Organocatalyzed Dearomative Cycloaddition of 2-Nitrobenzofurans

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Organocatalyzed Dearomative Cycloaddition of 2‑Nitrobenzofurans and Isatin-Derived Morita−Baylis−Hillman Carbonates: Highly Stereoselective Construction of Cyclopenta[b]benzofuran Scaffolds Jian-Qiang Zhao,† Lei Yang,‡,§ Xiao-Jian Zhou,‡,§ Yong You,† Zhen-Hua Wang,† Ming-Qiang Zhou,‡ Xiao-Mei Zhang,‡ Xiao-Ying Xu,‡ and Wei-Cheng Yuan*,‡ †

Institute for Advanced Study, Chengdu University, Chengdu 610106, China National Engineering Research Center of Chiral Drugs, Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China § University of Chinese Academy of Sciences, Beijing 100049, China

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ABSTRACT: The first organocatalyzed asymmetric dearomative cycloaddition between 2-nitrobenzofurans and isatin-derived Morita−Baylis−Hillman carbonates has been developed. Using a modified cinchona alkaloid as the catalyst, a series of structurally diverse cyclopenta[b]benzofuran derivatives with three contiguous stereocenters, including a spiro-quaternary chiral center, could be smoothly obtained in excellent results (all cases >20:1 dr, up to 99% yield and 98% ee). The utility of this method was showcased by the versatile transformations of the product.

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Scheme 1. Previous Studies on the Asymmetric Dearomative Cycloaddition of 2-Nitrobenzofurans with Transition-Metal Catalysts and Organocatalysts Described Herein

earomatization processes involving cycloadditions have emerged as one type of highly reliable strategy for the generation of polycyclic three-dimensional molecules from readily available planar aromatic compounds.1 Many efforts have been devoted to the development of efficient methods for the asymmetric dearomative cycloaddition to access the structure complicated and well-defined chiral molecules.2 The vast majority of the existing reports typically focus on the dearomative cycloadditions of electron-rich aromatic substrates, such as indoles, pyrroles, naphthols, and phenols.3 In contrast, the study of electron-deficient aromatic substrates for the corresponding transformations remains relatively underdeveloped, although the electron-deficient aromatics, such as nitroindoles and nitrobenzofurans, can be readily available. However, in this research area, abundant examples are primarily about the asymmetric dearomative cycloaddition of 3-nitroindoles.4−6 It is especially noteworthy that there were only two reports about the enantioselective dearomative cycloaddition of 2-nitrobenzofurans. In 2017, You and coworkers described the first Pd-catalyzed asymmetric dearomative [3 + 2] cycloaddition of 2-nitrobenzofurans (Scheme 1, (1)).7 After that, our group reported Zn-catalyzed asymmetric dearomative [3 + 2] cycloaddition of 2-nitrobenzoheterocycles with 3-isothiocyanato oxindoles for generating structurally diverse spirooxindoles (Scheme 1, (2)).6,8 And, more remarkable, stereoselective dearomative cycloaddition of 2nitrobenzofurans by organocatalyst is undefined so far, even though the organocatalytic asymmetric cycloaddition reactions have attracted considerable attention over the past decade because of their high efficiency and excellent stereoselectivity in the construction of elaborated polycyclic skeletons.9 In this © XXXX American Chemical Society

context, the realization of asymmetric organocatalytic dearomative cycloaddition of 2-nitrobenzofurans remains highly desirable and an important goal. The chiral cyclopenta[b]benzofuran ring system is commonly found as the core in a wide variety of biologically active natural products and pharmaceuticals (Figure 1).10 Their immense biological importance continually inspired organic chemists to develop new strategies for the construction of such Received: November 27, 2018

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DOI: 10.1021/acs.orglett.8b03786 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Table 1. Optimization of Reaction Conditionsa

Figure 1. Bioactive compounds containing the cyclopenta[b]benzofuran core.

oxygen-containing tricyclic scaffolds.11 However, effective methods are limited because of the difficulty of construction of multiple continuous stereogenic carbon centers with high stereoselectivity via one-step ring-closing processes. Therefore, developing efficient and powerful asymmetric approaches to access these tricyclic scaffolds is an active target extensively pursued by chemists. On the other hand, the isatin-derived Morita−Baylis−Hillman (MBH) carbonates have proved to be a class of highly reactive three-carbon synthons for various cycloaddition reactions catalyzed by Lewis bases to give spirocyclic oxindoles,12 which are ubiquitously occurring heterocyclics found in many alkaloid natural products. As part of our research program aimed at developing new methods for the asymmetric dearomative cycloaddition of electron-deficient aromatics,5,6,8 we reasoned that an asymmetric reaction between 2-nitrobenzofurans and isatin-derived MBH carbonates catalyzed by organocatalyst could occur through a dearomative cycloaddition process, thereby forming cyclopenta[b]benzofuran scaffolds with three contiguous stereocenters, including a spiro-quaternary stereocenter (Scheme 1, (3)). Moreover, these products are characterized by an intriguing fusion of two privileged motifs, including the cyclopenta[b]benzofuran and spirocyclic oxindole substructures, that makes them potentially promising candidates for drug discovery. Herein, we attempt to address our preliminary studies on this subject by presenting the first organocatalytic dearomatization process involving the cycloaddition of 2nitrobenzofurans. We initiated our investigation with the reaction of 2nitrobenzofuran 1a and isatin-derived MBH carbonate 2a in CH2Cl2 at 18 °C for screening catalysts. As shown in Table 1, in the presence of β-isocupreidine (β-ICD, A),13 the reaction gave the dearomative cycloaddition product 3a with excellent diastereoselectivity and 71% ee, but in only 39% yield (Table 1, entry 1). And then with the methoxyl protected β-ICD B as a catalyst, 3a could be obtained in 87% yield with >20:1 dr and 87% ee (Table 1, entry 2). Using cinchonidine-derived C as a catalyst, no obvious improvement was observed in the enantioselectivity (Table 1, entry 3). To our delight, changing the hydroxyl group of β-ICD to phenyl group, the corresponding catalyst D could provide 3a in 90% yield with >20:1 dr and 97% ee (Table 1, entry 4). However, the naphthyl-substituted catalyst E gave 3a in slightly reduced yield, without fluctuation in the dr and ee values (Table 1, entry 5). With D as the selected catalyst, experiments were carried out with different solvents, and it revealed that CHCl3 was better than other solvents (Table 1, entry 6 vs entries 7−9 and 4). Lowering the reaction temperature to 0 °C, the reaction gave 3a in 97% yield with >20:1 dr and 98% ee (Table 1, entry 10). Decreasing the catalyst loading from 10 to 5 mol %, 3a was isolated in a reduced yield along with a prolonged reaction time, but the dr and ee values were excellent (Table 1,

entry

cat.

solvent

time (h)

yieldb (%)

drc

eed (%)

1 2 3 4 5 6 7 8 9 10e 11e,f 12e,g

A B C D E D D D D D D D

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CHCl3 DCEh toluene MTBEi CHCl3 CHCl3 CHCl3

26 18 9 23 23 18 18 24 24 48 72 60

39 78 86 90 81 94 79 71 45 97 86 94

>20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1

71 87 88 97 96 97 98 98 98 98 98 98

a

Unless otherwise noted, the reactions were carried out with 1a (0.11 mmol), 2a (0.1 mmol), and 10 mol % of catalyst in 1.0 mL of solvent at 18 °C. bIsolated yield. cDetermined by 1H NMR. dDetermined by chiral HPLC. eRun at 0 °C. f5 mol % catalyst was used. gThe reaction was scaled up to 1.0 mmol for 2a, which is 10 times larger than the scale of the model reaction. hDCE = 1,2-dicholorethane. iMTBE = methyl tert-butyl ether.

entry 11). At this point, a set of optimal reaction conditions as used in entry 10 were established. More importantly, the model reaction could be scaled up to 1.0 mmol for 2a, and the product 3a could be obtained in 94% yield without compromising on the diastereo- and enantioselectivity (Table 1, entry 12). With the optimized conditions in hand, different 2nitrobenzofurans were first tested by reacting with isatinderived MBH carbonate 2a. As summarized in Scheme 2, all of the reactions catalyzed by D could afford their corresponding products with excellent diastereoselectivities and enantioselectivities (>20:1 dr, 95−98% ee). It was found that the reactivity and stereoselectivity were almost unaffected by the incorporation of varied electron-withdrawing substituents at different positions on the aryl ring of 2-nitrobenzofurans (products 3b−g). Meanwhile, this protocol also allowed the diverse electron-donating substituents at different positions on the aryl ring, affording the corresponding products in virtually pure stereoisomers with very high yields (products 3h−l). The more sterically hindered 2-nitronaphtho[2,1-b]furan was also proved to be amenable to this developed protocol, and the expected product 3m was obtained in 53% yield with >20:1 dr and 95% ee. The absolute configuration of product 3c was assigned as (C7R,C8R,C11S) with single-crystal X-ray analysis (see Scheme 5). The stereochemistry of the other products was assigned by analogy. Further exploration of the substrate scope was focused upon isatin-derived MBH carbonates by reacting with 1a (Scheme 3). Changing the methyl group substitution of N-1 in 2a to an ethyl or benzyl group, the corresponding dearomative cycloaddition reactions could proceed smoothly and provided products 3n and 3o with acceptable results. These results suggest that increasing the size of the N-protecting group had B

DOI: 10.1021/acs.orglett.8b03786 Org. Lett. XXXX, XXX, XXX−XXX

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in good to excellent yields (87−98%) with excellent diastereoselectivities (>20:1 dr) and ee values (97−98% ee) (products 3p−v). In addition, the ester moiety of MBH carbonates could be varied from ethyl ester to methyl ester, and the reaction gave 3w with the same excellent results as for ethyl ester substrate. Importantly, MBH carbonate derived from isatin and acrylonitrile also furnished product 3x with excellent diastereoselectivity and high enantioselectivity, but in moderate yield, probably due to the easy decomposition of substrate 2x under the standard reaction conditions. Although the reaction of isatin-derived MBH carbonate 2a with 1a proceeded very well through the asymmetric dearomative cycloaddition process with the developed catalytic system, unfortunately, we found that 2a was not able to react with 3-methyl-2-nitrobenzofuran 4 under the standard conditions, probably due to the steric hindrance at the C3position of 4 (Scheme 4). Nevertheless, we also observed that

Scheme 2. Substrate Scope of Different 2-Nitrobenzofurans with 2aa

Scheme 4. Reactions of 2a with Different 2Nitrobenzoheterocycles under the Standard Conditions

a

Reaction conditions: The reactions were carried out with 1 (0.11 mmol), 2a (0.1 mmol), and 10 mol % catalyst D in 1.0 mL of CHCl3 at 0 °C. The dr values were determined by 1H NMR, and the ee values were determined by chiral HPLC.

the reactions of 2a with other 2-nitrobenzoheterocycles, such as 2-nitroindole 5 and 2-nitrobenzothiophene 6, did not occur under the standard conditions. These suggest that 2-nitroindole and 2-nitrobenzothiophene had a significantly lower reactivity than 2-nitrobenzofuran on the dearomative cycloaddition with isatin-derived MBH carbonates (Scheme 4). In order to demonstrate the synthetic value of this methodology, we explored various transformations of product 3a to give some key heterocyclic compounds (Scheme 5).14

Scheme 3. Substrate Scope of 1a with Different IsatinDerived MBH Carbonatesa

Scheme 5. Different Transformations of Compound 3a and the X-ray Crystal Structure of 9 and 3c

a Reaction conditions: The reactions were carried out with 1a (0.11 mmol), 2 (0.1 mmol), and 10 mol % catalyst D in 1.0 mL of CHCl3 at 0 °C. The dr values were determined by 1H NMR, and the ee values were determined by chiral HPLC.

Treating optically pure 3a with tributyltin hydrogen and AIBN in toluene, the radical denitration process provided compound 7 in 70% yield without loss of the diastereo- and enantioselectivity. The rearomatization compound 8 was obtained with 56% yield and >99% ee from 3a through the DBU-promoted elimination of nitrous acid. The dihydroxylation of 3a could be conducted by using ruthenium trichloride and sodium periodate, and the corresponding product 9 containing five contiguous chiral centers was obtained in 58% yield as a virtually pure stereoisomer. The

almost no influence on the reactivity and stereoselectivity. Moreover, by installing an electron-donating or electronwithdrawing group into the oxindole skeleton of MBH carbonates, regardless of their electronic nature and the positions at the aryl ring, the asymmetric dearomative cycloaddition reactions also could afford the desired products C

DOI: 10.1021/acs.orglett.8b03786 Org. Lett. XXXX, XXX, XXX−XXX

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absolute configuration of 9 was determined to be (C2S,C16S,C17S,C18S,C19S) by single-crystal X-ray analysis. On the basis of previous literature reports and our own results,12 a proposed catalytic cycle for the asymmetric dearomative cycloaddition process is outlined in Scheme 6.

CCDC 1821158 and 1828916 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.

Scheme 6. Proposed Catalytic Cycle for the Organocatalyzed Asymmetric Dearomative Cycloaddition of 2-Nitrobenzofurans

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AUTHOR INFORMATION

Corresponding Author

*E-mail:[email protected]. ORCID

Wei-Cheng Yuan: 0000-0003-4850-8981 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National NSFC (Nos. 21572223, 21572224, 21602217, 21871252), Sichuan Youth Science and Technology Foundation (2016JQ0024), and the Start-up Fund of Chengdu University (2081916044)

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Catalyst D (*NR3) acting as tertiary amine first attacked the MBH carbonate 2 to generate the nucleophilic allylic nitrogen−ylide intermediate I with release of CO2 and t BuOH. Then the C3-position of 2-nitrobenzofuran 1 was attacked from the Re face by the intermediate I to form intermediate II. Subsequently, an intramolecular cyclization in intermediate II occurred to give the intermediate III, which could release catalyst D and thus furnish the expected dearomative cycloaddition product 3 with high stereocontrol. In conclusion, we have developed the first organocatalyzed asymmetric dearomative cycloaddition between 2-nitrobenzofurans and isatin-derived MBH carbonates. Using a modified cinchona alkaloid catalyst, structurally diverse cyclopenta[b]benzofuran derivatives, bearing three contiguous stereocenters including a spiro-quaternary chiral center, could be smoothly obtained with excellent results (up to 99% yield, 98% ee and all cases >20:1 dr) under mild conditions. The reaction displays a general scope for both 2-nitrobenzofurans and isatin-derived MBH carbonates. The synthetic value of the protocol has also been demonstrated by the versatile transformations of the product into other key heterocyclic compounds. Notably, this methodology represents the first example regarding to organocatalytic enantioselective reactions of 2-nitrobenzofurans. Further investigations employing 2-nitrobenzoheterocycles (2-nitrobenzofurans, 2-nitroindoles, and 2-nitrobenzothiophenes) in asymmetric dearomative reactions are currently being pursued in our laboratories and will be published in due course.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03786. Experimental details, characterization data for new compounds, and X-ray crystal structures of 3c and 9 (PDF) D

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DOI: 10.1021/acs.orglett.8b03786 Org. Lett. XXXX, XXX, XXX−XXX