A simple and green approach for the synthesis of polyfunctionalized mono- and bis-dihydro-2-oxopyrroles catalyzed by trityl chloride

Abstract

A simple synthesis of ployfunctionalized mono- and bis-dihydro-2-oxopyrrole derivatives is described by one-pot multi-component reaction of amines, dialkyl acetylenedicarboxylates and formaldehyde in the presence of trityl chloride (10 mol%) at room temperature in EtOH. The features of this procedure are mild and green reaction conditions, good to high yields, short reaction times, high atom economy, operational simplicity and no need for column chromatography.

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Introduction

Green chemistry and development of new processes that reduce pollution in chemical synthesis have received considerable attention due to increasing environmental concerns. Among other factors, two major adverse effects on the environment are consumption of energy for heating and cooling of reactions and using volatile organic solvents. The performance of reactions at ambient temperature in green solvents such as water and ethanol may be a viable option.^1,2 On the other hand, one active area to access green chemistry is multi-component reactions (MCRs). MCRs have merits over conventional multistage syntheses in several aspects including flexibility, no separation of intermediates and simple purification of products, time and energy saving, and rapid access to complex molecules.^3,4 Therefore, discovery and development of new MCRs is highly desirable.

Dihydro-2-oxopyrroles (dihydropyrrol-2-ones) display a broad spectrum of biological and pharmacological activity as antitumor and anticancer,⁵ HIV integrase,⁶ DNA polymerase inhibitors,⁷ human cytomegalovirus (HCMV) protease inhibitors,⁸ and inhibitors of human cytosolic carbonic anhydrase isozymes.⁹ Dihydro-2-oxopyrrole moiety was also found in various natural bioactive products including pyrrocidine A, talaroconvolutin A, thiomarinol A4, oteromycin, PI-091, EBPC, UCS1025A, Jatropham and (Z) pulchellalactam (Fig. 1).^10–16

Fig. 1 Bioactive compounds containing dihydro-2-oxopyrrole unit.

These heterocycles and derivatives are also active and important reagents as a material for the synthesis of organic complexes.¹⁷ As a result, several methods have been developed to synthesize these useful heterocycles.^16,18–22 Recently, Zhu et al. have reported an efficient multi-component synthesis of highly functionalized dihydro-2-oxopyrroles via reaction of amines, acetylenic esters and aldehydes in the presence of acetic acid as catalyst. However, the reactions were carried out in the presence of 200 mol% of catalyst at 70 °C and products purified by preparative TLC.²³ Next, literature reveals only a few methods to synthesis polyfunctionalized dihydro-2-oxopyrroles via MCRs catalyzed by I₂,²⁴ benzoic acid,²⁵ TiO₂ nanopowder,²⁶ and Cu(OAc)₂·H₂O.²⁷ The aforesaid methods have some of the disadvantages such as long reaction times,²⁵ utilization of chlorinated solvent under reflux conditions and need to column chromatography for products purification.²⁶ Therefore, development of an efficient, milder, green and more ecofriendly method for the preparation of these compounds is still in demand.

The utility of organic catalysts in organic synthesis has received a great deal of interest because of their unique properties such as, the possibility to perform reactions for acid-sensitive substrates, performing reactions in milder conditions and selectivity. In the recent years, triarylmethyl chlorides (Ar₃CCl) have been used as novel organic catalyst for the synthesis of many organic compounds.^28–30 Triarylmethyl chlorides are inexpensive and can be obtained commercially or easily prepared by a known procedure.³¹

Results and discussion

As a part of our continued interest in the development of efficient procedure for the preparation of dihydro-2-oxopyrroles,^32–35 herein we report a green, very simple and efficient route for the synthesis of ployfunctionalized dihydro-2-oxopyrroles via one-pot domino four-component reaction of amines, dialkyl acetylenedicarboxylates and formaldehyde. The reaction was carried out in the presence of the catalytic amount of trityl chloride (TrCl) in EtOH at ambient temperature (Scheme 1).

Scheme 1 Synthesis of highly functionalized dihydro-2-oxopyrroles 5.

Initially, a test reaction using aniline, dimethyl acetylenedicarboxylate (DMAD) and formaldehyde was performed in the absence of catalyst in EtOH at room temperature. The corresponding product 5a was obtained in trace amounts after 24 h. To optimize the reaction conditions, the above reaction was examined under different conditions and the results are presented in Table 1. The best result was obtained in the presence of 10 mol% TrCl in EtOH. As shown in Table 1, higher percentage loading of the catalyst neither increased the yield of product nor decreased the reaction time. Additionally, the effect of different solvents including MeOH, MeCN and THF was also investigated on the yield and time of reaction, which found to be ineffective. A similar result was also obtained in the synthesis of 5k when benzylamine was treated with diethyl acetylenedicarboxylate, formaldehyde and aniline.

Table 1 Optimization of the reaction conditions for the synthesis of 5a^a and 5k^b

Substrate	Compound	Catalyst (mol%)/solvent	Time (h)	Yield^c (%)
a Aniline (1 mmol), DMAD (1 mmol), formaldehyde (1.5 mmol), aniline (1 mmol), r.t.b Benzyl amine (1 mmol), Diethyl acetylenedicarboxylate (1 mmol), formaldehyde (1.5 mmol), aniline (1 mmol), r.t.c Isolated yield.
		—/EtOH	24	Trace
		5/EtOH	6	60
		10/EtOH	4	86
		15/EtOH	4	86
		20/EtOH	4	85
		10/MeOH	6	80
		10/MeCN	9	52
		10/THF	9	39
		5/EtOH	7	63
		10/EtOH	4	84
		15/EtOH	4	82
		20/EtOH	4	81
		10/MeOH	8	79
		10/MeCN	10	51
		10/THF	10	40

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Therefore, we investigated several reactions between variety of anilines, dimethyl and/or diethylacetylenedicarboxylate and formaldehyde under optimized reaction conditions. The results are summarized in Table 2. Anilines with substituents Me, OMe, F, Cl, and Br were reacted efficiently to generate the corresponding polyfunctionalized dihydro-2-oxopyrroles 5a–i in good to high yields (Table 2, entries 1–9). Moreover, to evaluate the generality and versatility of this method, the optimized conditions were used for the synthesis of different highly substituted dihydro-2-oxopyrroles 5j–y. As is clear in Table 2, the reactions of aliphatic amines such as benzyl amine, 1-(pyridin-2-yl)methanamine, n-butyl amine and n-propyl amine with dialkyl acetylenedicarboxylates, formaldehyde and aromatic amines were proceeded smoothly to give the expected products in good yields (Table 2, entries 10–25).

Table 2 Synthesis of polyfunctionalized dihydro-2-oxopyrroles 5

Entry	R′	R′′	Ar	Product	Time (h)	Yield^a (%)	M.p. (°C)	Lit. m.p. (°C)
a Isolated yield.
1	Ph	Me	Ph	5a	4	86	153–155	155–156 (ref. 24)
2	Ph	Et	Ph	5b	4	84	137–139	138–140 (ref. 23)
3	4-F-C6H4	Me	4-F-C6H4	5c	4	86	163–165	163–165 (ref. 33)
4	4-Cl-C6H4	Et	4-Cl-C6H4	5d	4	83	168–170	168–170 (ref. 34)
5	4-Br-C6H4	Me	4-Br-C6H4	5e	4	87	175–177	179–180 (ref. 24)
6	4-Br-C6H4	Et	4-Br-C6H4	5f	4	84	170–172	169–171 (ref. 23)
7	4-OMe-C6H4	Et	4-OMe-C6H4	5g	4	74	152–154	152–154 (ref. 32)
8	4-Me-C6H4	Me	4-Me-C6H4	5h	3.5	86	175–177	177–178 (ref. 24)
9	4-Me-C6H4	Et	4-Me-C6H4	5i	4	85	133–135	131–132 (ref. 23)
10	Ph-CH2	Me	Ph	5j	3	89	138–140	140–141 (ref. 23)
11	Ph-CH2	Et	Ph	5k	4	84	127–129	130–132 (ref. 23)
12	Ph-CH2	Me	4-Me-C6H4	5l	4	84	144–146	144–146 (ref. 35)
13	Ph-CH2	Me	4-Cl-C6H4	5m	3.5	88	145–147	147–148 (ref. 24)
14	Ph-CH2	Me	4-Br-C6H4	5n	3	89	117–118	120–121 (ref. 24)
15	Ph-CH2	Me	4-F-C6H4	5o	4	85	166–168	166–168 (ref. 32)
16	C5H4N-2-CH2	Me	4-Cl-C6H4	5p	8	74	156–158	—
17	C5H4N-2-CH2	Me	4-Me-C6H4	5q	7	70	106–108	106–108 (ref. 34)
18	n-C4H9	Me	Ph	5r	5.5	77	62–63	60 (ref. 24)
19	n-C4H9	Me	4-Br-C6H4	5s	4	82	103–105	108–109 (ref. 24)
20	n-C4H9	Et	4-Br-C6H4	5t	4	85	94–96	94–96 (ref. 33)
21	n-C4H9	Me	4-F-C6H4	5u	4	79	81–83	81–83 (ref. 35)
22	n-C4H9	Me	3,4-Cl2-C6H4	5v	4	80	97–99	97–99 (ref. 32)
23	n-C4H9	Me	4-Me-C6H4	5w	5	83	89–91	—
24	n-C3H7	Et	Ph	5x	4.5	86	76–78	78–79 (ref. 23)
25	n-C3H7	Me	3,4-Cl2-C6H3	5y	6	82	125–127	—

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Interestingly, this approach can be employed for the synthesis of new class of ployfunctionalized bis-dihydro-2-oxopyrroles 7 when ethane-1,2-diamine 6 was used instead of amine 1 (Scheme 2).

Scheme 2 Synthesis of polyfunctionalized bis-dihydro-2-oxopyrroles 7.

The one-pot four-component (pseudo seven-component) reaction of ethane-1,2-diamine 6, dialkyl acetylenedicarboxylate 2, aromatic amine 3 and formaldehyde 4 was carried out cleanly under optimized reaction conditions to generate the corresponding products 7a–g. The results are displayed in Table 3.

Table 3 Synthesis of polyfunctionalized bis-dihydro-2-oxopyrroles 7

Entry	R′′	Ar	Product	Time (h)	Yield^a (%)	M.p. (°C)	Lit. m.p. (°C)
a Isolated yields.
1	Et	Ph	7a	6	79	159–161	—
2	Me	Ph	7b	6	81	150–152	149–151 (ref. 34)
3	Et	4-OMe-C6H4	7c	8	71	222–224	—
4	Et	4-Me-C6H4	7d	6	77	210–212	210–212 (ref. 34)
5	Me	4-Cl-C6H4	7e	6	82	202–204	—
6	Et	3,4-Cl2-C6H3	7f	7	80	206–208	206–208 (ref. 34)
7	Me	4-F-C6H4	7g	6	83	198–200	199–201 (ref. 34)

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In general, at the beginning of the reaction, the substrates were completely soluble in reaction medium to form a homogeneous mixture. But, at the end of the reaction, the system became a suspension and finally the product precipitated. The solid precipitate was filtered off and washed with EtOH and purified by recrystallization from EtOH, if necessary. The structures of compounds were fully characterized by IR, ¹H and ¹³C NMR, mass spectra as well as elemental analysis. The structural elucidation of 7a is discussed as an example. The ¹H NMR spectrum of 7a showed a triplet at δ 1.27 ppm (J = 6.8 Hz) for methyl protons of ethoxy groups and a multiplet at δ 4.12–4.16 for 4CH₂ of ethoxy groups and ethane-1,2-diamine moiety. The methylene protons of dihydro-2-oxopyrrole ring were appeared at δ 4.36 ppm as a singlet. A fairly broad singlet was observed at δ 6.74 ppm for two amine NH groups. The aromatic protons were shown as two triplets and a doublet at δ 7.19 (J = 7.6 Hz), 7.39 (J = 8.0 Hz) and 7.75 ppm (J = 7.6 Hz), respectively. The ¹H decoupled ¹³C NMR spectrum of 7a exhibited 11 distinct signals in agreement with the proposed structure. The mass spectrum of 7a displayed the molecular ion peak (M⁺) at m/z = 518, which was consistent with the 1 : 2 : 2 : 2 adduct of ethane-1,2-diamine, diethyl acetylenedicarboxylate, aniline and formaldehyde, respectively. The IR spectrum of compound 7a showed frequencies expected for the NH and carbonyl groups at 3295 cm⁻¹ (NH) and 1698 and 1638 cm⁻¹ (C=O).

A possible reaction mechanism is suggested in Scheme 3. At first, the reaction of amine 1 (or 6) with dialkyl acetylenedicarboxylate 2 lead to intermediate I and condensation between amine 3 and formaldehyde 4 in the presence of TrCl produce imine II. Next, intermediate I undergoes Mannich type reaction with imine II to furnish reactive intermediate III, which converted to intermediate IV by cyclization reaction. Finally, intermediate IV tautomerizes to the corresponding mono- or bis-dihydro-2-oxopyrroles 5 or 7.

Scheme 3 Proposed mechanism for the synthesis dihydro-2-oxopyrroles 5 and 7.

Experimental

General

Melting points and IR spectra were taken on an Electrothermal 9100 apparatus and a JASCO FT/IR-460 plus spectrometer, respectively. The ¹H and ¹³C NMR spectra were recorded on a Bruker DRX-400 Avanve instrument with CDCl₃ as solvent at 400 and 100 MHz, respectively. Elemental analyses for C, H and N were performed using a Heraeus CHN-O-Rapid analyzer. The mass spectra were recorded on an Agilent Technology (HP) mass spectrometer, operating at an ionization potential of 70 eV. All chemicals were obtained from Merck (Darmastadt, Germany), Acros (Geel, Belgium) and Fluka (Buchs, Switzerland), and use without further purification.

General procedure for the synthesis of highly functionalized mono- and bis-dihydro-2-oxopyrroles 5 and 7

For product 5, a mixture of amine 1 (1 mmol) and dialkyl acetylenedicarboxylate 2 (1 mmol) in EtOH (3 mL) was stirred for 20 min. Next, aromatic amine 3 (1 mmol), formaldehyde 4 (37% solution, 1.5 mmol) and TrCl (10 mol%) were added successively. For compound 7, the stoichiometric amounts of amine 6, dialkyl acetylenedicarboxylate 2, amine 3 and formaldehyde 4 are 1, 2, 3 and 3 mmol, respectively. The reaction mixture was stirred at room temperature for appropriate time. The reaction progress was monitored by TLC. After completion, the solid precipitate was filtered off and washed with EtOH (2 mL) and recrystallized from EtOH (if necessary), to give the pure products 5 or 7. The structures of the known products were confirmed by comparison of their melting points and spectral data (IR, ¹H and ¹³C NMR) with literature.^{23,24,32–35} Physical and chemical data for unknown compounds are presented below.

Methyl 3-((pyridin-2-yl)methylamino)-1-(4-chlorophenyl)-2,5-dihydro-2-oxo-1H-pyrrole-4-carboxylate (5p). White solid; mp: 156–158 °C; IR (KBr, cm⁻¹) ν = 3306 (NH), 1698 (C=O), 1628 (C=O); ¹H NMR (400 MHz, CDCl₃): δ = 3.84 (s, 3H, OCH₃), 4.45 (s, 2H, CH₂–N), 5.25 (d, J = 5.2 Hz, 2H, CH₂–NH), 7.23–7.77 (m, 8H, NH and ArH), 8.64 (d, J = 4.4 Hz, 1H, ArH); ¹³C NMR (100 MHz, CDCl₃): δ = 47.1, 48.0, 51.2, 120.3, 122.0, 122.4, 129.1, 130.1, 137.2, 137.3, 148.7, 157.1, 164.7, 169.5; MS (EI, 70 eV): m/z (%) = 359 (M²⁺, 21), 357 (M⁺, 65), 327 (17), 325 (48), 298 (10), 279 (4), 247 (5), 192 (24) 190 (32), 171 (33), 158 (82), 144 (88), 111 (19), 93 (100), 65 (36); anal. calcd for C₁₈H₁₆ClN₃O₃: C, 60.42; H, 4.51; N, 11.74. Found: C, 60.71, H, 4.60, N, 11.87.

Methyl 3-(butylamino)-1-p-tolyl-2,5-dihydro-2-oxo-1H-pyrrole-4-carboxylate (5w). White solid; mp: 89–91 °C; IR (KBr, cm⁻¹) ν = 3353 (NH), 1685 (C=O), 1634 (C=O); ¹H NMR (400 MHz, CDCl₃): δ = 0.97 (t, J = 7.2 Hz, 3H, CH₃), 1.44 (sextet, J = 7.2 Hz, 2H, CH₂), 1.62 (quintet, J = 7.2 Hz, 2H, CH₂), 2.36 (s, 3H, CH₃), 3.80 (s, 3H, OCH₃), 3.89 (t, J = 6.8 Hz, 2H, CH₂–NH), 4.39 (s, 2H, CH₂–N), 6.76 (br s, 1H, NH), 7.21 (d, J = 8.4 Hz, 2H, ArH), 7.64 (d, J = 8.4 Hz, 2H, ArH); ¹³C NMR (100 MHz, CDCl₃): δ = 13.8, 19.8, 20.8, 33.4, 42.5, 48.0, 50.9, 97.5, 119.4, 129.6, 134.7, 136.3, 164.3, 165.5; MS (EI, 70 eV): m/z, (%) = 302 (M⁺, 74), 287 (4), 271 (8), 259 (57), 243 (100), 241 (62), 227 (73) 199 (14), 187 (17), 172 (9), 159 (19), 118 (30), 91 (42), 80 (21), 66 (34), 55 (19); anal. calcd for C₁₇H₂₂N₂O₃: C, 67.53; H, 7.33; N, 9.26. Found: C, 67.42, H, 7.39, N, 9.33.

Methyl 1-(3,4-dichlorophenyl)-3-(propylamino)-2,5-dihydro-2-oxo-1H-pyrrole-4-carboxylate (5y). White solid; mp: 125–127 °C; IR IR (KBr, cm⁻¹) ν = 3328 (NH), 1707 (C=O), 1643 (C=O); ¹H NMR (400 MHz, CDCl₃): δ = 1.00 (t, J = 7.2 Hz, 3H, CH₃), 1.64 (sextet, J = 7.2 Hz, 2H, CH₂), 3.81 (s, 3H, OCH₃), 3.81–3.83 (m, 2H, CH₂–NH), 4.37 (s, 2H, CH₂–N), 6.72 (br s, 1H, NH), 7.45 (d, J = 8.8 Hz, 1H, ArH), 7.66 (dd, J = 8.4, 2.4 Hz, 1H, ArH), 8.00 (d, J = 2.4 Hz, 1H, ArH); ¹³C NMR (100 MHz, CDCl₃): δ = 11.1, 24.5, 44.4, 47.7, 51.0, 96.3, 117.9, 120.5, 128.1, 130.5, 133.0, 138.2, 164.5, 165.5; MS (EI, 70 eV): m/z, (%) = 344 (M²⁺, 24), 342 (M⁺, 34), 315 (26), 313 (45), 295 (15), 285 (47), 283 (100), 241 (10), 226 (5), 187 (10), 174 (15), 172 (18), 145 (13), 112 (16), 80 (23), 66 (33), 53 (16); anal. calcd for C₁₅H₁₆Cl₂N₂O₃: C, 52.49; H, 4.70; N, 8.16. Found: C, 52.76, H, 4.81, N, 8.23.

Bis-(ethyl 3-(methyleneamino)-1-phenyl-2,5-dihydro-2-oxo-1H-pyrrole-4-carboxylate) (7a). White solid; mp: 159–161 °C; IR (KBr, cm⁻¹) ν = 3295 (NH), 1698 (C=O), 1638 (C=O); ¹H NMR (400 MHz, CDCl₃): δ = 1.27 (t, J = 6.8 Hz, 6H, 2OCH₂CH₃), 4.12–4.16 (m, 8H, 2OCH₂CH₃ and 2CH₂–NH), 4.36 (s, 4H, 2CH₂–N), 6.74 (br s, 2H, 2NH), 7.19 (t, J = 7.6 Hz, 2H, ArH), 7.39 (t, J = 8.0 Hz, 4H, ArH), 7.75 (d, J = 7.6 Hz, 4H, ArH); ¹³C NMR (100 MHz, CDCl₃): δ = 14.4, 43.8, 47.9, 59.8, 98.0, 119.3, 124.9, 129.0, 138.7, 164.5, 165.0; MS (EI, 70 eV): m/z, (%) = 518 (M⁺, 6), 472 (4), 426 (6), 272 (100), 259 (26), 213 (68), 199 (58), 187 (36), 173 (13), 158 (9), 104 (25), 66 (32), 55 (20); anal. calcd for C₂₈H₃₀N₄O₆: C, 64.85; H, 5.83; N, 10.80. Found: C, 65.04, H, 5.90, N, 10.95.

Bis-(ethyl 1-(4-methoxyphenyl)-3-(methyleneamino)-2,5-dihydro-2-oxo-1H-pyrrole-4-carboxylate) (7c). White solid; mp: 222–224 °C; IR (KBr, cm⁻¹) ν = 3306 (NH), 1695 (C=O), 1634 (C=O); ¹H NMR (400 MHz, CDCl₃): δ = 1.28 (t, J = 7.2 Hz, 6H, 2OCH₂CH₃), 3.84 (s, 6H, 2OCH₃), 4.16–4.19 (m, 8H, 2OCH₂CH₃ and 2CH₂–NH), 4.33 (s, 4H, 2CH₂–N), 6.70 (br s, 2H, 2NH), 6.92 (d, J = 9.2 Hz, 4H, ArH), 7.63 (d, J = 9.2 Hz, 4H, ArH); ¹³C NMR (100 MHz, CDCl₃): δ = 14.4, 41.5, 49.0, 55.5, 59.7, 114.2, 121.3, 131.9, 156.9, 164.1, 165.0; MS (EI, 70 eV): m/z, (%) = 578 (M⁺, 1), 368 (1), 272 (5), 213 (5), 149 (5), 123 (66), 108 (100), 80 (56), 65 (11), 53 (26); anal. calcd for C₃₀H₃₄N₄O₈: C, 62.27; H, 5.92; N, 9.68. Found: C, 62.59, H, 6.05, N, 9.80.

Bis-(methyl 1-(4-chlorophenyl)-3-(methyleneamino)-2,5-dihydro-2-oxo-1H-pyrrole-4-carboxylate) (7e). White solid; mp: 202–204 °C; IR (KBr, cm⁻¹) ν = 3308 (NH), 1698 (C=O), 1637 (C=O); ¹H NMR (400 MHz, CDCl₃): δ = 3.69 (s, 6H, 2OCH₃), 4.14–4.16 (m, 4H, 2CH₂–NH), 4.29 (s, 4H, 2CH₂–N), 6.75 (br s, 2H, 2NH), 7.34 (d, J = 8.8 Hz, 4H, ArH), 7.69 (d, J = 8.8 Hz, 4H, ArH); ¹³C NMR (100 MHz, CDCl₃): δ = 43.5, 47.9, 51.1, 98.0, 120.2, 129.1, 130.1, 137.2, 164.4, 165.2; MS (EI, 70 eV): m/z, (%) = 560 (M²⁺, 1), 558 (M⁺, 2), 441 (10), 336 (34), 304 (21), 292 (35), 280 (34), 266 (38), 247 (60), 221 (100), 138 (30), 111 (35), 91 (77), 69 (48), 57 (82), 55 (60); anal. calcd for C₂₆H₂₄Cl₂N₄O₆: C, 55.82; H, 4.32; N, 10.02. Found: C, 56.10, H, 4.40, N, 9.13.

Conclusions

In conclusion, we have developed a highly efficient and green approach for the synthesis of polyfunctionalized mono- or bis-dihydro-2-oxopyrroles via one-pot, four-component reaction of amines, dialkyl acetylenedicarboxylates and formaldehyde using TrCl as a homogeneous catalyst in EtOH at ambient temperature. The presented method offers several advantages including operational simplicity, high atom economy, easy work-up and no need to column chromatography, high yields, mild and environmentally benign reaction conditions.

Acknowledgements

Financial support from the Research Council of the Payame Noor University and the University of Sistan and Baluchestan is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Spectral data, FT-IR, ¹H, ¹³C NMR and mass spectra for the presented compounds. See DOI: 10.1039/c4ra06923d

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