Enantioselective [3 + 2] Formal Cycloaddition of 1-Styrylnaphthols with

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Enantioselective [3 + 2] Formal Cycloaddition of 1‑Styrylnaphthols with Quinones Catalyzed by a Chiral Phosphoric Acid Wang Feng,† Hui Yang,† Zhe Wang, Bo-Bo Gou, Jie Chen,* and Ling Zhou* Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Department of Chemistry & Materials Science, Northwest University, Xi’an 710127, P. R. China S Supporting Information *

ABSTRACT: The first highly enantioselective [3 + 2] formal cycloaddition of 1-styrylnaphthols (or phenol) with quinones catalyzed by a chiral phosphoric acid has been reported. A class of trans-2,3-diarylbenzofurans were prepared efficiently (up to 99% yield, >20:1 dr, 99% ee). This organocatalytic procedure allows lowering of the catalyst loading to 0.5 mol % without considerable loss in reactivity and enantioselectivity.

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the intrinsic challenge of implementing inverse-electrondemand 1,3-dipolar cycloadditions.6 Only a few examples of the [3 + 2] reaction of N,N’-cyclic azomethine imines and nitrile oxides to N-heterocycles have been reported, and these are due to the pioneering work of the Shi6a and Suga6b groups (Scheme 1, eqs 2, 3). Nonetheless, cycloaddition of ovinylphenols with quinones for the synthesis of chiral benzofurans is, to the best of our knowledge, unprecedented. trans-2,3-Diarylbenzofuran moieties commonly occur in a large number of natural products and drug molecules, such as εviniferin, carasiphenol C, ampelopsin B, and alopecurone C.7 Each display numerous biological actions including antifungal, antiviral, antioxidant, and anticancer activities.8 Consequently, the total synthesis of such natural products and derivatives has attracted great attention from synthetic chemists.9 To date, three main approaches have been established for the construction of 2,3-disubstituted benzofuran skeleta: (i) intramolecular cyclodehydration of alcohols,9,10 (ii) metalcatalyzed intramolecular C−H insertion reactions of donor− donor carbenes,11 including recent enantioselective examples;12 and (iii) [3 + 2] cycloaddition of quinones and alkenes.13−16 Of these transformations, the third strategy is the most straightforward method, because the intermolecular reaction of two molecules provides unique access to numerous benzofuran skeleta. Indeed, many remarkable endeavors have been made to develop [3 + 2] catalytic systems for the cycloaddition of quinones and alkenes, using Lewis acid,14 Brønsted acid,15 and oxidative strategies.16 Additionally, quinone derivatives such as quinone monoacetals, monoimides, and monoimines have also been employed in these reactions.17 However, although numerous protocols have been developed, the catalytic asymmetric construction of 2,3-diarylbenzofurans by this strategy is still lacking.13−17

inylphenols are important organic starting materials in industry and academia, undergoing many useful transformations. 1 In recent years, the catalytic asymmetric functionalization of the double bond in vinylphenol and its derivatives have received considerable attention, because such transformations offer a rapid buildup of molecular complexity from readily available compounds.2−6 o-Vinylphenols are usually used as substrates, because the ortho hydroxy group can participate in the stereocontrol. In addition to the well documented metal-catalyzed functionalization of o-vinylphenols,3 an organocatalytic strategy has also emerged. Elegant approaches include acyclic additions and [2 + n] cycloadditions of o-vinylphenols (e.g., Povarov reaction, Scheme 1, eq 1).4 Among these addition reactions, o-quinone methide intermediates are commonly generated by acid catalyzed protonation of o-vinylphenols.5 On the other hand, the catalytic asymmetric [3 + 2] reaction of o-vinylphenols has been met with less success, it having limited scope and application due to Scheme 1. Cycloaddition of o-Vinylphenols

Received: March 27, 2018

© XXXX American Chemical Society

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DOI: 10.1021/acs.orglett.8b00988 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

Interestingly, excellent yields and enantioselectivities were obtained after longer reaction times (entries 14, 15). Further decreasing the reaction temperature dramatically decreased the reaction efficiency and caused a measurable decrease in ee values (entries 16−18). Having identified the optimum conditions (Table 1, entry 15), we tested this method in a series of substituted (E)-1styrylnaphthols. As shown in Scheme 2, this protocol was

Inspired by our recent work on the asymmetric arylative dearomatization of β-naphthols catalyzed by a chiral phosphoric acid,18 we envisaged that 1-styrylnaphthols bearing an ortho hydroxyl group could enantioselectively attack quinones to generate o-quinone methide intermediates via a chiral phosphoric acid catalyzed dearomatization strategy. Subsequent intramolecular cyclization would then generate 2,3-diarylbenzofurans. Herein, we report the first chiral phosphoric acid catalyzed [3 + 2] cycloaddition of 1,4-quinones with 1styrylnaphthols via in situ generated transient o-quinone methide intermediates, which has resulted in the formation of enantioenriched trans-2,3-diarylbenzofurans. We initially investigated the reaction between (E)-1styrylnaphthol (1a) and 1,4-benzoquinone (2a) using a chiral phosphoric acid as the catalyst (Table 1). To our delight, the

Scheme 2. Substrate Scope of (E)-1-Styrylnaphtholsa

Table 1. Optimization of the Reaction Conditionsa

entry

catalyst

solvent

t (°C)

time (h)

yield (%)b

ee (%)c

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

4a 4b 4c 4d 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e

DCE DCE DCE DCE DCE DCM CHCl3 CCl4 toluene acetone Et2O CH3CN CH3CN DCE DCM DCM DCM DCM

rt rt rt rt rt rt rt rt rt rt rt rt −20 −20 −20 −40 −78 −10

36 36 36 36 36 36 36 36 36 50 50 36 72 60 72 50 96 72

80 90 86 73 98 98 93 90 85 75 78 93 95 98 96 38 17 99

53 65 69 75 88 87 80 71 81 80 83 90 88 93 98 98 94 90

a

For the standard reaction conditions, see Table 1, entry 15. The yields shown are for isolated products. bGram scale (1.23 g of 1a).

amenable to a wide range of 1 substrates, and the reactions all proceeded smoothly to yield the corresponding 2,3-diarylbenzofurans in excellent yields with excellent enantio- and diastereoselectivities (all >20:1). Substrates with the substituents Cl, Br, and CN at the ortho-position of the phenyl ring all resulted in the desired products with excellent enantioselectivities (3b−d: 88−96% ee). A meta-bromo substituted phenyl substrate returned a 91% yield with 90% ee. Next, substrates with various substituents at the para-position of the phenyl ring were extensively investigated, and the results indicated that both electron-withdrawing (3f−l) and electron-donating (3m, 3n) groups were well tolerated; the corresponding trans-2,3diarylbenzofurans were obtained in high yields (81−95%) and excellent enantioselectivities (87−91% ee). Furthermore, disubstituted substrates were suitable and provided the desired products in excellent yields and enantioselectivities (3o, 3p). The absolute configuration of 3 was assigned on the basis of the X-ray crystallographic structure of 3h.19 Satisfactorily, the reaction was readily scalable without losing any efficiency (Scheme 2, 3a).

a

Reactions were carried out with 1a (0.2 mmol), 2a (0.24 mmol), and catalyst (0.02 mmol) in solvent (2.0 mL) under N2. bIsolated yield. c Determined by HPLC analysis.

desired chiral 2,3-diarylbenzofuran 3a was obtained smoothly in good yield (80%) and excellent diastereoselectivity (dr >20:1), albeit with moderate enantioselectivity (53% ee) by using 10 mol % 4a as the catalyst in 1.2-dichloroethane at room temperature for 36 h (entry 1). To increase the enantioselectivity of this reaction, a series of catalysts were screened (entries 2−5). The chiral phosphoric acid 4e, bearing a 9anthryl group at the 3,3′-position of BINOL, provided the desired product with quantitative yield and good enantioselectivity (98%, 88% ee, entry 5). Then various solvents were screened; CH3CN gave the best results at room temperature (entries 6−12). However, reducing the temperature in CH3CN had a negative effect on the enantioselectivity (entry 13 vs 12). Meanwhile, both DCM and DCE were tested at −20 °C. B

DOI: 10.1021/acs.orglett.8b00988 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Next, the substrate scope with regard to the naphthol moiety was examined. The results are listed in Scheme 3.

Scheme 4. Reaction of (E)-2-Styrylphenols with 2a

Scheme 3. Substrate Scope of Naphthols and Quinonesa

Having established a highly efficient construction of 2,3diarylbenzofurans, we evaluated the chiral phosphoric acid catalyst loading (Table 2). Starting from 10 mol % of catalyst Table 2. Catalyst Loading of the [3 + 2] Reactiona

entry

loading of 4e (mol %)

time (h)

yield (%)b

ee (%)c

1 2 3 4 5

10 5 2 1 0.5

72 72 72 72 72

95 95 95 94 88

98 98 98 98 97

a Reactions were performed with 1a (0.20 mmol), 2a (0.24 mmol), and catalyst (4e) in CH2Cl2 (2.0 mL) at −20 °C under N2. bIsolated yields. cDetermined by HPLC.

4e, we were able to decrease the catalyst loading to 0.5 mol % without a considerable loss in enantioselectivity and reactivity (Table 2, entries 1−5). Importantly, 0.5 mol % of 4e was sufficient to catalyze the reaction, furnishing product 3a in 88% yield with 97% ee (entry 5). To shed some light on the reaction mechanism, one control experiment was conducted under the optimal conditions. No reaction occurred when the hydroxyl group of 1a was protected. This result demonstrated that the hydroxy group plays an important role in the process of generating the oquinone methide intermediate. On the basis of the experimental results mentioned above, and the previous literature,20 we propose a reaction pathway that explains the stereochemistry of this reaction. As illustrated in Scheme 5, the

a

Reactions were carried out with 1 (0.2 mmol), 2a (0.24 mmol), and 4e (0.02 mmol) in CH2Cl2 (2.0 mL) at −20 °C under N2. The yields shown are for isolated products. The ee and dr values were determined by HPLC analysis. bdr = 10:1.

Substrates with substituents at the 6-position of the naphthol all returned excellent enantioselectivities, diastereoselectivities, and good to excellent yields (3q−v, 80−96%, dr >20:1, 90− 99% ee). Notably, electron-withdrawing groups appear to give the best stereoselectivity control (3q−r vs 3t−u). When different substituents were employed at the 7-position of the naphthol, the corresponding products 3w−z were also obtained in excellent yields and enantioselectivities (86−95% yield, 92− 95% ee). In addition, substituted 1,4-quinones (2) were tested in this reaction. The reaction of 1a with 1,4-quinones containing various substituents occurred smoothly to give the desired products in excellent yields (3aa−ac, 90−96%) with excellent enantioselectivities (86−96% ee) and good to excellent diastereoselectivities (dr, 10:1 → 20:1). Satisfactorily, substrates bearing halogen, CN, NO2, ester, TMS, and OTf functionalities were all suitable for the catalytic asymmetric [3 + 2] cycloaddition exhibiting very impressive performance (Schemes 2, 3) and thus offering handles for further synthetic transformations. To further demonstrate the utility of this newly developed protocol, 2-styrylphenols were examined. Gratifyingly, the desired 2,3-diphenylbenzofuran 3ad could be prepared in 95% yield with 95% ee (Scheme 4). Unfortunately, 2styrylphenol 1ae returned no reaction under these conditions.

Scheme 5. Proposed Mechanism

chiral phosphoric acid serves as a Brønsted acid/Lewis base bifunctional catalyst to simultaneously activate the (E)-1styrylnaphthol and the 1,4-benzoquinone to generate intermediate I. Subsequent dearomatization of 2-naphthol forms the transient o-quinone methide intermediate II, which further undergoes an intramolecular oxa-Michael addition by hydroC

DOI: 10.1021/acs.orglett.8b00988 Org. Lett. XXXX, XXX, XXX−XXX

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gen-bond activation with the chiral phosphoric acid to afford the final product. In summary, we have developed the first chiral phosphoric acid catalyzed [3 + 2] formal cycloaddition of 1-styrylnaphthols and 1,4-quinones. This method provides an efficient strategy to construct trans-2,3-diarylbenzofurans with two adjacent stereogenic centers in high yields, with excellent diastereo- and enantioselectivities (up to 99% yield, >20:1 dr, up to 99% ee) under mild reaction conditions. In addition, this organocatalytic procedure allows lowering of the catalyst loading to 0.5 mol % without considerable loss of reactivity and enantioselectivity. Further investigations on the mechanism and other chiral phosphoric acid catalyzed reactions are underway in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00988. Experimental procedures; characterization data for all new compounds; and copies of 1H, 13C NMR spectra (PDF) Accession Codes

CCDC 1821589 contains 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 data_ [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

Ling Zhou: 0000-0002-6805-2961 Author Contributions †

W.F. and H.Y. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (NSFC-21672170), the Key Science and Technology Innovation Team of Shaanxi Province (2017KCT-37) and the Northwest University for financial support.

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REFERENCES

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