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Copper-catalyzed solvent-free redox condensation of benzothiazoles with aldehydes or benzylic alcohols
Accepted Manuscript Copper-Catalyzed Sovent-Free Redox Condensation of Benzothiazoles with Aldehydes or Benzylic Alcohols Mingliang Zhang, Wen-Ting Lu, Wenqing Ruan, Hui-Jun Zhang, Ting-Bin Wen PII: DOI: Reference:
S0040-4039(14)00166-X http://dx.doi.org/10.1016/j.tetlet.2014.01.120 TETL 44153
To appear in:
Tetrahedron Letters
Received Date: Revised Date: Accepted Date:
9 November 2013 19 January 2014 27 January 2014
Please cite this article as: Zhang, M., Lu, W-T., Ruan, W., Zhang, H-J., Wen, T-B., Copper-Catalyzed Sovent-Free Redox Condensation of Benzothiazoles with Aldehydes or Benzylic Alcohols, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.01.120
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Copper-Catalyzed Solvent-Free Redox Condensation of Benzothiazoles with Aldehydes or Benzylic Alcohols
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Mingliang Zhang, Wen-Ting Lu, Wenqing Ruan, Hui-Jun Zhang* and Ting-Bin Wen*
1
Tetrahedron Letters j o ur n al h om e p a g e : w w w . e l s e v i er . c o m
Copper-Catalyzed Sovent-Free Redox Condensation of Benzothiazoles with Aldehydes or Benzylic Alcohols Mingliang Zhang, Wen-Ting Lu, Wenqing Ruan, Hui-Jun Zhang∗ and Ting-Bin Wen∗ Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, P. R. China. Fax: (+86) 592-218-6295; Tel: (+86) 592-218-6295; E-mail: [email protected],cn, [email protected]
A R T IC LE IN F O
A B S TR A C T
Article history: Received Received in revised form Accepted Available online
An efficient and practical method for the construction of 2-aryl- and 2-alkyl-substituted benzothiazoles via a copper-catalyzed redox condensation process between benzothiazoles and aldehydes or benzylic alcohols has been developed. The reaction proceeded under mild reaction conditions using enviromentally benign tert-butyl hydroperoxide (TBHP) as the oxidant. A reaction mechanism involving the ring-opening of benzothiazoles followed by amidation and intramolecular condensation is proposed based on the isolation of an anilide disulfide intermediate.
Keywords: copper benzothiazoles aldehydes benzylic alcohols C-C bond formation
1. Introduction Benzothiazole derivatives, especially 2-substituted benzothiazoles, are of particular interest due to their important biological activities and diverse applications in pharmaceutical chemistry and material science.[1,2] Until now, a large number of synthetic routes have been developed for the formation of 2substituted benzothiazoles, including the condensation of 2aminothiophenol with aldehydes (Scheme 1, pathway a),[3,4] carboxylic acids,[5] orthoesters,[6] or nitriles,[7] and the intramolecular cyclization of thioanilides[8] (Scheme 1, pathway b) etc. However, these methods suffered from several limitations, such as the use of readily oxidized 2-aminothiophenols, [9] the difficulties involved in the preparation of functionalized thioamides,[10] and the relatively harsh reaction conditions. Therefore, several pathways starting directly from benzothiazoles were also developed.[11-13] Transition metal-catalyzed crosscoupling of benzothiazoles with different kinds of arylating reagents appeared as a powerful route for the preparation of 2arylsubstituted benzothiazoles (Scheme 1, pathway c).[12] Recently, the research groups of Li, Tan and Wu respectively reported the facile and direct oxidative condensations of 2unsubstituted benzothiazoles with aromatic aldehydes or aryl ketones for the construction of 2-aryl- or 2-acylbenzothiazoles (Scheme 1, pathway d).[13] In these transformations, a ringopening process involving the formation of aldimine intermediates followed by their intramolecular cyclization was proposed.[13,14] However, under both Li and Tan’s reaction
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conditions, aliphatic aldehydes are not suitable substrates perhaps due to their easy oxidation to carboxylic acids.[13a,b] Recently, Itoh and Ma’s groups developed a general and operationally simple route for the construction of benzothiazole moieties via the coupling of 2-haloanilides with thiol surrogates followed by intramolecular condensation of the anilide intermediates (Scheme 1, pathway e).[15] Inspired by these results, we envisioned that amide intermediate II, which could be obtained through the transition-metal catalyzed oxidative amidation of various aldehydes with 2-aminobenzenethiols I derived from benzothiazoles,[14,16] might undergo further intramolecular condensation to form the corresponding 2-aryl- and 2-alkylsubstituted benzothiazoles (Scheme 2).
Scheme 1. Major routes to 2-substituted benzothiazoles.
∗ Hui-Jun Zhang. Tel.: +86-592-218-6295; fax: +86-592-218-6295; e-mail: [email protected],cn ∗ Ting-Bin Wen. Tel.: +86-592-218-6085; fax: +86-592-218-6295; e-mail: [email protected]
2
Tetrahedron reaction of 4-nitrobenzaldehyde 2e with benzothiazole 1a afforded the desired product 3ae in only 24% yield. Notably, aliphatic aldehydes are also favorable substrates for this reaction process (3ai-ak), which represent an advantage of this catalytic system. Moreover, substituents on the phenyl ring of benzothiazoles had some effects on the reaction yields (3ba-ga). Table 2. Investigation of the substrate scope.a)
Scheme 2. A new route to 2-substituted benzothiazoles. Herein, we report the efficient and solvent-free CuCl2/TBHP promoted redox condensation of benzothiazoles with both aromatic and aliphatic aldehydes or benzylic alcohols under mild reaction conditions. We initially explored the reaction of benzothiazole 1a with pmethylbenzaldehyde 2a in the presence of 0.3 equiv of CuCl 2 and 1.4 equiv of TBHP.[16] To our delight, the condensation reaction between 1a and 2a did occur at 80 oC in toluene affording the 2aryl benzothiazole 3aa in 62% yield after 36 h (Table 1, entry 1). Screening with different solvents suggested that the reaction proceeded much faster without solvent, and 3aa was obtained in 71 and 80% yields after 12 h and 24 h respectively (Table 1, entry 2-5). Control experiments showed that no reaction took place in the absence of either CuCl2 or TBHP (Table 1, entry 67). Increasing the reaction temperature to 100 oC led to the formation of 3aa in only 39% yield (Table 1, entry 8). Several other transition metal catalysts, such as Cu(OAc)2, CuCl, and NiCl2, could also promote the same transformation, whereas they proved to be much less efficient than CuCl2 (Table 1, entry 9-11). In addition, the amount of CuCl2 is crucial for the process, and the reaction in the presence of either 0.2 or 0.4 equiv of CuCl2 gave 3aa in lower yield.
Table 1. Optimization of the reaction conditions.a)
Entry Cat. t (h) Solvent Yield (%)b) CuCl2 toluene 62 1 36 CuCl2 chloroform 74 2 36 CuCl2 DCE 59 3 36 CuCl2 71 4 12 CuCl2 80 5 24 0 6 24 CuCl2 0 7c) 24 CuCl2 39 8d) 24 Cu(OAc)2 24 8 9 CuCl 33 10 24 NiCl2 39 11 24 a) Reaction conditions: 1a (1.86 mmol), 2a (1.69 mmol), 70% TBHP (2.37 mmol), catalyst (0.3 equiv), solvent (2 mL) or without solvent, 80 oC. b) Yield of 3aa. c) Reaction in the absence of TBHP. d) Reaction at 100 oC. With the optimized reaction conditions in hand, we then investigated the scope of this transformation with various aldehydes and several benzothiazoles (Table 2). Both electronrich and electron-deficient aromatic aldehydes 2b-d and 2f-h reacted with benzothiazole 1a producing the desired products 3ab-ad and 3af-ah in moderate to excellent yields. However, the
a)
Reaction conditions: 1 (1.86 mmol), 2 (1.69 mmol), 70% TBHP (2.37 mmol), and CuCl2 (0.51 mmol), 24 h, 80 oC. b) tBuOH (2 mL). It has been reported that, under oxidative conditions, alcohols could be directly used instead of aldehydes in several reactions.[4,17] Consequently, the direct reactions of benzylic alcohols 4 with benzothiazole 1a were conducted, which led to the formation of 3aa, 3ab, and 3al-ao in moderate yields (Table 3). Table 3. Arylation of benzothiazoles with several benzylic alcohols.
a) Reaction conditions: 1a (1.86 mmol), 2 (1.69 mmol), 70% TBHP (4.06 mmol), CuCl2 (0.51 mmol), without solvent, 80 oC, 24 h. b) chloroform (2 mL).
To understand the reaction mechanism, several experiments were performed. Initially the reaction between 4methylbenzoaldehyde 2a and 2-aminothiophenol 5 under standard reaction conditions was conducted giving rise to the formation of 3aa in 77% yield (eq 1), which indicated that the transformation might proceed through a ring-opening pathway.13 Furthermore, a careful study on the reaction between 1a and 2a suggested that a disulfide intermediate 6 was formed in the
3 beginning and could be separated in 55% yield after 40 min (for the crystal structure of 6, see supporting information), which was then converted to 3aa in 88% yield in the presence of 0.3 equiv of CuCl2 and 1.4 equiv TBHP in t-BuOH under N2 (Scheme 3). Notably, control experiments showed that, in the absence of 4methylbenzoaldehyde 2a or using TBHP in decane instead of TBHP aqueous solution, no ring-opening products from benzothiazole 1a were detected at all. Further investigation on the direct reaction of 2-aminopenyl disulfide 7 with 2a under similar reaction conditions (eq 2) indicated that CuCl2/TBHP could also be an efficient catalytic system for the construction of 2-substituted benzothiazoles starting from stable 2-aminophenyl disulfides and aldehydes. Based on the above observations, a plausible mechanism can be proposed as shown in Scheme 4. Initially, the attack of acyl radical on the thiazole ring led to the formation of intermediate A which then reacted with water to form intermediate B.[16,18] The ring-opening of B followed by the oxidation of intermediate C gave the disulfide intermediate D.[9,18,19] The cleavage of the disulfide and subsequent intramolecular condensation of intermediate D were expected to afford the desired 2-substituted benzothiazole.[19-21]
benzothiazoles. Anilide disulfides instead of aldimine were formed as intermediates, which might explain the efficient condensation between even aliphatic aldehydes and benzothiazoles. Moreover, CuCl2/TBHP proved also efficient in the condensation of 2-aminophenyl disulfides with aldehydes. Further investigation on the scope, exact mechanism and applications of this method are underway in our laboratory. Experimental Section Typical procedure Benzothiazole 1 (1.86 mmol, 1.1 equiv), aldehyde 2 (1.69 mmol) or benzylic alcohol 4 (1.69 mmol), TBHP (2.36 or 4.06 mmol, 1.4 or 2.4 equiv, 70% solution in water), CuCl 2 (0.51 mmol, 0.3 equiv) were placed in a 25-mL reaction vessel fitted with a magnetic stirring bar under nitrogen. The reaction mixture was first stirred in an ice bath for 0.5 h, and then stirred at 80 oC for 24 h. After cooling to room temperature, the crude product was purified by column chromatography to afford the 2-substituted benzothiazoles 3.
Acknowledgements This work was financially supported by Natural Science Foundation of China (Nos. 21072161 and 21302157), National Basic Research Program of China (973 Program, No. 2012CB821600), Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT), and the Fundamental Research Funds for the Central Universities.
References and notes 1.
Scheme 3. A detailed reaction study.
H2N
S S
NH2
7
CuCl2 (30 mol%) 70%TBHP (1.4 equiv)
N
+
CHCl3, N2, 80 oC
S
Me
CHO
Me (2)
3aa, 76%
2a
2.
Scheme 4. Tentative mechanism. In summary, we have developed a practical copper-catalyzed redox approach for the formation of 2-aryl- or 2-alkyl-substituted benzothiazoles starting from aldehydes and benzothiazoles under mild reaction conditions. The involvement of TBHP as an oxidant also allowed the direct reaction of benzylic alcohols with
3.
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S0040-4039(14)00166-X http://dx.doi.org/10.1016/j.tetlet.2014.01.120 TETL 44153
To appear in:
Tetrahedron Letters
Received Date: Revised Date: Accepted Date:
9 November 2013 19 January 2014 27 January 2014
Please cite this article as: Zhang, M., Lu, W-T., Ruan, W., Zhang, H-J., Wen, T-B., Copper-Catalyzed Sovent-Free Redox Condensation of Benzothiazoles with Aldehydes or Benzylic Alcohols, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.01.120
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Copper-Catalyzed Solvent-Free Redox Condensation of Benzothiazoles with Aldehydes or Benzylic Alcohols
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Mingliang Zhang, Wen-Ting Lu, Wenqing Ruan, Hui-Jun Zhang* and Ting-Bin Wen*
1
Tetrahedron Letters j o ur n al h om e p a g e : w w w . e l s e v i er . c o m
Copper-Catalyzed Sovent-Free Redox Condensation of Benzothiazoles with Aldehydes or Benzylic Alcohols Mingliang Zhang, Wen-Ting Lu, Wenqing Ruan, Hui-Jun Zhang∗ and Ting-Bin Wen∗ Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, P. R. China. Fax: (+86) 592-218-6295; Tel: (+86) 592-218-6295; E-mail: [email protected],cn, [email protected]
A R T IC LE IN F O
A B S TR A C T
Article history: Received Received in revised form Accepted Available online
An efficient and practical method for the construction of 2-aryl- and 2-alkyl-substituted benzothiazoles via a copper-catalyzed redox condensation process between benzothiazoles and aldehydes or benzylic alcohols has been developed. The reaction proceeded under mild reaction conditions using enviromentally benign tert-butyl hydroperoxide (TBHP) as the oxidant. A reaction mechanism involving the ring-opening of benzothiazoles followed by amidation and intramolecular condensation is proposed based on the isolation of an anilide disulfide intermediate.
Keywords: copper benzothiazoles aldehydes benzylic alcohols C-C bond formation
1. Introduction Benzothiazole derivatives, especially 2-substituted benzothiazoles, are of particular interest due to their important biological activities and diverse applications in pharmaceutical chemistry and material science.[1,2] Until now, a large number of synthetic routes have been developed for the formation of 2substituted benzothiazoles, including the condensation of 2aminothiophenol with aldehydes (Scheme 1, pathway a),[3,4] carboxylic acids,[5] orthoesters,[6] or nitriles,[7] and the intramolecular cyclization of thioanilides[8] (Scheme 1, pathway b) etc. However, these methods suffered from several limitations, such as the use of readily oxidized 2-aminothiophenols, [9] the difficulties involved in the preparation of functionalized thioamides,[10] and the relatively harsh reaction conditions. Therefore, several pathways starting directly from benzothiazoles were also developed.[11-13] Transition metal-catalyzed crosscoupling of benzothiazoles with different kinds of arylating reagents appeared as a powerful route for the preparation of 2arylsubstituted benzothiazoles (Scheme 1, pathway c).[12] Recently, the research groups of Li, Tan and Wu respectively reported the facile and direct oxidative condensations of 2unsubstituted benzothiazoles with aromatic aldehydes or aryl ketones for the construction of 2-aryl- or 2-acylbenzothiazoles (Scheme 1, pathway d).[13] In these transformations, a ringopening process involving the formation of aldimine intermediates followed by their intramolecular cyclization was proposed.[13,14] However, under both Li and Tan’s reaction
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conditions, aliphatic aldehydes are not suitable substrates perhaps due to their easy oxidation to carboxylic acids.[13a,b] Recently, Itoh and Ma’s groups developed a general and operationally simple route for the construction of benzothiazole moieties via the coupling of 2-haloanilides with thiol surrogates followed by intramolecular condensation of the anilide intermediates (Scheme 1, pathway e).[15] Inspired by these results, we envisioned that amide intermediate II, which could be obtained through the transition-metal catalyzed oxidative amidation of various aldehydes with 2-aminobenzenethiols I derived from benzothiazoles,[14,16] might undergo further intramolecular condensation to form the corresponding 2-aryl- and 2-alkylsubstituted benzothiazoles (Scheme 2).
Scheme 1. Major routes to 2-substituted benzothiazoles.
∗ Hui-Jun Zhang. Tel.: +86-592-218-6295; fax: +86-592-218-6295; e-mail: [email protected],cn ∗ Ting-Bin Wen. Tel.: +86-592-218-6085; fax: +86-592-218-6295; e-mail: [email protected]
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Tetrahedron reaction of 4-nitrobenzaldehyde 2e with benzothiazole 1a afforded the desired product 3ae in only 24% yield. Notably, aliphatic aldehydes are also favorable substrates for this reaction process (3ai-ak), which represent an advantage of this catalytic system. Moreover, substituents on the phenyl ring of benzothiazoles had some effects on the reaction yields (3ba-ga). Table 2. Investigation of the substrate scope.a)
Scheme 2. A new route to 2-substituted benzothiazoles. Herein, we report the efficient and solvent-free CuCl2/TBHP promoted redox condensation of benzothiazoles with both aromatic and aliphatic aldehydes or benzylic alcohols under mild reaction conditions. We initially explored the reaction of benzothiazole 1a with pmethylbenzaldehyde 2a in the presence of 0.3 equiv of CuCl 2 and 1.4 equiv of TBHP.[16] To our delight, the condensation reaction between 1a and 2a did occur at 80 oC in toluene affording the 2aryl benzothiazole 3aa in 62% yield after 36 h (Table 1, entry 1). Screening with different solvents suggested that the reaction proceeded much faster without solvent, and 3aa was obtained in 71 and 80% yields after 12 h and 24 h respectively (Table 1, entry 2-5). Control experiments showed that no reaction took place in the absence of either CuCl2 or TBHP (Table 1, entry 67). Increasing the reaction temperature to 100 oC led to the formation of 3aa in only 39% yield (Table 1, entry 8). Several other transition metal catalysts, such as Cu(OAc)2, CuCl, and NiCl2, could also promote the same transformation, whereas they proved to be much less efficient than CuCl2 (Table 1, entry 9-11). In addition, the amount of CuCl2 is crucial for the process, and the reaction in the presence of either 0.2 or 0.4 equiv of CuCl2 gave 3aa in lower yield.
Table 1. Optimization of the reaction conditions.a)
Entry Cat. t (h) Solvent Yield (%)b) CuCl2 toluene 62 1 36 CuCl2 chloroform 74 2 36 CuCl2 DCE 59 3 36 CuCl2 71 4 12 CuCl2 80 5 24 0 6 24 CuCl2 0 7c) 24 CuCl2 39 8d) 24 Cu(OAc)2 24 8 9 CuCl 33 10 24 NiCl2 39 11 24 a) Reaction conditions: 1a (1.86 mmol), 2a (1.69 mmol), 70% TBHP (2.37 mmol), catalyst (0.3 equiv), solvent (2 mL) or without solvent, 80 oC. b) Yield of 3aa. c) Reaction in the absence of TBHP. d) Reaction at 100 oC. With the optimized reaction conditions in hand, we then investigated the scope of this transformation with various aldehydes and several benzothiazoles (Table 2). Both electronrich and electron-deficient aromatic aldehydes 2b-d and 2f-h reacted with benzothiazole 1a producing the desired products 3ab-ad and 3af-ah in moderate to excellent yields. However, the
a)
Reaction conditions: 1 (1.86 mmol), 2 (1.69 mmol), 70% TBHP (2.37 mmol), and CuCl2 (0.51 mmol), 24 h, 80 oC. b) tBuOH (2 mL). It has been reported that, under oxidative conditions, alcohols could be directly used instead of aldehydes in several reactions.[4,17] Consequently, the direct reactions of benzylic alcohols 4 with benzothiazole 1a were conducted, which led to the formation of 3aa, 3ab, and 3al-ao in moderate yields (Table 3). Table 3. Arylation of benzothiazoles with several benzylic alcohols.
a) Reaction conditions: 1a (1.86 mmol), 2 (1.69 mmol), 70% TBHP (4.06 mmol), CuCl2 (0.51 mmol), without solvent, 80 oC, 24 h. b) chloroform (2 mL).
To understand the reaction mechanism, several experiments were performed. Initially the reaction between 4methylbenzoaldehyde 2a and 2-aminothiophenol 5 under standard reaction conditions was conducted giving rise to the formation of 3aa in 77% yield (eq 1), which indicated that the transformation might proceed through a ring-opening pathway.13 Furthermore, a careful study on the reaction between 1a and 2a suggested that a disulfide intermediate 6 was formed in the
3 beginning and could be separated in 55% yield after 40 min (for the crystal structure of 6, see supporting information), which was then converted to 3aa in 88% yield in the presence of 0.3 equiv of CuCl2 and 1.4 equiv TBHP in t-BuOH under N2 (Scheme 3). Notably, control experiments showed that, in the absence of 4methylbenzoaldehyde 2a or using TBHP in decane instead of TBHP aqueous solution, no ring-opening products from benzothiazole 1a were detected at all. Further investigation on the direct reaction of 2-aminopenyl disulfide 7 with 2a under similar reaction conditions (eq 2) indicated that CuCl2/TBHP could also be an efficient catalytic system for the construction of 2-substituted benzothiazoles starting from stable 2-aminophenyl disulfides and aldehydes. Based on the above observations, a plausible mechanism can be proposed as shown in Scheme 4. Initially, the attack of acyl radical on the thiazole ring led to the formation of intermediate A which then reacted with water to form intermediate B.[16,18] The ring-opening of B followed by the oxidation of intermediate C gave the disulfide intermediate D.[9,18,19] The cleavage of the disulfide and subsequent intramolecular condensation of intermediate D were expected to afford the desired 2-substituted benzothiazole.[19-21]
benzothiazoles. Anilide disulfides instead of aldimine were formed as intermediates, which might explain the efficient condensation between even aliphatic aldehydes and benzothiazoles. Moreover, CuCl2/TBHP proved also efficient in the condensation of 2-aminophenyl disulfides with aldehydes. Further investigation on the scope, exact mechanism and applications of this method are underway in our laboratory. Experimental Section Typical procedure Benzothiazole 1 (1.86 mmol, 1.1 equiv), aldehyde 2 (1.69 mmol) or benzylic alcohol 4 (1.69 mmol), TBHP (2.36 or 4.06 mmol, 1.4 or 2.4 equiv, 70% solution in water), CuCl 2 (0.51 mmol, 0.3 equiv) were placed in a 25-mL reaction vessel fitted with a magnetic stirring bar under nitrogen. The reaction mixture was first stirred in an ice bath for 0.5 h, and then stirred at 80 oC for 24 h. After cooling to room temperature, the crude product was purified by column chromatography to afford the 2-substituted benzothiazoles 3.
Acknowledgements This work was financially supported by Natural Science Foundation of China (Nos. 21072161 and 21302157), National Basic Research Program of China (973 Program, No. 2012CB821600), Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT), and the Fundamental Research Funds for the Central Universities.
References and notes 1.
Scheme 3. A detailed reaction study.
H2N
S S
NH2
7
CuCl2 (30 mol%) 70%TBHP (1.4 equiv)
N
+
CHCl3, N2, 80 oC
S
Me
CHO
Me (2)
3aa, 76%
2a
2.
Scheme 4. Tentative mechanism. In summary, we have developed a practical copper-catalyzed redox approach for the formation of 2-aryl- or 2-alkyl-substituted benzothiazoles starting from aldehydes and benzothiazoles under mild reaction conditions. The involvement of TBHP as an oxidant also allowed the direct reaction of benzylic alcohols with
3.
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