Abstract

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Sp³-enriched small molecules play a critical role in developing drug candidates. While designing analogues with greater sp³ character, a methodology utilizing a less explored cyclic-aziridine amide ring-opening reaction to generate sp³-enriched scaffolds has been developed and reported. This methodology enables rapid access to substructures with higher fsp³ values, attracting greater attention within the past few decades. The reaction exhibits a wide reaction scope, featuring a highly sterically hindered phenolic ether, thiophenolic ethers, protected aniline formations, and aliphatic/heteroaromatic ring-containing aziridine amides as substrates. Additionally, this reaction provides access to congested tertiary ether formations through regioselective transformation, applicable to an extensive range of drug discovery targets, construction of complex small molecules, and natural product syntheses. The scaffolds developed show improved physicochemical properties.

Introduction

Fsp³, a fraction of sp³ carbon atoms, has a significant impact on the drug-likeness of small molecules and is well known as a verified indicator of physicochemical properties in drug discovery.¹ A higher fsp³ could reduce the promiscuity of a drug candidate to increase the success rate of its clinical trial.² However, the enantioselective or diastereoselective introduction of higher fsp³ motifs can be challenging due to their selectivity control. Meanwhile, aniline substructures are frequently encountered in pharmacologically active compounds,³ such as natural products and small molecule probes, since carbon–nitrogen bonds on aromatic rings are highly transformative. However, the motif has been well known as an alert structure with the potential for mutagenic character, resulting from a high tendency of bioactivation by CYP450 and reactive metabolite formation.⁴ Thus, an alternative aniline motif, including bicyclo[1.1.1]pentane (BCP), bicyclo[2.2.2]octane (BCO), and cubane (CUB), as benzene isosteres has been explored and examined for decades to achieve a higher safety profile as drug candidates.⁵

Case in point, a methodology for the replacement of o-substituted benzamides with trans-substituted cyclic analogues has been explored to prevent the generation of mutagenic structural motifs and to obtain enriched sp³ motifs (Scheme 1a). Moreover, this transformation is favorable to secure significant intellectual property. As depicted in Scheme 1b, the 1,2-trans-disubstituted cyclic structural motif has a diversity of dihedral angles and orientations between substituents depending on the steric bulk, ring size, and nature of the substituted functional groups. Since this motif can fill a space in a unique and tunable direction with its 3D structure, it has been an attractive scaffold in drug discovery for decades. Consequently, this motif is present in a wide range of marketed drugs, including the neuramidase inhibitor Oseltamivir,⁶ which contains a multisubstituted cyclohexane central core, Carmegliptin,⁷ a dipeptidyl peptidase 4 (DPP-4) inhibitor, and fused-piperidine core analogue, and κ-opioid receptor (KOR) agonists Spiradoline⁸ and Nalfurafine.⁹ (Scheme 1c).

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Scheme 1

Significance of Trans 1,2-Disubsituted Cycloaliphatic Motifs in Drug Discovery

However, due to the sterically congested nature of this type of substitution, there are limited synthetic routes to afford ortho-substituted aryloxy trans cyclic benzamide (Scheme 2).

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Scheme 2

Previously Reported Methods for Trans 1,2-Disubsituted Cycloaliphatic Motifs and This Work

Typically, known synthetic approaches employ N-protected reagents and intermediates and include the use of Mitsunobu inversion of cis-substituted N-Boc protected alcohols (A), followed by deprotection of the protecting group and a coupling reaction (Scheme 2a).¹⁰ Additionally, a simple nonsubstituted aryloxy trans cyclic benzamide can be accessed via protected cyclic aziridine ring cleavage (Scheme 2).

Although several N-protected cyclic aziridine ring-opening reactions, which are exemplified as applications of an aliphatic phosphine reagent P(n-Bu)₃,¹¹ Lewis acid B(OPh)₃,¹² BF₃OEt₂,¹³ and Sn(OTf)₂,¹³ are known in the literature,¹⁴ such transformations tend to be limited in scope. Specifically, reports for this type of transformation tend to be restricted to simple nucleophiles with minimal steric encumbrance and are often confined to symmetric azidirine substrates. And, under acidic conditions such as B(OPh)₃, the reaction shows a potential issue of incomplete cis/trans diastereoselectivity as a result of its cationic mechanism.

Importantly, the reactions have been limited to aziridines protected with classical protecting groups (Boc or Cbz) and arylsulfonamide-type substitution patterns (Ts or Ns) without expanding their scope to arylamide-type substitution.¹⁵ This explored substrate bias comes from the higher reactivity of sulfonylated or Boc/Cbz protected substitutions due to their higher electron withdrawing properties, and its inherent nature of the aziridine benzamide substructure. As shown in Scheme 2c, it is well known that the aziridine benzamide substructure (H) tends to undergo the Heine reaction,¹⁶ which can be easily induced by acidic¹⁷ or nucleophilic¹⁸ conditions; therefore, the detailed studies have been hampered.

Additionally, these types of transformations typically require long reaction times and relatively harsh conditions, which often limit the reaction scope. Therefore, by utilizing basic conditions, we sought to develop a methodology for an aziridine ring-opening reaction that would address these issues and focus specifically on underexplored diverse amide-substituted aziridine substrates. Herein, we report the detailed study of an efficient sp³-enriched scaffold generation through a cyclic symmetrical and nonsymmetrical aziridine amide ring-opening reaction, as well as a feature of generated molecules. This reaction features: (1) a wide range of reaction scope including sterically hindered o-substituted nucleophiles and aliphatic/heteroaromatic ring-containing aziridine amides as substrates; (2) the selective formation of a tertiary ether; (3) direct trans 1,2-disubstituted amide scaffold generation without a multistep deprotection/functionalization; (4) stereoselective conversion and regioselective conversion for the nonsymmetric cyclic aziridine amide; and (5) a short reaction time (10–20 min). These goals were ultimately realized using a 2-step sequence (Rh₂(esp)₂¹⁹-catalyzed nitrene transfer reaction from cyclic olefins²⁰ and an amide coupling reaction), which allows for the rapid generation of a wide range of sp³-enriched cyclic motifs (Scheme 2d).

Results and Discussion

Since regioselectivity of nonsymmetric 6-membered cyclic aziridine rings with o-substituted phenol nucleophiles has been underexplored, we conducted an extensive reaction optimization campaign with a piperidine-based aziridine benzamide. We employed various operationally simple bases, the sterically hindered o-cresol as a nucleophile, and a variety of solvents while considering the pK_a value (pK_a = 10.3) of o-cresol (Table 1).

Table 1

Optimization of the Reaction Conditions^a

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entry	base	solvent	temp (°C)	time (min)	4aa + 4ab (%)b	ratio (4aa/4ab)
1	none	THF	110	20	0	-
2	Cs2CO3	THF	110	20	96	3.0/1
3	DBU	THF	110	20	3	-
4	Li2CO3	DMF	110	20	10	2.9/1
5	Na2CO3	DMF	110	20	22	4.0/1
6	K2CO3	DMF	110	20	82	4.4/1
7	Cs2CO3	DMF	110	20	85	4.0/1
8	K2CO3	CH3CN	110	20	29	3.9/1
9	K2CO3	acetone	110	20	22	3.8/1
10	K2CO3	toluene	110	20	0	-
11	K2CO3	DCE	110	20	0	-
12	K2CO3	DMF	80	20	43	4.9/1
13	K2CO3	DMF	140	20	88	3.7/1
14	Cs2CO3	THF	140	20	95	2.7/1
15	K2CO3	DMF	110	10	51	4.5/1
16	Cs2CO3	THF	140	10	93	2.8/1

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^aReactions were performed with 3 (1.0 equiv), base (1.2 equiv), and o-cresol (1.2 equiv) in solvent (0.10 M) under microwave irradiation.

^bYield determined via LCMS using a calibration curve and naphthalene as an internal standard.

During the preliminary study, we tested various bases [Li₂CO₃, Na₂CO₃, K₂CO₃, Cs₂CO₃, N,N-diisopropylethylamine (DIPEA), pyridine, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), Supporting Information, Table S1 and entry 3] in tetrahydrofuran (THF) under microwave irradiation to determine that Cs₂CO₃ was the most effective base, resulting in a high yield (96%) in a time-efficient manner (20 min) (entry 2). Gratifyingly, no epimerization was observed in this study. As for regioselectivity, nucleophilic addition occurred predominantly at the C4 position, which is consistent with the results observed for other nucleophiles with 6-membered piperidines.²¹ As a control experiment, a reaction without a base was conducted to show the necessity for the reaction to proceed (entry 1). In the next step, other solvent systems were explored for their effects on regioselectivity. To our pleasure, utilizing several bases with N,N-dimethylformamide (DMF) boosted regioselectivity up to 4.4/1 (entry 4–7). Other solvents [acetonitrile (CH₃CN), acetone, toluene, and 1,2-dichloroethane (DCE)] were also tested to show that DMF was the most efficient in terms of both yield and selectivity (entry 6, 8–11). Next, optimization of reaction temperature using K₂CO₃/DMF was conducted to show that 110 °C was well balanced in both yield and selectivity (entry 6, 12, and 13). In a similar manner, using Cs₂CO₃ in THF at higher temperatures gave comparable yields but less selectivity (entry 2 and 14). Finally, a shorter 10 min reaction time favorable to rapid library generation, was performed (entry 15 and 16). The Cs₂CO₃/THF protocol was preferred in terms of the high yield (93%) although regioselectivity was moderate (2.4/1). As a control experiment, the previously reported phosphine conditions using P(n-Bu)₃¹¹ was attempted, but no product formation was observed after 10 min (Table S1). Considering the high regioselectivity and yield in an efficient reaction time for nonsymmetric aziridine formation, we selected the condition described in entry 6 as the standard condition for nonsymmetric aziridines such as 3. For a symmetric cyclic aziridine 1 (5-membered ring; n = m = 1, 7-membered ring; n = m = 2, Scheme 2), the condition described in entry 16 was selected as a standard high yielding reaction condition since regioselectivity is unnecessary due to its symmetric plane.

With optimized conditions for cyclic aziridine amide ring openings in hand, we sought to explore the substrate and nucleophile scope for these reaction conditions (Schemes 3 and 4). As a general method for preparing aziridine amide substrates, 2-step procedures, including an aziridination reaction for the corresponding cyclic olefin and the subsequent coupling reaction with variety of carboxylic acids, were utilized successfully. This was suitable to increase accessibility for a broader range of substrates (Scheme S1). N–H free aziridination was achieved through a nitrene transfer reaction by employing Du Bois’ catalyst Rh₂(esp)₂,¹⁹ which conditions were originally reported by Kürti et al.²⁰ The amide formation was conducted through a 1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxid hexafluorophosphate (HATU) coupling or acylation with acid anhydride. For symmetric 5-membered pyrrolidine aziridine amides, a broad range of nucleophiles could be applied to give trans substituted pyrrolidine analogues (Scheme 3). Importantly, substituted pyrrolidines are one of the lead structures in many drugs and drug-like small molecules,²² where the motif is ranked among the top five common nitrogen-containing heterocycles in a survey of FDA-approved drugs.²³ Gratifyingly, the o-, m-, and p-Me substituted cresols gave the desired products in high yields (2a–2c, 85–88%). The o-Me-substituted phenolic ether 2a was confirmed by X-ray analysis to demonstrate clear relative trans stereochemistry (CCDC; 2280912). In addition, further sterically hindered o-substituted phenols, including di-o-substituted phenols, provided an excellent yield (2d–2f). These results highlight the utility of the methodology since this sterically congested phenolic ether formation can be challenging from coupling between a corresponding sterically hindered alcohol and an electron-rich arylhalide or arylboronic acid. On a 1.0 mmol scale, the ring-opening reaction provided o-iPr-substituted phenolic ether (2d) in a comparable yield (92%) compared to the 89.2 μmol scale reaction (95%). In a similar manner, an electron-deficient pyridinol and phenols gave the desired ring-opened products (2g–2i). In addition to phenols, 4-nitrobenzenesulfonyl (4-Ns) protected aniline, and an o-substituted thiophenol provided the desired products in high yields (2j and 2k), which exemplifies a broad range of nucleophile scopes. Additionally, a resulting 4-Ns group in 2j could be deprotected orthogonal to the Boc group to afford 2j′ (89%), which demonstrates the possibility for increased functionalization of the scaffold (Supporting Information). The substrate scope of the aziridine amide ring-opening reaction could be expanded to a symmetric 7-membered azepane aziridine amide. In a similar manner, all nucleophiles, including sterically hindered phenols, electron-deficient phenols, and a pyridinol, afforded the corresponding trans-substituted azepane analogues in good yields (2l–2r, 69–95%).

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Scheme 3

Substrate/Nucleophile Scope of a Selective Aziridine Amide Ring-Opening Reaction

All reactions for symmetric aziridine amides were performed with 1 (1.0 equiv), Cs₂CO₃ (1.2 equiv), and o-substituted nucleophile (1.2 equiv) in THF (0.10 M) at 140 °C for 10 min under microwave irradiation; isolated yield. ^bAll reactions for nonsymmetric aziridine amides were performed with 3 (1.0 equiv), K₂CO₃ (1.2 equiv), and o-substituted phenol (1.2 equiv) in DMF (0.10 M) at 110 °C for 20 min under microwave irradiation; isolated yield. ^cIsolated yield on a 1.0 mmol scale. ^dYield indicated as a combined amount of regio isomers. ^eYield determined via LCMS. ORTEP, ellipsoids are set at a 50% probability.

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Scheme 4

Reaction Scope of Various Amide Groups and Various Cyclic Aziridines for Aziridine Ring Opening

Unless otherwise specified, all reactions were performed with 5 (1.0 equiv), Cs₂CO₃ (1.2 equiv), and o-cresol (1.2 equiv) in THF (0.10 M) at 140 °C for 10 min under microwave irradiation; isolated yield. ^bYield indicated as a combined amount of regio isomers. ^cCs₂CO₃ (5.0 equiv), o-cresol (5.0 equiv) for 40 min. ^dCs₂CO₃ (5.0 equiv), o-cresol (5.0 equiv) for 80 min.

As for nonsymmetric piperidine aziridine amides (3), the optimal condition, which was identified in Table 1, entry 6, could also be applied in excellent yields while maintaining regioselectivity (4.4–7.1:1) to afford trans substituted piperidine analogues (Scheme 3). In addition to a pyrrolidine ring, a piperidine ring is one of the lead scaffolds in drug discovery development. The piperidine ring was the most prevalent nitrogen ring system and also found in over 70 unique small-molecule drugs in FDA-approved drugs.²³ The utilization of cresols and sterically hindered phenols maintained regioselectivity with good yields (4aa–4fa, 56–82%, 4.4–7.2:1) Notably from these analogues, an o-iPr substitution demonstrated greater regioselectivity than other substituents (7.2:1). In addition, the o-thiocresol gave the ring-opened product in an excellent yield with high regioselectivity (4ga, 88%, 5.7:1) as similar results were observed for an electron-deficient pyridinol and phenols (4ha–4ja, 75–83%, 5.2–5.9:1).

Encouraged by these results, we further explored the reaction scope regarding various amide groups and cyclic aziridines, including congested trisubstituted patterns (Scheme 4). A pyrrolidine aziridine amide, with an electron-rich benzamide and electron-deficient pyridine amide, showed robust reactivity to afford ring-opened products (6a–6f, 63%-quant). Pleasingly, a 5-membered heteroaromatic ring and an aliphatic aziridine amide also generated the desired product in high yields (6g, 85%, 6h, 77%), which features a novel and broad substrate scope. In addition, a 4-Ns functionalized aziridine and a Boc-protected azetidine spirocycle bearing aziridine amides produced ring-opened products (6i, 65%, and 6j, 88%). The 4-Ns protecting group can be further deprotected in the presence of the Boc group, and spiroazetidine ring products allow for further functionalization with high fsp³ fragments.

Lastly, we explored aziridine amides that include trisubstituted cyclic aziridine amides (5k–5m). Interestingly, the reaction for them afforded the tertiary ether in moderate to good yield in a selective manner (6ka–6ma, 4.4:1 to single isomer, 48–73%). Notably, these findings are the first reported examples of tertiary ether formation in cyclic aziridine amide ring-opening reactions, and their utility could be helpful for constructing sterically congested structural motifs.²⁴ Meanwhile, the 6-membered lactam aziridine amide substrate (5n in Supporting Information) did not provide any ring-opening product. This may be due to the acidity of the α-proton, which can result in aziridine ring decomposition via β-elimination. Limitations with nucleophiles were observed as reactions with an aliphatic alcohol, and aniline nucleophiles did not undergo the ring-opening reaction due to lack of nucleophile acidity.

As a mechanistic hypothesis for the ring-opening reaction, including tertiary ether formation, it is hypothesized that a nucleophilic SN₂ substitution via a 6-membered transition state occurs predominantly or exclusively at the more substituted site in the aziridine amide ring (Scheme 5a), considering the resulting stereochemical relationships (6ka–6ma) in Scheme 4. To investigate the regioselectivity, each bond length within the aziridine amide ring was measured by X-ray crystallographic analysis (5m; CCDC 2280913, 7; CCDC 2280914). As depicted in Scheme 5b, the C4–N bond in 7 was longer (1.4836 Å) than the C3–N bond (1.4602 Å), while a slight difference was observed in 5m (C3–N bond; 1.4813 Å vs C4–N bond; 1.4706 Å). This means that a longer bond is weaker and more reactive, which is consistent with its regioselectivity for tertiary ether formation for 6l (corresponding 7) and 6ma (corresponding 5m). The longer bond length at the more substituted carbon may be due to the electron-donating property of alkyl groups, suggesting that more alkyl substitutions in the aziridine ring would donate electrons at the more substituted carbon and induce an electron density shift from a more substituted carbon to the nitrogen atom of the aziridine (Scheme 5b, C4–N bond in 7 and C3–N bond in 5m). Consequently, a more substituted carbon would increase the C–N bond length and its reactivity for nucleophiles. Second, this bond length difference could also be understood from the standpoint of a stereoelectronic effect. Presumably, an overlapping of the filled σ_C–H orbital of the alkyl group or Me group at the more substituted carbon with the empty σ*_C–N orbital of the aziridine ring can contribute to stabilization of the longer bond length.²⁴ Third, the longer bond in the aziridine ring could be caused by steric repulsion between the Me group of the substituted site and the aziridine N-Bz moiety. In the analysis of X-ray crystal structures of 5m and 7, the Me group at the aziridine ring is closely located with the N(aziridine)–C(C=O of benzoyl group) moiety, generating steric repulsion.

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Scheme 5

Mechanistic Hypothesis for the Regioselectivity

Finally, we compared the calculated physicochemical properties between newly generated aliphatic scaffolds (Figure ​Figure11a, e–k) and corresponding aromatic core analogues as a reference (Figure ​Figure11a, a–d). The calculation for the generated aliphatic scaffolds was performed for the Boc deprotected form (Figure ​Figure11a, a–d, Supporting Information, Table S6). As displayed in Figure ​Figure11a, fsp³ of the aliphatic central core analogues is significantly increased (e–k; 0.334 vs a–d; 0.053) compared to the aromatic central core analogues. Notably, the aliphatic central core scaffolds maintain a sufficient molecular weight (average; e–k; 318 vs a–d; 301) and exhibit slightly lower clog P (e–k; 3.23 vs a–d; 3.97). In addition to this calculation, kinetic solubility for both selected generated aliphatic scaffolds (e–g) and corresponding aromatic analogue (d) was assessed. As shown in Figure ​Figure11b, the available 1,2-trans disubstituted cyclic scaffolds showed significantly improved kinetic solubility at neutral pH (e; a pyrrolidine ring, 79.07 μM, f; a piperidine ring, 90.67 μM, g; an azepane ring; 92.17 μM) compared to aromatic core analogues (d; 5.74 μM). The increased fsp³ and the additional amine functionality would contribute to increase their solubility significantly. These results highlight that this methodology is practical for obtaining higher fsp³ scaffolds with improved kinetic solubility but also avoids the production of a mutagenic alert structure to improve the potential tox profile.

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Figure 1

Evaluation of generated scaffolds.

Conclusions

In summary, we have described the rapid generation of sp³-enriched scaffolds by developing a novel selective cyclic aziridine amide ring-opening reaction with high regioselectivity. This methodology is highlighted by a wide reaction scope range, which encompasses the formation of a sterically hindered phenolic ether, aliphatic and heteroaromatic rings containing aziridine amides as substrates, and the regioselective formation of a tertiary ether. Additionally, the direct generation of trans-substituted amide scaffolds can be obtained without the use of a protecting group for aziridine functionalization. Further, this methodology allows for a stereoselective and regioselective conversion of aziridine amides, is operationally straightforward, and can be performed in a short reaction time. Efforts to understand the extensive regioselective reaction mechanics associated with this novel methodology, expand the current reaction scope, and further applications in drug discovery and natural product synthesis are underway in our laboratory and will be reported in due course.

Experimental Section

Acknowledgments

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.joc.3c02952.

• Experimental procedures, characterization data, X-ray crystallographic data and detailed physicochemical property data (PDF)

Author Contributions

M.A. conceived this work and performed synthetic chemistry with J.S.C. and C.C.P. N.D.S. conducted X-ray analysis. M.A. wrote the manuscript with input and revisions from all authors.

Notes

The authors declare no competing financial interest.

References