study compared five drugs and three agrochemicals with their corresponding saturated analogs. The reported solubility was experimental kinetic solubility in phosphate-buffered saline at pH 7.4 rather than equilibrium aqueous solubility in the general sense. Kinetic solubility increased in six of the eight matched pairs, and the calculated octanol/water partition coefficient (cLogP) decreased in all eight pairs. Among the five pairs that could be measured reliably using the experimental method, the octanol/buffer distribution coefficient, logD, at pH 7.4 decreased by approximately 0.5 units [1].

These changes are consistent with removal of the hydrophobic aromatic surface and introduction of the bridging oxygen. However, solubility is not determined by polarity alone. Solid-state lattice energy, molecular conformation, and other functional groups also affect dissolution, and individual compounds may therefore deviate from the overall trend.

In human liver microsome assays, changes in metabolic stability showed clear compound dependence. The analogs corresponding to Sonidegib, Lomitapide, and Bixafen showed lower metabolic stability, whereas those corresponding to Conivaptan, Fluxapyroxad, and Boscalid showed higher metabolic stability. No clear change was observed for the remaining two pairs because both the parent compounds and their analogs were intrinsically stable [1]. The researchers therefore limited their overall conclusion to the following: 2-oxa-BCH replacement tends to increase water solubility and reduce lipophilicity, whereas its effect on metabolic stability has no uniform direction.

Optimization of physicochemical properties and validation of bioisosterism are distinct dimensions of assessment. Reducing lipophilicity or increasing water solubility may enhance the development value of a structural replacement, but it does not demonstrate that the original molecular-recognition properties have been retained. Conversely, a replacement that retains good biological activity may not improve every physicochemical property simultaneously.

6. Geometric Isosterism, Bioisosterism, and Practical Utility

A systematic review of replacements for ortho- and meta-substituted benzenes distinguished between geometric isosteres and bioisosteres. The former are defined by geometric properties similar to those of the parent structure, whereas the latter additionally require experimental evidence of corresponding biological activity [4].

The series of studies on 2-oxa-BCH further shows that aromatic-ring replacement can be divided into three successive but noninterchangeable levels:

Practical medicinal chemistry is also constrained by synthetic accessibility. In the 2024 study, the key 2-oxa-BCH intermediate 2 was prepared on a 377 g scale in a single batch, and subsequent transformations afforded more than 100 functionalized 2-oxa-BCH derivatives in gram quantities [1]. These results show that the scaffold has an experimental foundation for use in structure–activity relationship studies and medicinal chemistry design–synthesis–testing cycles. However, scalable preparation demonstrates synthetic practicality and does not, by itself, establish a bioisosteric relationship.

Geometric analysis of 2-oxa-BCH is first used to determine whether a particular substitution pattern provides a spatial basis for replacing the corresponding disubstituted benzene, but such geometric similarity is only the starting point for structural design. After incorporation into a specific bioactive molecule, it remains necessary to determine whether aromatic-ring replacement alters key molecular-recognition properties and to establish through actual biological activity whether the replacement is successful. Changes in physicochemical properties are used to evaluate the development benefits of the replacement, whereas synthetic accessibility determines whether the design can proceed into subsequent structure–activity relationship studies. Neither can substitute for biological functional validation.

7. Bioactive Compounds, Analytical Standards, and Functionalized Building Blocks Relevant to 2-Oxabicyclo[2.1.1]hexane Bioisostere Research

Table 1. Bioactive Compounds and Analytical Standards

Table 2. Monofunctionalized 2-Oxabicyclo[2.1.1]hexane Building Blocks

Table 3. Bifunctionalized 2-Oxabicyclo[2.1.1]hexane Building Blocks

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References

[1] Levterov V. V., Panasiuk Y., Shablykin O., et al. 2-Oxabicyclo[2.1.1]hexanes: Synthesis, Properties, and Validation as Bioisosteres of ortho- and meta-Benzenes. Angewandte Chemie International Edition, 2024, 63: e202319831. DOI: 10.1002/anie.202319831.

[2] Levterov V. V., Panasyuk Y., Pivnytska V. O., Mykhailiuk P. K. Water-Soluble Non-Classical Benzene Mimetics. Angewandte Chemie International Edition, 2020, 59: 7161–7167. DOI: 10.1002/anie.202000548.

[3] Denisenko A., Garbuz P., Voloshchuk N. M., et al. 2-Oxabicyclo[2.1.1]hexanes as saturated bioisosteres of the ortho-substituted phenyl ring. Nature Chemistry, 2023, 15: 1155–1163. DOI: 10.1038/s41557-023-01222-0.

[4] Diepers H. E., Walker J. C. L. (Bio)isosteres of ortho- and meta-substituted benzenes. Beilstein Journal of Organic Chemistry, 2024, 20: 859–890. DOI: 10.3762/bjoc.20.78.

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