Organocatalyzed Skeletal Editing of Unstrained Rings via Wolff-type Rearrangement: Modular Synthesis of BN- Isosteres and [1,2]- Azaborines

doi:10.21203/rs.3.rs-5055197/v1

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Organocatalyzed Skeletal Editing of Unstrained Rings via Wolff-type Rearrangement: Modular Synthesis of BN- Isosteres and [1,2]- Azaborines

https://doi.org/10.21203/rs.3.rs-5055197/v1

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Herein, we report a rare example of organocatalyzed skeletal editing of unstrained rings (n = 4–8) to ring-expansion via Wolff-type rearrangement and through regioselective fixation of carbon and nitrogen atom. Strategy employed 2-formylarylboronic acid as C-B surrogate and TMSN₃ as an exogenous single nitrogen source; allowed the de novo rapid synthesis of BN isosteres by forging C-C, C-N and B-N bonds under single operation. The developed method proved to be compatible with wide substrate scope (50 examples) including cyclic ketones and diverse heterocycles afforded 1C ring expanded [1,2]-azaborines. Reaction was also effective to acyclic ketones to give BN naphthalene isosteres. Control experiments and DFT study dictate the plausible reaction pathways following [1,2]-C-C/C-H shift, analogous to Wolff rearrangement.

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The diversification of privileged molecular scaffolds for improving properties is key in drug design and the development of novel pharmaceutical candidates.¹ In order to achieve desired transformations of the underlying key structural core, de novo multi-step synthesis is usually required, which inevitably compromises the overall synthetic efficiency and time. Skeletal editing of core scaffolds, which involves insertion, deletion, or exchange of atoms, has emerged as a contemporary concept in structural modification to quickly prepare analogues of drug molecules or natural products.^2,3 Recently, cut-and-sew methods have been emerged as efficient tool for the direct editing of cyclic scaffolds. Ring expansion occurs with an insertion of external partners (CO, CO₂, alkyne, alkene, etc) via the oxidative addition of low-valent metals (Rh, Ni and other few) into the ring.^4–11 However, there are some key challenges and limitations in developing such reactions, such as limited to strained rings (n = 3 and 4), exotic conditions, and insertion of external nucleophiles takes place mostly at α-position to carbonyls (Fig. 1a).^12–15 Whereas, skeletal editing of unstrained rings and C-insertion at β-position to carbonyls are rare in literature probably due to high C-C bond dissociation energy (Fig. 1b). In addition, covalent B-N bond formation (isoelectronic with C = C) towards to generation of novel boron-containing heterocycles drew much attention of wide research community, comprised of synthetic, medicinal and materials chemists.^16–23 Particularly, [1,2]-benzazaborine, a class of boron–nitrogen (BN) heterocycles are viewed as unique BN isosteres of naphthalenes or quinolines (Fig. 1c).^24–30 They often exhibit better therapeutic features, such as improved metabolic stability and aqueous solubility, probably because the NH groups of the 1,2-benzazaborines can act as hydrogen-bond donors for better binding to proteins.^31–33 This strategy can not only improve biological activity of lead carbonaceous molecules, but also expand intellectual property space. Despite the great promises of BN isosteres, only a limited number of strategies have been reported so far with certain limitations.^34–36 Mostly two component strategies were studied with limited reactant scope and required harsh conditions.^37–44 The seminal recent work by Dong et al reported an three components two step synthesis of monocyclic 1,2-azaborines via ring-opening of cyclopropyl ketone/imines (Fig. 2).⁴⁵ Given the importance of skeletal editing strategy to streamline the rapid production of molecules and great potential of using benzazaborines as arene bioisosteres in drug discovery, herein we disclose an organocatalyzed synthesis of 1,2-benzazaborines via [1,2]-sigmatropic shift/ring expansion of cycloalkanones/ketones (Fig. 2). This strategy represents a true three component modular synthesis of structurally diverse ring-fused [1,2] azaborines (3–6) from easily accessible ketones (1), 2-formylarylboronic acid (2) and TMSN₃. The present innovation could be appealing in aspects of novelty in chemistry pertaining organocatalyzed ring (unstrained; n = 4–8) expansion, exploitation of TMSN₃ as exogenous single nitrogen source,⁴⁶ and regioselective fixation of N and C atoms at α and β position of C = O (Fig. 1e).

Our proposed hypothesis (Fig. 2) was based on the formation of an aldol condensation product (I) through imine/enamine catalysis between ketones (1) and an active formyl group of phenylboronic acid 2 (due to intramolecular activation of aromatic C = O group with precisely located B-center). Reaction on activated alkene with azide would undergo either dihydrotriazole formation (II) via [3 + 2] cycloaddition or aziridine formation (II’).^47–50 Subsequent B-N bond formation due to electrophilic nature of boron could lead to spiro azoborine (III) or azaborole (III’) with the extrusion of molecular nitrogen. These possible reactive intermediates; short lived carbene (III) and ^aReaction was carried out in 0.2 mmol scale. ¹H NMR yields are mentioned.

To test our hypothesis, an experiment was conducted at first with relatively strained ring, cyclobutanone (1a) and 2-formylphenylboronic acid (2a) as model substrates in the presence of different azide source and activator/catalyst. Gratifyingly, TMSN₃ (1.5 equiv.) in the presence of 20 mol% pyrrolidine led to the exclusive formation of cyclopentanone ring fused [1,2]-azaborine 3a in 81% yield (Figure 2) as confirmed by NMR of reaction mixture.⁵² Optimization study further revealed that pyrrolidine is essential for the desired transformation. Other catalytic secondary amines were not effective or delivered inferior chemical yields (Figure 2). Whereas, the reaction with NaN₃ in the presence of catalytic pyrrolidine, resulted merely 12% yield of 3a. Structure of 3a was further unambiguously confirmed by NMR and a single crystal X-ray (CCDC no 2264750, Table 1) analyses, ascertained the regioselective formation 3a (not 3a’). Medium to large cyclic (n = 5 to 8) ketones also afforded the corresponding 1C expanded (n = 6 to 9) ring fused benzazaborines (3b-3e; 25-77% yields, Table 1; CCDC no 2352207 for 3c). Given the importance of skeletal editing or diversification of privileged molecular structures in the discovery of novel pharmaceuticals,⁴ azaborine formation was explored with bioactive scaffolds, such as 2-oxindoles and 2-coumaranones. Under optimum conditions using 20 mol% pyrrolidine, 2-oxindoles gave the corresponding 1C expansion congener, quinolinone fused azaborines 4a-4h in very good to excellent yields (84-90%). This process is highly efficient and didn’t require any column chromatography and competent with electronically diverse free (NH) and N-alkyl oxindoles (Table 1). Similarly, 2-coumaranone afforded the corresponding coumarine ring fused azaborines in moderate to good yields (5a-5c; 52-65%). Such examples of rapid access to novel azaborine analogues of privileged scaffolds could be of great potential in drug development.

After the successful illustration of three component strategy with a range of cyclic ketones, unstrained rings, 2-oxindoles and 2-coumaranone for the synthesis of 1C-expanded ring fused azaborines, acyclic ketones were also investigated to form corresponding [1,2]-azaborines (Table 2). Notably, acyclic ketone was initially anticipated to be difficult under the line of proposed hypothesis (1,2-H or aryl or alkyl shift) due to no ring strain in comparison to cyclic congeners. Despite the fact the reaction was found efficient with acetone under the standard conditions in the presence of 20 mol% pyrrolidine, resulted the selective formation of [1,2]-azaborine 6a in 58% yield. Other aliphatic ketones, such as butanone and 2-hexanone, gave the corresponding azaborines (6b and 6c) in moderate to good yields (58-70%; Table 2). Cyclopropyl methyl ketone was also found compatible to give desired product 6d in 74% yield. Notably, there was no ring opening/closure of cyclopropyl ketone, as reported previously under Lewis acid catalysis.⁴⁵ Reaction with methyl pyruvate afforded the corresponding ethyl ester of azaborine 6e in 62% yield, due to trans-esterification with solvent ethanol. Whereas, pyruvic acid gave the corresponding decarboxylated azaborine 6g (30% yield). However, 5-oxohexanoic acid was compatible to retain its distant CO₂H group in the desired transformation (6f, 58% yield, however required excess pyrrolidine (300 mol%). While studying with aromatic ketones; acetophenone resulted a trace amount of desired product 6h with 20 mol% of pyrrolidine, which can be attributed to low reactivity of aromatic ketones in comparison to aliphatic ketones. Thus, further efforts (See SI for detail optimization)⁵² were made to increase the efficiency of the desired transformation, led to optimum results with pyrrolidine (300 mol%) and TMSN₃ (5 equiv.), resulted 78% yield (75% isolated) for 6h (CCDC no 2236657). High amount of catalyst and TMSN₃ are accounted to felicitate the desired transformation to azaborine 6 and retard the competitive polymerization of 2-formylphenylboronic acid.⁵² This modified optimum condition was scalable as demonstrated at 4.4 mmol scale (isolated 6h, 0.8 g, 72% yield).⁵² Under the above modified conditions, different aromatic ketones, bearing electron withdrawing and electron donating groups at different positions were examined (6h-6ze). 4-Substituted (Br, I, Me, OMe, OH, n-Bu, CF₃ and NO₂) phenyl and dimethoxyphenyl methyl ketones gave the corresponding azaborines (6i-6q) in 65-90% yields. Electron-withdrawing group (NO₂ and CF₃) bearing aryl ketones underwent reaction faster (14 h, 80 ^oC) in comparison to electron-donating counterparts (22-36h, 90 ^oC). This result advocates that reaction might proceeds via enolate formation, following iminium/enamine chemistry. Polyarenes (2-naphthyl and phenanthrenyl) and heteroaromatic (4-pyridinyl, 2-furanyl, and 2-thiophenyl) methyl ketones also worked well to give corresponding azaborines 6r-6v in moderate to very good yields (45-85%). Reactions were proficiently compatible with substituted 2-formylarylboronic acid having electron withdrawing and electron donating group at different position such as CH₃, OMe, F, Cl, CF₃, methylenedioxy, dimethoxy (6w-6zc; 64-75% yields). Bis-azaborines, 6zd and 6ze can be easily synthesized in good yields (76% and 65%) from 1,4- and 1,3-diacetylbenzenes, respectively. After [1,2-H] shift, we intended to check aryl or alkyl shift. To our delight, reaction worked proficiently with 2-phenylacetophenone and propiophenone gave 6zf and 6zg (72% and 62% yields, respectively) with the migration of phenyl and methyl group. Reactions were notably slow as compared to acetophenone and required longer reaction time and slightly elevated temperature.⁵²

Finally, the strategy was employed to late stage modification of drug and complex molecules towards the development of novel potential boron containing drug candidates (Table 3). Ziprasidone is US-FDA approved atypical antipsychotic drug contain 2-oxindole structural core was converted into corresponding quinolinone fused azaborine congener (7a) in 70% yield. Similarly, estrone molecule was transformed into 7b by converting its cyclopentanone nucleus to cyclohexanone ring fused [1,2]-benzazborine. Norcamphor, a bicyclo[2.2.1]heptan-2-one also underwent molecular transformation to bicyclo[2.3.1]octatan-2-one 7c. Reaction overall cover wide range of cyclic and acyclic ketones, and containing bioactive molecules to furnish novel BN- isosteres of privileged scaffolds under very mild reaction conditions.

Mechanistic insights

In order to understand the mechanistic pathways for the developed three-component azaborine synthesis and ring expansion, a series of control experiments were conducted. Initially, we attempted to isolate a proposed aldol/enone intermediate (I, Fig. 2) from 2-formylphenylboronic acid and cyclohexanone/cyclopentanone under base mediated aldol condensation conditions. However, our endeavor failed to isolate it. Pleasingly, we could isolate a model aldol intermediate 8 from 2-formylphenylboronic acid and Wittig ylide of acetophenone.^52,53 Subsequently, it was exposed to TMSN₃ under standard conditions in the presence of pyrrolidine afforded the desired product 6h in 68% yield (Eq. 1, Fig. 3). To our surprise, other anticipated azide Michael addition product (9) was not observed at all.^50,51 As chalcone/aldol product 8 is easily convertible to benzoxaborole 10, can be anticipated to be another intermediate.⁵³ Thus, intermediate 10 was synthesized by stirring 8 in the presence of acidic alumina, and exposed it to an identical condition, resulted the formation of desired product 6h in 36% yield (Eq. 2, ^aReaction was carried out in 0.2 mmol scale. NMR yields are mentioned.

Figure 3. Control experiments and mechanistic Insight (Eq. 1–7) and identifications of molecular ion peaks in MS.

Figure 3). Notably, these intermediates (8 or 10) didn’t give any product in the absence of pyrrolidine (Eq. 3). These results suggested that pyrrolidine plays an important role in the subsequent step (8 / 10 to 6h). To find out the slowest and rate determining step in the present transformations, a model reaction was conducted at lower temperature (45 ^oC), independently with chalcone (8, Eq. 4), benzoxaborole (10, Eq. 5), and standard three component reactions using acetophenone and 2-formylphenylboronic acid (Eq. 6) under standard conditions (Fig. 3). It was observed that intermediates 8 and 10 underwent complete reaction in 16 h and 20 h, respectively (Eqs. 4 and 5). Whereas, the third set of reaction was sluggish and found to undergo merely 20% conversion in 36 h (Eq. 6). Above three set of reactions revealed that first step that is formation of intermediate (8 or 10) might be the slowest step in the present process. The reaction with acetone-d₆ in dry dioxane gave the corresponding desired product 11 (D:H, 1:1), suggested that there is [1,2]-deutrium transfer from CD₃ group of acetone-D₆. Furthermore, trimethylsilyl group was proved to be crucial, as confirmed by control experiments, showed poor yield with NaN₃.⁵² To identify the possible intermediates, electrospray ionization mass spectrometry (ESI-MS) analysis experiments were also conducted with time intervals. Characterization of molecular ion peaks suggested the formation of adducts 8, 10, dihydrotriazole (Int. C) and 6-membered carbene (Int. D) intermediates (see ESI for detail).⁵²

An initial DFT computational study was also conducted to get further insight into our hypothesis (Fig. 4a). The mechanism has been studied with the cyclobutanone as a benchmark. As enone (analogous to 8, Fig. 3) was proved to be the key intermediate for the above transformation, we began our DFT study from an iminium compound (A), formed in situ in the presence of pyrrolidine and computed different possible intermediates (Fig. 4a). 1,3-Dipolar cycloaddition of the olefinic moiety of iminium intermediate (A) and TMSN₃ gave spiro dihydrotrizazole (B).^49,50 The regioselectivity Fig. 4. Mechanism and Energy Profile: a) Energy Profile based on DFT, b) General proposed Mechanism

of the addition of the TMSN₃ is mainly under kinetic control with the difference of 49.3 KJ.mol-1 (TS-a; G = + 102.7 KJ.mol^-1 vs TS-a’; 152.0 KJ.mol^-1 for other isomer).⁵² Intermediate B undergoes B-N bond formation due to electrophilic nature of boron leads to a bridged type intermediate C with the elimination of Me₃Si-OH. Subsequent opening of strained and unstable dihydrotriazole ring C and nitrogen extrusion leads to carbene or diazo-type intermediate D, which in turns leads to [1,2]- C-C or C-H shift, analogous to Wolff-type rearrangement to an intermediate E.^51,54 [1,2]-Migration is energetically driven by the formation of thermodynamically stable benzazaborine E.^24–30 We have not been able to determine any other potential transition states between B to E. One of the explanations could be the impossibility of taking correctly into account the influence of proton and hydroxyl fragments in the concerted rearrangement steps (Fig. 4b). The hydrolysis of the iminium E led to the final isolated products. Accordingly, the general mechanism has been proposed in Fig. 4b.

In summary, a rare organocatalyzed skeletal editing approach has been devised for ring expansion of unstrained cyclic ketones (n = 4–8) via Wolff-type rearrangement. The present strategy represents an efficient three-component synthesis of ring-fused benzoazaborines employing TMSN₃ as an exogenous single “N” source. The strategy was also compatible with acyclic ketones to give BN naphthalene isosteres via [1,2]-a shift of H/aryl/alkyl groups. Reaction was highly efficient and scalable with diverse substrate scopes and functional groups tolerance. Strategy was well-suited to tune privileged scaffolds and drug molecules to potential high valued azaborine congeners. Control experiments and DFT study were conducted to draw the putative reaction mechanism for selective formation of azaborines.

X-ray crystallographic data. CCDC no 2264750 contains the crystallographic data for compound 3a,CCDC no 2352207 contains the crystallographic data for compound 3c and CCDC no 2236657 contains the crystallographic data for compound 6h.These data can be obtained free of charge from Cambridge Crystallographic Data Center (www. ccdc.cam.ac.uk).

Acknowledgement

Financial support from SERB, New Delhi grant no-CRG/2018/002147 and CRG/2023/003756 are highly acknowledged. AS thank UGC for providing fellowship. Authors acknowledge SAIF, CSIR-CDRI for providing analytical supports. CDRI communication number XXXX.

Author contributions

R.K. has conceived and designed the synthetic routes. A.S. has carried out the experiments. Ruchir analyzed the X-rays. M.G. has done DFT calculations. All the authors discussed and co-wrote the manuscript.

Additional information

Supplementary and chemical compound information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to R.K.

Competing financial interests

The authors declare no competing financial interests.

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Tables 1 to 3 are available in the Supplementary Files section.

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Organocatalyzed Skeletal Editing of Unstrained Rings via Wolff-type Rearrangement: Modular Synthesis of BN- Isosteres and [1,2]- Azaborines

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