DABCO-catalysed highly diastereoselective synthesis of 1,3-indandione containing fully saturated spirocyclopentanes involving 2-(2′-ketoalkyl)-1,3-indandiones and nitrostyrenes

Abstract

Highly functionalized 1,3-indandione containing spirocyclopentanes has been synthesized by reacting nitrostyrenes with 2-(2′-ketoalkyl)-1,3-indandiones using a catalytic amount of DABCO. The resultant products possess three consecutive chiral centres in addition to a spirocyclic centre and are obtained in a highly diastereoselective manner using a catalytic amount of the organic base, DABCO.

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2-(2′-Ketoalkyl)-1,3-indandiones (1) were easily accessed from the commercially available 1,3-indandione, and this substrate has more electrophilic and nucleophilic sites compared to the parent compound 1,3-indanedione.^1–4 Accordingly, these compounds have been used for the synthesis of various carbo-, heterocycles in our laboratory (Scheme 1). In particular, the reaction of 2-(2′-ketoalkyl)-1,3-indandiones provided bicyclo[3.2.1]octadienones (Scheme 1A),¹ spiro[4.4]nonanes, pentafulvene dyes (Scheme 1B and C),^2,3 and azabicyclo[3.1.0]hexanes (Scheme 1D)⁴ by reacting it with arynes, dimethylacetylenedicarboxylate (DMAD) or its derivatives and vinyl azides, respectively. As part of our ongoing research interest in exploring the reactivity of 2-(2′-ketoalkyl)-1,3-indandiones, we became interested in utilising the nitrostyrenes as an alternative reacting partner, because they serve as effective dipolarophiles, and have been extensively utilized in reactions with a 1,3-dipole to produce five-membered cyclic compounds that contain nitro groups.⁵ Therefore, the reaction of nitrostyrenes with 2-(2′-ketoalkyl)-1,3-indandiones would provide fully saturated cyclopentanes embedded within the 1,3-indandione skeleton. However, there were similar systems that were synthesized by cycloaddition of vinyl cyclopropane derived from 1,3-indanedione with nitroalkenes in the presence of Pd⁰ catalyst, in an asymmetric manner.⁶ Therefore, we focused on developing transition metal-free conditions to access fully saturated spirocyclopentyl indane-1,3-dione compounds in our study. It is noteworthy to mention that 1,3-indandiones and their derivatives containing spirocyclopentane have garnered considerable interest due to their potential for creating architectural themes. 1,3-indandione structural motifs have been found in diverse fields, ranging from bioactive compounds^7–11 to functionalized materials.¹² For instance, fredericamycin A, which was isolated from a soil bacterium of Streptomyces griseus (FCRC-48), and found to be antibacterial, antifungal and cytotoxic.¹³ Therefore, developing a novel method to construct 1,3-indandione containing spirocyclopentanes would be vital, specifically under transition metal-free conditions.

Scheme 1 Previous and current approaches by utilizing 2-(2′-ketoalkyl)-1,3-indandiones.

Accordingly, we started our investigation using 2-(2′-ketoalkyl)-1,3-indandiones 1a and (E)-(2-nitrovinyl)benzene 2a as a model substrate to check our initial hypothesis. Consequently, we have treated 1a and 2a with Et₃N as a base in MeCN at room temperature. To our delight, we found the expected 1,3-indandione containing spirocyclopentane 3a in 46% yield (Table 1, entry 1). Subsequently, the product 3a was isolated and confirmed by the meticulous NMR experiments. However, the formation of a compound 3a motivated us to screen the reaction conditions further to get better results. Accordingly, several bases were tested, and the results are described in Table 1. Potassium fluoride (KF) provided the required product, 3a, in only 40% yield (Table 1, entry 2), while prolonged conditions up to 24 h by increasing the equivalents up to 3 decreased the yield to 29% (Table 1, entry 3). On the other hand, caesium fluoride failed to give the expected product (Table 1, entry 4). Next, the Carbonate bases such as K₂CO₃ and Cs₂CO₃ were also tested; surprisingly, they failed to give the desired product (Table 1, entries 5 & 6). Thereafter, we have tried the reaction with Na₂CO_3, but it resulted in only 50% yield of the product (Table 1, entry 7). These results forced us to switch back to the usage of organic bases for fruitful results. Consequently, a stoichiometric amount of DABCO was tested, which furnished the expected product in 40% yield (Table 1, entry 8), and further increasing the equivalents of base provided the required product 3a in 55% yield (Table 1, entry 9). This persuaded us to employ a catalytic amount, allowing the reaction to proceed for longer hours by varying the temperature. To our astonishment, the required product 3a was obtained in 50%, 61% & 70% yield (Table 1, entries 10, 11 & 12) in the presence of 20 mol%. The reaction did not benefit from reducing the catalyst loading to 10 mol%, which only produced a 53% yield (Table 1, entry 13). Next, DABCO was used in two different solvents, DMF and THF, at higher temperatures, which did not help us in providing better results in terms of yield (Table 1, entries 14 & 15). Furthermore, DBU and pyridine were also tested, which did not provide satisfactory results (Table 1, entries 16 & 17). Subsequently, DMAP was used at ambient and elevated temperatures, but the results were not satisfactory (Table 1, entries 18 & 19), even with the higher equivalents of base (Table 1, entry 20). Therefore, the optimised conditions for the synthesis of compound 3a are 20 mol% DABCO, at 80 °C for 2 h in acetonitrile solvent (Table 1, entry 12). Afterwards, we explored the generality of the transformation with a series of 2-(2′-ketoalkyl)-1,3-indandiones and nitrostyrenes. The substituent on the aryl moiety of 2-(2′-ketoalkyl)-1,3-indandiones was summarised in Scheme 2.

Table 1 Optimization of reaction conditions^a

S. No.	Base (equiv.)	Time (h)	Temperature (°C)	Yield^b (%)
a Reactions were performed on a 0.1 mmol scale of 2-(2′-ketoalkyl)-1,3-indandiones (1a) and 1.5 equiv. of nitrostyrenes (2a) in acetonitrile (0.1 M) solvent unless otherwise stated.b Isolated yields after silica gel column chromatography. ND: Not Detected.c DMF (0.1 M) was used as a solvent.d THF (0.1 M) was used as a solvent.
1	Et3N (1.1)	1	25	46
2	KF (1.5)	3	25	40
3	KF (3.0)	24	25	29
4	CsF (3)	1	25	ND
5	K2CO3 (1.5)	22	25	ND
6	Cs2CO3 (1.5)	22	25	ND
7	Na2CO3 (1.5)	5	25	50
8	DABCO (1.1)	3	25	40
9	DABCO (3)	3	25	55
10	DABCO (0.2)	8	25	50
11	DABCO (0.2)	6	60	61
12	DABCO (0.2)	2	80	70
13	DABCO (0.1)	2	80	53
14^c	DABCO (0.2)	2	100	32
15^d	DABCO (0.2)	2	65	28
16	DBU (1.1)	12	25	6
17	Pyridine (1.1)	12	25	ND
18	DMAP (1.1)	12	25	5
19	DMAP (1.1)	3	80	ND
20	DMAP (3)	3	Rt	Traces

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Scheme 2 Substrate scope of 2-(2′-ketoalkyl)-1,3-indandiones.

Accordingly, various halogen-containing compounds were tested for this method under standard reaction conditions and provided the required compounds 3b, 3c and 3d in 45%, 54% and 55%, respectively. The lower yields might be due to the electron-withdrawing character of halogens. However, the methyl group at the para position exhibited smooth reactivity, yielding 65% of product 3e. Next, the ortho substituent of the aryl group of 2-(2′-ketoalkyl)-1,3-indandiones was altered with the halogens to produce 3f and 3g in 50% and 40%, respectively. Compound 3g was further characterised by its single-crystal X-ray diffraction analysis, confirming the relative configuration of substituents (CCDC 2456358). Further, an electron-withdrawing group at the meta position was well tolerated in the developed method and furnished 58% of the product 3h in 58% yield. To our delight, dichlorinated 2-(2′-ketoalkyl)-1,3-indandiones also yielded product 3i in good yield of 58%. The diastereomeric ratio was determined by NMR analysis of the crude reaction mixture.

Next, we moved our attention towards the installation of the diverse substituents on nitrostyrenes (Scheme 3). Several groups at para-position were varied, furnishing respective products in moderate to good yields. Accordingly, the methoxy, benzyloxy, and hydroxyl groups were well tolerated in the developed method and provided the required products, 3j, 3k, and 3l, in yields of 60%, 67%, and 63%, respectively. Next, fluoro- and chloro-containing nitrostyrenes were tested under standard conditions and provided the corresponding products, 3m and 3n, in 71% and 57% yield, respectively. Thereafter, a strong electron-withdrawing group, such as a cyano group, was endured, affording the product 3o in 69% yield. Furthermore, the feasibility of the reaction was checked for ortho-substituted nitrostyrenes. Methyl-substituted nitrostyrene delivered corresponding spirocycle 3p in 63% yield. Bromide or chloride substitution at the ortho position of the phenyl ring of the nitrostyrene gave 3q–3s in moderate to good yields. Additionally, ortho-nitro nitrostyrene was further tested with a variety of 2-(2′-ketoalkyl)-1,3-indandiones, which yielded 3t–3w in good yields. Interestingly, tri-nitro spirocyclopentane 3v was synthesized in 66% yield. Meta-fluoro substituted nitrostyrene was also analysed in the developed method, and the corresponding spirocyclopentane 3x was isolated with 60% yield. Notably, a variety of di- and tri-functionalized nitrostyrenes at various positions on the benzene moiety were well accommodated in this method. Moreover, the dimethoxy group located at two distinct positions was evaluated, yielding the respective spirocyclopentane compounds 3y and 3z in moderate yields. Dichloro-substituted spirocyclopentane 3za was also synthesized in 61% yield. To our delight, nitrostyrene containing hydroxyl and ethoxy groups at the 1,2-position on the benzene moiety provided compound product, 3zb, in 65% yield. Finally, trifunctionalized spirocyclopentanes 3zc and 3zd were also synthesised from corresponding nitrostyrenes under standard conditions (Scheme 3).

Scheme 3 Substrate scope of nitrostyrenes.

A plausible reaction mechanism for the formation of 1,3-indandione-containing spirocyclopentane is depicted in Scheme 4. The reaction pathway starts with the formation of enolate I from 1a in the presence of DABCO, which undergoes Michael addition with nitrostyrene 2a, leading to intermediates II. The intermediate II undergoes intramolecular annulation via Henry reaction. To our surprise, the intramolecular Henry reaction proceeds in a highly diastereoselective manner via Re-face attack of the carbonyl functionality provided the intermediate III, which upon protonation furnishes the 1,3-indandione containing spirocyclopentane 3a. It was possible to isolate and thoroughly characterized IIa, the protonated form of II. The formation of side product IIa is inevitable in some cases, where the reaction is slow. This might be the reason for the lower yields for some substrates. However, our attempts were unsuccessful in converting IIa to the required product 3a under different conditions (see the SI for details). The syn relationship between the two phenyl groups was confirmed by single-crystal X-ray diffraction analysis of one of the examples, 3g (CCDC 2456358).

Scheme 4 Plausible reaction mechanism for the formation of 1,3-indandione containing spirocyclopentanes.

Conclusions

In conclusion, we have divulged a facile method for the synthesis of highly functionalized 1,3-indandione containing spirocyclopentane under mild, organocatalytic conditions. The resultant products possess three consecutive chiral centres in addition to a spirocyclic centre. They are obtained in a highly diastereoselective manner using a catalytic amount of the organic base DABCO. The high degree of functional group tolerance was observed in both reacting partners.

General information

All reagents were used as supplied commercially without further purification unless otherwise stated. All the solvents are distilled before use and stored over molecular sieves. Air-sensitive reactions were carried out with the oven-dried glass apparatus with the flow of N₂ gas. All the column purifications were done with either silica gel 60–120 mesh or 230–400 mesh, in most cases eluent were a mixture of ethyl acetate and hexane unless mentioned. Analytical thin-layer chromatography (TLC) was performed on TLC Silica gel 60 F254 plates purchased from Merck, Germany. Visualisation was accomplished with UV light (254 nm) and exposure to either ethanolic phosphomolybdic acid (PMA), anisaldehyde or KMnO4 solution, CeSO4 + ammonium phosphomolybdate + 10% H₂SO₄, followed by heating. Melting points are uncorrected. 1H NMR spectra were acquired on a Bruker AVANCE NEO Ascend 400 (at 400 MHz), and chemical shifts are reported relative to the residual solvent peak (acetone-d6, δ = 2.05 ppm, CDCl3, δ = 7.26 ppm, methanol-d4, δ = 3.31 ppm). 13C NMR spectra were acquired on Bruker AVANCE (at 100 MHz), and chemical shifts are reported in ppm relative to the residual solvent peak. Unless noted, NMR spectra were acquired in CDCl3, acetone-d6 and methanol-d4; individual peaks are reported as multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, dd = doublet of doublet); coupling constants were in Hz. All IR spectra were obtained on Bruker Platinum ATR, and selected absorbance is reported in cm⁻¹. Melting points were determined with a digital melting point apparatus EI–India while keeping the sample on a melting point capillary and are uncorrected. High-resolution mass data were acquired using a Mass Spectrometer (HRMS)Xevo G2-XS QTOF Quadrupole Time of Flight Mass Spectrometer Waters, where MeOH or MeCN was used as a solvent. Chiral HPLC was performed using an Agilent 1260 Infinity II by using chiral phase columns and hexane, isopropanol are eluents. X-ray diffraction measurements were performed to determine the crystal structure of compounds at 273 K using APEX3 (Bruker, 2016; Bruker D8 Venture photon 100 CMOS detector) diffractometer having graphite-monochromatized (MoKα = 0.71073 Å).

General Procedure for the synthesis of spirocyclic compounds (3)

To a solution of compound 1 (0.2 mmol, 1 equiv.) and 1,4-diazabicyclo[2.2.2]octane (DABCO) in acetonitrile (1 mL), a solution of 2 (1.1 equiv.) in acetonitrile (1 mL) was added at room temperature. The reaction mixture was then refluxed for 2 h at 80 °C in an oil bath. After completion of the reaction (monitored by TLC), the solvent was removed under reduced pressure. The resultant crude compound was diluted with CH₂Cl₂ (15 mL) and water (5 mL). The organic layer was separated, and the aqueous layer was extracted with CH₂Cl₂ (3 × 10 mL). The combined organic layer was washed with brine (2 × 10 mL), dried over anhydrous Na₂SO₄, filtered, and concentrated under reduced pressure. The resultant crude product was purified over silica gel column chromatography using ethyl acetate/hexane mixture as eluent afforded spirocyclic compounds 3.

Characterization of spirocyclic compounds (3a–3zd)

(±) (2S,3R,4R)-4-Hydroxy-3-nitro-2,4-diphenylspiro[cyclopentane-1,2′-indene]-1′,3′-dione (3a)

The title compound 3a was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (70% yield; 96 : 4 dr). The spectral data are for the major diastereomer.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 198–200 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.00 (d, J = 7.6 Hz, 1H), 7.80 (dd, J = 12.6, 7.5 Hz, 3H), 7.72 (t, J = 7.4 Hz, 1H), 7.65 (d, J = 7.5 Hz, 1H), 7.48 (t, J = 7.6 Hz, 2H), 7.38 (t, J = 7.3 Hz, 1H), 7.11–7.03 (m, 5H), 6.14 (d, J = 12.4 Hz, 1H), 4.95 (d, J = 12.4 Hz, 1H), 2.85 (d, J = 14.9 Hz, 1H), 2.58 (d, J = 15.0 Hz, 1H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 201.8, 201.2, 142.2, 141.6, 139.7, 136.6, 136.2, 131.9, 128.7, 128.5, 128.4, 128.1, 125.3, 123.6, 123.4, 93.4, 81.5, 60.4, 54.4, 46.5.

IR (ATR) ṽ (cm⁻¹): 3430, 3062, 3011, 2920, 2851, 1741, 1694, 1594, 1520, 1498, 1448.

HRMS (SI, m/z): calculated for C₂₅H₁₉NO₅Na ([M + Na]⁺): 436.1161; found: 436.1157.

(±) (2S,3R,4R)-4-(4-Fluorophenyl)-4-hydroxy-3-nitro-2-phenylspiro[cyclopentane-1,2′-indene]-1′,3′-dione (3b)

The title compound 3b was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (45% yield; 98 : 2 dr) The spectral data are for the major diastereomer.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 195–197 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.00 (d, J = 7.7 Hz, 1H), 7.85–7.76 (m, 3H), 7.73 (t, J = 7.3 Hz, 1H), 7.65 (d, J = 7.6 Hz, 1H), 7.16 (t, J = 8.3 Hz, 2H), 7.09 (s, 3H), 7.05 (s, 2H), 6.07 (d, J = 12.4 Hz, 1H), 4.90 (d, J = 12.4 Hz, 1H), 4.02 (s, 1H), 2.83 (d, J = 15.0 Hz, 1H), 2.56 (d, J = 15.0 Hz, 1H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 201.8, 201.3, 163.9, 161.4, 142.2, 141.6, 136.5, 136.2, 136.3, 132.0, 128.7, 128.5, 128.1, 127.4, 127.3, 123.5 (d, J = 26.3 Hz), 115.6 (d, J = 21.6 Hz), 93.5, 81.2, 60.4, 54.3, 46.4.

IR (ATR) ṽ (cm⁻¹): 3405, 3064, 3012, 2801, 1741, 1694, 1593, 1541, 1511, 1448, 1269.

HRMS (SI, m/z): calculated for C₂₅H₁₈NO₅FNa ([M + Na]⁺): 454.1067; found: 454.1063.

(±) (2S,3R,4R)-4-(4-Bromophenyl)-4-hydroxy-3-nitro-2-phenylspiro[cyclopentane-1,2′-indene]-1′,3′-dione (3c)

The title compound 3c was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (54% yield; 94 : 6 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 205–207 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.00 (d, J = 7.5 Hz, 1H), 7.80 (t, J = 7.3 Hz, 1H), 7.75–7.63 (m, 4H), 7.60 (d, J = 8.2 Hz, 2H), 7.09 (s, 3H), 7.04 (d, J = 2.7 Hz, 2H), 6.05 (d, J = 12.4 Hz, 1H), 4.89 (d, J = 12.4 Hz, 1H), 4.06 (s, 1H), 2.81 (d, J = 15.0 Hz, 1H), 2.55 (d, J = 14.9 Hz, 1H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 201.8, 201.2, 142.2, 141.6, 139.7, 136.6, 136.3, 131.9, 128.7, 128.5, 128.1, 127.3, 123.7, 123.4, 122.6, 93.4, 81.2, 60.4, 54.3, 46.3.

IR (ATR) ṽ (cm⁻¹): 3367, 3044, 2953, 2920, 2849, 1745, 1695, 1589, 1550, 1487, 1461, 698.

HRMS (SI, m/z): calculated for C₂₅H₁₈NO₅NaBr ([M + Na]⁺): 514.0266; found: 514.0266.

(±) (2S,3R,4R)-4-(4-chlorophenyl)-4-hydroxy-3-nitro-2-phenylspiro[cyclopentane-1,2′-indene]-1′,3′-dione (3d)

The title compound 3d was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (55% yield, 95 : 5 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 190–192 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.00 (d, J = 7.5 Hz, 1H), 7.82–7.71 (m, 4H), 7.65 (d, J = 7.5 Hz, 1H), 7.44 (d, J = 7.6 Hz, 2H), 7.08 (s, 3H), 7.03 (s, 2H), 6.06 (d, J = 12.4 Hz, 1H), 4.90 (d, J = 12.4 Hz, 1H), 4.07 (s, 1H), 2.82 (d, J = 15.0 Hz, 1H), 2.56 (d, J = 15.0 Hz, 1H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 201.8, 201.3, 142.2, 141.6, 139.1, 136.6, 136.2, 134.4, 131.8, 128.9, 128.7, 128.5, 128.1, 126.9, 123.7, 123.4, 93.4, 81.2, 60.4, 54.3, 46.4.

IR (ATR) ṽ (cm⁻¹): 3282, 2979, 2947, 2881, 2855, 1750, 1695, 1591, 1552, 1491, 1474, 768.

HRMS (SI, m/z): calculated for C₂₅H₁₈NO₅NaCl ([M + Na]⁺): 470.0771; found: 470.0780.

(±) (2S,3R,4R)-4-Hydroxy-3-nitro-2-phenyl-4-(p-tolyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3e)

The title compound 3e was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (65% yield; 98 : 2 dr). The spectral data are for the major diastereomer.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 175–177 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.00 (d, J = 7.4 Hz, 1H), 7.79 (t, J = 7.1 Hz, 1H), 7.73–7.62 (m, 5H), 7.29 (s, 1H), 7.07 (d, J = 8.8 Hz, 5H), 6.11 (d, J = 12.4 Hz, 1H), 4.93 (d, J = 12.3 Hz, 1H), 2.83 (d, J = 14.8 Hz, 1H), 2.55 (d, J = 14.9 Hz, 1H), 2.39 (s, 3H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 201.8, 201.5, 142.1, 141.7, 138.2, 137.5, 136.4, 136.1, 132.1, 129.4, 128.6, 128.3, 128.1, 125.2, 123.6, 123.3, 93.4, 81.5, 60.4, 54.4, 46.4, 21.1.

IR (ATR) ṽ (cm⁻¹): 3436, 2914, 1743, 1707, 1554, 1538, 1495, 1455.

HRMS (SI, m/z): calculated for C₂₆H₂₁NO₅Na ([M + Na]⁺): 450.1317; found: 450.1311.

(±) (2S,3R,4R)-4-(2-Bromophenyl)-4-hydroxy-3-nitro-2-phenylspiro[cyclopentane-1,2′-indene]-1′,3′-dione (3f)

The title compound 3f was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (50% yield; 95 : 5 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 219–221 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.16 (dd, J = 8.0, 1.4 Hz, 1H), 8.06 (d, J = 7.7 Hz, 1H), 7.83 (t, J = 7.4 Hz, 1H), 7.77–7.71 (m, 2H), 7.60 (d, J = 7.6 Hz, 1H), 7.45–7.43 (m, 1H), 7.24 (dd, J = 7.6, 1.4 Hz, 1H). 7.14–7.13 (m, 3H), 7.07 (dd, J = 6.5, 2.8 Hz, 2H), 6.97 (d, J = 11.3 Hz, 1H), 4.95 (s, 1H), 4.86 (d, J = 11.3 Hz, 1H), 3.78 (d, J = 15.1 Hz, 1H), 2.35 (d, J = 15.1 Hz, 1H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 204.2, 199.0, 143.0, 140.8, 137.3, 136.8, 136.0, 135.2, 132.9, 130.1, 130.0, 128.8, 128.6, 128.3, 127.9, 123.9, 123.5, 120.2, 91.1, 82.1, 61.1, 53.9, 42.2.

IR (ATR) ṽ (cm⁻¹): 3401, 3064, 3010, 2955, 2918, 1758, 1695, 1589, 1543, 1469, 1452, 692.

HRMS (SI, m/z): calculated for C₂₅H₁₉BrNO₅ ([M + H]⁺): 492.0447; found: 492.0467.

(±) (2S,3R,4R)-4-(2-Chlorophenyl)-4-hydroxy-3-nitro-2-phenylspiro[cyclopentane-1,2′-indene]-1′,3′-dione (3g)

The title compound 3g was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (40% yield; 93 : 7 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 198–200 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.11 (d, J = 7.7 Hz, 1H), 8.06 (d, J = 7.6 Hz, 1H), 7.83 (t, J = 7.4 Hz, 1H), 7.75 (t, J = 7.4 Hz, 1H), 7.60 (d, J = 7.6 Hz, 1H), 7.51 (d, J = 7.6 Hz, 1H), 7.37 (dt, J = 19.6, 7.2 Hz, 2H), 7.13 (d, J = 2.6 Hz, 3H), 7.06 (d, J = 3.1 Hz, 2H), 6.84 (d, J = 11.4 Hz, 1H), 4.87 (d, J = 13.8 Hz, 2H), 3.73 (d, J = 14.9 Hz, 1H), 2.30 (d, J = 14.9 Hz, 1H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 204.2, 198.9, 143.0, 140.8, 136.8, 136.0, 135.8, 132.8, 131.6, 131.0, 129.9, 129.7, 128.8, 128.6, 128.3, 127.4, 123.9, 123.5, 91.1, 81.7, 61.1, 54.0, 42.1.

IR (ATR) ṽ (cm⁻¹): 3530, 3066, 3035, 2954, 1741, 1695, 1592, 1544, 1495, 1467, 761.

HRMS (SI, m/z): calculated for C₂₅H₁₈NO₅NaCl ([M + Na]⁺): 470.0771; found: 470.0757.

(±) (2S,3R,4R)-4-Hydroxy-3-nitro-4-(3-nitrophenyl)-2-phenylspiro[cyclopentane-1,2′-indene]-1′,3′-dione (3h)

The title compound 3h was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (58% yield; 97 : 3 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 198–200 °C.

¹H NMR (400 MHz, DMSO-d₆) δ 8.66 (s, 1H), 8.32 (d, J = 7.8 Hz, 1H), 8.25 (d, J = 8.1 Hz, 1H), 7.94 (d, J = 7.5 Hz, 1H), 7.89–7.76 (m, 3H), 7.68 (d, J = 7.5 Hz, 1H), 7.13 (s, 1H), 7.05 (d, J = 10.7 Hz, 5H), 6.24 (d, J = 12.2 Hz, 1H), 4.78 (d, J = 12.2 Hz, 1H), 2.80 (d, J = 15.5 Hz, 1H), 2.73 (d, J = 15.6 Hz, 1H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 203.1, 200.8, 148.3, 147.4, 142.3, 141.8, 136.9, 133.6, 133.1, 130.3, 128.9, 128.3, 127.9, 123.4, 123.0, 121.2, 93.6, 80.3, 60.7, 53.9, 47.6.

IR (ATR) ṽ (cm⁻¹): 3320, 3062, 2976, 1741, 1687, 1588, 1553, 1526, 1441.

HRMS (SI, m/z): calculated for C₂₅H₁₈N₂O₇Na ([M + Na]⁺): 481.1012; found: 481.1025.

(±) (2S,3R,4R)-4-(2,4-Dichlorophenyl)-4-hydroxy-3-nitro-2-phenylspiro[cyclopentane-1,2′-indene]-1′,3′-dione (3i)

The title compound 3i was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (58% yield; 96 : 4 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 150–152 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.06 (dd, J = 7.8, 4.7 Hz, 2H), 7.84 (t, J = 7.4 Hz, 1H), 7.76 (t, J = 7.4 Hz, 1H), 7.60 (d, J = 7.6 Hz, 1H), 7.53 (s, 1H), 7.36 (d, J = 8.5 Hz, 1H), 7.13 (d, J = 2.7 Hz, 3H), 7.03 (d, J = 3.6 Hz, 2H), 6.75 (d, J = 11.3 Hz, 1H), 5.01 (s, 1H), 4.83 (d, J = 11.3 Hz, 1H), 3.73 (d, J = 14.9 Hz, 1H), 2.23 (d, J = 14.9 Hz, 1H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 204.4, 198.6, 143.0, 140.6, 137.0, 136.1, 135.1, 134.5, 132.7, 131.5, 131.2, 131.0, 128.8, 128.7, 128.4, 127.6, 124.0, 123.5, 91.2, 81.4, 61.1, 53.9, 41.6.

IR (ATR) ṽ (cm⁻¹): 3372, 3118, 3088, 3040, 3040, 3012, 1745, 1693, 1591, 1550, 1471, 1417, 763.

HRMS (SI, m/z): calculated for C₂₅H₁₈Cl₂NO₅ ([M + H]⁺): 482.0562; found: 482.0571.

(±) (2S,3R,4R)-4-Hydroxy-2-(4-methoxyphenyl)-3-nitro-4-(3-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3j)

The title compound 3j was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (60% yield; 92 : 8 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 218–220 °C.

¹H NMR (400 MHz, DMSO-d₆) δ 8.65 (s, 1H), 8.31 (d, J = 7.7 Hz, 1H), 8.25 (d, J = 8.4 Hz, 1H), 7.95 (d, J = 7.5 Hz, 1H), 7.88 (t, J = 7.2 Hz, 1H), 7.85–7.80 (m, 2H), 7.73 (d, J = 7.4 Hz, 1H), 7.09 (s, 1H), 6.98 (d, J = 8.4 Hz, 2H), 6.63 (d, J = 8.4 Hz, 2H), 6.16 (d, J = 12.3 Hz, 1H), 4.73 (d, J = 12.3 Hz, 1H), 3.57 (s, 3H), 2.74 (q, J = 15.6 Hz, 2H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 203.3, 201.0, 159.1, 148.3, 147.4, 142.3, 141.8, 136.9, 133.1, 130.3, 129.2, 125.2, 123.4, 123.0, 121.2, 114.3, 94.0, 80.2, 60.7, 55.4, 53.3, 47.6.

IR (ATR) ṽ (cm⁻¹): 3319, 3020, 2986, 2971, 2934, 1742, 1688, 1612, 1587, 1554, 1527.

HRMS (SI, m/z): calculated for C₂₆H₂₀N₂O₈Na ([M + Na]⁺): 511.1118; found: 511.1111.

(±) (2S,3R,4R)-2-(4-(Benzyloxy)phenyl)-4-hydroxy-3-nitro-4-(3-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3k)

The title compound 3k was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (67% yield; 93 : 7 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 190–192 °C.

¹H NMR (400 MHz, DMSO-d₆) δ 8.65 (s, 1H), 8.28 (dd, J = 25.6, 8.0 Hz, 2H), 7.95–7.80 (m, 4H), 7.72 (d, J = 7.4 Hz, 1H), 7.36–7.28 (m, 5H), 7.10 (s, 1H), 6.99 (d, J = 8.1 Hz, 2H), 6.71 (d, J = 8.0 Hz, 2H), 6.18 (d, J = 12.3 Hz, 1H), 4.91 (s, 2H), 4.73 (d, J = 12.3 Hz, 1H), 2.74 (q, J = 15.5 Hz, 2H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 203.3, 201.0, 158.2, 148.2, 147.4, 142.3, 141.8, 137.2, 136.9, 133.1, 130.3, 129.2, 128.8, 128.2, 128.1, 125.5, 123.4, 123.0, 121.2, 115.1, 93.9, 80.2, 69.5, 60.8, 53.3, 47.6.

IR (ATR) ṽ (cm⁻¹): 3299, 3083, 3064, 3025, 2970, 2939, 1745, 1698, 1611, 1593, 1544, 1529.

HRMS (SI, m/z): calculated for C₃₂H₂₄N₂O₈Na ([M + Na]⁺): 587.1430; found: 587.1424.

(±) (2S,3R,4R)-4-Hydroxy-2-(4-hydroxyphenyl)-3-nitro-4-(3-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3l)

The title compound 3l was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (63% yield; 95 : 5 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 219–221 °C.

¹H NMR (400 MHz, DMSO-d₆) δ 9.28 (s, 1H), 8.66 (t, J = 1.7 Hz, 1H), 8.32–8.23 (m, 2H), 7.94–7.80 (m, 4H), 7.73 (d, J = 7.4 Hz, 1H), 7.07 (s, 1H), 6.84 (d, J = 8.5 Hz, 2H), 6.43 (d, J = 8.5 Hz, 2H), 6.09 (d, J = 12.3 Hz, 1H), 4.68 (d, J = 12.3 Hz, 1H), 2.74 (q, J = 15.5 Hz, 2H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 203.5, 201.1, 157.2, 148.3, 147.5, 142.4, 141.9, 136.9, 136.8, 133.0, 130.3, 129.0, 123.4, 123.3, 123.0, 121.1, 115.7, 94.0, 80.1, 60.9, 53.5, 47.4.

IR (ATR) ṽ (cm⁻¹): 3438, 3278, 3086, 3036, 2979, 2942, 1734, 1693, 1615, 1592, 1549, 1523.

HRMS (SI, m/z): calculated for C₂₅H₁₈N₂O₈Na ([M + Na]⁺): 497.0961; found: 497.0957.

(±) (2S,3R,4R)-2-(4-Chlorophenyl)-4-hydroxy-3-nitro-4-(3-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3m)

The title compound 3m was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (71% yield; 92 : 8 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 246–248 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.16 (d, J = 7.6 Hz, 1H), 7.88 (t, J = 7.5 Hz, 1H), 7.78 (t, J = 7.5 Hz, 1H), 7.73–7.63 (m, 6H), 7.44 (d, J = 8.5 Hz, 2H), 7.39 (t, J = 7.7 Hz, 1H), 5.77 (d, J = 11.6 Hz, 1H), 5.41 (s, 1H), 5.36 (d, J = 11.6 Hz, 1H), 3.06 (d, J = 14.9 Hz, 1H), 2.41 (d, J = 14.9 Hz, 1H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 201.8, 199.6, 147.2, 146.2, 141.2, 140.6, 136.0, 132.0, 131.9, 131.7, 129.2, 128.9, 127.8, 122.4, 121.9, 120.1, 92.3, 79.2, 59.5, 52.2, 46.5.

IR (ATR) ṽ (cm⁻¹): 3445, 3091, 2979, 2907, 1741, 1691, 1588, 1543, 1524, 1494, 727.

HRMS (SI, m/z): calculated for C₂₅H₁₇N₂O₇NaCl ([M + Na]⁺): 515.0622; found: 515.0631.

(±) (2S,3R,4R)-2-(4-Fluorophenyl)-4-hydroxy-3-nitro-4-(3-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3n)

The title compound 3n was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (57% yield; 99 : 1 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 195–197 °C.

¹H NMR (400 MHz, CD₂Cl₂) δ 8.70 (s, 1H), 8.26 (d, J = 8.3 Hz, 1H), 8.21 (d, J = 7.9 Hz, 1H), 8.02 (d, J = 7.5 Hz, 1H), 7.86 (t, J = 7.4 Hz, 1H), 7.80 (t, J = 7.3 Hz, 1H), 7.73–7.69 (m, 2H), 7.08 (dd, J = 8.1, 5.4 Hz, 2H), 6.84 (t, J = 8.5 Hz, 2H), 6.03 (d, J = 12.2 Hz, 1H), 4.86 (d, J = 12.2 Hz, 1H), 4.32 (s, 1H), 2.93 (d, J = 15.3 Hz, 1H), 2.60 (d, J = 15.3 Hz, 1H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 203.01, 200.73, 161.89 (d, J = 244.7 Hz), 148.26, 147.36, 142.27, 141.73, 136.99, 133.13, 130.29, 130.13 (d, J = 8.2 Hz), 129.92 (d, J = 2.9 Hz), 123.38 (d, J = 1.6 Hz), 122.98, 121.21, 115.73 (d, J = 21.5 Hz), 93.58, 80.29, 60.76, 53.27, 47.42.

¹⁹F NMR (377 MHz, CD₂Cl₂) δ −113.54.

IR (ATR) ṽ (cm⁻¹): 3387, 3096, 3075, 2988, 2966, 1739, 1686, 1590, 1555, 1529, 1511, 729.

HRMS (SI, m/z): calculated for C₂₅H₁₇FN₂O₇Na ([M + Na]⁺): 499.0918; found: 499.0906.

(±) 4-((3R,4R,5S)-3-Hydroxy-4-nitro-3-(3-nitrophenyl)-1′,3′-dioxo-1′,3′-dihydrospiro[cyclopentane-1,2′-inden]-5-yl)benzonitrile (3o)

The title compound 3o was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (69% yield; 99 : 1 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 260–262 °C.

¹H NMR (400 MHz, DMSO-d₆) δ 8.66 (s, 1H), 8.31 (d, J = 7.8 Hz, 1H), 8.25 (d, J = 8.1 Hz, 1H), 7.96 (d, J = 7.5 Hz, 1H), 7.90 (t, J = 7.3 Hz, 1H), 7.85–7.81 (m, 2H), 7.70 (d, J = 7.5 Hz, 1H), 7.57 (d, J = 7.9 Hz, 2H), 7.35 (d, J = 8.0 Hz, 2H), 7.18 (s, 1H), 6.44 (d, J = 12.1 Hz, 1H), 4.85 (d, J = 12.1 Hz, 1H), 2.81–2.72 (m, 2H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 202.4, 200.3, 148.2, 147.2, 142.2, 141.5, 139.7, 137.2, 137.2, 133.2, 132.7, 130.3, 129.3, 123.5, 123.4, 123.0, 121.2, 118.6, 111.1, 92.9, 80.4, 60.6, 53.7, 47.6.

IR (ATR) ṽ (cm⁻¹): 3457, 3088, 2998, 2930, 2102, 1739, 1698, 1589, 1526, 1483, 1432.

HRMS (SI, m/z): calculated for C₂₅H₁₇N₃O₉Na ([M + Na]⁺): 506.0964; found: 506.0965.

(±) (2S,3R,4R)-4-Hydroxy-3-nitro-4-(3-nitrophenyl)-2-(o-tolyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3p)

The title compound 3p was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (63% yield; 97 : 3 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 195–197 °C.

¹H NMR (400 MHz, CD₂Cl₂) δ 8.71 (s, 1H), 8.24 (dd, J = 14.4, 8.0 Hz, 2H), 8.03 (d, J = 7.6 Hz, 1H), 7.85 (t, J = 7.4 Hz, 1H), 7.78 (t, J = 7.4 Hz, 1H), 7.72 (t, J = 8.0 Hz, 1H), 7.67 (d, J = 7.6 Hz, 1H), 7.33 (d, J = 7.8 Hz, 1H), 7.15 (t, J = 7.5 Hz, 1H), 7.03 (t, J = 7.4 Hz, 1H), 6.90 (d, J = 7.5 Hz, 1H), 5.94 (d, J = 11.9 Hz, 1H), 5.17 (d, J = 11.9 Hz, 1H), 4.74 (s, 1H), 3.05 (d, J = 15.2 Hz, 1H), 2.56 (d, J = 15.2 Hz, 1H), 2.09 (s, 3H).

¹³C{1H} NMR (100 MHz, CD₂Cl₂) δ 203.4, 199.8, 148.6, 142.9, 142.3, 140.9, 137.6, 137.1, 136.4, 131.8, 130.8, 130.3, 129.7, 128.2, 128.0, 126.2, 123.6, 123.4, 123.2, 121.0, 95.8, 81.2, 60.4, 49.4, 45.9, 19.5.

IR (ATR) ṽ (cm⁻¹): 3367, 3118, 3088, 3068, 3027, 2983, 2910, 1746, 1691, 1591, 1551, 1524, 1438.

HRMS (SI, m/z): calculated for C₂₆H₂₀N₂O₇Na ([M + Na]⁺): 495.1169; found: 495.1169.

(±) (2R,3R,4R)-2-(2-Bromophenyl)-4-hydroxy-3-nitro-4-phenylspiro[cyclopentane-1,2′-indene]-1′,3′-dione (3q)

The title compound 3q was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (55% yield; 97 : 3 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 230–232 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.08 (d, J = 7.6 Hz, 1H), 7.87–7.79 (m, 3H), 7.75 (t, J = 7.4 Hz, 1H), 7.62 (d, J = 7.6 Hz, 1H), 7.48 (t, J = 7.5 Hz, 2H), 7.40 (dd, J = 17.2, 7.9 Hz, 2H), 7.29 (dd, J = 11.6, 6.7 Hz, 2H), 7.01 (t, J = 7.6 Hz, 1H), 5.89 (d, J = 11.9 Hz, 1H), 5.52 (d, J = 11.9 Hz, 1H), 4.80 (s, 1H), 3.03 (d, J = 14.9 Hz, 1H), 2.47 (d, J = 14.9 Hz, 1H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 203.0, 199.7, 142.0, 141.8, 139.5, 136.6, 136.3, 133.3, 132.2, 129.8, 129.7, 128.7, 128.3, 127.7, 126.2, 125.4, 124.1, 123.2, 95.2, 81.8, 60.2, 52.1, 45.6.

IR (ATR) ṽ (cm⁻¹): 3401, 3064, 3010, 2955, 2918, 2853, 1758, 1695, 1589, 1543, 1469, 1452, 692.

HRMS (SI, m/z): calculated for C₂₅H₁₈NO₅NaBr ([M + Na]⁺): 514.0266; found: 514.0231.

(±) (2R,3R,4R)-2-(2-Bromophenyl)-4-hydroxy-3-nitro-4-(3-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3r)

The title compound 3r was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (61% yield; 95 : 5 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 214–216 °C.

¹H NMR (400 MHz, DMSO-d₆) δ 8.68 (s, 1H), 8.35 (d, J = 7.9 Hz, 1H), 8.25 (dd, J = 8.1, 2.1 Hz, 1H), 7.95 (d, J = 7.6 Hz, 1H), 7.87–7.81 (m, 2H), 7.76 (t, J = 7.4 Hz, 1H), 7.59 (d, J = 7.6 Hz, 2H), 7.28 (d, J = 8.0 Hz, 1H), 7.22 (t, J = 7.6 Hz, 1H), 7.11 (s, 1H), 7.00–6.95 (m, 1H), 6.26 (d, J = 11.9 Hz, 1H), 5.37 (d, J = 11.9 Hz, 1H), 2.80 (s, 2H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 201.6, 199.0, 147.8, 146.6, 142.0, 141.2, 136.1, 136.1, 132.8, 132.8, 129.8, 129.6, 129.5, 127.6, 125.2, 123.0, 122.5, 122.4, 120.8, 94.0, 80.0, 60.4, 51.6, 45.7.

IR (ATR) ṽ (cm⁻¹): 3457, 3116, 2998, 2930, 1738, 1698, 1590, 1548, 1528, 1477, 725.

HRMS (SI, m/z): calculated for C₂₅H₁₇N₂O₇NaBr ([M + Na]⁺): 559.0117; found: 559.0112.

(±) (2R,3R,4R)-2-(2-chlorophenyl)-4-hydroxy-3-nitro-4-(3-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3s)

The title compound 3s was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (66% yield; 96 : 4 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 242–244 °C.

¹H NMR (400 MHz, DMSO-d₆) δ 8.68 (s, 1H), 8.34 (d, J = 7.8 Hz, 1H), 8.25 (d, J = 8.1 Hz, 1H), 7.95 (d, J = 7.6 Hz, 1H), 7.84 (dt, J = 11.9, 7.7 Hz, 1H), 7.76 (t, J = 7.4 Hz, 1H), 7.65–7.52 (m, 1H), 7.18 (t, J = 7.4 Hz, 1H), 7.13–7.01 (m, 2H), 6.28 (d, J = 12.0 Hz, 1H), 5.35 (d, J = 12.0 Hz, 1H), 2.79 (s, 1H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 202.2, 199.6, 148.3, 147.0, 142.4, 141.5, 136.7, 136.6, 134.4, 133.2, 131.7, 130.2, 129.8, 127.6, 123.4, 123.0, 122.9, 121.3, 94.1, 80.5, 60.7, 49.7, 46.3.

IR (ATR) ṽ (cm⁻¹): 3457, 3088, 2998, 2930, 1739, 1698, 1590, 1549, 1528, 1483, 724.

HRMS (SI, m/z): calculated for C₂₅H₁₇ClN₂O₇Na ([M + Na]⁺): 515.0622; found: 515.0574.

(±) (2S,3R,4R)-4-Hydroxy-3-nitro-2-(2-nitrophenyl)-4-phenylspiro[cyclopentane-1,2′-indene]-1′,3′-dione (3t)

The title compound 3t was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (63% yield; 95 : 5 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 214–216 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.16 (d, J = 7.7 Hz, 1H), 7.88 (t, J = 7.4 Hz, 1H), 7.77 (t, J = 7.5 Hz, 3H), 7.68 (d, J = 15.4, 13.1, 7.9 Hz, 4H), 7.48 (t, J = 7.7 Hz, 2H), 7.39 (t, J = 7.8 Hz, 2H), 5.84 (d, J = 11.6 Hz, 1H), 5.38 (d, J = 11.6 Hz, 1H), 5.30 (s, 1H), 3.12 (d, J = 14.9 Hz, 1H), 2.44 (d, J = 14.9 Hz, 1H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 203.6, 199.3, 150.2, 141.7, 141.4, 139.1, 136.7, 136.6, 133.0, 131.2, 129.4, 128.6, 128.4, 128.2, 125.5, 125.0, 124.4, 123.5, 96.6, 81.8, 59.9, 47.8, 46.4.

IR (ATR) ṽ (cm⁻¹): 3469, 3083, 2955, 2917, 2851, 1736, 1690, 159, 1551, 1522, 1446.

HRMS (SI, m/z): calculated for C₂₅H₁₈N₂O₇Na ([M + Na]⁺): 481.1012; found: 481.1015.

(±) (2S,3R,4R)-4-(4-Chlorophenyl)-4-hydroxy-3-nitro-2-(2-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3u)

The title compound 3u was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (64% yield; 94 : 6 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 210–212 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.16 (d, J = 7.6 Hz, 1H), 7.88 (t, J = 7.5 Hz, 1H), 7.78 (t, J = 7.5 Hz, 1H), 7.73–7.63 (m, 6H), 7.44 (d, J = 8.5 Hz, 2H), 7.39 (t, J = 7.7 Hz, 1H), 5.77 (d, J = 11.6 Hz, 1H), 5.41 (s, 1H), 5.36 (d, J = 11.6 Hz, 1H), 3.06 (d, J = 14.9 Hz, 1H), 2.41 (d, J = 14.9 Hz, 1H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 203.6, 199.1, 150.2, 141.7, 141.4, 137.8, 136.8, 136.7, 134.3, 133.0, 131.1, 129.5, 128.8, 128.1, 127.0, 125.0, 124.5, 123.5, 96.4, 81.5, 59.8, 47.6, 46.3.

IR (ATR) ṽ (cm⁻¹): 3476, 3114, 3082, 2922, 2854, 1736, 1697, 1594, 1550, 1519, 1437, 729.

HRMS (SI, m/z): calculated for C₂₅H₁₇N₂O₇ClNa ([M + Na]⁺): 515.0622; found: 515.0635.

(±) (2S,3R,4R)-4-Hydroxy-3-nitro-2-(2-nitrophenyl)-4-(3-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3v)

The title compound 3v was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (66% yield; 95 : 5 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 215–217 °C.

¹H NMR (700 MHz, DMSO-d₆) δ 8.67 (t, J = 2.0 Hz, 1H), 8.33 (d, J = 7.9 Hz, 1H), 8.26–8.24 (m, 1H), 7.92 (d, J = 7.6 Hz, 1H), 7.87–7.85 (m, 2H), 7.82 (t, J = 8.0 Hz, 1H), 7.78–7.76 (m, 1H), 7.64 (d, J = 7.6 Hz, 1H), 7.57–7.55 (m, 2H), 7.36–7.33 (m, 1H), 7.12 (s, 1H), 6.38 (d, J = 11.7 Hz, 1H), 5.25 (d, J = 11.7 Hz, 1H), 2.81 (d, J = 15.6 Hz, 1H), 2.76 (d, J = 15.7 Hz, 1H).

¹³C{1H} NMR (177 MHz, DMSO-d₆) δ 201.5, 198.6, 150.1, 147.8, 146.3, 141.8, 141.0, 136.4, 136.2, 132.8, 132.6, 130.1, 129.7, 129.4, 127.0, 124.2, 123.2, 122.6, 122.6, 120.8, 93.9, 79.9, 60.5, 47.3, 45.9.

IR (ATR) ṽ (cm⁻¹): 3368, 3116, 3094, 3068, 3046, 2983, 1743, 1690, 1593, 1551, 1522, 1439.

HRMS (SI, m/z): calculated for C₂₅H₁₇N₃O₉Na ([M + Na]⁺): 526.0863; found: 526.0870.

(±) (2S,3R,4R)-4-(4-Bromophenyl)-4-hydroxy-3-nitro-2-(2-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3w)

The title compound 3w was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (65% yield; 89 : 11 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 25% EtOAc/Hexane.

Melting point = 208–210 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.16 (d, J = 7.6 Hz, 1H), 7.88 (t, J = 7.3 Hz, 1H), 7.80–7.76 (m, 1H), 7.68 (dd, J = 17.9, 7.8 Hz, 6H), 7.60 (d, J = 8.6 Hz, 2H), 7.39 (t, J = 7.6 Hz, 1H), 5.77 (d, J = 11.5 Hz, 1H), 5.35 (d, J = 11.5 Hz, 1H), 3.05 (d, J = 14.9 Hz, 1H), 2.41 (d, J = 14.9 Hz, 1H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 203.6, 199.0, 150.2, 141.7, 141.4, 138.3, 136.8, 136.7, 133.0, 131.8, 131.1, 129.5, 128.1, 127.4, 125.0, 124.5, 123.5, 122.6, 96.4, 81.5, 59.8, 47.6, 46.2.

IR (ATR) ṽ (cm⁻¹): 3480, 3111, 3079, 2953, 2923, 2853, 2849, 1737, 1696, 1591, 1551, 1520, 1488, 715.

HRMS (SI, m/z): calculated for C₂₅H₁₇N₂O₇NaBr ([M + Na]⁺): 559.0117; found: 559.0137.

(±) (2S,3R,4R)-2-(3-Fluorophenyl)-4-hydroxy-3-nitro-4-phenylspiro[cyclopentane-1,2′-indene]-1′,3′-dione (3x)

The title compound 3x was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (60% yield; 95 : 5). The spectral data are for the major diastereomer.

R_f = 0.3, eluent = 20% EtOAc/Hexane.

Melting point = 245–247 °C.

¹H NMR (400 MHz, CDCl₃) δ 8.02 (d, J = 7.5 Hz, 1H), 7.83–7.71 (m, 5H), 7.49 (t, J = 7.5 Hz, 2H), 7.41–7.37 (m, 1H), 7.08 (dd, J = 14.8, 7.6 Hz, 1H), 6.86 (d, J = 7.7 Hz, 1H), 6.79 (d, J = 8.3 Hz, 2H), 6.10 (d, J = 12.4 Hz, 1H), 4.94 (d, J = 12.3 Hz, 1H), 2.83 (d, J = 15.0 Hz, 1H), 2.58 (d, J = 14.9 Hz, 1H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 201.37, 201.30, 163.77, 161.30, 141.96, 141.66, 140.34, 136.59, 136.39, 134.74, 134.67, 130.36, 130.27, 128.79, 128.46, 125.28, 123.9 (d, J = 3.0 Hz), 123.75, 123.49, 115.41 (dd, J = 21.7, 14.4 Hz)., 93.29, 81.46, 60.06, 53.68, 46.74.

IR (ATR) ṽ (cm⁻¹): 3337, 2899, 2885, 1702, 1650, 1552, 1449, 1427.

HRMS (SI, m/z): calculated for C₂₅H₁₈NO₅NaF ([M + Na]⁺): 454.1067; found: 454.1053.

(±) (2S,3R,4R)-2-(2,4-Dimethoxyphenyl)-4-hydroxy-3-nitro-4-(3-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3y)

The title compound 3y was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (66% yield; 95 : 5 dr). The spectral data are for the major diastereomer.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 207–209 °C.

¹H NMR (400 MHz, CD₂Cl₂) δ 8.65 (s, 1H), 8.24 (d, J = 8.1 Hz, 1H), 8.17 (d, J = 7.8 Hz, 1H), 8.11 (d, J = 7.7 Hz, 1H), 7.89 (t, J = 7.4 Hz, 1H), 7.80 (t, J = 7.4 Hz, 1H), 7.69 (t, J = 8.5 Hz, 2H), 7.10 (d, J = 8.5 Hz, 1H), 6.45 (d, J = 8.3 Hz, 1H), 6.09 (s, 1H), 5.96 (d, J = 12.1 Hz, 1H), 5.22 (s, 1H), 5.04 (d, J = 12.1 Hz, 1H), 3.71 (s, 3H), 3.15 (s, 3H), 2.90 (d, J = 14.9 Hz, 1H), 2.43 (d, J = 14.9 Hz, 1H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 203.4, 199.3, 160.9, 157.3, 148.6, 142.4, 141.6, 141.4, 136.4, 136.0, 131.9, 129.6, 127.3, 123.2, 123.2, 121.0, 113.7, 104.3, 98.1, 93.3, 81.5, 59.5, 55.3, 54.0, 46.7, 45.8.

IR (ATR) ṽ (cm⁻¹): 3432, 3075, 3001, 2932, 2837, 1745, 1696, 1614, 1586, 1547, 1529.

HRMS (SI, m/z): calculated for C₂₇H₂₂N₂O₉Na ([M + Na]⁺): 541.1223; found: 541.1220.

(±) (2S,3R,4R)-2-(2,5-Dimethoxyphenyl)-4-hydroxy-3-nitro-4-(3-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3z)

The title compound 3z was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (67% yield; 98 : 2 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 204–206 °C.

¹H NMR (400 MHz, DMSO-d₆) δ 8.63 (s, 1H), 8.31 (d, J = 7.8 Hz, 1H), 8.24 (d, J = 8.1 Hz, 1H), 7.99 (d, J = 7.6 Hz, 1H), 7.88 (t, J = 7.4 Hz, 1H), 7.81 (t, J = 8.0 Hz, 1H), 7.75 (t, J = 7.4 Hz, 1H), 7.56 (d, J = 7.6 Hz, 1H), 6.97 (s, 1H), 6.84 (s, 1H), 6.59 (d, J = 8.9 Hz, 1H), 6.43 (d, J = 9.0 Hz, 1H), 6.22 (d, J = 12.2 Hz, 1H), 5.05 (d, J = 12.2 Hz, 1H), 3.62 (s, 3H), 3.29 (s, 3H), 2.75–2.67 (m, 2H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 202.5, 199.3, 153.3, 150.9, 148.2, 147.1, 142.8, 140.7, 136.4, 136.1, 133.2, 130.2, 123.2, 122.9, 122.8, 121.3, 114.7, 113.0, 110.9, 93.06, 80.7, 59.8, 56.0, 55.0, 47.3, 46.4.

IR (ATR) ṽ (cm⁻¹): 3367, 3129, 3068, 3049, 3014, 2908, 2833, 1736, 1696, 1590, 1551, 1529, 1498, 1348.

HRMS (SI, m/z): calculated for C₂₇H₂₂N₂O₉Na ([M + Na]⁺): 541.1223; found: 541.1241.

(±) (2R,3R,4R)-2-(2,4-Dichlorophenyl)-4-hydroxy-3-nitro-4-(3-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3za)

The title compound 3za was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (61% yield; 90 : 10 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 210–213 °C.

¹H NMR (400 MHz, DMSO-d₆) δ 8.67 (s, 1H), 8.32 (d, J = 7.9 Hz, 1H), 8.25 (d, J = 8.1 Hz, 1H), 7.97 (d, J = 7.6 Hz, 1H), 7.89 (t, J = 7.4 Hz, 1H), 7.82 (dd, J = 13.6, 7.2 Hz, 2H), 7.70 (d, J = 8.4 Hz, 1H), 7.65 (d, J = 7.6 Hz, 1H), 7.29 (d, J = 10.6 Hz, 2H), 7.13 (s, 1H), 6.34 (d, J = 11.9 Hz, 1H), 5.30 (d, J = 11.9 Hz, 1H), 2.78 (s, 2H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 202.0, 199.6, 148.3, 146.9, 142.3, 141.5, 136.9, 136.8, 135.4, 133.6, 133.2, 131.4, 131.1, 130.2, 129.2, 127.7, 123.5, 123.0, 121.3, 93.9, 80.6, 60.5, 49.3, 46.4.

IR (ATR) ṽ (cm⁻¹): 3438, 3278, 3086, 3036, 2979, 2942, 1734, 1693, 1615, 1592, 1549, 1523.

HRMS (SI, m/z): calculated for C₂₅H₁₆N₂O₇NaCl₂ ([M + Na]⁺): 549.0232; found: 549.0225.

(±) (2S,3R,4R)-2-(3-Ethoxy-4-hydroxyphenyl)-4-hydroxy-3-nitro-4-(3-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3zb)

The title compound 3zb was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (65% yield; 96 : 4 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 35% EtOAc/Hexane.

Melting point = 231–233 °C.

¹H NMR (400 MHz, DMSO-d₆) δ 8.76 (s, 1H), 8.65 (s, 1H), 8.32 (d, J = 7.7 Hz, 1H), 8.24 (d, J = 8.0 Hz, 1H), 7.95–7.81 (m, 4H), 7.72 (d, J = 7.4 Hz, 1H), 7.04 (s, 1H), 6.50 (s, 1H), 6.44 (s, 2H), 6.11 (d, J = 12.6 Hz, 1H), 4.66 (d, J = 12.4 Hz, 1H), 3.86–3.82 (m, 1H), 3.76–3.73 (m, 1H), 2.73 (q, J = 16.2 Hz, 2H), 1.18 (t, J = 6.9 Hz, 3H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 203.5, 201.1, 148.2, 147.6, 146.9, 146.6, 142.4, 141.9, 136.9, 136.9, 133.1, 130.3, 124.0, 123.3, 123.2, 123.0, 121.2, 120.6, 115.9, 113.7, 93.8, 80.2, 64.3, 61.0, 54.0, 47.2, 15.0.

IR (ATR) ṽ (cm⁻¹): 3460, 3017, 2983, 2906, 1744, 1697, 1595, 1545, 1529, 148, 737.

HRMS (SI, m/z): calculated for C₂₇H₂₂N₂O₉Na ([M + Na]⁺): 541.1223; found: 541.1220.

(±) (2S,3R,4R)-4-Hydroxy-3-nitro-4-(3-nitrophenyl)-2-(3,4,5-trimethoxyphenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3zc)

The title compound 3zc was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (56% yield; 98 : 2 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 226–228 °C.

¹H NMR (400 MHz, DMSO-d₆) δ 8.64 (t, J = 1.9 Hz, 1H), 8.31 (d, J = 8.1 Hz, 1H), 8.25 (dd, J = 8.1, 1.6 Hz, 1H), 7.97 (d, J = 7.6 Hz, 1H), 7.87–7.74 (m, 3H), 7.57 (d, J = 7.6 Hz, 1H), 7.02 (s, 1H), 6.95 (d, J = 8.8 Hz, 1H), 6.58 (d, J = 8.8 Hz, 1H), 6.09 (d, J = 12.3 Hz, 1H), 5.01 (d, J = 12.3 Hz, 1H), 3.65 (s, 3H), 3.61 (s, 3H), 3.21 (s, 3H), 2.74 (s, 2H).

¹³C{1H} NMR (100 MHz, DMSO-d₆) δ 207.6, 204.4, 158.0, 156.2, 153.0, 152.1, 147.7, 146.0, 141.1, 140.9, 137.9, 135.0, 128.1, 127.7, 127.5, 127.1, 125.9, 124.1, 112.5, 98.5, 85.2, 65.2, 65.2, 65.1, 60.9, 52.2, 51.2.

IR (ATR) ṽ (cm⁻¹): 3457, 3234, 3009, 2969, 2936, 2836, 1747, 1700, 1638, 1596, 1551, 1500, 1306.

HRMS (SI, m/z): calculated for C₂₈H₂₄N₂O₁₀Na ([M + Na]⁺): 571.1329; found: 571.1318.

(±) (2R,3R,4R)-2-(6-Bromo-2,3-dimethoxyphenyl)-4-hydroxy-3-nitro-4-(3-nitrophenyl)spiro[cyclopentane-1,2′-indene]-1′,3′-dione (3zd)

The title compound 3zd was prepared according to general procedure A on a 0.2 mmol scale. The desired product was obtained as a white solid (61% yield; 91 : 9 dr). The minor diastereomer was separated by silica gel column chromatography.

R_f = 0.3, eluent = 30% EtOAc/Hexane.

Melting point = 238–240 °C.

¹H NMR (400 MHz, DMSO-d₆) δ 8.67 (s, 1H), 8.37 (d, J = 7.7 Hz, 1H), 8.25 (d, J = 8.1 Hz, 1H), 7.92 (d, J = 7.5 Hz, 1H), 7.83 (dt, J = 13.5, 7.2 Hz, 3H), 7.70 (d, J = 7.5 Hz, 1H), 7.02 (d, J = 13.8 Hz, 2H), 6.79 (s, 1H), 6.25 (d, J = 11.9 Hz, 1H), 5.29 (d, J = 11.9 Hz, 1H), 3.77 (s, 3H), 3.57 (s, 3H), 2.85–2.69 (m, 2H).

¹³C{1H} NMR (100 MHz, CDCl₃) δ 207.8, 204.2, 153.7, 153.1, 153.0, 152.1, 147.1, 146.6, 141.4, 138.2, 135.0, 129.6, 128.1, 127.7, 127.6, 126.1, 120.8, 120.7, 118.1, 99.3, 85.2, 65.8, 61.4, 60.9, 57.0, 50.9.

IR (ATR) ṽ (cm⁻¹): 3320, 3095, 3007, 2938, 283, 1731, 1690, 1591, 1542, 1511, 1464, 725.

HRMS (SI, m/z): calculated for C₂₇H₂₁BrN₂O₉Na ([M + Na]⁺): 619.0328; found: 619.0324.

Conflicts of interest

There are no conflicts to declare.

Data availability

CCDC 2456358 contains the supplementary crystallographic data for this paper.¹⁴

The data supporting this article have been included in the SI. Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre (https://www.ccdc.cam.ac.uk/services/structures) with CCDC reference number 2456358 for compound 3g. See DOI: https://doi.org/10.1039/d5ra04224k.

Acknowledgements

The authors acknowledge DST-SERB for financial support in the form of a Core Research Grant (CRG/2022/003143). We are grateful for the financial aid in the form of an initiation grant from IISER Berhampur, Central Advanced Instrument Facility (CAIF) at IISER Berhampur. G.M. thanks CSIR, New Delhi, India, for the financial assistance in the form of fellowships.

Notes and references

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