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International Journal of Molecular Sciences logoLink to International Journal of Molecular Sciences
. 2022 Oct 20;23(20):12639. doi: 10.3390/ijms232012639

Selective and Reversible 1,3-Dipolar Cycloaddition of 2-(2-Oxoindoline-3-ylidene)acetates with Nitrones in the Synthesis of Functionalized Spiroisoxazolidines

Dmitriy D Karcev 1, Mariia M Efremova 1, Alexander P Molchanov 1, Nikolai V Rostovskii 1,*, Mariya A Kryukova 1, Alexander S Bunev 2, Dmitry A Khochenkov 2,3
Editor: Elena K Beloglazkina
PMCID: PMC9603865  PMID: 36293495

Abstract

The 1,3-dipolar cycloaddition of 2-(2-oxoindoline-3-ylidene)acetates with functionalized aldo- and ketonitrones proceeds with good selectivity to provide new highly functionalized 5-spiroisoxazolidines. A characteristic feature of these reactions is reversibility that allows for the control of the diastereoselectivity of cycloaddition. The reduction of obtained adducts using zinc powder in acetic acid leads to 1,3-aminoalcohols or spirolactones. For a number of the spiro compounds obtained, anticancer activity was found.

Keywords: 1,3-dipolar cycloaddition; spiroisoxazolidines; nitrones; regioselectivity; diastereoselectivity

1. Introduction

In recent years, spiroheterocyclic scaffolds have attracted increased interest in the search for promising candidates for the development of new medicines [1,2,3,4]. Moreover, spirocyclic motifs can often be found in natural compounds [5]. It is known that spiro derivatives of indolin-2-one are privileged structures in medicinal chemistry [6,7,8], due to their wide range of biological properties, including anti-inflammatory [9], anticancer [10,11], and antimycobacterial activity [12]. In addition, this structural fragment is common to indolinone alkaloids [13].

In the last decade, the number of methodologies for the synthesis of spiroindolin-2-one derivatives has been growing rapidly [14,15]. One of the most effective among them is based on 1,3-dipolar cycloaddition reactions [7,16,17]. The most investigated cycloadditions using indolin-2-one-based dipolarophiles are reactions with azomethine ylides, which make it possible to obtain spiropyrazolines in high yields and selectivity [18,19,20,21,22,23,24]. Promising biological properties were noted for the obtained cycloadducts [25,26,27,28,29,30]. However, the cycloaddition reactions of nitrones to indolin-2-one-based dipolarophiles are represented by only a few examples [31,32,33,34,35,36,37] (Scheme 1). It was found that such reactions can proceed with different regio- and stereoselectivity, depending not only on the structure of the starting compounds but also on the reaction conditions. The prediction of selectivity in the case of 2-(2-oxoindoline-3-ylidene)acetates is complicated due to the reversibility of the nitrone cycloaddition reactions [34,37]. Previously, the reactions with aldonitrones that did not contain additional functional groups were most studied. However, it is known that the transition from aldo- to ketonitrones or the introduction of additional functional groups in the α-position of the nitrone can dramatically affect the regio- and stereoselectivity of the 1,3-dipolar cycloaddition reactions [38,39,40].

Scheme 1.

Scheme 1

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The 1,3-dipolar cycloaddition of 2-(2-oxoindoline-3-ylidene)acetates with nitrones [31,33,35,36].

Isoxazolidine derivatives, the products of the cycloaddition of nitrones to the C=C bond, are of considerable interest due to the wide range of their biological properties [41,42,43,44,45,46,47,48,49,50,51]. Moreover, due to the ease of the N–O bond cleavage in the isoxazolidine ring under reductive conditions, these compounds can be used for the synthesis of 1,3-aminoalcohols [52,53,54], biologically valuable β-lactams [55,56,57,58,59], and amino acids [60]. The presence of additional functional groups in the isoxazolidine ring significantly expands the possibilities of using these substrates in organic synthesis [61,62].

In this work, the regio- and stereoselectivity of the 1,3-dipolar cycloaddition reactions of functionalized aldo- and ketonitrones with 2-(2-oxoindoline-3-ylidene)acetates were investigated for the first time. As a result, a series of novel 5-spiroisoxazolidines were synthesized with controllable diastereoselectivity. The introduction of the ester group at the C3 of isoxazolidines makes it possible to redirect the reduction from 1,3-aminoalcohols to spirolactones. In addition, the anticancer activity of the obtained functionalized spiroisoxazolidine derivatives was evaluated.

2. Results and Discussion

2.1. Synthesis and Structural Characterization

First, we studied the reactions of 2-(2-oxoindoline-3-ylidene)acetates 1 with C-carbamoyl aldonitrones 2. A short optimization of the reaction conditions was carried out for the reaction of 1a with nitrone 2a (Table 1). We obtained the mixtures of isomeric cycloadducts with a predominance of 5-spiroisoxazolidines 3 in all variants of the tested reaction conditions. The stereoselectivity of the reactions was found to be dependent on the conditions. Heating in toluene at 80 °C was optimal for obtaining diastereomer 3a as the main product (Table 1, Entry 2). At a higher temperature, 3a remained the main reaction product, but its yield decreased due to the considerable tarring of the reaction mixture (Table 1, Entry 3). We obtained diastereomer 3′a as the main product of the reactions conducted at room temperature in dichloromethane (DCM). In this case, we had to change the solvent from toluene to DCM due to the low solubility of compounds 1a and 2a in toluene (Table 1, Entry 4).

Table 1.

Optimization of cycloaddition reaction conditions.

graphic file with name ijms-23-12639-i001.jpg

Entry Conditions Ratio 3a:3′a:4a 1 Isolated Yield, %
3a 3′a 4a
1 CH3CN, 82 °C, 14 h - 2 67 - -
2 PhMe, 80 °C, 14 h 4:1:0.3 70 18
3 PhMe, 110 °C, 7 h - 2 45 - -
4 CH2Cl2, rt, 7 h 1:4:0 11 73 -
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1—from 1H NMR spectra of the crude reaction mixtures; 2—analysis of the 1H NMR spectrum of the reaction mixture is difficult due to tarring.

It was found that the reactions of 2-(2-oxoindoline-3-ylidene)acetates 1a,b with C-carbamoyl nitrones 2a–d proceed regio- and stereoselectively in toluene at 80 °C, predominantly obtaining 5-spiroisoxazolidines 3 (Table 2). In most experiments, the minor products 3′ and 4 could not be separated chromatographically, and in the table their yields are presented as summary.

Table 2.

Cycloaddition of 2-(2-oxoindoline-3-ylidene)acetates 1a,b with C-carbamoyl nitrones 2a-d under heating.

graphic file with name ijms-23-12639-i002.jpg

Entry Dipolarophile R Nitrone R’ Products Ratio 3a:3′a:4a 1 Isolated Yield, %
3 (3′ + 4)
1 1a H 2a H 3a, 3′a, 4a 4:1:0.3 70 18
2 1a H 2b Cl 3b, 3′b, 4b 5:1:0.2 70 16
3 1a H 2c Me 3c, 3′c, 4c 5:1:0.5 57 24 2
4 1a H 2d MeO 3d, 3′d, 4d 4:1:0.3 69 19
5 1b Me 2a H 3e, 3′e, 4e 5:1:0.2 74 13
6 1b Me 2b Cl 3f, 3′f, 4f 5:1:0.2 65 17
7 1b Me 2c Me 3g, 3′g, 4g 3.5:1:0.3 55 20
8 1b Me 2d MeO 3h, 3′h, 4h 3.2:1:0.2 58 12 3
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1—from 1H NMR spectra of the crude reaction mixtures. 2—8% of the (3′c + 4c) mixture and 16% of pure 3′c. 3—pure 3′h.

At the same time, it was possible to selectively obtain diastereomers 3′ when carrying out the reaction in DCM at room temperature (Table 3). In this case, we observed no signals of regioisomeric products 4 in the 1H NMR spectra of the crude reaction mixtures. The compound 3′e was also obtained as a major product (yield 54%) in toluene at 55 °C for 7 h.

Table 3.

Cycloaddition of 2-(2-oxoindoline-3-ylidene)acetates 1a,b with C-carbamoyl nitrones 2a,b at rt.

graphic file with name ijms-23-12639-i003.jpg

Entry Dipolarophile R Nitrone R’ Products Ratio 3a:3′a 1 Isolated Yield, %
3 3′
1 1a H 2a H 3a, 3′a 1:4 11 73
2 1a H 2b Cl 3b, 3′b 1:4 17 58
3 1b Me 2b Cl 3f, 3′f 1:4 14 61
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1—from 1H NMR spectra of the crude reaction mixtures.

To confirm the stereochemistry of adducts 3 and 4, X-ray analysis data for compound 3a (Figure 1) and 1H-1H NOESY NMR spectra for compounds 3h, 3′h (Figures S1 and S2 in SI) and 4c (Figure S6 in SI) were used.

Figure 1.

Figure 1

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X-ray structures of products 3a and 6e.

Reversibility was observed for the 1,3-dipolar cycloaddition reactions of nitrones [50,63]. In our case, the ratio of isomeric compounds 3 and 3′ depends on the reaction temperature. Furthermore, the isolated compounds 3′ were noticed to undergo transformation when kept in a CDCl3 solution overnight. The signals of reagents 1 and 2 along with the signals of stereoisomers 3 appeared in the 1H NMR spectrum of the sample. In this regard, we further inspected the reversibility of the cycloaddition reactions of C-carbamoyl nitrones 2 under the reaction conditions. 1H NMR spectra were recorded before and after heating individual compounds 3h and 3′h at 80 °C for 4 h (two times for 2 h) in C6D6. After heating, the spectra of both samples contained the signals of all three isomers 3h, 3′h, and 4h, as well as the signals of reagents 1b and 2d, and the ratio of the compounds 3h:3′h:4h:2d:1b was 5:1:0.05:1:1 for 3h and 3:1:0.3:1.6:1.6 for 3′h (Figure 2). In both experiments, the product 3h was the main component of the mixture and the ratio of compounds 2d:1b was 1:1. The obtained results provide evidence for the studied reaction to proceed reversibly under the applied conditions.

Figure 2.

Figure 2

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1H NMR monitoring of thermal interconversion between isoxazolidines 3h and 3′h.

Next, we investigated the reactions of dipolarophiles 1a,b with ketonitrones 5a–c containing two ester groups (Table 4). As noted in the introduction, the regioselectivity of their cycloadditions can often be different compared to aldonitrones. The reaction with more sterically hindered C,C-bis(methoxycarbonyl)nitrones 5 required more harsh conditions, and the cycloaddition was carried out at 110 °C. The reactions proceeded to obtain only 5-spiro regioisomers 6a–f in good-to-high yields. The signals of regioisomeric products were not observed in the 1H NMR spectra of crude reaction mixtures. The regioselectivity of the reaction in this case can be explained by steric factors. The structure of the cycloadducts 6 was further confirmed using X-ray analysis data for compound 6e (Figure 1).

Table 4.

Cycloaddition of 2-(2-oxoindoline-3-ylidene)acetates 1a,b with ketonitrones 5a–c.

graphic file with name ijms-23-12639-i004.jpg

Entry Dipolarophile R Nitrone R’ Product Isolated Yield of 6, %
1 1a H 5a H 6a 95
2 1a H 5b Me 6b 66
3 1a H 5c MeO 6c 72
4 1b Me 5a H 6d 92
5 1b Me 5b Me 6e 77
6 1b Me 5c MeO 6f 82
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Compounds 6 showed greater stability than compounds 3 and 3′: the 1H NMR spectrum of 6 did not change when the sample was kept in a CDCl3 solution for two days at room temperature. However, the noticeable reversibility was indicated under the used cycloaddition reaction conditions. When pure compound 6b was heated in toluene at 110 °C for 2 h, the 1H NMR spectrum and TLC showed the presence of the starting compounds 1a and 5b in the solution along with the signals of compound 6b. Notably, isomerization or cycloreversion was not observed for solutions of compounds 3h and 6b in DMSO-d6 at room temperature within two days.

2.2. Transformations of the Cycloadducts

Next, we studied the possibility of selective opening of the isoxazolidine ring under the action of zinc in acetic acid. The selective conversion to the corresponding amino alcohols was shown for both stereoisomeric adducts with aldonitrones 3 and 3′, at room temperature for 1 h (Scheme 2). Notably, the reduction of diastereomeric isoxazolidines 3e and 3′e made it possible to obtain diastereomeric amino alcohols 7a and 7′a, correspondingly.

Scheme 2.

Scheme 2

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Synthesis of 1,3-aminoalcohols 7 from 3 and 3′.

The relative configuration of the stereocenters in 7a and 7′a was confirmed by 1H-1H NOESY spectra (see Figures S3 and S4 in SI) and the comparison of these data with those of isoxazolidines 3e and 3′e.

A similar reaction of cycloadduct 6d gave spirolactone 8 (Scheme 3). Presumably, in this case, the amino alcohol 9 is formed at the first stage, which is converted to lactone 8 in an acidic medium. The structure of compound 8 was confirmed by 1H-1H NOESY spectra (see Figure S5 in SI) and X-ray analysis data (Scheme 3).

Scheme 3.

Scheme 3

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Synthesis of spirolactone 8 from 6d.

2.3. Antiproliferative Activity

Given the high potential of spiro derivatives of indolin-2-one in medicinal chemistry, it was of keen interest to test the bioactivity potential of the novel synthesized spiro compounds 3, 3′, and 6, as well as amino alcohols 7. They were evaluated for antiproliferative activity on several tumor cell lines: A549 (lung carcinoma), MCF7 (breast cancer), MDA-MB-231 (triple-negative breast cancer), Caki-2 (kidney clear cell carcinoma), and T98G (glioblastoma multiforme) (Figure 3). The highest cytotoxicity was observed for compound 3b, against the MCF7 (breast cancer) and A549 (lung carcinoma) cell lines. At a concentration of 50 μM, a 32% inhibition for MCF7 was achieved relative to the control (etoposide). Compounds 3a,d–h, 6, and 7 showed no inhibitory activity on all tested cell lines.

Figure 3.

Figure 3

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Cytotoxic activity (cell viability, %) of selected compounds against several cancer cell lines at 50 μM concentration.

3. Materials and Methods

3.1. General Information

All the cycloaddition reactions were performed in anhydrous solvents under an argon atmosphere. Toluene was distilled over sodium. Reaction progress was monitored using thin layer chromatography (TLC) on precoated Silufol UV–254 plates. 1H and 13C NMR spectra were recorded in CDCl3, benzene-d6, DMSO-d6 using a Bruker Avance 400 spectrometer (see 1H and 13C NMR spectra in SI). HRMS spectra were obtained with a Bruker-maXis (QTOF). Xcalibur, Eos diffractometer was used for X-ray analysis. (E)-methyl 2-(2-oxoindolin-3-ylidene)acetates 1a,b and nitrones 2a–d, 5a–c were prepared using known procedures [61,64,65].

3.2. Synthetic Methods and Analytic Data of Compounds

3.2.1. General Procedure for Obtaining Cycloadducts 3, 3′, and 4

A mixture of nitrone 2a–d (1.5 eqv.) and dipolarophile 1a,b (1 eqv.) was stirred in toluene (10 mL) at 80 °C for 14 h. The solvent was removed under reduced pressure. Products were separated by column chromatography (silica gel, hexane:ethyl acetate 3:1).

rac-(3R,3′R,4′S)-methyl 2-oxo-2′-phenyl-3′-(phenylcarbamoyl)spiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3a), rac-(3R,3′S,4′S)-methyl 2-oxo-2′-phenyl-3′-(phenylcarbamoyl)spiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3′a) and rac-(3R,3′S,5′R)-methyl 2-oxo-2′-phenyl-3′-(phenylcarbamoyl)spiro[indoline-3,4′-isoxazolidine]-5′-carboxylate (4a) were obtained from (2-oxoindoline-3-ylidene)acetate 1a (102 mg, 0.5 mmol) and nitrone 2a (180 mg, 0.75 mmol).

Isomer 3a. Yield 155 mg (70%). Colorless solid, m.p. 178–180 °C. 1H NMR (400 MHz, CDCl3): δ 9.41 (s, 1H, NH), 8.85 (s, 1H, NH), 7.67 (d, J = 7.7 Hz, 2HAr), 7.39–7.27 (m, 4HAr), 7.26–7.19 (m, 1HAr), 7.19–7.04 (m, 4HAr), 6.94–6.75 (m, 3HAr), 5.12 (d, J = 5.8 Hz, 1H, CH), 4.28 (d, J = 5.8 Hz, 1H, CH), 3.37 (s, 3H, CO2CH3). 13C NMR (101 MHz, CDCl3): δ 175.4 (C), 168.5 (C), 167.6 (C), 148.7 (C), 141.4 (C), 137.6 (C), 131.4 (CH), 129.2 (2CH), 129.2 (2CH), 126.5 (CH), 124.8 (CH), 123.7 (CH), 123.4 (C), 123.2 (CH), 120.1 (2CH), 115.8 (2CH), 111.0 (CH), 84.7 (C-O), 70.9 (CH), 59.8 (CH), 52.7 (CH3). HRMS (ESI): (M + Na)+, found 466.1379. [C25H21N3O5Na]+ calculated 466.1373. Appropriate crystals for X-ray analysis were obtained from hexane/ethyl acetate solution. Crystallographic data for 3a have been deposited with the Cambridge Crystallographic Data Centre, no. CCDC 2177789.

Isomers 3′a and 4a. Yield 40 mg (18%). Ratio: 4:1. Yellowish solid. 1H NMR (400 MHz, CDCl3): 3′a: δ 9.00 (s, 1H, NH), 8.20 (s, 1H, NH), 5.34 (d, J = 7.9 Hz, 1H, CH), 4.25 (d, J = 7.9 Hz, 1H, CH), 3.64 (s, 3H, CH3), the aromatic signals of the isomers overlap. 4a: δ 9.12 (s, 1H, NH), 8.54 (s, 1H, NH), 5.12 (s, 1H, CH), 4.95 (s, 1H, CH), 3.41 (s, 3H, CH3), The aromatic signals of the isomers overlap. 13C NMR (101 MHz, DMSO-d6): 3′a: δ 174.2 (C), 167.5 (C), 166.7 (C), 150.0 (C), 143.0 (C), 138.3 (C), 131.0 (CH), 128.6 (2CH), 128.6 (2CH), 126.5 (CH), 123.9 (CH), 123.4 (C), 122.1 (CH), 121.9 (CH), 120.3 (2CH), 114.6 (2CH), 110.2 (CH), 83.5 (C-O), 68.9 (CH), 56.6 (CH), 51.8 (CO2CH3). HRMS (ESI): (M + H)+, found 444.1559. [C25H21N3O5H]+ calculated 444.1554.

rac-(3R,3′R,4′S)-methyl 3′-((4-chlorophenyl)carbamoyl)-2-oxo-2′-phenylspiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3b), rac-(3R,3′S,4′S)-methyl 3′-((4-chlorophenyl)carbamoyl)-2-oxo-2′-phenylspiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3′b) and rac-(3R,3′S,5′R)-methyl 3′-((4-chlorophenyl)carbamoyl)-2-oxo-2′-phenylspiro[indoline-3,4′-isoxazolidine]-5′-carboxylate (4b) were obtained from (2-oxoindoline-3-ylidene)acetate 1a (102 mg, 0.5 mmol) and nitrone 2b (206 mg, 0.75 mmol).

Isomer 3b. Yield 167 mg (70%). Yellowish solid, m.p. 184–186 °C. 1H NMR (400 MHz, DMSO-d6): δ 10.95 (s, 1H, NH), 10.60 (s, 1H, NH), 7.76–7.69 (m, 2HAr), 7.45–7.36 (m, 2HAr), 7.33–7.22 (m, 3HAr), 7.10–6.97 (m, 3HAr), 6.91 (d, J = 7.7 Hz, 1HAr), 6.86–6.77 (m, 1HAr), 6.62 (d, J = 7.7 Hz, 1HAr), 5.17 (d, J = 7.5 Hz, 1H, CH), 4.40 (d, J = 7.5 Hz, 1H, CH), 3.25 (s, 3H, CO2CH3). 13C NMR (101 MHz, DMSO-d6): δ 172.4 (C), 167.5 (C), 167.0 (C), 149.2 (C), 142.3 (C), 137.2 (C), 130.9 (CH), 128.7 (2CH), 128.6 (2CH), 127.8 (C), 125.6 (CH), 124.7 (C), 122.7 (CH), 121.7 (CH), 121.7 (2CH), 115.2 (2CH), 110.5 (CH), 83.9 (C-O), 69.0 (CH), 59.4 (CH), 52.3 (CO2CH3). HRMS (ESI): (M + Na)+, found 500.0990. [C25H20ClN3O5Na]+ calculated 500.0984.

Isomers 3′b and 4b. Yield: 38 mg (16%). Ratio: 1.7:1. Yellow oil. 1H NMR (400 MHz, CDCl3): 3′b: δ 9.00 (s, 1H, NH), 7.86 (s, 1H, NH) 5.30 (d, J = 7.9 Hz, 1H, CH), 4.22 (d, J = 7.9 Hz, 1H, CH), 3.61 (s, 3H, CO2CH3), the aromatic signals of the isomers overlap. 4b: δ 9.09 (s, 1H, NH), 8.18 (s, 1H, NH) 5.08 (s, 1H, CH), 4.90 (s, 1H, CH), 3.39 (s, 3H, CO2CH3), the aromatic signals of the isomers overlap. 13C NMR (101 MHz, DMSO-d6): 3′b: δ 174.2 (C), 167.5 (C), 167.0 (C), 150.0 (C), 143.0 (C), 137.3 (C), 131.1 (CH), 128.6 (2CH), 128.6 (2CH), 127.7 (C), 126.5 (CH), 123.4 (C), 122.2 (CH), 122.0 (2CH), 121.9 (CH), 114.6 (2CH), 110.3 (CH), 83.5 (C-O), 69.0 (CH), 56.6 (CH), 51.8 (CO2CH3). HRMS (ESI): (M + Na)+, found 500.0990. [C25H20ClN3O5Na]+ calculated 500.0984.

rac-(3R,3′R,4′S)-methyl 2-oxo-2′-phenyl-3′-(p-tolylcarbamoyl)spiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3c), rac-(3R,3′S,4′S)-methyl 2-oxo-2′-phenyl-3′-(p-tolylcarbamoyl)spiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3′c) and rac-(3R,3′S,5′R)-methyl 2-oxo-2′-phenyl-3′-(p-tolylcarbamoyl)spiro[indoline-3,4′-isoxazolidine]-5′-carboxylate (4c) were obtained from (2-oxoindoline-3-ylidene)acetate 1a (28 mg, 0.133 mmol) and nitrone 2c (52 mg, 0.204 mmol).

Isomer 3c. Yield 35 mg (57%). Yellowish solid, m.p. 178–181 °C. 1H NMR (400 MHz, CDCl3): δ 9.33 (s, 1H, NHCO), 9.02 (s, 1H, NHCO), 7.55 (d, J = 7.9 Hz, 2HAr), 7.36–7.27 (m, 2HAr), 7.25–7.18 (m, 1HAr), 7.17–7.04 (m, 5HAr), 6.92–6.80 (m, 2HAr), 6.77 (d, J = 7.9 Hz, 1HAr), 5.12 (d, J = 5.8 Hz, 1H, CH), 4.29 (d, J = 5.8 Hz, 1H, CH), 3.36 (s, 3H, CO2CH3), 2.33 (s, 3H, H3C-C6H4). 13C NMR (101 MHz, CDCl3): δ 175.4 (C), 168.6 (C), 167.4 (C), 148.8 (C), 141.4 (C), 135.0 (C), 134.4 (C), 131.3 (CH), 129.6 (2CH), 129.2 (2CH), 126.5 (CH), 123.6 (CH), 123.4 (C), 123.1 (CH), 120.1 (2CH), 115.7 (2CH), 111.0 (CH), 84.8 (C-O), 70.8 (CH), 59.8 (CH), 52.7 (CO2CH3), 21.1 (H3C-C6H4). HRMS (ESI): (M + Na)+, found 480.1536. [C26H23N3O5Na]+ calculated 480.1530.

Isomers 3′c and 4c. Yield: 5 mg (8%). Ratio: 1.2:1. Beige solid. 1H NMR (400 MHz, CDCl3): 3′c: δ 8.92 (s, 1H, NHCO), 7.56 (s, 1H, NHCO), 5.49 (d, J = 7.9 Hz, 1H, CH), 4.42 (d, J = 7.9 Hz, 1H, CH), 3.62 (s, 3H, CO2CH3), 2.33 (s, 3H, H3C-C6H4), the aromatic signals of the isomers overlap. 4c: δ 9.00 (s, 1H, NHCO), 7.97 (s, 1H, NHCO), 5.09 (s, 1H, CH), 4.90 (s, 1H, CH), 3.39 (s, 3H, CO2CH3), 2.27 (s, 3H, H3C-C6H4), the aromatic signals of the isomers overlap. 13C NMR (101 MHz, CDCl3): 3′c: δ 173.3 (C), 167.6 (C), 165.4 (C), 150.2 (C), 141.1 (C), 134.7 (C), 134.7 (C), 131.6 (CH), 129.8 (2CH), 129.6 (2CH), 124.9 (C), 124.7 (CH), 124.0 (CH), 122.9 (CH), 120.4 (2CH), 114.6 (2CH), 110.4 (CH), 83.8 (C-O), 69.6 (CH), 59.9 (CH), 52.5 (CO2CH3), 21.1 (H3C-C6H4). 4c: δ 174.0 (C), 168.1 (C), 164.6 (C), 149.6 (C), 141.9 (C), 135.0 (C), 133.7 (C), 130.1 (CH), 129.7 (2CH), 128.9 (2CH), 125.1 (CH), 124.6 (C), 123.7 (CH), 123.0 (CH), 120.1 (2CH), 114.5 (2CH), 110.6 (CH), 85.8 (C-O), 78.5 (CH), 64.8 (CH), 53.2 (CO2CH3), 21.0 (H3C-C6H4). HRMS (ESI): (M + Na)+, found 480.1536. [C26H23N3O5Na]+ calculated 480.1530.

Isomer 3′c. Yield 10 mg (16%). Pale yellow solid, m.p. 163–166 °C (recrystallized from Et2O). 1H NMR (400 MHz, DMSO-d6): 1H NMR (400 MHz, DMSO-d6) δ 10.71 (s, 1H, NHCO), 10.04 (s, 1H, NHCO), 7.61–7.48 (m, 3HAr), 7.38–7.22 (m, 3HAr), 7.17–7.09 (m, 2HAr), 7.04–6.85 (m, 5HAr), 5.06 (d, J = 8.4 Hz, 1H, CH), 4.30 (d, J = 8.4 Hz, 1H, CH), 3.39 (s, 3H, CO2CH3), 2.27 (s, 3H, H3C-C6H4). 13C NMR (101 MHz, DMSO-d6) δ 174.2 (C), 167.5 (C), 166.4 (C), 150.0 (C), 143.0 (C), 135.8 (C), 133.0 (C), 131.0 (CH), 129.0 (2CH), 128.6 (2CH), 126.5 (CH), 123.5 (C), 122.1 (CH), 121.9 (CH), 120.4 (2CH), 114.6 (2CH), 110.2 (CH), 83.5 (C-O), 69.0 (CH), 56.5 (CH), 51.8 (CO2CH3), 20.5 (H3C-C6H4). HRMS (ESI): (M + Na)+, found 480.1537. [C26H23N3O5Na]+ calculated 480.1530.

rac-(3R,3′R,4′S)-methyl 3′-((4-methoxyphenyl)carbamoyl)-2-oxo-2′-phenylspiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3d), rac-(3R,3′S,4′S)-methyl 3′-((4-methoxyphenyl)carbamoyl)-2-oxo-2′-phenylspiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3′d) and rac-(3R,3′S,5′R)-methyl 3′-((4-methoxyphenyl)carbamoyl)-2-oxo-2′-phenylspiro[indoline-3,4′-isoxazolidine]-5′-carboxylate (4d) were obtained from (2-oxoindoline-3-ylidene)acetate 1a (102 mg, 0.5 mmol) and nitrone 2d (203 mg, 0.75 mmol).

Isomer 3d. Yield 164 mg (69%). Beige solid, m.p. 167–170 °C. 1H NMR (400 MHz, CDCl3): δ 9.28 (s, 1H, NH), 8.83 (s, 1H, NH), 7.56 (d, J = 7.9 Hz, 2HAr), 7.35–7.27 (m, 2HAr), 7.25–7.19 (m, 1HAr), 7.16–7.03 (m, 3HAr), 6.92–6.84 (m, 3HAr), 6.84–6.76 (m, 2HAr), 5.12 (d, J = 5.5 Hz, 1H, CH), 4.28 (d, J = 5.5 Hz, 1H, CH), 3.79 (s, 3H, OCH3), 3.36 (s, 3H, CO2CH3). 13C NMR (101 MHz, CDCl3): δ 175.3 (C), 168.5 (C), 167.3 (C), 156.8 (C), 148.8 (C), 141.4 (C), 131.3 (CH), 130.8 (C), 129.2 (2CH), 126.5 (CH), 123.6 (CH), 123.5 (C), 123.1 (CH), 121.7 (2CH), 115.7 (2CH), 114.3 (2CH), 111.0 (CH), 84.8 (C-O), 70.7 (CH), 59.9 (CH), 55.6 (OCH3), 52.8 (CO2CH3). HRMS (ESI): (M + H)+, found 474.1665. [C26H23N3O6H]+ calculated 474.1660.

Isomers 3′d and 4d. Yield: 45 mg (19%). Ratio: 1.7:1. Beige solid. 1H NMR (400 MHz, CDCl3): 3′d: δ 8.87 (s, 1H, NH), 8.04 (s, 1H, NH), 5.29 (d, J = 7.9 Hz, 1H, CH), 4.21 (d, J = 7.9 Hz, 1H, CH), 3.80 (s, 3H, OCH3), 3.61 (s, 3H, CO2CH3), the aromatic signals of the isomers overlap. 4d: δ 8.96 (s, 1H, NH), 5.08 (s, 1H, CH), 4.89 (s, 1H, CH), 3.74 (s, 3H, OCH3), 3.39 (s, 3H, CO2CH3), the aromatic signals of the isomers overlap. 13C NMR (101 MHz, CDCl3): 3′d: δ 174.6 (C), 168.6 (C), 166.9 (C), 157.0 (C), 151.1 (C), 141.8 (C), 131.9 (CH), 130.3 (C), 129.1 (2CH), 126.2 (CH), 123.5 (CH), 122.8 (CH), 122.2 (2CH), 121.1 (C), 114.3 (2CH), 114.0 (2CH), 110.8 (CH), 84.8 (C-O), 71.0 (CH), 57.4 (CH), 55.6 (OCH3), 52.6 (CO2CH3). HRMS (ESI): (M + H)+, found 474.1665. [C26H23N3O6H]+ calculated 474.1660.

rac-(3R,3′R,S-methyl 1-methyl-2-oxo-2′-phenyl-3′-(phenylcarbamoyl)spiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3e), rac-(3R,3′S,4′S)-methyl 1-methyl-2-oxo-2′-phenyl-3′-(phenylcarbamoyl)spiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3′e) and rac-(3R,3′S,5′R)-methyl 1-methyl-2-oxo-2′-phenyl-3′-(phenylcarbamoyl)spiro[indoline-3,4′-isoxazolidine]-5′-carboxylate (4e) were obtained from (2-oxoindoline-3-ylidene)acetate 1b (109 mg, 0.5 mmol) and nitrone 2a (180 mg, 0.75 mmol).

Isomer 3e. Yield 169 mg (74%). Colorless solid, m.p. 155–157 °C. 1H NMR (400 MHz, CDCl3): δ 9.55 (s, 1H, NH), 7.69 (d, J = 7.9 Hz, 2HAr), 7.40–7.27 (m, 5HAr), 7.17–7.10 (m, 3HAr), 7.10–7.03 (m, 1HAr), 7.00–6.90 (m, 2HAr), 6.87 (d, J = 7.9 Hz, 1HAr), 5.05 (d, J = 5.8 Hz, 1H, HC), 4.20 (d, J = 5.8 Hz, 1H, HC), 3.37 (s, 3H, CO2CH3), 3.27 (s, 3H, NCH3). 13C NMR (101 MHz, CDCl3): δ 173.4 (C), 168.7 (C), 167.6 (C), 148.6 (C), 144.3 (C), 137.8 (C), 131.4 (CH), 129.2 (2CH), 129.1 (2CH), 126.1 (CH), 124.7 (CH), 123.7 (CH), 123.2 (CH), 122.9 (C), 120.0 (2CH), 116.0 (2CH), 109.0 (CH), 83.9 (C-O), 71.1 (CH), 59.8 (CH), 52.7 (CO2CH3), 26.8 (NCH3). HRMS (ESI): (M + H)+, found 458.1713. [C26H23N3O5H]+ calculated 458.1711.

Isomers 3′e and 4e. Yield: 30 mg (13%). Ratio: 10:1. Yellow oil. 1H NMR (400 MHz, CDCl3): 3′e: δ 8.96 (s, 1H, NH), 7.59 (d, J = 7.8 Hz, 2HAr), 6.86 (d, J = 7.8 Hz, 1HAr), 5.34 (d, J = 7.8 Hz, 1H, CH), 4.18 (d, J = 7.8 Hz, 1H, CH), 3.61 (s, 3H, CO2CH3), 3.12 (s, 3H, NCH3), the aromatic signals of the isomers overlap. 4e: δ 9.05 (s, 1H, NH), 5.09 (s, 1H, CH), 4.90 (s, 1H, CH), 3.36 (s, 3H, CH3), 3.31 (s, 3H, CH3), the aromatic signals of the isomers overlap. 13C NMR (101 MHz, CDCl3): 3′e: δ 173.3 (C), 168.6 (C), 167.1 (C), 151.0 (C), 144.8 (C), 137.2 (C), 131.9 (CH), 129.2 (2CH), 129.1 (2CH), 125.7 (CH), 125.0 (CH), 123.4 (CH), 122.9 (CH), 120.8 (C), 120.3 (2CH), 114.1 (2CH), 109.1 (CH), 84.7 (C-O), 70.9 (CH), 57.7 (CH), 52.5 (CO2CH3), 26.5 (NCH3). HRMS (ESI): (M + H)+, found 458.1714. [C26H23N3O5H]+ calculated 458.1711.

rac-(3R,3′R,4′S)-methyl 3′-((4-chlorophenyl)carbamoyl)-1-methyl-2-oxo-2′-phenylspiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3f), rac-(3R,3′S,4′S)-methyl 3′-((4-chlorophenyl)carbamoyl)-1-methyl-2-oxo-2′-phenylspiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3′f) and rac-(3S,3′R,5′S)-methyl 3′-((4-chlorophenyl)carbamoyl)-1-methyl-2-oxo-2′-phenylspiro[indoline-3,4′-isoxazolidine]-5′-carboxylate (4f) were obtained from (2-oxoindoline-3-ylidene)acetate 1b (109 mg, 0.5 mmol) and nitrone 2b (206 mg, 0.75 mmol).

Isomer 3f. Yield 160 mg (65%). Colorless solid, m.p. 153–155 °C. 1H NMR (400 MHz, CDCl3): δ 9.63 (s, 1H, NHCO), 7.68–7.60 (m, 2HAr), 7.40–7.34 (m, 1HAr), 7.34–7.27 (m, 4HAr), 7.13–7.05 (m, 3HAr), 7.01–6.92 (m, 2HAr), 6.88 (d, J = 7.8 Hz, 1HAr), 5.03 (d, J = 5.6 Hz, 1H, CH), 4.16 (d, J = 5.6 Hz, 1H, CH), 3.38 (s, 3H, CO2CH3), 3.27 (s, 3H, NCH3). 13C NMR (101 MHz, CDCl3): δ 173.6 (C), 168.7 (C), 167.7 (C), 148.4 (C), 144.3 (C), 136.4 (C), 131.5 (CH), 129.6 (C), 129.2 (2CH), 129.1 (2CH), 126.1 (CH), 123.8 (CH), 123.3 (CH), 122.8 (C), 121.30 (2CH), 116.1 (2CH), 109.1 (CH), 83.9 (C-O), 71.0 (CH), 59.7 (CH), 52.7 (CO2CH3), 26.8 (NCH3). HRMS (ESI): (M + Na)+, found 514.1139. [C26H22ClN3O5Na]+ calculated 514.1140.

Isomers 3′f and 4f. Yield: 42 mg (17%). Ratio: 5:1. Yellowish solid. 1H NMR (400 MHz, CDCl3): 3′f: δ 8.98 (s, 1H, NHCO), 7.58–7.51 (m, 2HAr), 7.48–7.39 (m, 1HAr), 7.36–7.27 (m, 5HAr), 7.13–6.98 (m, 4HAr), 6.87 (d, J = 7.9 Hz, 1HAr), 5.33 (d, J = 7.8 Hz, 1H, CH), 4.18 (d, J = 7.8 Hz, 1H, CH), 3.60 (s, 3H, CO2CH3), 3.11 (s, 3H, NCH3). 4f: δ 9.08 (s, 1H, NHCO), 5.08 (s, 1H, CH), 4.89 (s, 1H, CH), 3.35 (s, 3H, CH3), 3.31 (s, 3H, CH3), the aromatic signals of the isomers overlap. 13C NMR (101 MHz, CDCl3): 3′f: δ 173.2 (C), 168.7 (C), 167.3 (C), 151.0 (C), 144.8 (C), 135.8 (C), 132.0 (CH), 130.1 (C), 129.2 (2CH), 129.2 (2CH), 125.6 (CH), 123.5 (CH), 123.0 (CH), 121.6 (2CH), 120.7 (C), 114.0 (2CH), 109.2 (CH), 84.8 (C-O), 70.8 (CH), 57.7 (CH), 52.5 (CO2CH3), 26.5 (NCH3). HRMS (ESI): (M + Na)+, found 514.1139. [C26H22ClN3O5Na]+ calculated 514.1140

rac-(3R,3′R,4′S)-methyl 1-methyl-2-oxo-2′-phenyl-3′-(p-tolylcarbamoyl)spiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3g), rac-(3R,3′S,4′S)-methyl 1-methyl-2-oxo-2′-phenyl-3′-(p-tolylcarbamoyl)spiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3′g) and rac-(3R,3′S,5′R)-methyl 1-methyl-2-oxo-2′-phenyl-3′-(p-tolylcarbamoyl)spiro[indoline-3,4′-isoxazolidine]-5′-carboxylate (4g) were obtained from (2-oxoindoline-3-ylidene)acetate 1b (109 mg, 0.5 mmol) and nitrone 2c (191 mg, 0.75 mmol).

Isomer 3g. Yield 130 mg (55%). Colorless solid, m.p. 143–146 °C. 1H NMR (400 MHz, CDCl3): δ 9.44 (s, NHCO), 7.60–7.52 (m, 2HAr), 7.39–7.32 (m, 1HAr), 7.32–7.27 (m, 2HAr), 7.17–7.03 (m, 5HAr), 6.99–6.89 (m, 2HAr), 6.87 (d, J = 7.9 Hz, 1HAr), 5.05 (d, J = 6.0 Hz, 1H, CH), 4.21 (d, J = 6.0 Hz, 1H, CH), 3.37 (s, 3H, CO2CH3), 3.27 (s, 3H, NCH3), 2.32 (s, 3H, H3C-C6H4). 13C NMR (101 MHz, CDCl3): δ 173.3 (C), 168.7 (C), 167.4 (C), 148.7 (C), 144.3 (C), 135.2 (C), 134.3 (C), 131.4 (CH), 129.6 (2CH), 129.2 (2CH), 126.2 (CH), 123.6 (CH), 123.2 (CH), 123.0 (C), 120.1 (2CH), 116.0 (2CH), 108.9 (CH), 84.0 (C-O), 71.0 (CH), 59.9 (CH), 52.7 (CO2CH3), 26.8 (NCH3), 21.1 (H3C-C6H4). HRMS (ESI): (M + Na)+, found 494.1688. [C27H25N3O5Na]+ calculated 494.1686.

Isomers 3′g and 4g. Yield: 47 mg (20%). Ratio 2:1. Beige solid. 1H NMR (400 MHz, CDCl3): 3′g: δ 8.91 (s, 1H, NH), 5.34 (d, J = 7.8 Hz, 1H, CH), 4.18 (d, J = 7.8 Hz, 1H, CH), 3.61 (s, 3H, CO2CH3), 3.11 (s, 3H, NCH3), 2.33 (s, 3H, H3C-C6H4), the aromatic signals of the isomers overlap. 4g: δ 9.00 (s, 1H, NH), 6.98–6.91 (m, 1HAr), 5.08 (s, 1H, CH), 4.89 (s, 1H, CH), 3.35 (s, 3H, CH3), 3.31 (s, 3H, CH3), 2.27 (s, 3H, H3C-C6H4), the aromatic signals of the isomers overlap. 13C NMR (101 MHz, CDCl3): 3′g: δ 173.3 (C), 168.5 (C), 166.9 (C), 151.1 (C), 144.7 (C), 134.7 (C), 134.6 (C), 131.8 (CH), 129.6 (2CH), 129.1 (2CH), 125.7 (CH), 123.4 (CH), 122.9 (CH), 120.9 (C), 120.6 (2CH), 114.1 (2CH), 109.1 (CH), 84.7 (C-O), 70.9 (CH), 57.6 (CH), 52.5 (CO2CH3), 26.5 (NCH3), 21.0 (H3C-C6H4). 4g: δ 171.9 (C), 165.2 (C), 164.3 (C), 150.1 (C), 144.1 (C), 134.7 (C), 133.7 (C), 130.0 (CH), 129.6 (2CH), 129.4 (2CH), 124.5 (C), 124.2 (CH), 123.8 (CH), 122.7 (CH), 120.4 (2CH), 114.4 (2CH), 108.5 (CH), 83.7 (CH), 78.0 (C), 64.3 (CH), 52.3 (CO2CH3), 27.0 (NCH3), 20.9 (H3C-C6H4). HRMS (ESI): (M + Na)+, found 472.1868. [C26H23N3O5Na]+ calculated 472.1867.

rac-(3R,3′R,4′S)-methyl 3′-((4-methoxyphenyl)carbamoyl)-1-methyl-2-oxo-2′-phenylspiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3h) and rac-(3R,3′S,4′S)-methyl 3′-((4-methoxyphenyl)carbamoyl)-1-methyl-2-oxo-2′-phenylspiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3′h) were obtained from (2-oxoindoline-3-ylidene)acetate 1b (109 mg, 0.5 mmol) with nitrone 2c (203 mg, 0.75 mmol).

Isomer 3h. Yield 111 mg (58%). Colorless solid, m.p. 145–147 °C 1H NMR (400 MHz, CDCl3): δ 9.43 (s, 1H, NH), 7.64–7.56 (m, 2HAr), 7.40–7.28 (m, 3HAr), 7.16–7.05 (m, 3HAr), 6.99–6.86 (m, 5HAr), 5.08 (d, J = 5.9 Hz, 1H, CH), 4.23 (d, J = 5.9 Hz, 1H, CH), 3.82 (s, 3H, OCH3), 3.39 (s, 3H, CO2CH3), 3.29 (s, 3H, NCH3). 13C NMR (101 MHz, CDCl3): δ 173.3 (C), 168.6 (C), 167.2 (C), 156.7 (C), 148.6 (C), 144.3 (C), 131.4 (CH), 130.9 (C), 129.1 (2CH), 126.1 (CH), 123.6 (CH), 123.2 (CH), 123.0 (C), 121.6 (2CH), 115.9 (2CH), 114.3 (2CH), 108.9 (CH), 84.0 (C-O), 70.9 (CH), 59.9 (CH), 55.6 (OCH3), 52.6 (CO2CH3), 26.8 (NCH3). HRMS (ESI): (M + Na)+, found 510.1638. [C27H25N3O6Na]+ calculated 510.1636.

Isomer 3′h. Yield 23 mg (12%). Dark yellow solid, m.p. 119–122 °C. 1H NMR (400 MHz, CDCl3): δ 8.86 (s, 1H), 7.53–7.47 (m, 2H), 7.45–7.39 (m, 1H), 7.36–7.28 (m, 3H), 7.13–7.00 (m, 4H), 6.92–6.82 (m, 3H), 5.33 (d, J = 7.9 Hz, 1H), 4.18 (d, J = 7.8 Hz, 1H), 3.80 (s, 3H), 3.61 (s, 3H), 3.11 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 173.3 (C), 168.6 (C), 166.9 (C), 157.0 (C), 151.1 (C), 144.8 (C), 131.9 (CH), 130.4 (C), 129.1 (2CH), 125.7 (CH), 123.4 (CH), 122.9 (CH), 122.1 (2CH), 120.9 (C), 114.4 (2CH), 114.1 (2CH), 109.1 (CH), 84.7 (C-O), 70.8 (CH), 57.6 (CH), 55.6 (OCH3), 52.5 (CO2CH3), 26.5 (NCH3). HRMS (ESI): (M + Na)+, found 510.1638. [C27H25N3O6Na]+ calculated 510.1636.

3.2.2. General Procedure for Obtaining Cycloadducts 3′

A mixture of nitrone 2 (1.5 eqv.) and dipolarophile 1 (1 eqv.) was stirred in DCM (10 mL) at room temperature for 7 h. The solvent was removed under reduced pressure. Products were separated by column chromatography (silica gel, hexane:ethyl acetate 3:1). Isomers 3a (25 mg, 11%), 3b (20 mg, 17%), and 3f (34 mg, 14%) were also obtained in corresponding experiments.

rac-(3R,3′S,4′S)-methyl 2-oxo-2′-phenyl-3′-(phenylcarbamoyl)spiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3′a) was obtained from (2-oxoindoline-3-ylidene)acetate 1a (102 mg, 0.5 mmol) and nitrone 2a (180 mg, 0.75 mmol). Yield 163 mg (73%). Colorless solid, m.p. 165–167 °C. 1H NMR (400 MHz, DMSO-d6): δ 10.72 (s, 1H, NHCO), 10.14 (s, 1H, NHCO), 7.66 (d, J = 8.2 Hz, 2HAr), 7.59 (d, J = 7.2 Hz, 1HAr), 7.38–7.31 (m, 3HAr), 7.30–7.23 (m, 2HAr), 7.14–7.08 (m, 1HAr), 7.05–6.93 (m, 4HAr), 6.90 (d, J = 7.7 Hz, 1HAr), 5.08 (d, J = 8.5 Hz, 1H, CH), 4.34 (d, J = 8.4 Hz, 1H, CH), 3.39 (s, 3H, CO2CH3). 13C NMR (101 MHz, DMSO-d6): δ 174.2 (C), 167.5 (C), 166.7 (C), 150.0 (C), 143.0 (C), 138.3 (C), 131.0 (CH), 128.6 (2CH), 128.6 (2CH), 126.5 (CH), 123.9 (CH), 123.4 (C), 122.1 (CH), 121.9 (CH), 120.3 (2CH), 114.6 (2CH), 110.2 (CH), 83.5 (C-O), 68.9 (CH), 56.6 (CH), 51.8 (CO2CH3). HRMS (ESI): (M + H)+, found 444.1559. [C25H21N3O5H]+ calculated 444.1554.

rac-(3R,3′S,4′S)-methyl 3′-((4-chlorophenyl)carbamoyl)-2-oxo-2′-phenylspiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3′b) was obtained from (2-oxoindoline-3-ylidene)acetate 1a (51 mg, 0.25 mmol) and nitrone 2b (103 mg, 0.375 mmol). Yield 70 mg (58%). Colorless solid, m.p. 163–165 °C. 1H NMR (400 MHz, DMSO-d6): δ 10.72 (s, 1H, NHCO), 10.31 (s, 1H, NHCO), 7.74–7.67 (m, 2HAr), 7.59–7.53 (m, 1HAr), 7.42–7.37 (m, 2HAr), 7.36–7.31 (m, 1HAr), 7.30–7.22 (m, 2HAr), 7.06–6.94 (m, 4HAr), 6.93–6.86 (m, 1HAr), 5.08 (d, J = 8.5 Hz, 1H, CH), 4.34 (d, J = 8.5 Hz, 1H, CH), 3.38 (s, 3H, CO2CH3). 13C NMR (101 MHz, DMSO-d6): δ 174.2 (C), 167.5 (C), 167.0 (C), 150.0 (C), 143.0 (C), 137.3 (C), 131.1 (CH), 128.6 (2CH), 128.6 (2CH), 127.7 (C), 126.5 (CH), 123.4 (C), 122.2 (CH), 122.0 (2CH), 121.9 (CH), 115.6 (2CH), 110.3 (CH), 83.5 (C-O), 69.0 (CH), 56.6 (CH), 51.8 (CO2CH3). HRMS (ESI): (M + Na)+, found 500.0989. [C25H20ClN3O5Na]+ calculated 500.0984.

rac-(3R,3′S,4′S)-methyl 3′-((4-chlorophenyl)carbamoyl)-1-methyl-2-oxo-2′-phenylspiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3′f) was obtained from (2-oxoindoline-3-ylidene)acetate 1b (109 mg, 0.5 mmol) and nitrone 2b (206 mg, 0.75 mmol). Yield 150 mg (61%). Colorless solid, m.p. (dec.) 120–123 °C. 1H NMR (400 MHz, CDCl3): δ 8.98 (s, 1H, NHCO), 7.58–7.51 (m, 2HAr), 7.48–7.39 (m, 1HAr), 7.36–7.27 (m, 5HAr), 7.13–6.98 (m, 4HAr), 6.87 (d, J = 7.9 Hz, 1HAr), 5.33 (d, J = 7.8 Hz, 1H, CH), 4.18 (d, J = 7.8 Hz, 1H, CH), 3.60 (s, 3H, CO2CH3), 3.11 (s, 3H, NCH3). 13C NMR (101 MHz, CDCl3): δ 173.2 (C), 168.7 (C), 167.3 (C), 151.0 (C), 144.8 (C), 135.8 (C), 132.0 (CH), 130.1 (C), 129.2 (2CH), 129.2 (2CH), 125.6 (CH), 123.5 (CH), 123.0 (CH), 121.6 (2CH), 120.7 (C), 114.0 (2CH), 109.2 (CH), 84.8 (C-O), 70.8 (CH), 57.7 (CH), 52.5 (CO2CH3), 26.5 (NCH3). HRMS (ESI): (M + Na)+, found 514.1138. [C26H22ClN3O5Na]+ calculated 514.1140.

3.2.3. Procedure for Obtaining Cycloadduct 3′e

A mixture of nitrone 2a (360 mg, 1.5 mmol) and dipolarophile 1b (218 mg, 1 mmol) was stirred in toluene at 55 °C for 7 h. The solvent was removed under reduced pressure. Products were separated by column chromatography (silica gel, hexane:ethyl acetate 3:1). Isomer 3e was also obtained in the reaction in 37% yield (170 mg).

rac-(3R,3′S,4′S)-methyl 1-methyl-2-oxo-2′-phenyl-3′-(phenylcarbamoyl)spiro[indoline-3,5′-isoxazolidine]-4′-carboxylate (3′e). Yield 250 mg (54%). Yellow solid, m.p. (dec.) 117–120 °C. 1H NMR (400 MHz, CDCl3): δ 8.97 (s, 1H, NH), 7.63–7.55 (m, 2HAr), 7.47–7.40 (m, 1HAr), 7.39–7.28 (m, 5HAr), 7.20–7.00 (m, 5HAr), 6.86 (d, J = 7.9 Hz, 1HAr), 5.35 (d, J = 7.9 Hz, 1H, CH), 4.19 (d, J = 7.9 Hz, 1H, CH), 3.61 (s, 3H, CO2CH3), 3.11 (s, 3H, NCH3). 13C NMR (101 MHz, CDCl3): δ 173.3 (C), 168.6 (C), 167.1 (C), 151.0 (C), 144.8 (C), 137.2 (C), 131.9 (CH), 129.2 (2CH), 129.1 (2CH), 125.7 (CH), 125.0 (CH), 123.4 (CH), 122.9 (CH), 120.8 (C), 120.3 (2CH), 114.1 (2CH), 109.1 (CH), 84.7 (C-O), 70.9 (CH), 57.7 (CH), 52.5 (CO2CH3), 26.5 (NCH3). HRMS (ESI): (M + H)+, found 458.1714. [C26H23N3O5H]+ calculated 458.1711.

3.2.4. General Procedure for Obtaining Cycloadducts 6

A mixture of nitrone 5 (1.5 eqv.) and dipolarophile 1 (1 eqv.) was stirred in toluene at 110 °C for 14 h. The solvent was removed under reduced pressure. The products were purified by column chromatography (silica gel, hexane:ethyl acetate 3:1).

rac-(3R,4′S)-trimethyl 2-oxo-2′-phenylspiro[indoline-3,5′-isoxazolidine]-3′,3′,4′-tricarboxylate (6a) was obtained from the reaction of 1a (102 mg, 0.5 mmol) with nitrone 5a (178 mg, 0.75 mmol). Yield 210 mg (95%). Colorless solid, m.p. 156–158 °C. 1H NMR (400 MHz, CDCl3): δ 8.30 (s, 1H, NH), 7.96 (d, J = 7.8 Hz, 1HAr), 7.48 (d, J = 7.8 Hz, 2HAr), 7.32–7.21 (m, 3HAr), 7.18–7.10 (m, 1HAr), 7.09–7.01 (m, 1HAr), 6.90 (d, J = 7.8 Hz, 1HAr), 5.28 (s, 1H, CH), 3.84 (s, 3H, CH3), 3.39 (s, 3H, CH3), 3.25 (s, 3H, CH3). 13C NMR (101 MHz, CDCl3): δ 172.6 (C), 167.0 (C), 166.6 (C), 166.3 (C), 145.5 (C), 141.0 (C), 130.7 (CH), 128.4 (2CH), 127.9 (CH), 126.5 (C), 126.2 (CH), 123.5 (CH), 121.3 (2CH), 110.1 (CH), 82.1 (C-O), 78.7 (C(CO2CH)2), 61.5 (CH), 53.2 (CO2CH3), 53.0 (CO2CH3), 52.3 (CO2CH3). HRMS (ESI): (M + H)+, found 441.1297. [C22H20N2O8H]+ calculated 441.1292.

rac-(3R,4′S)-trimethyl 2-oxo-2′-(p-tolyl)spiro[indoline-3,5′-isoxazolidine]-3′,3′,4′-tricarboxylate (6b) was obtained from the reaction of 1a (102 mg, 0.5 mmol) with nitrone 5b (188 mg, 0.75 mmol). Yield 150 mg (66%). Colorless solid, m.p. 98–100 °C. 1H NMR (400 MHz, CDCl3): δ 7.96 (dd, J = 7.7, 1.2 Hz, 1HAr), 7.85 (s, 1H, NH), 7.41–7.34 (m, 2HAr), 7.25 (m, 1HAr), 7.11–7.00 (m, 3HAr), 6.85 (d, J = 7.7 Hz, 1HAr), 5.26 (s, 1H, CH), 3.82 (s, 3H, CH3), 3.42 (s, 3H, CH3), 3.23 (s, 3H, CH3), 2.29 (s, 3H, H3C-C6H4). 13C NMR (101 MHz, CDCl3): δ 172.9 (C), 167.1 (C), 166.6 (C), 166.4 (C), 142.8 (C), 141.1 (C), 136.2 (C), 130.6 (CH), 128.9 (2CH), 127.9 (CH), 126.6 (C), 123.4 (CH), 121.6 (2CH), 110.2 (CH), 82.0 (C-O), 78.5 (C(CO2CH3)2), 61.5 (CH), 53.1 (CO2CH3), 53.0 (CO2CH3), 52.3 (CO2CH3), 21.1 (H3C-C6H4). HRMS (ESI): (M + Na)+, found 477.1271. [C23H22N2O8Na]+ calculated 477.1268.

rac-(3R,4′S)-trimethyl 2′-(4-methoxyphenyl)-2-oxospiro[indoline-3,5′-isoxazolidine]-3′,3′,4′-tricarboxylate (6c) was obtained from the reaction of 1a (102 mg, 0.5 mmol) with nitrone 5c (200 mg, 0.75 mmol). Yield 170 mg (72%). Pale yellow solid, m.p. 164–167 °C (recrystallized from Et2O). 1H NMR (400 MHz, CDCl3): δ 8.62 (s, 1H, NH), 7.95 (dd, J = 7.7, 1.3 Hz, 1HAr), 7.50–7.43 (m, 2HAr), 7.29–7.20 (m, 1HAr), 7.09–7.00 (m, 1HAr), 6.90 (d, J = 7.7 Hz, 1HAr), 6.84–6.77 (m, 2HAr), 5.26 (s, 1H, CH), 3.81 (s, 3H, CH3), 3.76 (s, 3H, CH3), 3.44 (s, 3H, CH3), 3.22 (s, 3H, CH3). 13C NMR (101 MHz, CDCl3): δ 172.9 (C), 167.1 (C), 166.6 (C), 166.4 (C), 158.4 (C), 141.1 (C), 138.2 (C), 130.6 (CH), 127.9 (CH), 126.6 (C), 123.9 (2CH), 123.4 (CH), 113.5 (2CH), 110.2 (CH), 82.0 (C-O), 78.5 (C(CO2CH)2), 61.4 (CH), 55.6 (CH3), 53.1 (CH3), 53.0 (CH3), 52.2 (CH3). HRMS (ESI): (M + H)+, found 471.1403. [C23H22N2O9H]+ calculated 471.1398.

rac-(3R,4′S)-trimethyl 1-methyl-2-oxo-2′-phenylspiro[indoline-3,5′-isoxazolidine]-3′,3′,4′-tricarboxylate (6d) was obtained from the reaction of 1b (109 mg, 0.5 mmol) with nitrone 5a (178 mg, 0.75 mmol). Yield 209 mg (92%). Colorless solid, m.p. 139–141 °C. 1H NMR (400 MHz, CDCl3): δ 7.96 (d, J = 7.7 Hz, 1HAr), 7.46 (d, J = 7.7 Hz, 2HAr), 7.36–7.22 (m, 3HAr), 7.17–7.03 (m, 2HAr), 6.81 (d, J = 7.8 Hz, 1HAr), 5.27 (s, 1H, CH), 3.84 (s, 3H, CH3), 3.40 (s, 3H, CH3), 3.27 (s, 3H, CH3), 3.21 (s, 3H, CH3). 13C NMR (101 MHz, CDCl3): δ 170.6 (C), 167.0 (C), 166.7 (C), 166.1 (C), 145.5 (C), 144.0 (C), 130.6 (CH), 128.4 (2CH), 127.5 (CH), 126.2 (C), 126.1 (CH), 123.5 (CH), 121.2 (2CH), 108.0 (CH), 81.8 (C-O), 78.7 (C(CO2CH)2), 61.4 (CH), 53.1 (CO2CH3), 53.0 (CO2CH3), 52.2 (CO2CH3), 26.9 (NCH3). HRMS (ESI): (M + H)+, found 455.1454. [C23H22N2O8H]+ calculated 455.1449.

rac-(3R,4′S)-trimethyl 1-methyl-2-oxo-2′-(p-tolyl)spiro[indoline-3,5′-isoxazolidine]-3′,3′,4′-tricarboxylate (6e) was obtained from the reaction of 1b (54 mg, 0.25 mmol) with nitrone 5b (94 mg, 0.375 mmol). Yield 90 mg (77%). Amber solid, m.p. 108–111 °C. 1H NMR (400 MHz, CDCl3): δ 7.96 (dd, J = 7.7, 1.3 Hz, 1HAr), 7.39–7.27 (m, 3HAr), 7.11–7.02 (m, 3HAr), 6.79 (d, J = 7.7 Hz, 1HAr), 5.25 (s, 1H, CH), 3.82 (s, 3H, CO2CH3), 3.43 (s, 3H, CO2CH3), 3.26 (s, 3H, CO2CH3), 3.19 (s, 3H, NCH3). 13C NMR (101 MHz, CDCl3): δ 170.7 (C), 167.1 (C), 166.7 (C), 166.2 (C), 144.0 (C), 142.9 (C), 136.0 (C), 130.5 (CH), 128.9 (2CH), 127.5 (CH), 126.2 (C), 123.4 (CH), 121.5 (2CH), 108.0 (CH), 81.7 (C-O), 78.6 (C(CO2Me)2), 61.4 (CH), 53.0 (CO2CH3), 53.0 (CO2CH3), 52.1 (CO2CH3), 26.9 (NCH3), 21.1 (H3C-C6H4). HRMS (ESI): (M + H)+, found 469.1608. [C24H24N2O8H]+ calculated 469.1605. Appropriate crystals for X-ray analysis were obtained from hexane/ethyl acetate solution. Crystallographic data for 6e have been deposited with the Cambridge Crystallographic Data Centre, no. CCDC 2177790.

rac-(3R,4′S)-trimethyl 2′-(4-methoxyphenyl)-1-methyl-2-oxospiro[indoline-3,5′-isoxazolidine]-3′,3′,4′-tricarboxylate (6f) was obtained from the reaction of 1b (109 mg, 0.5 mmol) with nitrone 5c (200 mg, 0.75 mmol). Yield 199 mg (82%). Pale orange solid, m.p. 151–153 °C. 1H NMR (400 MHz, CDCl3): δ 7.96 (dd, J = 7.5, 1.3 Hz, 1HAr), 7.49–7.41 (m, 2HAr), 7.35–7.27 (m, 1HAr), 7.11–7.02 (m, 1HAr), 6.84–6.75 (m, 3HAr), 5.25 (s, 1H, CH), 3.80 (s, 3H, CH3), 3.76 (s, 3H, CH3), 3.44 (s, 3H, CH3), 3.25 (s, 3H, CH3), 3.18 (s, 3H, NCH3). 13C NMR (101 MHz, CDCl3): δ 170.7 (C), 167.2 (C), 166.7 (C), 166.3 (C), 158.3 (C), 144.0 (C), 138.3 (C), 130.6 (CH), 127.5 (CH), 126.2 (C), 123.8 (2CH), 123.4 (CH), 113.5 (2CH), 108.0 (CH), 81.6 (C-O), 78.6 (C(CO2CH)2), 61.2 (CH), 55.5 (CH3), 53.0 (CH3), 53.0 (CH3), 52.1 (CH3), 26.8 (NCH3). HRMS (ESI): (M + H)+, found 485.1558. [C24H24N2O9H]+ calculated 485.1555.

3.2.5. General Procedure for Obtaining 1,3-Aminoalcohols 7 and 7′

Activated zinc dust and glacial acetic acid were added to the solution of isoxazolidine 3 (or 3′) in methanol. The mixture was stirred at room temperature for 1 h. Then, the zinc dust was filtered out. The filtrate was neutralized with saturated NaHCO3 solution until pH = 8. The resulting solution was extracted with DCM (2 × 100 mL). Organic layers were combined and dried with anhydrous sodium sulfate. The solvent was removed under reduced pressure. The product was purified by column chromatography (silica gel, hexane:ethyl acetate 2:1).

rac-(2R,3S)-methyl 2-((R)-3-hydroxy-1-methyl-2-oxoindolin-3-yl)-4-oxo-3,4-bis(phenylamino)butanoate (7a) was obtained from the reaction of 3e (125 mg, 0.27 mmol) with 390 mg of zinc dust and 0.9 mL of glacial acetic acid in methanol (4 mL). Yield 72 mg (58%). Colorless oil. 1H NMR (400 MHz, C6D6): δ 9.03 (s, 1H, NHCO), 7.59–7.51 (m, 2HAr), 7.44 (d, J = 7.7 Hz, 1HAr), 7.14–6.99 (m, 5HAr), 6.96–6.85 (m, 2HAr), 6.79–6.69 (m, 2HAr), 6.69–6.61 (m, 2HAr), 6.14 (d, J = 7.7 Hz, 1HAr), 5.95 (s, 1H, OH), 5.84 (d, J = 9.7 Hz, 1H, NHPh), 5.01 (dd, J = 9.7, 3.5 Hz, 1H, CH), 3.92 (d, J = 3.5 Hz, 1H, CH), 3.12 (s, 3H, CO2CH3), 2.51 (s, 3H, NCH3). 13C NMR (101 MHz, C6D6): δ 175.6 (C), 173.7 (C), 169.7 (C), 147.0 (C), 143.5 (C), 138.6 (C), 131.5 (C), 129.8 (CH), 129.8 (2CH), 129.1 (2CH), 124.4 (CH), 124.3 (CH), 123.1 (CH), 120.2 (2CH), 120.1 (CH), 115.3 (2CH), 108.9 (CH), 76.8 (C-OH), 60.1 (CH), 52.0 (CO2CH3), 50.8 (CH), 25.8 (NCH3). HRMS (ESI): (M + H)+, found 460.1872. [C26H25N3O5H]+ calculated 460.1867.

rac-(2R,3R)-methyl 2-((R)-3-hydroxy-1-methyl-2-oxoindolin-3-yl)-4-oxo-3,4-bis(phenylamino)butanoate (7′a) was obtained from the reaction of 3′e (50 mg, 0.11 mmol) with 130 mg of zinc dust and 0.3 mL of glacial acetic acid in methanol (2 mL). Yield 25 mg (50%). Pale yellow oil. 1H NMR (400 MHz, C6D6): δ 8.38 (s, 1H, NHCO), 7.43–7.36 (m, 2HAr), 7.29 (dd, J = 7.8, 1.3 Hz, 1HAr), 7.01–6.94 (m, 2HAr), 6.88–6.69 (m, 5HAr), 6.53–6.46 (m, 1HAr), 5.84–5.74 (m, 3HAr), 5.11 (s, 1H, OH), 4.99 (d, J = 9.3 Hz, 1H, NH), 4.84 (d, J = 2.9 Hz, 1H, CH), 4.01 (dd, J = 9.3, 2.9 Hz, 1H, CH), 3.36 (s, 3H, CO2CH3), 2.23 (s, 3H, NCH3). 13C NMR (101 MHz, C6D6): δ 176.0 (C), 172.2 (C), 169.9 (C), 145.9 (C), 144.6 (C), 138.3 (C), 130.2 (CH), 129.5 (2CH), 129.1 (2CH), 128.4 (C) 124.6 (CH), 124.3 (CH), 123.4 (CH), 119.7 (2CH), 118.8 (CH), 112.4 (2CH), 109.1 (CH), 76.2 (C-OH), 56.8 (CH), 53.4 (CO2CH3), 52.7 (CH), 25.5 (NCH3). HRMS (ESI): (M + H)+, found 460.1872. [C26H25N3O5H]+ calculated 460.1867.

rac-(2R,3S)-methyl 4-((4-chlorophenyl)amino)-2-((R)-3-hydroxy-1-methyl-2-oxoindolin-3-yl)-4-oxo-3-(phenylamino)butanoate (7b) was obtained from the reaction of 3f (50 mg, 0.11 mmol) with 130 mg of zinc dust and 0.3 mL of glacial acetic acid in methanol (2 mL). Yield 31 mg (62%). Pale brown oil. 1H NMR (400 MHz, C6D6): δ 8.97 (s, 1H, NHCO), 7.42 (d, J = 7.8 Hz, 1HAr), 7.32–7.25 (m, 2HAr), 7.09–7.00 (m, 4HAr), 6.97–6.87 (m, 1HAr), 6.80–6.70 (m, 2HAr), 6.64 (d, J = 7.8 Hz, 2HAr), 6.14 (d, J = 7.8 Hz, 1HAr), 5.87 (s, 2H, OH, NHPh), 4.95 (s, 1H, CH), 3.89 (d, J = 3.0 Hz, 1H, CH), 3.10 (s, 3H, CO2CH3), 2.50 (s, 3H, NCH3). 13C NMR (101 MHz, C6D6): δ 175.4 (C), 174.1 (C), 169.7 (C), 146.9 (C), 143.3 (C), 137.1 (C), 131.6 (C), 129.9 (CH), 129.8 (2CH), 129.3 (C), 129.2 (2CH), 124.3 (CH), 123.1 (CH), 121.3 (2CH), 120.3 (CH), 115.3 (2CH), 109.0 (CH), 76.7 (C-OH), 60.2 (CH), 52.0 (CO2CH3), 50.2 (CH), 25.8 (NCH3). HRMS (ESI): (M + Na)+, found 516.1297. [C26H24ClN3O5Na]+ calculated 516.1297.

3.2.6. Procedure for Obtaining Lactone 8

Activated zinc dust (130 mg) was added to the solution of isoxazolidine 6d (50 mg, 0.11 mmol) in glacial acetic acid (2 mL). The mixture was stirred at room temperature for 2 h. Then, the zinc dust was filtered out. The filtrate was neutralized with saturated NaHCO3 water solution so that pH = 8. The resulting solution was extracted with DCM (2 × 100 mL). Organic layers were combined and dried over anhydrous sodium sulfate. The solvent was removed under reduced pressure. The product was purified by column chromatography (silica gel, hexane:ethyl acetate 2:1).

rac-(2R,3S,4S)-dimethyl 1′-methyl-2′,5-dioxo-4-(phenylamino)-4,5-dihydro-3H-spiro[furan-2,3′-indoline]-3,4-dicarboxylate (8). Yield 25 mg (55%). Pale brown oil. 1H NMR (400 MHz, C6D6): δ 7.83 (dd, J = 7.7, 1.2 Hz, 1HAr), 7.21–7.17 (m, 2HAr), 7.12–7.03 (m, 2HAr), 6.94–6.84 (m, 1HAr), 6.82–6.73 (m, 1HAr), 6.72–6.62 (m, 1HAr), 6.08–6.00 (m, 1HAr), 5.89 (s, 1H, NHPh), 4.91 (s, 1H, CH), 3.29 (s, 3H, CO2CH3), 2.81 (s, 3H, CO2CH3), 2.41 (s, 3H, NCH3). 13C NMR (101 MHz, C6D6): δ 172.3 (C), 169.6 (C), 167.3 (C), 167.1 (C), 145.0 (C), 144.2 (C), 131.6 (CH), 129.3 (2CH), 127.3 (CH), 123.5 (C), 123.4 (CH), 121.6 (CH), 119.1 (2CH), 108.9 (CH), 81.6 (C-O), 68.3 (C), 55.5 (CO2CH3), 51.9 (CH), 26.1 (NCH3). HRMS (ESI): (M + Na)+, found 447.1161. [C22H20N2O7Na]+ calculated 447.1163. Appropriate crystals for X-ray analysis were obtained from ethanol solution. Crystallographic data for 8 have been deposited with the Cambridge Crystallographic Data Centre, no. CCDC 2177791.

3.3. Bioactivity Assay

3.3.1. Cell Culture

MCF7 breast adenocarcinoma, A549 lung cancer, MDA-MB-231 breast adenocarcinoma, Caki-2 kidney clear cell carcinoma, and T98-G glioblastoma cell lines were purchased from the American Type Culture Collection (ATCC, Manassas, VA, USA). The MCF7 cells were maintained in Advanced MEM (Gibco, Paisley, UK) supplemented with 5% fetal bovine serum (FBS, Gibco, UK), penicillin (100 UI/mL), streptomycin (100 µg/mL), insulin (0.01 mg/mL), and GlutaMax (1.87 mM, Gibco, UK). The A549 cells were maintained in F12-K media (Gibco, Grand Island, NY, USA) supplemented with 10% fetal bovine serum (FBS, Origin: Brazil, Gibco, UK), penicillin (100 UI/mL), streptomycin (100 µg/mL), and GlutaMax (2 mM, Gibco, UK). The MDA-MB-231 cells were maintained in Advanced DMEM/F12 (Gibco, USA) supplemented with 5% fetal bovine serum (FBS, Gibco, UK), penicillin (100 UI/mL), streptomycin (100 µg/mL), and GlutaMax (2.5 mM, Gibco, UK). The T98-G cells were maintained in Advanced MEM (Gibco, UK) supplemented with 5% fetal bovine serum (FBS, Origin: Brazil, Gibco, UK), penicillin (100 UI/mL), streptomycin (100 µg/mL), and GlutaMax (1.87 mM, Gibco, UK). All the cell lines were cultivated under a humidified atmosphere of 95% air/5% CO2 at 37 °C. Subconfluent monolayers, in the log growth phase, were harvested by a brief treatment with TrypLE Express solution (Gibco, USA) in phosphate-buffered saline (PBS, Capricorn Scientific, Germany) and washed three times with serum-free PBS. The number of viable cells was determined by trypan blue exclusion.

3.3.2. Antiproliferative Assay

The effects of the synthesized compounds on cell viability were determined using the MTT colorimetric test. All the examined cell lines were diluted with the growth medium to 3.5 × 104 cells per mL, and the aliquots (7 × 103 cells per 200 μL) were placed in individual wells in 96-well multiplates (Eppendorf, Germany) and incubated for 24 h. The cells were treated with the synthesized compounds separately at a concentration of 50 μM and incubated for 72 h at 37 °C in a 5% CO2 atmosphere. Each compound was tested in triplicate. The cells were treated with 40 μL of an MTT solution (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, 5 mg/mL in PBS) and incubated for 8 h. The medium with the MTT was removed and DMSO (150 μL) was added to dissolve the formazan crystals. The plates were shaken for 10 min. The optical density of each well was determined at 560 nm using a GloMax Multi+ (Promega, Madison, WI, USA) microplate reader. The cytotoxicity of each compound was evaluated in three separate experiments.

4. Conclusions

It has been shown that the 1,3-dipolar cycloaddition of 2-(2-oxoindoline-3-ylidene)acetates with aldo- and ketonitrones is an effective method for the selective synthesis of new highly functionalized spiroisoxazolidines. The reaction of C-carbamoyl aldonitrones predominantly obtains 5-spiroisoxazolidines. In this case, varying the reaction conditions makes it possible to selectively obtain certain diastereomers in good yields due to the reversibility of the reaction. The reaction of C,C-bis(methoxycarbonyl) ketonitrones obtains only 5-spiroisoxazolidines in good-to-high yields. The reduction of the obtained cycloadducts can obtain aminoalcohols or spirolactones depending on the structure of the starting cycloadduct. Cytotoxicity screening against several cancer cell lines revealed that several cycloadducts exhibit antiproliferative activity.

Acknowledgments

We are grateful to all staff members of the Research Park of SPSU, who carried out the analyses for the current research. The research used resources of the SPSU Resource Centres: Magnetic Resonance Research Centre, Chemical Analysis and Materials Research Centre, and Centre for X-ray Diffraction Studies.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ijms232012639/s1.

Click here for additional data file. (3MB, zip)

Author Contributions

Conceptualization, M.M.E., A.P.M. and N.V.R.; Methodology, M.M.E.; Validation, M.M.E. and D.D.K.; Investigation, D.D.K. and M.M.E. (chemistry), M.A.K. (X-ray analysis), A.S.B. and D.A.K. (bioassay); Writing—Original Draft Preparation, M.M.E., D.D.K., A.S.B. and A.P.M.; Writing—Review and Editing, M.M.E., A.P.M. and N.V.R.; visualization, M.M.E., D.D.K. and A.S.B.; supervision, M.M.E.; project administration, M.M.E. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

Funding Statement

This research was supported by the Russian Science Foundation, grant number 22-73-10184.

Footnotes

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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