Accepted Manuscript Multicatalytic Beckmann Rearrangement of 2-Hydroxylarylketone Oxime: Switchable Synthesis of Benzo[d]oxazoles and N-(2-Hydroxylaryl)amides Zhen Li, Chengtao Fang, Yannan Zheng, Guanyinsheng Qiu, Xiaofang Li, Hongwei Zhou PII: DOI: Reference:
S0040-4039(18)31133-X https://doi.org/10.1016/j.tetlet.2018.09.043 TETL 50282
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Received Date: Revised Date: Accepted Date:
6 July 2018 12 September 2018 17 September 2018
Please cite this article as: Li, Z., Fang, C., Zheng, Y., Qiu, G., Li, X., Zhou, H., Multicatalytic Beckmann Rearrangement of 2-Hydroxylarylketone Oxime: Switchable Synthesis of Benzo[d]oxazoles and N-(2Hydroxylaryl)amides, Tetrahedron Letters (2018), doi: https://doi.org/10.1016/j.tetlet.2018.09.043
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Multicatalytic Beckmann Rearrangement of 2-Hydroxylarylketone Switchable Synthesis of Benzo[d]oxazoles and N-(2-Hydroxylaryl)amides
Zhen Li,a Chengtao Fang, a Yannan Zheng,a Guanyinsheng Qiu,a, Xiaofang Li, b and Hongwei Zhou*,a a b
College of Biological, Chemical Science and Engineering, Jiaxing University, 118 Jiahang Road, Jiaxing 314001, China. School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Hunan 411201, China
A RT I C L E I N F O Article history: Received Received in revised form Accepted Available online
A BS T RA C T A switchable synthesis route is developed for benzo[d]oxazole derivatives and (2hydroxylaryl)benzamide from 2-hydroxylbenzeneketoxime using organomolecules (BOP-Cl, and CNC) and Lewis acid cocatalyzed Beckmann rearrangement (BR) reaction. Further, this reaction is switched using different organocatalysts. 2009 Elsevier Ltd. All rights reserved .
Keywords: Beckmann Rearrangement Switchable Synthesis Benzo[d]oxazoles Multicatalysis 2-Hydroxylarylketone Oxime
As a well-known name reaction, Beckmann rearrangement (BR) represents an important route to synthesize substitutedamides.1 Compared to other synthetic reactions towards amidecontaining compounds, the BR reaction is featured with high atom efficiency, thus received significant attention by the synthetic chemists. The initial endeavour focused intensively on exploiting strong acid-mediated BR transformations.2 The established achievements suggested that a series of inorganic acids enabled the BR reaction. It has been reported that acidassisted removal of hydroxyl group produces nitrene as an intermediate;2f in addition, the removal of hydroxyl group and migration of group at ortho-site was in a stepwise manner.2g Acid-mediated BR reaction produced a mixture of amide with high efficiency, which probably resulted from isomerisation of Z/E oxime in the presence of acid; though it suffered by using acid-resistant reaction equipments. To achieve functional group migration selectivity under mild reaction conditions, it is highly desirable for the BR reaction to design a synthetic route without using strong acids. Recent advances in BR reaction indicated that an array of organocatalysts were engaged in BR reaction.3-10 As shown in Scheme 1, TsCl,3 BOP-Cl,4 CNC,5 TAPC,6 CPI-Cl,7 BAC,8 and DCID9 etc. served as efficient catalysts to promote BR reaction (Scheme 1, eq 1a). Distinctively, the acid-mediated BR reaction was enabled by generating the nitrene as an intermediate, while a self-propagating mechanism11 was postulated mainly for the use Scheme 1 Proposed synthesis route for benzo[d]oxazole of organomolecules promoted BR reaction. More importantly, derivatives via organocatalytic Beckmann rearrangement reaction. G. Qiu, Tel.: +86-0573-83642057; fax: +86-0573-83642057; e-mail: [email protected]
functional group migration in the organocatalytic BR reaction was specific, proceeding in an anti fashion. It is believed that the development of organocatalytic BR reaction represents a huge advance in amide synthesis. Based on progress of organocatalytic BR reaction, numerous useful functional architectures were achieved.12 For example, Wu and co-workers reported the synthesis of indole from 2alkynylarylketoxime through a tandem CNC/InCl3-promoted BR reaction and palladium-catalyzed 5-endo aza-cyclization.12a In light of this finding, our continuous interests in the development of synthetic methodology for organocatalytic BR reaction with privileged structures,13 we design an alternative synthetic route toward benzo[d]oxazole core 2 from 2-hydroxylarylketoxime 1. Particularly, the reaction proceeded through an organocatalytic self-propagating pathway to produce a 2-hydroxylphenylimine cation A (Scheme 1, eq1b). It was expected that the benzo[d]oxazole core was formed via an intramolecular annulation. Sardarian and co-workers reported that the stoichiometric loading of diethyl chlorophosphate ((EtO)2POCl) enabled the synthesis of benzo[d]oxazole core.14 Considering potential usage of benzo[d]oxazole core in many functional molecules15, it is of highly synthetic significance to develop a catalytic version on organomolecular-promoted BR reaction for the synthesis of benzo[d]oxazole core.
and provided the desired product 2a in 81% yield (entry 1, Table 4). It is important mentioned here that the use of CNC catalyst did not give the desired product 2a, instead, surprisingly resulted the product of N-(2-hydroxylphenyl)benzamide 3a in 70% yield (entry 1, Table 3). This interesting outcome probably provided the possibility that switchable synthesis of benzo[d]oxazole 2a and amide 3a could be reached by adopting different organocatalysts and Lewis acid additives. Inferior results were observed when either FeCl3 or AlCl3 was used (entries 5-6, Table 1). Reduction of reaction temperature was not favourable for the BR reaction, but it resulted in the product 2a in 67% yield (entry 7, Table 1). In addition, change of reaction solvent was also not favourable for BR reaction. For instance, the use of DMF and MeCN as solvent, resulted low product yields (entries 8-9, Table 1). A blank experiment was conducted without MgSO4 and displayed the yield of desired product 2a to be 44%, suggesting that the importance of MgSO4 in BR reaction (entry 10, Table 1). Based on the above facts, we optimized the reaction conditions as follows: BOP-Cl (10 mol%), ZnCl2 (10 mol%), MgSO4 (2.0 equiv), toluene, and reflux. Table 2 Synthesis of benzo[d]oxazole derivatives using BOPCl/ZnCl2-catalyst in Beckmann rearrangement reaction a
Table 1 Optimization of organocatalytic Beckmann rearrangement for the synthesis of benzo[d]oxazole derivatives a
yield of 2a (%)a,b
Reaction conditions: 1a (0.2 mmol ), organocatalyst (0.1 equiv), additive (0.1 equiv), 8 h. b Isolated yield based on 2hydroxylbenzoketoxime 1a. c 2.0 equiv MgSO4. d Amide 3a was observed in 70% yield. e no MgSO4 was added.
Based on this ideal strategy, various organocatalysts are initially evaluated and optimized for BR reaction. Tosylating reagent, p-methylbenzenesulfonyl chloride (TsCl) was readily available and employed as an organocatalyst to commence the reaction condition optimization. The results obtained from the model reaction of 2-hydroxylbenzeneketoxime 1a, inspired us to reconsider the reaction (entry 1, Table 1). Enlightened by Yamamoto’s findings,5a we used ZnCl2 as an additional additive when TsCl was selected as an organocatalyst (entry 2, Table 1). To reduce hydrolysis of the starting material 1a, 2.0 equivalent amount of MgSO4 was also added. To our delight, a desired benzo[d]oxazole 2a was achieved as expected in a 20% isolated yield (entry 1, Table 2). With aid of this promising result, other organocatalysts for BR reaction were also screened. Based on the obtained results, BOP-Cl was found to be the best organocatalyst,
Isolated yield based on 2-hydroxylketone oxime 1.
With the optimized reaction conditions, (entry 4, Table 1), we then examined the yield of benzo[d]oxazole derivatives and obtained results as illustrated in Table 2. The substituent R1 could be replaced by aryl and alkyl functional groups. For example, under optimized conditions the reaction of 1-(2hydroxyphenyl)propan-1-one oxime 1b offered benzo[d]oxazole 2b in 87% yield. Next, the substituent R2 functional group tolerance was also highly favourable in BR reaction. For instance, the reaction of 1-(1-hydroxynaphthalen-2-yl)ethan-1-one oxime 1c under optimized conditions delivered a corresponding product 2c in 67% yield. Methyl-, methoxyl-, chloro-, bromo-, and fluoro-substituted substrates were also compatible in BR reaction. Particularly, substrates (2e and 2f) with sensitive hydroxyl group were also merely suitable for BR reaction. For example, the reaction of 1-(2,5-dihydroxyphenyl)ethan-1-one oxime 1e provided the desired product of 2e in 75% yield, while that of 1(2,4-dihydroxyphenyl)ethan-1-one oxime 1f produced the
targeted molecule 2f in 73% yield. As we know, hydroxyl and bromo groups in benzo[d]oxazole core were highly useful for structural elaboration via substitution and transition metalcatalyzed cross-coupling reactions. Table 3 Synthesis of N-(2-hydroxylaryl)amide derivatives using CNC/ZnCl2-catalyst in Beckmann rearrangement reaction a
the organocatalyst B was proposed as a key intermediate in BR reaction. In summary, we have developed a switchable route for organocatalytic Beckmann rearrangement of 2hydroxylbenzeneketoxime using BOP-Cl/ZnCl2 and CNC/ZnCl2. A series of benzo[d]oxazole derivatives and N-(2hydroxylaryl)amides were also synthesized with high reaction efficiency and offered a broad scope in synthetic organic chemistry. It is believed that, the hydroxyl group in final products is useful for structural elaboration through substitution and transition metal-catalyzed cross-coupling reactions.
Isolated yield based on 1.
As mentioned above, the amide product 3a was produced when CNC/ZnCl2 was used as organocatalyst/additive in BR reaction. Recently, Yamamoto and co-workers developed CNC/ZnCl2-promoted BR reaction for the synthesis of amides.5b In particular, the CNC/ZnCl2 system exhibited an excellent reaction efficiency for alkyl ketoxime-based Beckmann rearrangement. However, they5b did not exploit the compatibility of 2-hydroxylbenzeneketoxime derivatives 1 under optimized condition. Therefore, we would like to disclose Yamamoto’s results using 2-hydroxylbenzeneketoxime derivatives 1 as starting materials. The reaction optimization was already presented in entry 3, Table 1 indicated by using 10 mol% CNC/ZnCl2 in toluene and the amide 3a was afforded in 70% yield. Changing solvent from toluene to MeCN at a refluxing temperature (80C), resulted in a drastic improvement of reaction efficiency, leading to the desired amide 3a in a yield of 90%. Consequently, we then explored the reaction tolerance for producing various amides 3 under optimized conditions indicated as follows: CNC (10 mol%), ZnCl2 (10 mol%), MgSO4 (2.0 equiv), MeCN, reflux. As illustrated in Table 3, various 2-hydroxylbenzeneketoxime 1 were compatible for the amide-forming reaction under optimized conditions, and the desired amides 3 were achieved in good yields. Replenishing the results from Yamamoto, the combination of CNC and ZnCl2 also enabled BR reaction of 2hydroxylbenzeneketoxime 1 and it produced N-(2hydroxyphenyl)benzamide 3 when the concentration of CNC/ZnCl2 was increased into from 2 mol%16 to 10 mol%.
Scheme 2 Control experiments To gain insight into the mechanism, two control experiments were further carried out (Scheme 2). As illustrated in Scheme 2,
Financial supports from the Natural Science Foundation of China (Nos: 21502069 and 21772067) is gratefully acknowledged. Authors also gratefully thank Dr. Kannan Palanisamy (Jiaxing University, China) for English editing of this manuscript. References and notes 1.
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Multicatalytic Beckmann Rearrangement of 2-Hydroxylarylketone Oxime: Switchable Synthesis of Benzo[d]oxazoles and N-(2Hydroxylaryl)amides Zhen Li,a Chengtao Fang, a Yannan Zheng,a Guanyinsheng Qiua, Xiaofang Li,b and Hongwei Zhou*a College of Biological, Chemical Science and Engineering, Jiaxing University, 118 Jiahang Road, Jiaxing 314001, China. School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Hunan 411201, China
G. Qiu, Tel.: +86-0573-83642057; fax: +86-0573-83642057; email: [email protected]
Tetrahedron Letters Research Highlights: 1) A BOP-Cl catalyzed Beckmann rearrangement is described toward benzo[d]oxazole. 2) By using CNC as catalyst, N-(2-hydroxylaryl)amides is afforded as well. 3) A switchable synthesis is reached by the use of different organocatalysts.