Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Synthesis of Chiral Bispirotetrahydrofuran Oxindoles by Cooperative Bimetallic-Catalyzed Asymmetric Cascade Reaction Meng-Meng Liu, Xiao-Chao Yang, Yuan-Zhao Hua, Jun-Biao Chang,* and Min-Can Wang* College of Chemistry and Molecular Engineering, Zhengzhou University, 100 Science Avenue, Zhengzhou, Henan 450001, People’s Republic of China
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S Supporting Information *
ABSTRACT: A new, efficient route for the enantioselective construction of bispirotetrahydrofuran oxindoles is described via the cooperative dinuclear zinc−AzePhenol catalyst. Under mild conditions, a broad range of bispirotetrahydrofuran oxindoles have been synthesized with excellent stereoselectivities through the cascade Michael/hemiketalization/Friedel−Crafts reaction of β,γ-unsaturated α-ketoamide and 2-hydroxy-1-indanone. The reaction can be performed on a gram scale with low catalyst loading (2 mol %) without impacting its efficiency.
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Scheme 1. Strategies for the Catalytic Asymmetric Synthesis of Bispirotetrahydrofuran Oxindoles
symmetric catalytic cascade reactions have been one of the most powerful and economical methods to prepare complex chiral molecules.1 Although organocatalysts have received much more attention in this field,2 chiral metal catalysts still retain an indispensable role due to their special ability to activate a broad scope of chemical bonds.3 Among these elegant chiral metal catalysts, bimetallic cooperative catalysts endowed with a synergistic activation mode have been intensively investigated because of the advantages in terms of catalytic activity, stereoselectivity, and substrate generality.4 However, the utility of bimetallic cooperative catalysts in asymmetric catalytic cascade transformations. is very limited.5 In order to enrich the application of bimetallic cooperative catalysts in asymmetric catalytic cascade reactions, continuing research is still necessary and valuable. Spirooxindoles are ubiquitous among many natural products and biologically active molecules.6 Optically active spirooxindoles containing a tetrahydrofuran framework have been proven to have anticancer activity.7 In this context, efficient approaches to build chiral spirotetrahydrofuran oxindoles are extremely significant. Nevertheless, catalytic asymmetric methods for the synthesis of spirotetrahydrofuran oxindoles are quite rare.8 In addition, all of the current progress relied on the transformations of substrates bearing a oxindole motif such as isatin,8a,b 3-hydroxy-2-oxindoles,8c or 3-alkylidene-2-oxindols.8d Unfortunately, only one example was included on how to obtain spirotetrahydrofuran oxindoles possessing two spirostereocenters. In the presence of squaramide organocatalyst, spirotetrahydrofuran bispirooxindoles were obtained through the catalytic asymmetric reaction of 3-substituted oxindole and 3-olefinic oxindoles (Scheme 1a).8d Therefore, it is highly desirable to develop a new catalytic system and new © XXXX American Chemical Society
methodology for the enantioselective construction of chiral spirotetrahydrofuran oxindoles, especially the chiral bispirotetrahydrofuran oxindoles with two spiro-stereocenters, from substrates which do not contain a oxindole skeleton. In recent years, our work has focused on the application of dinuclear zinc cooperative catalysts in catalytic asymmetric synthesis.9 As part of our ongoing efforts toward these catalysts, herein we demonstrate a flexible and efficient protocol promoted by dinuclear zinc cooperative catalyst for the catalytic synthesis of bispiro[3,2′-tetrahydrofuran-5′,2′′indanone] oxindoles through the cascade reaction of β,γunsaturated α-ketoamide and 2-hydroxy-1-indanone (Scheme 1b). Notably, compared with reported ways, the spirooxindole Received: January 30, 2019
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DOI: 10.1021/acs.orglett.9b00386 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
entries 9−12). In particular, tetrahydrofuran gave the best result (76% yield, 8.0:1 dr, 88% ee) (Table 1, entry 12). Temperature proved to have a great effect on the ee value and yield (Table 1, entries 13−16). Lowering the temperature from 25 to 0 °C, higher ee value (95%) was obtained (Table 1, entry 14). However, the stereoselectivities as well as the yields were reduced when the temperature was further decreased to −10 °C and −20 °C, even if the reaction time was prolonged (Table 1, entries 15−16). Furthermore, adding Et3N as an additive led to significant advance in diastereoselectivity, the ee value was up to 98% with the dr value of up to 17:1 (Table 1, entry 17). Further optimization, including the loading of L1c, the different kinds of additive, and the amount of additive were also screened (For details, see Supporting Information, Table S1). With the optimized reaction conditions in hand, we explored the generality of this reaction (Scheme 2). The investigation began with the evaluation of substituents on the nitrogen atom of β,γ-unsaturated α-ketoamide. First, the effect of R1 groups was examined. It was found that those ketoamides bearing NMe, N-Et, and N-Bn are well tolerated, giving the corresponding products (3a−c) with 12:1−18:1 dr values and 97−98% ee. Then the influence of substituents R2 was investigated. Under optimized conditions, the substrates containing electron-withdrawing groups or electron-donating groups at the para-position of the benzene ring reacted smoothly with 2a, providing the expected products (3d−h) with high diastereoselectivities (up to dr >20:1) and enantioselectivities (up to 98% ee). Next, we evaluated the R3 substituents on β,γ-unsaturated αketoamide. Both electron-withdrawing groups (−F, −Cl, −Br) and electron-donating groups (−Me, −OMe) on the benzene ring performed well under the same reaction conditions and delivered the corresponding products (3i−k, 3m−q) in good diastereoselectivities (up to >20:1) and excellent enantioselectivities (up to 98% ee). It should be pointed out that substrate 1l with a strong electron-withdrawing group (−NO2) showed relatively lower reactivity and led to a decreasing of both yield and enantioselectivity (32% yield, 87% ee). Moreover, by varying the R3 groups to 2-naphthyl and piperonyl, the reaction proceeded well under the reaction conditions, affording products 3r and 3s in 10:1−21:1 dr values and 95−99% ee. Besides, this protocol could be also broadened to heterocyclic substrates and provided the products (3t and 3u) with excellent stereoselectivities. Substrates 1v and 1w with two or more Br groups were tested under the same conditions, and the corresponding products 3v and 3w were obtained in excellent ee values. In addition, the relative and absolute configuration of product 3q was determined unambiguously by X-ray crystallographic analysis (2′S,4′S,5′R). Finally, we investigated the influence of substituents R4 on 2hydroxy-1-indanone (Table 2). 2-Hydroxy-1-indanone bearing either electron-withdrawing groups or electron-donating groups at the 5-position of the benzene ring were successfully employed to give the desired products with excellent enantioand diastereoselectivities (Table 2, entries 1−4). When 4bromo-substituted 2-hydroxy-1-indanone 2f was examined, the corresponding product was produced with slightly lower enantioselectivity (89% ee) (Table 2, entry 5). 6-Bromosubstituted 2-hydroxy-1-indanone 2g could also participate in the transformation to furnish the product with >20:1 dr value and 93% ee (Table 2, entry 6).
motif of products is built spontaneously from easy prepared linear substrates. In the initial investigation, β,γ-unsaturated α-ketoamide 1a and 2-hydroxy-1-indanone 2a were employed as the model substrates in the presence of 10 mol % of ligands L1a and 20 mol % of ZnEt2 in dichloromethane at room temperature. To our delight, the devised reaction proceeded smoothly and delivered the desired product 3a with moderate yield and stereoselectivity (Table 1, entry 1). Encouraged by this Table 1. Optimization of Reaction Conditionsa
entry
ligand
solvent
temp (°C)
yieldb (%)
drc
eed (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17e
L1a L1b L1c L2a L2b L2c L2d L2e L1c L1c L1c L1c L1c L1c L1c L1c L1c
CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 toluene 1,4-dioxane CH3CN THF THF THF THF THF THF
25 25 25 25 25 25 25 25 25 25 25 25 40 0 −10 −20 0
75 59 79 56 69 61 72 50 75 68 74 76 85 74 53 32 73
3:1 3:1 3:1 4:1 3:1 3:1 5:1 4:1 6:1 8:1 7:1 8:1 7:1 8:1 6:1 5:1 17:1
65 32 69 2 9 37 50 41 70 76 85 88 76 95 90 82 98
a
Unless otherwise noted, all reactions were conducted with 0.275 mmol of 1a, 0.25 mmol of 2a, 10 mol % of ligands, and 20 mol % of ZnEt2 in solvent (2.0 mL). After evaporation of the solvent, DCM (4.0 mL) and TFA (2.0 mL) were added. bIsolated yield. c Determined by 1H NMR analysis. dDetermined by chiral HPLC analysis. e2.0 equiv of E3N was added as an additive.
promising result, we then screened a series of chiral ligands with different substituents and backbones to improve the stereoselectivities (Table1, entries 2−8). The reaction proceeded well and afforded the corresponding products with moderate diastereoselectivities and different levels of enantioselectivities. The results demonstrated that ligand L1c with an electron-deficient group showed a better performance in terms of enantioselectivity than ligand L1b with an electronrich group (Table 1, entries 2 vs 3). However, Trost’s ligand10 L2 bearing a similar structure gave the products with up to only 50% ee under the same conditions (Table 1, entries 4−8). Considering the enantioselectivity of the reaction, we choose L1c as the optimal chiral ligand for further optimization. Subsequently, the effect of solvents was evaluated. Commonly used solvents, such as toluene, 1,4-dioxane, CH3CN, and THF, were tolerated in this reaction (Table 1, B
DOI: 10.1021/acs.orglett.9b00386 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 2. Substrate Scope of 2-Hydroxy-1-indanonea
Scheme 2. Substrate Scope of β,γ-Unsaturated αKetoamidea
entry
R4
product
yieldb (%)
drc
eed (%)
1 2 3 4 5 6
5-F 5-Cl 5-Br 5-OMe 4-Br 6-Br
3ab 3ac 3ad 3ae 3af 3ag
75 70 77 76 75 68
>20:1 >20:1 >20:1 >20:1 >20:1 >20:1
97 95 94 96 89 93
a
Unless otherwise noted, all reactions were conducted with 0.275 mmol 1a, 0.25 mmol 2, 10 mol % ligand L1c and 20 mol % ZnEt2, in THF (2.0 mL) at 0 °C. After evaporation of the solvent, DCM (4.0 mL) and TFA (2.0 mL) were added. bIsolated yields. cThe dr values were determined by 1H NMR analysis. dThe ee values were determined by HPLC analysis.
Scheme 3. Gram-Scale Reaction and Further Derivatization of Products
and 97% ee. Then further elaborations of the obtained product 3a were investigated. Treatment 3a with hydroxylamine hydrochloride, oxime 4 was achieved in almost quantitative yield with retention of the stereogenic center. Further diastereoselective reduction of 3a with NaBH4 resulted in the formation of alcohol 5 possessing three contiguous stereocenters in 96% yield with 20:1 dr and 97% ee, and the absolute configuration of product 5 was determined by X-ray crystallographic analysis. In order to gain insight into the reaction mechanism, a nonlinear effect was also investigated (see the Supporting Information, Figure S1). The correlation between the ee values of ligand L1c and the ee values of product 3a was carefully examined, and a positive nonlinear effect11 was observed in the reaction of β,γ-unsaturated α-ketoamide with 2-hydroxy-1indanone. The results suggested that when ligand L1c was treated with 2 equiv of ZnEt2 there was a dynamic balance between the catalytically active species monomeric Zn2EtL1c and the inactive species oligomeric aggregates composed of monomeric Zn2EtL1c, which could lead to the generation of a nonlinear effect.
a
Unless otherwise noted, all reactions were conducted with 0.275 mmol of 1, 0.25 mmol of 2a, 10 mol % of ligand L1c, and 20 mol % of ZnEt2 in THF (2.0 mL) at 0 °C. After evaporation of the solvent, DCM (4.0 mL) and TFA (2.0 mL) were added. Isolated yields. The ee values were determined by HPLC analysis. The dr values were determined by 1H NMR analysis.
To further measure the synthetic applicability of this methodology for constructing chiral bispiro[3,2′-tetrahydrofuran-5′,2′′-indanone] oxindoles, the reaction was performed on a gram scale (Scheme 3). β,γ-Unsaturated α-ketoamide 1a reacted smoothly with 2-hydroxy-1-indanone 2a, although the catalyst loading was reduced to only 2 mol %, affording the corresponding product 3a in 74% yield (1.02 g) with 16:1 dr C
DOI: 10.1021/acs.orglett.9b00386 Org. Lett. XXXX, XXX, XXX−XXX
Organic Letters
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On the basis of the above results and the previous reports,9b,c a proposed catalytic cycle is depicted in Scheme 4. After the generation of dinuclear zinc−ligand monomeric
Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00386. Screening tables, experimental procedures, compound characterization data, NMR spectra, and HPLC traces (PDF)
Scheme 4. Proposed Catalytic Cycle
Accession Codes
CCDC 1550061 and 1887100 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Min-Can Wang: 0000-0002-3817-3607 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (NNSFC: 21871237, NNSFC: 81330075).
ZnEtL1c from ligand L1c and diethylzinc, coordination and deprotonation of nucleophilic 2-hydroxy-1-indanone 2a afforded intermediate 6. Subsequently, electrophilic β,γunsaturated α-ketoamide 1a is activated by zinc−oxygen coordination to give 7 from the most sterically accessible site. Then, the complex 7 undergoes Michael addition reaction to form intermediate 8 with the observed stereochemistry. Finally, proton transfer with another 2-hydroxy-1-indanone could release the Michael addition product 9 and restart the catalytic cycle. Compound 9 spontaneously partially transforms into its hemiketal product 10.12 Eventually, the desired product 3a is obtained with complete diastereocontrol by the intramolecular Fridel−Crafts alkylation reaction of 9 and 10 under acidic condition. In summary, we have developed an efficient bimetallic cooperative catalytic cascade Michael/hemiketalization/Friedel−Crafts reaction of β,γ-unsaturated α-ketoamide with 2hydroxy-1-indanone. This strategy allows the formation of a range of structurally novel bispiro[3,2′-tetrahydrofuran-5′,2′′indanone] oxindoles with three stereocenters, including two spiro-quaternary chiral centers, in excellent diastereo- and enantioselectivities under mild conditions. The reaction is highly step-economic and can be run on a gram scale with only 2 mol % catalyst loading without impacting its efficiency. Moreover, the product can be further elaborated to molecules possessing three contiguous chiral centers with a diastereoselective reduction. Furthermore, a positive nonlinear effect is observed. Finally, a possible mechanism of the reaction is proposed. The biological evaluation of these compounds is the major goal of our future efforts.
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DOI: 10.1021/acs.orglett.9b00386 Org. Lett. XXXX, XXX, XXX−XXX
