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Publicly Available Published by De Gruyter January 11, 2019

Reactivity of allenylphosphonates/allenylphosphine oxides – some new addition/cycloaddition and cyclization pathways

  • K. C. Kumara Swamy EMAIL logo , Mandala Anitha , Shubham Debnath and Mallepalli Shankar

Abstract

In this paper, we highlight some addition/cycloaddition reactions of allenylphosphonates and allenylphosphine oxides, mostly based on the work done in our laboratory. Thus the electrophilic addition of iodine monochloride (ICl) with allenylphosphine oxides affords cyclic phosphonium salts rather than γ-chloro-β-iodovinylphosphine oxides (NMR, HRMS, X-ray) that exhibit rather unusual downfield shifts in the 31P NMR spectra. These compounds undergo hydrolysis to afford γ-hydroxy-β-iodovinylphosphine oxides; the hydroxymethyl group in these compounds can be oxidized by Dess-Martin periodinane to afford the corresponding aldehyde-substituted vinylphosphine oxides. A [2+2] cycloaddition product of an allenylphosphonate has also been structurally characterized. Other reactions that are highlighted include those leading to (Z)/(E)-β-aminovinylphosphonates, β-ketophosphonates (and their utility in Horner-Wadsworth-Emmons reaction), indolyl/furanyl/isocoumaranyl/naphthyl phosphine oxides, thiophosphorylated phosphonates and azo-substituted coumarin phosphonates.

Introduction

Allenes of type 1 have always engrossed the minds of chemists because of the interesting feature of possessing two cumulative C=C double bonds and three reactive carbon centers. Allenylphosphonates 2 and allenylphosphine oxides 3 constitute a special class of allenes containing one of the substituents as –P(O)(OR)2 and –P(O)R2 group, respectively [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Due to the relative stability and low cost of preparation, these are valuable precursors for exploring allene chemistry. In the past decade, by using phosphorus based allenes, various phosphorus substituted heterocycles/homocycles like benzofurans [12], triazoles [13], tetrahydrofurans [14], indenes [15], isochromenes [15], indolo-pyrane-1-ones [16], pyrroles [17], indolinones [18], isocoumarins/indoles [19], dihydro[1,2]oxaphospholes [20], pyrrolines [21], tetrahydropyridines [21], cyclopentenes [21], substituted vinylphosphonates/allylphosphonates [22], [23], [24], [25], [26], [27], quinolines [28], and β-amino-phosphonates [29], [30] have been synthesized. Herein, literature on the following topics involving allenes with, mainly, contributions from our group will be reviewed along with a few new results: (i) Halogen addition, (ii) Cycloaddition and (iii) Nucleophilic addition/cyclization.

Halogen addition reactions of allenylphosphonates/allenylphosphine oxides

Halogen addition to allenes is less investigated and only a limited number of reports are available [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]. In an important contribution, Ma et al. explored the iodohydroxylation of allenylphosphine oxides 4 (Scheme 1a) that led to the β-iodo-γ-hydroxy-vinyl phosphine oxides 5 [34]. In reactions such as this, an initial electrophilic attack at the central carbon of the allene is envisaged. More recently, Zhou’s group developed a cooperative CuBr2/TBHP mediated protocol to synthesize β,γ-dibromo-vinyl-diphenylphosphine oxides 8 and β-bromo-γ-hydroxy-vinyldiphenylphosphine oxides 7 from allenylphosphine oxides 6 (Scheme 1b) [37]. In this transformation, bromohydroxylated product was formed due to neighboring-group participation of the diphenylphosphoryl group of allenylphosphine oxide. Purely nonphosphorus allenes do react with TMSX (X=Cl, Br, I) using selectfluor as the oxidant as shown by Laali’s group [38], [39]. Hammond et al. reported the addition of molecular iodine with the allene 9 that afforded β,γ-diiodo-α-fluoro-vinylphosphonate 10 readily (Scheme 1c) [40]. The diiodovinylphosphonate 10 undergoes substitution reaction with nucleoside bases such as purine and adenine leading to fluorinated acyclic phosphono-nucleosides 11. Not all the halogenations lead to simple addition as shown by Ma et al. who developed a protocol for the synthesis of halogeno-phosphono-heterocycles 13, by the reaction of monoesters of 1,2-allenyl phosphonic acids 12 with CuX2 (X=Cl, Br) (Scheme 1d) [41]. They have extended this study to include other allenylphosphonates also [42]. Very recently our group reported the dichloro-addition on allenylphosphonates 14 by treating them with AgNO3 and oxalyl chloride. From this reaction, α,β-vicinal dichloro-vinylphosphonates 15 as major products and an isomeric mixture of β,γ-vicinal dichloro vinylphosphonates 15′ as minor products were obtained (Scheme 1e) [43]. These reactions are in contrast to the reaction of AgNO3/oxalyl chloride with normal olefins wherein chloroamidated product was formed [44].

Scheme 1: Selected halogenation reactions of phosphorus-based allenes.
Scheme 1:

Selected halogenation reactions of phosphorus-based allenes.

In the interhalogen ICl, we can assign a partial positive charge on iodine and hence if the attack is electrophilic, iodine could be part of a three-membered ring in the intermediate. Later, iodine will be attached to the β-carbon and the chlorine may either bind to the terminal carbon of the allene or be present as anion of a cyclic phosphonium salt. In our work, we treated allenylphosphine oxides 16a–b with 1 M solution of ICl in CH3CN at 0°C and obtained 17–18 in good yields (Scheme 2) almost instantaneously. The 31P NMR spectrum of compound 17 shows a peak at δ 79.5 and in the 13C NMR spectrum a doublet at δ 118.0 [1J(PC=C)=102.6 Hz] indicating the presence of a P-C=C moiety. A surprising point here was that the 31P NMR chemical shifts of compounds 17–18 are unusually downfield which is consistent with phosphorus being part of a 5-membered ring (vide infra), but not with the initially assigned γ-chloro-β-iodovinylphosphine oxides 17′–18′ (peaks corresponding to these were observed in HRMS) [45]. It can be noted that a similar cyclic structure with a five-membered ring had been proposed earlier (cf. Scheme 1a) [33]. Presence of chlorine in the molecule was also confirmed by HRMS data. These compounds could be purified by recrystallization from DCM/hexane (1:2) mixture, but when we tried silica gel column chromatography, the hydrolyzed products 19–20 were obtained in good yields (Scheme 2a). In the 1H NMR spectra, the OCH2 protons appear as a singlet in 19 but a doublet of doublet in 20. The structure of compound 20 was confirmed by single crystal X-ray analysis (Fig. 1a) [46], [47], [48], [49]. In the solid state, this compound exhibits H-bonding involving the –CH2OH group and the phosphoryl group (Fig. 1b); this factor together with the presence of 2-methyl group on the aromatic ring at the α-position may be the reason for the observation of separate signals for the two CH2 protons in the 1H NMR spectrum. Compounds of type 19–20 are valuable precursors since the terminal –CH2OH group is amenable for oxidation. Thus we treated γ-hydroxy-β-iodovinylphosphine oxides 19–20 with Dess–Martin periodinane in DCM to obtain the corresponding functionalized aldehydes 21–22 in good yields (Scheme 2b). Absence of peaks due to allylic protons at δ 4.9–5.2 and appearance of aldehydic proton at δ 10.11 [3J=1.2 Hz] in the 1H NMR in conjunction with the observation of CHO signal at δ 187.2 [J=6.4 Hz] in the 13C NMR confirm that the primary alcohol is readily converted into aldehyde without affecting other bonds. These compounds have geminal functionalities (iodo, aldehyde and alkene) that could be gainfully employed for further derivatization.

Scheme 2: Reactions of phosphorus-based allenes 16a–b with ICl followed by derivatization of the products.
Scheme 2:

Reactions of phosphorus-based allenes 16a–b with ICl followed by derivatization of the products.

Fig. 1: (a) Molecular structure of Compound 20 (left). Selected bond lengths [Å] with esds are given in parentheses: P1 C13 1.846(5), C21 C13 1.324(7), I1 C21 2.113(5), C21 C22 1.507(7), O2 C22 1.393(6), C13 C14 1.490(7); (b) Molecular structure of compound 20 (right) showing H-bonding O1···H2O2 (1.832(4) Å). CCDC No. 1878710.
Fig. 1:

(a) Molecular structure of Compound 20 (left). Selected bond lengths [Å] with esds are given in parentheses: P1 C13 1.846(5), C21 C13 1.324(7), I1 C21 2.113(5), C21 C22 1.507(7), O2 C22 1.393(6), C13 C14 1.490(7); (b) Molecular structure of compound 20 (right) showing H-bonding O1···H2O2 (1.832(4) Å). CCDC No. 1878710.

In the above reaction of allenylphosphine oxides with ICl, although the compounds 17–18 could be isolated, we wanted to characterize such intermediates by single crystal X-ray structure and hence surmised that steric protection at the γ-carbon of the allene could render more stable species. Thus the reaction of allenylphosphine oxide 23a with ICl was conducted; however with only an excess of ICl being present, we were able to obtain a decent crystalline solid 24a (cf. Scheme 3) for which we could obtain an X-ray structure. In this case, the asymmetric unit has two cationic moieties and the corresponding anions are [ICl2] and [Cl(ICl)2] thus balancing the charge required. The structure is shown in Fig. 2; there is some disorder at the [ICl2] anion. In a similar manner we obtained compound 24b from the precursor 23b. It is interesting to note that somewhat similar phosphonium salt has also been reported recently in the reaction of an allenylphosphonate with triflic acid by Bogachenkov et al. [50] and Lozovskiy et al. [51] Equally important is that halogenation of allenic phosphine oxides does lead to the formation of stable phospholene salts as reported by Bogachenkov and Ionin [52]. However, to our knowledge, characterization of such a compound in the reaction using ICl is new.

Scheme 3: Isolation of products 24a–b from the reaction of ICl with allenylphosphine oxides 23a–b.
Scheme 3:

Isolation of products 24a–b from the reaction of ICl with allenylphosphine oxides 23a–b.

Fig. 2: Molecular structure of [Ph2P{O-C(C6H4-4-Me)2-C(I)=C(Ph)}]2[(ICl2)−, {Cl(ICl)2}−] (24a) showing two cationic moieties (left and center) and the two anionic species in the asymmetric unit; the [ICl2]− anion has some disorder for one of the chlorine atoms. Selected bond distances (Å): P1-O1 1.580(6), P1-C1 1.760(9), P1-C7 1.767(9), P1-C13 1.780(9), O1-C15 1.499(9), C14-C13 1.327(11), C14-C15 1.531(12), P2-O2 1.585(6), P2-C36 1.781(9), P2-C42 1.792(9), P2-C48 1.768(9), O2-C50 1.507(9), C49-C48 1.343(11) C49-C50 1.541(12). CCDC No. 1878711.
Fig. 2:

Molecular structure of [Ph2P{O-C(C6H4-4-Me)2-C(I)=C(Ph)}]2[(ICl2), {Cl(ICl)2}] (24a) showing two cationic moieties (left and center) and the two anionic species in the asymmetric unit; the [ICl2] anion has some disorder for one of the chlorine atoms. Selected bond distances (Å): P1-O1 1.580(6), P1-C1 1.760(9), P1-C7 1.767(9), P1-C13 1.780(9), O1-C15 1.499(9), C14-C13 1.327(11), C14-C15 1.531(12), P2-O2 1.585(6), P2-C36 1.781(9), P2-C42 1.792(9), P2-C48 1.768(9), O2-C50 1.507(9), C49-C48 1.343(11) C49-C50 1.541(12). CCDC No. 1878711.

Cycloaddition reactions

The two cumulative double bonds in allenes can be gainfully employed in cycloaddition reactions. Among such atom-economy reactions, [4+2]-cycloaddition is generally very facile and, in general, the allene can act as a dienophile [4], [5], [6], [53]. An example using anthracene as the diene resulting in the product 26 from the allene 25 is shown in Scheme 4a. However, what is perhaps unique for allenes is the self [2+2] cycloaddition reaction, and one such example involving [β,γ] double bonds was previously reported by us [29]. To know whether this is a common feature we heated the allene 27 at 140°C in p-xylene and found again that the [β,γ]-dimerization occurs to give rise to the 4-membered ring compound 28 (NMR/HRMS/X-ray; cf. Scheme 4b and Fig. 3).

Scheme 4: Examples of [4+2] and Self [2+2] cycloaddition reaction of allenes.
Scheme 4:

Examples of [4+2] and Self [2+2] cycloaddition reaction of allenes.

Fig. 3: Molecular structure of compound 28. The packing involves the two types of molecules with the result that there is disorder essentially at the central part, but not at the dioxaphosphorinane ring. Selected bond distances (Å): P1-C6 1.759(7), P1-C13′ 1.784(7), C6-C8 1.320(10), C8-C9 1.521(8), C8-C11 1.490(8), C9-C10 1.547(9), C10-C11 1.531(8), C11-C13 1.324(10). CCDC No. 1878712.
Fig. 3:

Molecular structure of compound 28. The packing involves the two types of molecules with the result that there is disorder essentially at the central part, but not at the dioxaphosphorinane ring. Selected bond distances (Å): P1-C6 1.759(7), P1-C13′ 1.784(7), C6-C8 1.320(10), C8-C9 1.521(8), C8-C11 1.490(8), C9-C10 1.547(9), C10-C11 1.531(8), C11-C13 1.324(10). CCDC No. 1878712.

Additional double bonds present intramolecularly can lead to several interesting cyclization reactions. Thus the alkylidene substituted propargylic alcohol 29 reacts with (OCH2CMe2CH2O)PCl to afford indene derivative 30, most likely via an allene intermediate (Scheme 5) [15]. Although it is not a formal cycloaddition, the cyclization process like the one depicted here has more potential for further exploration.

Scheme 5: Formation of an indene derivative 30 via allenylphosphonate.
Scheme 5:

Formation of an indene derivative 30 via allenylphosphonate.

Nucleophilic addition/cyclization reactions

As mentioned above, nucleophiles in general tend to attack the central carbon atom of the allene. However, depending on the amine, either vinyl- or allyl phosphonates may result. In the case of vinylphosphonates, an (E) configuration is generally favored, but in the case of product with ammonia, the (Z)-isomer is preferred because of the intramolecular H-bonding. These features are illustrated in Scheme 6 for the reaction of allenylphosphonate 31. Although equilibrium is expected between the (enamino)vinyl phosphonate and the corresponding (enamino)allyl phosphonate [54], thermal conversion of 32 to its (enamino)allyl form or of 33 to its (enamino)vinyl form did not occur. As regards compound 34, previous reports have also suggested that the intramolecular hydrogen bonding leads to the Z form, but structural proof was not available [55]. The X-ray structures for three types of products are shown in Fig. 4 [30].

Scheme 6: Differing reactivity of allenylphosphonates with amines.
Scheme 6:

Differing reactivity of allenylphosphonates with amines.

Fig. 4: Molecular structures of 32–34. Selected bond distances [Å] with esds in parentheses. Compound 32: P-C6 1.742(2), C6-C7 1.358(2), C7-C8 1.501(2), C7-N 1.377(2). Compound 33: P-C6 1.798(2), C6-C7 1.498(3), C7-C11 1.311(3), C7-N1 1.421(3). Compound 34: P-C6 1.718(6), C6-C7 1.353(7), C7-C8 C7 C8 1.471(8), C7-N1 1.346(7). Strucutres are redrawn from the coordinates from Ref. [30].
Fig. 4:

Molecular structures of 32–34. Selected bond distances [Å] with esds in parentheses. Compound 32: P-C6 1.742(2), C6-C7 1.358(2), C7-C8 1.501(2), C7-N 1.377(2). Compound 33: P-C6 1.798(2), C6-C7 1.498(3), C7-C11 1.311(3), C7-N1 1.421(3). Compound 34: P-C6 1.718(6), C6-C7 1.353(7), C7-C8 C7 C8 1.471(8), C7-N1 1.346(7). Strucutres are redrawn from the coordinates from Ref. [30].

Although amine addition products can be isolated as shown above, in the presence of water, they undergo hydrolysis to lead to β-ketophosphonates that can be conveniently used for Horner-Wadsworth-Emmons (HWE) reaction as shown in Scheme 7. Thus the β-ketophosphonate 36 obtained from allenylphosphonate 35 reacts with terephthalaldehyde to lead to the conjugated alkene 37 [29].

Scheme 7: Formation and utility of the β-ketophosphonates obtained from the base catalyzed hydrolysis of allenylphosphonates.
Scheme 7:

Formation and utility of the β-ketophosphonates obtained from the base catalyzed hydrolysis of allenylphosphonates.

In the above nucleophilic additions, in case the –NHR moiety is present in the same molecule, interesting intramolecular cyclization reactions can occur as shown in Scheme 8. From the first two reactions, phosphinoyl-indoles (cf. 39) or phosphinoyl-isocoumarins (cf. 42/42′) are obtained in pretty good yields. The allene intermediates (cf. 40) shown in Scheme 8a have also been individually isolated. The reactions shown in Scheme 8c lead to phosphinoyl benzofurans 44 or isochromenes 45, depending on whether the functional group X is OH or CH2OCH2OMe in the precursor allene 43 [15], [19].

Scheme 8: Intramolecular cyclization reactions involving nucleophilic addition to allenylphosphine oxides.
Scheme 8:

Intramolecular cyclization reactions involving nucleophilic addition to allenylphosphine oxides.

Cyclodiphosphazanes of the type [t-BuNHPN(t-Bu)]2 (46) can readily act as nucleophilic bases and in fact attack the β-carbon of the allene. One such reaction that we encountered is shown in Scheme 9. In this reaction, a new chiral center is created at the carbon α to the phosphorus, generally leading to racemic products (e.g. 47a–b). Other nucleophilic bases like Ph2P(NH-t-Bu) can also be used for this purpose. In a favorable case of 47c, we were able to separate the two enantiomeric forms by ‘spontaneous resolution by crystallization’ [56]. This was verified both by X-ray crystallography as well as the circular dichroism (CD) spectra of the enantiomers thus isolated. Interestingly, even in the case of phosphinoyl substituted allenes we were able to observe ‘spontaneous resolution by crystallization’ [57]. Another interesting reaction involves cyclodiphosphazane appended allene 49 as a possible intermediate; the oxygen of the –NO2 group attacks the β-carbon of the allene in an unusual reaction to lead to the indolinone product 48 (Scheme 10) [18].

Scheme 9: Cyclodiphosphazane addition to allenylphosphonates.
Scheme 9:

Cyclodiphosphazane addition to allenylphosphonates.

Scheme 10: Formation of indolinone 48 via cyclodiphosphazane appended allene 49.
Scheme 10:

Formation of indolinone 48 via cyclodiphosphazane appended allene 49.

In a manner not too different from that shown in Scheme 9, under [Pd]-catalyzed conditions, H-phosphonates of the type (RO)2P(O)H or phosphine oxides/sulfides Ph2P(X)H [X=O, S] readily add to phosphorylated allenes. By varying the reaction conditions, the yields of some of these products can be maximized. The product distribution appears to vary depending upon whether phosphite or thiophosphite is used. An example of this type of phosphonylation/thiophosphonylation is illustrated in Scheme 11 by starting with the allene 31 that leads to the products 50–55 [58]. This type of reaction offers a nice route to multiphosphonylated systems and has potential to be developed further. In these reactions, the thiophosphites or phosphine sulfide Ph2P(S)H appear to be more reactive; in a few instances, even the [Pd]-catalyst is not required. The molecular structures of 50 and 55 are shown in Fig. 5 along with relevant bond parameters.

Scheme 11: Reaction of allenylphosphonate 31 with cyclic phosphites/thiophosphites.
Scheme 11:

Reaction of allenylphosphonate 31 with cyclic phosphites/thiophosphites.

Fig. 5: Molecular structures of 50 and 55. Selected bond lengths [Å] with estimated standard deviations in parentheses: Compound 50 P1-C6 1.805(2), P2-C7 1.793(2), C6-C7 1.509(3), C7-C8 1.321(3). Compound 55 (there is some disorder in the ring methyl groups) P1-C6 1.800(2), P2-C7 1.824(2), P3-C8 1.787(3), P1-O3 1.471(2), P2-S1 1.913(9), P3-S2 1.892(1), C6-C7 1.533(3), C7-C8 1.546(3) (figure is redrawn using the coordinates taken from Ref. [58]).
Fig. 5:

Molecular structures of 50 and 55. Selected bond lengths [Å] with estimated standard deviations in parentheses: Compound 50 P1-C6 1.805(2), P2-C7 1.793(2), C6-C7 1.509(3), C7-C8 1.321(3). Compound 55 (there is some disorder in the ring methyl groups) P1-C6 1.800(2), P2-C7 1.824(2), P3-C8 1.787(3), P1-O3 1.471(2), P2-S1 1.913(9), P3-S2 1.892(1), C6-C7 1.533(3), C7-C8 1.546(3) (figure is redrawn using the coordinates taken from Ref. [58]).

Propargylate anion also can attack the β-carbon of the allene readily, but in this case, facile cyclization leading to the phosphinoyl substituted furan 56 occurs, by making use of the alkyne group (cf. Scheme 12) [59]. Zn(OTf)2 is a good catalyst for this transformation. The mechanistic pathway appears to be complicated, but may involve the transformation of the propargylic moiety into allenic system prior to the formation of the final product.

Scheme 12: Formation of phosphinoyl substituted furan 56 from allene.
Scheme 12:

Formation of phosphinoyl substituted furan 56 from allene.

Although the reaction of salicylaldehydes with allenylphosphonates had been reported in earlier work also [7], extension to substrates bearing chromophores may lead to products with useful properties. Thus the azo-substituted salicylaldehyde 57 reacts with the allenylphosphonates to afford red colored phosphono-chromans as (Z) and (E) isomeric mixtures (58a–61a, 58b–61b; Scheme 13). Both the (Z) and (E) isomers have been isolated in a pure state [60]. It can be noted that the oxygen of the phenolate ion attack the β-position of the allene in these reactions. In continuation of similar strategy, we performed the reaction of phosphorus-based allene 25 with dimethyl 2-(2-formylphenyl)malonate for the synthesis of (β,γ)-cyclized phosphono-naphthalene derivative 62 (Scheme 14) [61]. The yields of 62 and related products were quite good. In these reactions the nucleophile, methine-carbanion generated in the presence of the base, attacks the β-carbon of the allene in the initial step.

Scheme 13: Formation of azo-substituted chromans 58–61 from allenylphosphonates.
Scheme 13:

Formation of azo-substituted chromans 58–61 from allenylphosphonates.

Scheme 14: Formation of phosphono-naphthalene derivative 62 from allenylphosphonate 25.
Scheme 14:

Formation of phosphono-naphthalene derivative 62 from allenylphosphonate 25.

Conclusions

New [2+2] and [4+2] cycloaddition reactions using phosphorus-based allenes have opened up more avenues for exploration. Nitro, and alkylidene functionalized allenes act as intermediates in the formation of phosphono-indolinones and indenes. The first example of spontaneous resolution by crystallization in an allylphosphonate was demonstrated by using allenylphosphonate and a cyclodiphosphazane. Hydroxy and other functionalized allenes have been utilized for the generation of new phosphorus-substituted benzofurans and isochromenes. Azo-substituted salicylaldehydes and dimethyl 2-(2-formylphenyl)malonate react with phosphorylated allenes to form novel chromenes and naphthalenes, respectively. Electrophilic addition of iodine monochloride (ICl) to allenylphosphine oxides affords cyclic phosphonium salts. These salts on passing through silica gel column led to γ-hydroxy-β-iodovinylphosphine oxides.

Supporting information

Characterization data for the new compounds reported herein are available with the corresponding author (e-mail: kckssc@uohyd.ac.in; kckssc@yahoo.com).


Article note

A collection of invited papers based on presentations at the 22nd International Conference on Phosphorus Chemistry (ICPC-22) held in Budapest, Hungary, 8–13 July 2018.


Acknowledgments

We thank Council of Scientific and Industrial Research (CSIR), University Grants Commission (UGC-UPE-2), Department of Science and Technology (DST-FIST and PURSE) and AvH foundation for financial support and/or equipment. KCK thanks (i) Prof. Dr. Konstantin Karaghiosoff for support during his stay in Germany on AvH fellowship, and (ii) Dr. Regine Herbst-Irmer and Prof. Dr. Dietmar Stalke for help in resolving the disorder in the structure of 28.

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Supplementary Material

The online version of this article offers supplementary material (https://doi.org/10.1515/pac-2018-1111).


Published Online: 2019-01-11
Published in Print: 2019-05-27

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