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DOI: 10.1039/D5OB01012H (Communication) Org. Biomol. Chem., 2025, Advance Article

Diastereoselective synthesis of spiro[cyclopropane-1,3′-indolin]-2′-imines via a sequential [1 + 2] annulation reaction of indolin-2-imines with α-aryl vinylsulfonium salts†

Yan Goua, Shanling Houa, Qingyi Fub, Zhi Fanb and Yuming Li*a
aDepartment of Chemistry, College of Sciences, Tianjin University of Science and Technology, Tianjin 300457, People's Republic of China. E-mail: liyuming22@tust.edu.cn
bCollege of Chemical Engineering and Materials Science, Tianjin University of Science and Technology, Tianjin 300457, People's Republic of China

Received 20th June 2025 , Accepted 24th July 2025

First published on 25th July 2025

Abstract

A sequential [1 + 2] annulation reaction of indolin-2-imines with α-aryl vinylsulfonium salts has been achieved for the first time, leading to a series of functionalized spiro[cyclopropane-1,3′-indolin]-2′-imines in 51–95% yields with high chemoselectivity and diastereoselectivity. The method is easy to manipulate, operates under mild conditions, and shows wide substrate scope with good functional group tolerance. Moreover, some of the selected spiro-annulated products exhibit promising anticancer activity against the A549 cell line, highlighting their potential for application in drug screening.

Introduction

Spirocyclic oxindoles, privileged structural motifs within the nitrogen-containing heterocycle family, hold a special position both in synthetic and medical chemistry.1 In particular, spiro-3,3′-cyclopropyl oxindole skeletons have gained considerable research attention as they are widely distributed in diverse natural alkaloids (e.g., hapalindolinone A and B) and pharmaceutical compounds (e.g., anti-HIV drugs I–II, anti-RSV agents III, and adrenaline agonists IV),2–5 as shown in Fig. 1. Driven by the unique characteristic features and remarkable biological properties of spiro-cyclopropyl oxindoles, several efficient methods have been developed in the past decades, involving transition metal- and photo-catalyzed annulation reactions,6–8 organocatalyzed intramolecular [2 + 1] annulation reactions and others.9 Despite the impressiveness of these studies, there is still scope to develop an efficient and convenient protocol for the synthesis of valuable spirooxindole analogues under operationally simple conditions, which holds great significance.
image file: d5ob01012h-f1.tif
Fig. 1 Selected examples of bioactive spiro-3,3′-cyclopropyl oxindole motifs.

Regarding the three-membered ring-forming reactions, the reaction based on sulfur salts represents an efficient strategy in organic synthesis.10 Among the known transformations, α-substituted vinylsulfonium salts, first reported by Aggarwal in 2014,11a have emerged as useful tools for constructing biologically important cyclopropane-containing ring systems.11,12 However, to the best of our knowledge, the use of α-aryl vinylsulfonium salts as starting materials for the preparation of spirocyclopropyl oxindoles has not been exploited up to now. On the other hand, indolin-2-imines bearing 1,3-C,N-dinucleophilic sites have been proven to be versatile synthons in organic chemistry, enabling the construction of various functionalized heterocycles.13 For example, Smith,13a Li,13b Du13c and He13d et al. independently reported the organic small-molecule-catalyzed [3 + 3] cascade annulation reactions of indolin-2-imines with electrophilic substrates such as α,β-unsaturated p-nitrophenyl esters, N-acylated succinimides, 4-nitrophenyl 3-arylpropiolates and isatin-derived MBH carbonates. Moreover, in 2018, Li and co-workers developed a base-mediated three-component [3 + 1 + 3] cyclization of indolin-2-imines, aldehydes and crotonate-derived sulfur ylides for the synthesis of azepino[2,3-b]indole derivatives.13e Very recently, Wang and co-workers developed a catalyst-free [1 + 5] cycloaddition reaction of 2-sulfonyliminoindolines with 1,3,5-triazinanes, affording a series of pyrimidine-spirofused compounds.14 Notably, the majority of the previously reported annulation reactions involving indolin-2-imines were predominantly focused on constructing indole-fused skeletons, while the development of synthetic methods for new three-membered spiro-annulation products remains scarce. Based on the afore-mentioned motivations and our continued interest in organic sulfur chemistry,11f,15 we describe herein our study on the development of a base-mediated sequential [1 + 2] annulation of indolin-2-imines with vinylsulfonium salts to construct spiro-3,3′-cyclopropyl oxindole derivatives with broad substrate scope and excellent diastereoselectivity in a modular and efficient fashion (Scheme 1).


image file: d5ob01012h-s1.tif
Scheme 1 Reaction mode of indolin-2-imines and the present work.

Results and discussion

We started our investigation by employing 4-methyl-N-(1-methylindolin-2-ylidene)benzenesulfonamide 1a with α-phenylvinylsulfonium tetraphenylborate 2a as model substrates to optimize the annulation reaction conditions in DCM under an air atmosphere for 0.5–24 h using different bases, including K2CO3, Cs2CO3, K3PO4, KOH, t-BuOK, DMAP, Et3N, and DBU (Table 1, entries 1–8). To our delight, the reactions were found to be comparatively well promoted by Cs2CO3, yielding spiro[cyclopropane-1,3′-indolin]-2′-imine 3a in 89% yield with >20[thin space (1/6-em)]:[thin space (1/6-em)]1 dr (Table 1, entry 2). It is noteworthy that the aromatized indole ring 3a′ was not formed in this transformation, and such an abnormal but interesting case is seldom observed in the literature. Subsequent screening of solvents (Table 1, entry 7 vs. entries 9–13) suggested that DCM was superior to other solvents for this domino process, in which the expected sequential reaction was complete within 0.5 h. In addition, when 2.0 equiv. of 2a were used, an improvement in the yield to 95% was achieved (Table 1, entry 14). Further explorations such as increasing the amount of α-phenyl vinylsulfonium salt 2a, lowering or elevating the reaction temperature to 0 °C or 40 °C, changing the ratio of Cs2CO3, and increasing the concentration of substrates were ineffective for the cyclization process (Table 1, entries 15–19). Therefore, the optimal conditions were established as the use of 1a (1.0 equiv.), 2a (2.0 equiv.), and Cs2CO3 (3.0 equiv.) in DCM at room temperature, and product 3a was obtained in 95% isolated yield (Table 1, entry 14).
Table 1 Optimization of reaction conditionsa

image file: d5ob01012h-u1.tif
Entry Base Solvent Time (h) Yieldb (%) dr
a All reactions were carried out with 1a (0.2 mmol), 2a (0.24 mmol), and base (0.1–0.6 mmol) in solvent (2.0 mL) at room temperature unless otherwise stated, and the dr values were determined by 1H NMR analysis.b Isolated yields based on 1a.c The ratio of 1a[thin space (1/6-em)]:[thin space (1/6-em)]2a[thin space (1/6-em)]:[thin space (1/6-em)]base = 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]3.d The ratio of 1a[thin space (1/6-em)]:[thin space (1/6-em)]2a[thin space (1/6-em)]:[thin space (1/6-em)]base = 1[thin space (1/6-em)]:[thin space (1/6-em)]3[thin space (1/6-em)]:[thin space (1/6-em)]3.e Reaction temperature was 0 °C.f Reaction temperature was 40 °C.g 0.1 mmol of Cs2CO3 was used.h 0.2 mmol of Cs2CO3 were used.
1 K2CO3 DCM 2 82 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
2 Cs2CO3 DCM 0.5 89 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
3 K3PO4 DCM 12 72 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
4 KOH DCM 0.5 69 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
5 t-BuOK DCM 0.5 65 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
6 DMAP DCM 24 23 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
7 Et3N DCM 24 15 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
8 DBU DCM 0.5 80 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
9 Cs2CO3 MeCN 12 67 15[thin space (1/6-em)]:[thin space (1/6-em)]1
10 Cs2CO3 Toluene 12 78 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
11 Cs2CO3 THF 12 23 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
12 Cs2CO3 DCE 0.5 82 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
13 Cs2CO3 CHCl3 0.5 65 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
14c Cs2CO3 DCM 0.5 95 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
15d Cs2CO3 DCM 0.5 93 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
16e Cs2CO3 DCM 2 86 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
17f Cs2CO3 DCM 2 89 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
18g Cs2CO3 DCM 2 48 >20[thin space (1/6-em)]:[thin space (1/6-em)]1
19h Cs2CO3 DCM 2 77 >20[thin space (1/6-em)]:[thin space (1/6-em)]1

With the established conditions in hand, we then turned to explore the reaction scope. The scope of indolin-2-imines was first investigated using the Cs2CO3-mediated reaction of α-phenylvinylsulfonium tetraphenylborate 2a (Scheme 2). Substrates 1b–1h bearing an electron-withdrawing group (F, Cl, and Br) or an electron-donating group (Me) were well tolerated, furnishing the corresponding spiro-annulation products 3b–3h in 51–87% yields with high diastereoselectivity. No obvious electronic effect was observed. However, when 4-substituted indolin-2-imines with bulky halo or methyl substituents were utilized, no reaction occurred (3i and 3j). The results indicated that the steric effect had a critical influence on the reaction. Moreover, the substituents at the N-position of the indole ring were explored, and it was found that regardless of whether the substituent was H or Et, the desired products 3k and 3l could be obtained in 80% and 90% yields, respectively.


image file: d5ob01012h-s2.tif
Scheme 2 Scope of indolin-2-imines 1. Reactions were performed with 0.2 mmol of 1, 0.4 mmol of 2a, and 0.6 mmol of Cs2CO3 in DCM (2.0 mL), and the dr values were determined by 1H NMR analysis. Isolated yields. a[thin space (1/6-em)]Reaction time: 2.5 h.

With these encouraging data in hand, we then surveyed the scope of vinylsulfonium salts (Scheme 3). Various substituents including electron-deficient (F, Cl, and Br) and electron-rich (t-Bu and OAc) groups at the para- and meta-positions of the aryl moiety did not hamper the reaction process, affording the desired products 3m–3r in moderate to high yields (75%–91%). Additionally, the reaction of 1a with the bulky α-biphenylvinylsulfonium salt 2h and α-(1-naphthyl)-vinylsulfonium salt 2i could also generate products 3s and 3t in 83% and 85% yields, respectively. Gratifyingly, the vinylsulfonium salt 2j with the substituent at the ortho-position of the benzene ring was also compatible, giving the desired product 3u in 70% yield. Notably, only a single diastereomer was obtained in all cases, and the structure of 3u was unambiguously confirmed by single-crystal X-ray analysis (CCDC 2457160; see the ESI† for details).


image file: d5ob01012h-s3.tif
Scheme 3 Scope of vinylsulfonium salts 2. Reactions were performed with 0.2 mmol of 1a, 0.4 mmol of vinylsulfonium salts 2, and 0.6 mmol of Cs2CO3 in DCM (2.0 mL), and the dr values were determined by 1H NMR analysis. Isolated yields. a[thin space (1/6-em)]Reaction time: 2.5 h.

To demonstrate the potential synthetic utility of this method, a scale-up experiment was performed, which is shown in Scheme 4a. The Cs2CO3-mediated reaction of 2a with a loading of 3.0 mmol of 1a proceeded smoothly to give the target product 3a in 92% yield with >20[thin space (1/6-em)]:[thin space (1/6-em)]1 dr. Furthermore, we investigated the anticancer activity of all the synthesized products 3 by evaluating their cytotoxicity against human non-small cell lung cancer A549 cells. To our delight, compound 3k showed promising anti-cancer activity with a minimum inhibitory concentration (MIC) of 16.48 μg mL−1 (Table 2), indicating that a detailed study of the structure–activity relationship (SAR) of these novel scaffolds might improve their activity and potential application in drug discovery.


image file: d5ob01012h-s4.tif
Scheme 4 Scale-up synthesis experiment.
Table 2 lC50 values for compounds 3a–u in the human cancer A549 cell linea
Compounds MIC/(μM ± SD) Compounds MIC/(μM ± SD)
a 50% inhibitory concentration and the values are the average of three individual experiments after 48 h of drug treatment.
3a 40.71 ± 0.36 3m 33.77 ± 1.96
3b 31.57 ± 1.15 3n 38.25 ± 1.12
3c 35.61 ± 0.98 3o 69.86 ± 0.56
3d 38.79 ± 2.36 3p 55.89 ± 3.33
3e 27.92 ± 1.09 3q >100
3f 39.05 ± 5.36 3r 68.04 ± 0.87
3g 66.82 ± 0.74 3s 44.72 ± 2.23
3h 44.19 ± 0.38 3t 38.48 ± 0.96
3k 16.48 ± 2.34 3u 45.41 ± 2.64
3l 43.95 ± 1.77    

On the basis of the above results and previous references,12 a possible reaction mechanism is proposed in Scheme 5. Initially, the deprotonation of indolin-2-imine 1a in the presence of base generates the nucleophilic anion A, which can resonate with intermediate A′. The subsequent intermolecular Michael addition of A to vinylsulfonium salt 2a affords intermediate B, followed by an intramolecular 1,3-proton shift to give intermediate C. Finally, an intramolecular SN2 reaction occurs via intermediate C, yielding the expected polycyclic product 3a and releasing dimethyl sulfide simultaneously.


image file: d5ob01012h-s5.tif
Scheme 5 Proposed reaction mechanism.

Conclusions

In summary, we have developed a metal-free sequential [1 + 2] annulation reaction of indolin-2-imines with vinylsulfonium salts, affording 21 examples of spiro[cyclopropane-1,3′-indolin]-2′-imines up to 95% yields as single diastereomers. The reaction is characterized by simple operation, excellent functional group tolerance, and easy product scalability, making it highly valuable in synthetic chemistry. Furthermore, the synthesized compounds were screened against the human cancer A549 cell line, and compound 3k showed promising anticancer activity.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data underlying this study are available in the published article and its ESI.†

Acknowledgements

We gratefully acknowledge the financial support from the School-level Research Projects of Tianjin University of Science and Technology (202410057001), and we thank Dr Fengchao Wang (Instrumental Analysis and Research Center, Tianjin University of Traditional Chinese Medicine) for providing biological activity testing.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2457160. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5ob01012h
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