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DOI: 10.1055/a-1904-0420
Functionalization of Alkyl Groups Adjacent to Azoles: Application to the Synthesis of α-Functionalized Carboxylic Acids
We sincerely thank the Council of Scientific and Industrial Research, India (CSIR, CSMCRI project nos. MLP 0028 and MLP 0067 through CSIR-FIRST Scheme) and UGC (for a fellowship to RDS and JK) for financial support.
Abstract
A plethora of bioactive compounds and natural products bears an azole subunit within their complex structural frameworks. A footstep to realize those complex structures in atom economic fashion rely on the direct functionalization of C–H bonds adjacent to an azole group. In addition, the resulting functionalized azole compounds can be simply modified into practically significant genre of α-functionalized carboxylic acids that are otherwise inaccessible through a formal α-functionalization strategy. In this Account, we describe an up-to-date progress on the functionalization of a methyl and/or methylene group(s) adjacent to an azole ring enabled by late and earth-abundant transition metals. Contributions made by our group and that by others in the field are elaborated in this Account article.
1 Introduction
2 Mode of Reactivity of C–H Bonds Next to Azoles under Transition-Metal Catalysis
3 Pd-Catalyzed Functionalization of Alkyl Groups Adjacent to an Azole Ring
3.1 Functionalization through C–C Bond Formation
3.2 Functionalization through C–Heteroatom Bond Formation
4 3d-Metal-Catalyzed Functionalization of Alkyl Groups Adjacent to an Azole Ring
5 Other Metal-Catalyzed Functionalization of Alkyl Groups Adjacent to an Azole Ring
6 Conclusion and Future Prospects
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Key words
azoles - transition-metal catalysis - C–H functionalization - palladium - copper - iron - alpha-functionalization - carboxylic acidBiographical Sketches
From left to right: Mr. Anupam Kumar Singh, Ms. Rupali Dasharath Shinde, Mr. Jogendra Kumar, and Dr. Sukalyan Bhadra
Anupam Kumar Singh pursued his Master’s degree in chemistry (2017) from Indian Institute of Engineering Science and Technology, Shibpur (India). Since October 2018, he joined as a project assistant at CSIR-CSMCRI and subsequently joined PhD program under the supervision of Dr. S. Bhadra. His specific research interest is to develop synthetic procedures leading to fine and specialty chemicals.
Rupali Dasharath Shinde completed her BSc (2017) and MSc (2019) degrees from University of Mumbai (India) and topped in both degree exams (university 1st rank holder). Subsequently, she received a Junior Research Fellowship from UGC and joined the group of Dr. S. Bhadra as a PhD student in August 2021. Her PhD work is focused on the oxidative functionalization of common organic building blocks.
Jogendra Kumar received his Master’s degree with a specialization in organic chemistry (2015) from Kumaun University, Nainital (India). He had hold UGC-JRF and UGC-SRF positions in the group of Dr. S. Bhadra at CSIR-CSMCRI Bhavnagar. His PhD research topic is ‘development of catalytic procedures for the functionalizations of unactivated C–H bond by heteroatom-based substituents’. In 2022, he has been selected as a visiting research scholar by Ruhr-Universität Bochum (RUB), Germany under Research Explorer Ruhr Program (RER).
Sukalyan Bhadra received both his BSc (2004) and MSc (2006) degrees in chemistry from the University of Calcutta (India). In 2011, he was awarded with a PhD degree under the supervision of Professor Brindaban C. Ranu at Indian Association for the Cultivation of Science, India. He then moved to Germany, in the group of Professor Lukas J. Goossen at TU Kaiserslautern for a postdoctoral research. Later in 2013, he joined the group of Professor Hisashi Yamamoto at Chubu University, Japan as a JSPS postdoctoral fellow. In May 2016, he returned to India as a DST-INSPIRE faculty fellow to begin his independent career at CSIR-CSMCRI Bhavnagar, where he currently works as a senior scientist. His research interest revolves around development of new methodologies towards metal-promoted organic transformations, α-functionalization of common building blocks, cooperative catalysis, and asymmetric catalysis leading to the synthesis of fine chemicals and API molecules.
Introduction
Azoles are widely distributed as a ubiquitous building block among a myriad of bioactive compounds including agrochemicals, pharmaceuticals, and natural products (Figure [1]).[1] The direct functionalization of C–H bonds adjacent to azoles in the alkyl side chain of 2-alkylazoles is a synthetic transformation of prime significance in terms of both atom and step economy to realize a targeted azole-bearing complex structure. In addition, the resulting functionalized azole compounds can be simply modified into practically useful α-functionalized carboxylic acids and ketones that are otherwise inaccessible through a formal α-functionalization strategy.[2]
Owing to the presence of an engrained imine (C=N) counterpart, 1,3-azoles possess electron-withdrawing properties comparable to that of carbonyl functions. The C–H bond next to an azole ring is rather triggered by the existence of the electron-rich heterocycle. Nonetheless, the pKa value of those methylene C–H bonds is still high (>25 in DMSO) leading to inherently poor acidity.[3] As a consequence, the C–H abstraction with subsequent functionalization adjacent to an azole system calls for catalysis by a transition metal and involves an ionic or a radical mechanism. However, the metal-catalyzed C–H functionalization of 2-alkyl(benz)azoles is an intrinsically challenging task and highly substrate-specific, due to the known ligation of the azole N atom(s) to the transition-metal catalyst.
A literature survey indicates that examples of uncatalyzed functionalization of 2-alkylazoles have appeared rather sporadically, primarily relying on base-mediated strategies. For instance, the benzylation of 2-benzylbenzoxazole with relatively reactive benzylic electrophiles was achieved under basic conditions.[4] The selective metalation of MOM-protected 2-methylbenzimidazole with a novel hindered zinc amide base, (2,2,6,6-tetramethylpiperidyl)ZnOPiv·LiCl, led to an organozinc compound that underwent Negishi cross-coupling and allylic alkylation reactions under transition-metal catalysis.[5]
On the other hand, as early as in 1970s, Meyers et al. developed a strong base-mediated α-alkylation reaction of carboxylic acids via converting them into the corresponding nonaromatic oxazoline derivatives.[6] Later, Hansen et al. documented an oxygenation of the benzyl group adjacent to oxazoline subunits upon the treatment with stoichiometric n-butyllithium in the presence of oxygen bubbles.[7] Subsequently, analogous strategies were unveiled towards nitration, epoxidation, and aziridination processes to access numerous synthetic building blocks.[8] Although, these works indicated the principal viability of an α-functionalization of carboxylic acids via an azole intermediate, the catalytic variant of an analogous α-functionalization remained elusive until we recently established the concept.
In this Account, we describe an up-to-date progress on the functionalization of a methyl and/or methylene group(s) adjacent to an azole ring enabled by late and earth-abundant transition metals. Reactions that primarily depend on the oxidation of the title C–H bond by involving an organometallic species or a radical intermediate have been discussed. Of note, this article does not cover instances of the transition-metal-catalyzed and/or -free benzylic C–H bond oxidation processes that are sporadically applied to those of 2-alkylazoles.[9] Contributions made by our group and others in the field are elaborated in this Account article.[3] [10]
# 2
Mode of Reactivity of C–H Bonds next to Azoles under Transition-Metal Catalysis
2-Alkylazoles, specifically 2-alkylbenzazoles, are known to undergo functionalization at the C–H bond α-to-azole ring in the presence of a transition-metal catalyst. The reaction mechanism is dictated by the transition metal used. In general, a palladium catalyst promotes the tautomerization of 2-alkylbenzazole systems to their enamine-type form leading to an organopalladium intermediate, which readily undergoes functionalization through a subsequent ionic or radical process.[11] Alternatively, in the presence of a 3d transition metal, 2-alkylbenzazoles form a C-centered radical intermediate at the benzylic position with regard to the azole ring. The resulting carbo-radical species subsequently undergoes either an oxidation event to form a benzylic cation that reacts with a nucleophile or a radical–radical coupling process to give the desired functionalized product (Scheme [1]).[3]
# 3
Pd-Catalyzed Functionalization of Alkyl Groups Adjacent to an Azole Ring
3.1Functionalization through C–C Bond Formation
The palladium-catalyzed oxidative C–H bond functionalizations next to an azole ring have recently turned out to be an important strategy. In 2010, Miura et al. accomplished the decarboxylative benzylation of 2-benzylbenzazole with benzyl carbonates by means of an effective catalyst system comprising of Pd2(dba)3/dppp in DMSO without adding any external base.[12] A range of benzyl substituents was installed at the benzylic position with regard to the benzazole ring (Scheme [2]).
Literature report demonstrates that the methyl C–H bonds of 2-methylbenzazoles can be arylated with various aryl chlorides in the presence of Pd(OAc)2/PCy3 as the catalyst system and NaOtBu as the base.[13] Diarylated products were formed in good to excellent yield from 2-methylbenzazoles with numerous aryl chlorides (Scheme [3]). Control experiments suggested that H/D scrambling occurred among the methyl group of the starting material, 2-methyl-(N-methyl)imidazole, and the KIE value was estimated to be 1.3. These results indicate that the C–H bond-cleavage process is not likely to be involved in the rate-limiting step. This experimental finding was further supported by DFT study which suggests that the arylation proceed through a metalation-assisted intramolecular deprotonation process leading to a palladium η3-aza-allylic intermediate instead of a C–H cleavage process in the rate-determining step.
The triarylation of the methyl group adjacent to a benzazole system was achieved via palladium catalyzed deprotonative cross-coupling process (DCCP) using aryl chloride as the arylating agent (Scheme [4]).[14] The selectivity of the triarylation over the diarylation process was significantly improved by the use of a ligand, cataCXium A in the presence of a less polar solvent o-xylene. This tandem triarylation technique provides a wide range of sterically congested triaryl(heteroaryl)methane derivatives in high yield and selectivity and is scalable up to gram level.
Among others, palladium-catalyzed allylic alkylation has received tremendous attention in the context of C–C bond-forming transformations.[15] In 2011, Trost et al. reported the allylic alkylation of imine-containing N-heterocycles, including N-protected 2-alkylbenzoimidazoles.[16] In this study, a sterically hindered mesityl ester as the allylic leaving group was employed to accomplish the desired transformation in highly enantioselective fashion.[16] A chiral N,P,P,N-ligand 14 was found vastly effective for the palladium-catalyzed allylic alkylation process, however, a strong base such as LiHMDS had to be used in over stoichiometric proportion (3 equiv, Scheme [5]).
Later, Breinbauer and co-workers reported an additive-free palladium-catalyzed protocol that offered numerous α-allylated azoles in good to excellent yields under base and/or a Lewis acid free conditions.[17] The catalyst system comprising of [η3(C3H5)PdCl]2 and Xantphos is able to activate the α-C–H bonds of 2-alkylazoles via an N-allylation process (Scheme [6]). Thus, the reaction was proposed to involve an N-allylation leading to the formation of iminium ion complex 18, which subsequently led to the N-vinyl allylamine intermediate 19, followed by palladium-catalyzed aza-Claisen rearrangement to furnish the desired product 22 (Scheme [7]).
Recently, we have reported a convenient synthesis of ene-nitrile compounds through the catalytic direct cyanomethylenation of C(sp3)–H bonds adjacent to benzazoles with a ‘CNCH=’ equivalent.[18] In this process, DMF was used as a C-1 source (‘=CH’) and trimethylsilyl cyanide (TMSCN) was employed as the cyanating agent (‘CN’) to deliver ene-nitrile compounds via a one-step C=C double bond and C–CN bond formations in the presence of a palladium/copper bimetallic system (Scheme [8]). Control experiments suggested that the key catalyst component is constituted by combining PdCl2 in catalytic quantity (3 mol%) and CuCl2 in stoichiometric amount (1 equiv) in the presence of XPhos as a ligand (6 mol%). The cyanomethylenation was functional to a broad range of 2-alkyl and 2-benzylbenzazole substrates with diverse substitution patterns. A variety of common substituents, including fluoro, chloro, bromo, methyl, methoxy, trifluoromethyl, nitro, 2-thinyl, etc., were tolerant to the reaction conditions. Interestingly, the cyano group was found to be installed selectively at the position trans to the benzazole group in the cyanomethylenated products.
Extensive mechanistic studies revealed the following facts: 1. a (cyano)(methylamino)methyl radical intermediate is involved; 2. the KIE = 1.5 and no H/D scrambling in the starting material suggest an irreversible C–H bond cleavage; and 3. the palladium catalyst is responsible for the C(sp3)–H functionalization process. Based on these outcomes and literature precedence, we proposed a mechanism as depicted in Scheme [9]. In the mechanistic cycle, the 2-alkylbenzazole 23, first undergoes C–H palladation leading to the intermediate 25, presumably due to an imine–enamine tautomerization/palladation sequence, similar to the independent observation of Yoshikai and Glorius during the formation of an α-palladated imine species.[11] Concurrently, an in situ copper-enabled sequential oxidation and decarbonylation of DMF and subsequent nucleophilic addition of cyanide ion from TMSCN produce the (cyano)(methylamino)methyl radical species 28′.[19] The intermediate 25, in the presence of 28′ and molecular oxygen, gives the putative Pd(IV) intermediate 29, which subsequently undergoes reductive elimination leading to the palladium(II)-coordinated complex 30. Eventually, the cyanomethylenated compound 24, bearing the cyano group at the position trans to benzazole, is formed upon the palladium-assisted extrusion of MeNH2 from complex 30.
The synthetic practicality of the approach was demonstrated by a representative gram-scale synthesis and subsequent interconversion processes. The removal of the benzoxazole auxiliary led to an important class of branched acrylic acid 31 (Scheme [10]). Furthermore, the newly installed cyanomethylene group was converted into an amide 32 and an alkane 33.
Since 1,3-azoles hold an implanted ‘C=N’ function that exhibits electron-withdrawing properties similar to those of a carbonyl group, the cyanomethylenation approach was expanded toward ketones. Pleasingly, numerous (E)-4-oxo-2-enenitriles 35 were formed in good yields via α-cyanomethylenation of the corresponding alkyl phenyl ketones with DMF and TMSCN in a highly selective fashion (Scheme [11]).
# 3.2
Functionalization through C–Heteroatom Bond Formation
While palladium-catalyzed functionalization of C–H bonds next to an azole ring is primarily focused on various C–C bond-forming processes, we have unveiled a methoxylation reaction through a related C–O coupling process.[20] The privileged Pd(OAc)2/PhI(OAc)2/MeOH catalyst system, originally developed by Sanford for C–H alkoxylation reactions, was employed to accomplish the desired dehydrogenative methoxylation reaction between 2-alkylbenzazoles and methanol.[21] The scope of our methoxylation approach was found to be general in terms of both 2-benzylbenzoxazole and 2-benzylbenzothiazole substrates (Scheme [12]).
Soon it turned out that the reaction involves a radical process, wherein the C–H bond was likely to be cleaved at the rate-limiting step (KIE = 2.2–2.3) without the involvement of an acetoxyalkane intermediate. At first, 2-alkylazole undergoes swift tautomerization to give the enamine-type form 5′ that accelerates the formation of the palladated intermediate 37. Next, in the presence of methanol, PhI(OAc)2 oxidizes the Pd(II) intermediate 37 to furnish the putative Pd(IV) intermediate 38 that eventually produces the methoxyalkane 36a via reductive elimination (Scheme [13]).
The synthetic applicability of the methoxylation approach was established through 1. the multigram synthesis; 2. the removal of the benzoxazole auxiliary to provide the corresponding α-methoxyacetic acid; and 3. the synthesis of a stable precursor of an O-methylated Breslow intermediate (Scheme [14]).[22]
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# 4
3d-Metal-Catalyzed Functionalization of Alkyl Groups Adjacent to an Azole Ring
In general, 3d transition metals are relatively abundant compared to palladium and other 4d and 5d metals. Thus, 3d metals have recently appeared as an inexpensive alternative to 4d and 5d metal catalysts. However, due to smaller size, the catalytic activities of 3d transition metals differs considerably from that of a larger 4d or 5d metal catalysts. Among the 3d metals, copper and iron are known to serve as promising catalyst candidates for the functionalization of a C(sp3)–H bond adjacent to azoles.[3]
The required C–H functionalization takes place via the formation of a C-centered radical that is stabilized by its N-centered form as discussed in the previous sections. These C–H functionalizations are of fundamental importance as many azole moieties act as masked form of carboxylic acids. As such, aliphatic carboxylic acids are functionalized at the α-position with various heteroatom-based substituents via the corresponding benzazoles.
In 2014, Kanai et al. first reported that the C–H bond at the methyl or methylene group of 2-alkylbenzimidazoles can undergo intramolecular alkoxylation under copper(I) catalysis (Scheme [15]).[23] The addition of an N,N-ligand, such as 5,6-dimethylphenanthroline or 4,7-dimethoxyphenanthroline and an external oxidant, e.g. (tBuO)2, were beneficial for this transformation. The intramolecular approach was later extended for the intermolecular alkoxylation of benzimidazole and quinazolinone in the presence of butanol/2-phenylethanol as the alkoxylating agent. The mechanistic proposal includes the formation of an active copper intermediate and tert-butoxy radical upon the action of di-tert-butyl peroxide (DTPB) on CuBr. The azole substrate reacts with this active copper catalyst leading to an alkoxycopper intermediate and a benzyl radical which ultimately undergoes C–O bond formation to give the ether product (Scheme [16]).
Based on the documented ability of a copper-based system to form a carbo-radical intermediate at the benzylic site with regard to benzimidazole ring, we postulated that this class of radical species may react with a chalcogen counterpart to give the corresponding thio- and seleno-ether compounds. Screening of the thiolated counterpart and optimization of the reaction conditions revealed that indeed a copper-based catalyst system allowed phenylthiolation of 2-phenylbenzoxazoles with simple thiophenols (Scheme [17]).[24] The catalyst system is comprised of a Cu(I) salt (CuI) and a base (K3PO4) in DMSO (solvent). In this transformation, DMSO not only served as a solvent, but also acts as 2-fold oxidant. Gratifyingly, a wide range of 2-alkylbenzoxazole and -benzothiazole substrates underwent arylthiolation with diverse arenethiols providing an expedient access to a large variety of azole-decorated thioethers that are otherwise unattainable. These thioether compounds act as the precursor of biologically active compounds and important synthetic intermediates. The reaction scope was further broadened towards the phenylselenylation of 2-alkylbenzazoles by using diphenyldiselenide.
Mechanistic studies including radical scavenging and EPR experiments suggest the participation of a base-mediated single-electron-transfer (SET) process. As outlined in Scheme [18], the catalytic cycle begins with the oxidation of the Cu(I) catalyst by DMSO to produce an active Cu(II) species that undergoes a base-enabled SET process with 2-alkylazole and regenerates the Cu(I) species. A consequent reaction of the carbo-radical formed with the disulfide produced in situ upon the oxidation of thiols with DMSO furnished the thiolated product. An analogous mechanism is anticipated for the phenylselenylation process.
The synthetic applicability of this C–H chalcogenation approach was attained by the gram-scale synthesis of a thiolated benzoxazole 45d which was further transformed into the corresponding α-thiolated phenylacetic acid derivative 47. Finally, by using 47 as the building block, we accomplished the formal synthesis of a bioactive sphingosine 1-phosphate antagonist that is known to prevent glaucoma, dry eye, and angiogenesis-related diseases (Scheme [19]).[25] Furthermore, the transformation to a sulfone 49 and a ketal 50 gave rise to important class of benzothiazole derivatives.
In order to develop a related C–H amination reaction of 2-alkylbenzazoles, we relied on an iron-based catalyst system owing to the easy access, inexpensiveness, and documented abilities of those systems in promoting amination reactions as electron mediators.[26] We envisaged that the stabilized C-centered benzylic radical once formed from 2-alkylbenzazole upon the action of the iron catalyst might be trapped by aniline to form the corresponding (N-arylamino)-functionalized benzazoles.[27] Since benzazoles serve as an active carboxylate equivalent, the protocol gives an access to α-(N-arylamino)acetic acids that are largely distributed among bioactive compounds and pharmaceutical agents. To our delight, when a reasonably hindered benzoxazole comprising of two nonequivalent benzylic groups was treated with aniline in the presence of catalytic amount of FeCl3 and 2 equivalents of di-tert-butyl peroxide (DTBP) the desired amination took place regiospecifically at the methylene C–H group with no concurrent aromatic ring amination (Scheme [20]). An array of anilines bearing numerous functional groups including fluoro, chloro, bromo, iodo, methyl, ester, pyridyl, etc. with diverse substitution pattern were rapidly installed at the alkyl group of 2-alkylbenzoxazoles and -benzothiazoles. Notably, the amination allows structural modification with anilines for a few nonsteroidal anti-inflammatory drug derivatives, e.g., ibuprofen and naproxen.
As anticipated, the radical quenching studies indicated the formation of C-centered radical at the benzylic position in regard to benzazole moiety. Furthermore, it was believed that a stabilized amine radical species, such as aniline radical cation, might be involved, as the attempted amination with primary and/or secondary aliphatic amines was unsuccessful to furnish the desired aminated products. The experimental kinetic isotope effect (KIE) was found to be 1.6. Further, no H/D scrambling in the starting material during the amination with aniline in the presence of D2O suggested that the C–H abstraction occurs irreversibly.
These mechanistic investigations support a catalytic cycle as shown in Scheme [21]. At first, a carbo-radical intermediate 46′ is generated from the benzazole substrate upon the catalytic action of Fe2+ and DTBP.[28] Subsequently, a single-electron transfer takes place from the carbo-radical to Fe3+, leading to a benzylic cation 52 that is trapped with aniline to furnish the aminated product via path A. Instead, the carbo-radical may undergo radical coupling with in situ formed aniline radical cation to give the desired aminated compound 51 via path B.
The C–H amination strategy was finally applied to the synthesis of an α-amino acid, namely, 2-(phenylamino)-2-(o-tolyl)acetic acid via the removal of the benzoxazole ring (Scheme [22]). Further, the α-aminated phenylacetic acid was utilized in the preparation of a class of artificial peptide that are prevalent in bioactive scaffolds.[29] In another strategy, an aniline-substituted 2-benzylbenzothiazole derivative was found tolerant under the reaction conditions of N-protection with commonly used protecting groups.
As discussed above, the 3d-metal-catalyzed heterofunctionalization approaches proceed via the formation of a carbo-radical species from 2-alkyl(benz)azoles. We speculate that these carbo-radical formations can also be facilitated in the presence of an electrocatalyst or a photocatalyst by involving a single-electron-transfer cycle, which can be eventually merged with a desirable heterofunctionalization event. Overall, these tactics establish an evolving strategy for the construction of C–heteroatom bonds.
# 5
Other Metal-Catalyzed Functionalization of Alkyl Groups Adjacent to an Azole Ring
Recently, Fukumoto et al. has reported that a cationic iridium complex derived from the combination of (POCOPtBu)IrHCl and NaBArF 4 enables the silylation of a C(sp3)–H bond at the α-position in the 2-methyl group of 2-methylazoles (Scheme [23]).[30] Addition of 3,5-dimethylpyridine in catalytic quantity was mandatory for a successful catalytic cycle. The addition of a hydrogen acceptor showed better performance compared to without acceptor in terms of the efficiency of the reaction. During the process, an electrophilic silicon species is formed as a key intermediate for the transfer of the silyl group.
# 6
Conclusion and Future Prospects
In this Account, we have discussed the chemistry and an up-to-date progress of the functionalization of C–H bonds α to azoles. As can be seen, these functionalization processes have received current interest particularly given the fact that the functionalized azoles can be converted into numerous important building blocks including α-functionalized carboxylic acids that are otherwise inconvenient to access. In this venture, several metal-catalyzed new C–C and C–heteroatom bond-forming transformations of 2-alkylazoles were developed by our group and others. Despite considerable progress, the asymmetric variant of these reactions remained underdeveloped due to the incompatibility of optically active ligands under the functioning reaction conditions. Likewise, photocatalyzed and electrocatalyzed functionalization approaches are yet to be disclosed. These will be explored in the future. We firmly believe that these novel approaches will be progressively balancing classical organic synthesis and ultimately utilized by industries to achieve complex molecular scaffolds in a simpler manner.
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Conflict of Interest
The authors declare no conflict of interest.
Acknowledgment
CSIR-CSMCRI communication number 104/2022.
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Corresponding Author
Publication History
Received: 19 May 2022
Accepted after revision: 19 July 2022
Accepted Manuscript online:
19 July 2022
Article published online:
19 August 2022
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