1,2-Disubstituted bicyclo[2.1.1]hexanes as saturated bioisosteres of ortho-substituted benzene

Bicyclo[2.1.1]hexanes have been synthesized, characterized, and biologically validated as saturated bioisosteres of the ortho-substituted benzene ring. The incorporation of the 1,2-disubstituted bicyclo[2.1.1]hexane core into the structure of fungicides boscalid (BASF), bixafen (Bayer CS), and fluxapyroxad (BASF) gave saturated patent-free analogs with high antifungal activity.


Introduction
The benzene ring is a basic structural unit in chemistry, and we learn about it in school.It is the most popular ring in natural products 1 and bioactive compounds. 2,3he ortho-substituted benzene ring, in particular, is found in the structure of more than three hundred drugs and agrochemicals (Fig. 1). 4,5For example, the well-known drug aspirin contains an ortho-substituted benzene ring.Recently, we discovered that 1,5-disubstituted bicyclo[2.1.1]hexanes and their oxa-containing analogs can mimic the ortho-substituted benzene ring in bioactive compounds (Fig. 1). 6These scaffolds were synthesized as a mixture of two diastereomers.Some of them were subsequently separated by crystallization or column chromatography.In many cases, however, the separation of diastereomers failed, and the desired products were not obtained.

EDGE ARTICLE
uxapyroxad (BASF) to provide saturated patent-free analogs with high antifungal activity.

Synthesis
Despite numerous studies on the topic, 7-15 we needed a practical approach to bicyclo[2.1.1]hexaneswith only two substituents (two exit vectors) at the 1-and 2-positions of the core (Scheme 1) without additional (poly)substitution at other positions.Moreover, one substituent should be (hetero)aromatic, and another one should be the carboxylic group, which is needed for the subsequent modications of the core via amide coupling. 21,22n light of our previous experience, 6 we wondered if diene 1 (easily obtained from acetophenone, please see Scheme 1) would undergo an intramolecular photocycloaddition into the desired bicyclo[2.1.1]hexanecore.Direct irradiation of diene 1 in acetonitrile at different wavelengths gave only traces of products (entries 1-4, Table 1).Irradiation with a broad wavelength mercury lamp gave the target product in 35% yield along with the formation of unidentied side products (entry 5).We also tried available organic ketones for the triplet sensitization of the styrene moiety.Cleaner formation of the desired bicyclo[2.1.1]hexane1a was observed (entries 6-10).The best yield of 76% was obtained with benzophenone (entry 8), whereas thioxanthone also worked well (entry 7).Among all tested solvents (entries 11-14), the best result was obtained in acetonitrile.Without irradiation, the reaction did not take place (entry 15).In the dark, rt n.r.

Scalable synthesis
The entire optimized synthetic protocol is shown in Scheme 1.It was important for us to elaborate on a modular method that would provide bicyclo[2.1.1]hexanesemploying only available and inexpensive starting materials.The synthesis started from acetophenone.The Horner-Wadsworth-Emmons reaction of acetophenone smoothly gave alkene 2 in 90% yield aer distillation.Treatment of the latter with LDA in THF at −78 °C followed by the addition of allyl bromide gave diene 1 in 81% yield (ca. 90% purity).The compound contained ca.10% of the isomeric diene, as the undesired alkylation at the methyl group also took place (please see the ESI, † p. S5-S6).An analytically pure sample of diene 1 was obtained by purication with HPLC.In the next step, however, we used the crude material.An intramolecular photocycloaddition of diene 1 proceeded smoothly on scale to provide the desired bicyclo[2.1.1]hexane1a in 71% yield (ca. 90% purity) aer distillation.Saponication of the ester group in 1a followed by crystallization from hexane-tBuOMe gave pure carboxylic acid 1b as a white crystalline solid in 70% yield.The structure of compound 1b was conrmed by X-ray crystallographic analysis (Scheme 2). 23t is important to note that following this optimized sequence, we easily synthesized 23 g of product 1b in one run.No column chromatography was involved at any step.

Modications
Compounds 1b and 3b-23b possess one functional group (-CO 2 H).We aimed to perform some representative modications of these bicyclo[2.1.1]hexanesto obtain linkers for medicinal chemistrycompounds with two functional groups.

Crystallographic analysis
Next, we compared the geometric parameters of 1,2-disubstituted bicyclo[2.1.1]hexaneswith those of the ortho-substituted benzene ring.For this, we employed the exit vector plot tool.In this method, two substituents on the scaffold were simulated by two exit vectors n 1 and n 2 (Fig. 3).The relative spatial arrangement of the vectors is described by four geometric parameters: the distance between C-atoms r, the plane angles 4 1 (between vectors n 1 and C-C) and 4 2 (between vectors n 2 and C-C), and the dihedral angle q dened by vectors n 1 , C-C and n 2 .An additional representative parameterthe distance d between two carbon substituents (Fig. 3)was also measured.
In general, both distances r and d and angles 4 1 and 4 2 of 1,2-disubstituted bicyclo[2.1.1]hexaneswere similar to those of the ortho-substituted benzene ring.
Synthesis of the saturated analog of conivaptan was performed from carboxylic acid 1b (Scheme 3).Amide coupling with the corresponding para-substituted aniline gave the desired compound 26.Compound 27,a saturated analog of lomitapide,was obtained analogously from carboxylic acid 8b (Scheme 3).Synthesis of the saturated analog of boscalid was performed from carboxylic acid 5b (Scheme 4).The Curtius reaction followed by acylation of the intermediate amine with 2chloropyridine-3-carboxylic acid gave compound 28.Using an analogous strategy, compound 29,a saturated analog of bixafen,was obtained from carboxylic acid 13b (Scheme 4).The saturated analog of uxapyroxad, compound 30, was synthesized from carboxylic acid 14b.The structures of bicyclo[2.1.1]hexanes28 and 29 were conrmed by X-ray crystallographic analysis. 23

Physicochemical parameters
We studied the effect of the replacement of the orthosubstituted benzene ring by bicyclo[2.1.1]hexaneson the physicochemical properties of bioactive compounds (Schemes 3 and 4).
Replacement of the ortho-benzene ring by bicyclo[2.1.1]hexane in agrochemicals boscalid, bixafen, and uxapyroxad produced conicting results.In boscalid, such a replacement led to a signicant threefold increase in solubility: 11 mM (boscalid) vs. 35 mM (28).In bixafen, the opposite effect was observed, and the solubility was reduced: 30 mM (bixafen) vs. 4 mM (29). 26In uxapyroxad, such a replacement resulted in a slight increase in solubility: 25 mM (uxapyroxad) vs. 27 mM (30).In conclusion, in four out of the ve bioactive compounds, the replacement of the ortho-benzene ring by bicyclo[2.1.1]hexane led to an enhanced water solubility.Lipophilicity.To estimate the inuence of the replacement of the ortho-benzene ring with bicyclo[2.1.1]hexaneon lipophilicity, we used two parameters: calculated lipophilicity (c log P) 27 and experimental lipophilicity (log D).
Replacement of the ortho-benzene ring with bicyclo[2.1.1]hexane had only a small effect on the logD index.In four bioactive compounds (conivaptan, boscalid, bixafen, and uxapyroxad) such a replacement almost did not affect log D. Only in lomitapide, the saturated analog 27 had a signicantly lower log D index: >4.5 (lomitapide) vs. 3.9 mM (27).
In summary, in all ve bioactive compounds, replacement of the ortho-benzene ring by bicyclo[2.1.1]hexaneled to a decrease in calculated lipophilicity (c log P) by 0.7-1.2units; and in four of them it had little effect on the experimental lipophilicity (log D).
In brief summary, in four out of the ve bioactive compounds, replacement of the ortho-benzene ring with bicyclo [2.1.1]hexanedecreased the metabolic stability.

Bioactivity
Finally, we wanted to answer the key question,can 1,2disubstituted bicyclo[2.1.1]hexanesindeed act as bioisosteres of the ortho-benzene ring in bioactive compounds? 9Therefore, we measured antifungal activity of the marketed fungicides boscalid (BASF), bixafen (Bayer CS), uxapyroxad (BASF), and their saturated analogs 28-30.In strict contrast to medicinal chemistry, the use of racemic mixtures in agrochemistry is common; 5 therefore for the validation of the proof-of-concept, we directly studied the biological activity of the available racemic compounds 28-30 (Fig. 4).
We measured the antifungal activity of all six compounds against the fungal strain Aspergillus niger,using the disk diffusion method (ESI, p. S327-S333 †).Even though the original agrochemicals were more potent, all three saturated analogs 28-30 were active and showed a high inhibition of the growth of Aspergillus niger compared to the vehicle (Fig. 4; and ESI, p. S327-S333 †).

Conclusions
In this work, we have synthesized, characterized, and biologically validated 1,2-disubstituted bicyclo[2.1.1]hexanesas saturated bioisosteres of the ortho-substituted benzenes.These structures were obtained from readily available and inexpensive starting materials (acetophenone) on a multigram scale (Schemes 1 and 2).Physicochemical and geometric properties of bicyclo[2.1.1]hexaneswere measured and compared to those of the ortho-substituted benzenes (Fig. 3).The incorporation of the bicyclo[2.1.1]hexanecore into the structure of agrochemicals boscalid (BASF), bixafen (Bayer CS), and uxapyroxad (BASF) gave the saturated patent-free analogs 28-30 with high antifungal activity.
We believe that given the commonplace of the phenyl group in chemistry, its saturated bioisosteres developed here will become common in medicinal chemistry in the coming years.

a
100 mmol scale.b 1 H NMR yield (CH 2 Br 2 as an internal standard).c Isolated yield aer column chromatography.See the ESI for details.n.r.: no reaction.