Palladium-Catalyzed Oxidative [2 + 2 + 1] Annulation of 1,7-Diynes

Letter Cite This: Org. Lett. 2018, 20, 6765−6768

pubs.acs.org/OrgLett

Palladium-Catalyzed Oxidative [2 + 2 + 1] Annulation of 1,7-Diynes with H2O: Entry to Furo[3,4‑c]quinolin-4(5H)‑ones Xuan-Hui Ouyang,†,§ Fang-Lin Tan,†,§ Ren-Jie Song,*,† Wei Deng,† and Jin-Heng Li*,†,‡ †

Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, China ‡ State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China

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S Supporting Information *

ABSTRACT: A novel cascade annulation of 1,7-diynes with water has been developed for the synthesis of furo[3,4c]quinolin-4(5H)-one skeletons with high atom- and stepeconomy. The transformation was enabled by a palladium catalyst in the presence of copper salt as the promoter, involving the formation of one C−C bond and two C−O bonds. Moreover, the reaction exhibits good tolerance of functional groups and broad substrate scope. Notably, the control experiments support the incorporation of the new oxygen atom from water.

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Scheme 1. Annulation toward Furo[3,4-c]quinolin-4(5H)ones

uro[3,4-c]quinolin-4(5H)-ones are common motifs of many natural products and bioactive molecules that have gained increased attention from the synthetic chemistry and pharmaceutical industries in recent years.1 Such compounds have shown activity against bacteria,1a−e lymphoma,1f,g hepatocellular disease,1h and n-NOS1i (Figure 1). Current

be efficient for this transformation. For instance, the Hirao and Chan group reported the gold-catalyzed cycloisomerization of 1,6-diyne to furnish 2,4a-dihydro-1H-fluorenes.11a Hao, Tu, Jiang, and co-workers next established a metal-free domino cyclization of 1,7-diynes for the construction of fused azaheterocyclic synthesis in two steps under microwave conditions.11b Recently, we and other groups constructed a functionalized polycyclic ring through cascade annulation of enynes.12 With continued interest in the synthesis of polycyclic rings, herein we report a method that directly converted 1,7diynes to furo[3,4-c]quinolin-4(5H)-ones under a Pd/Cu catalyst system (Scheme 1b). Notably, it was confirmed that the newly formed oxygen atom came from water. We began our studies by using N-methyl-3-phenyl-N-(2(phenylethynyl)phenyl)propiolamide 1a as a model substrate, and a summary of results are provided in Table 1. Gratifyingly,

Figure 1. Important examples of bioactive furo[3,4-c]quinolin-4(5H)one derivatives.

methods for the synthesis of these compounds mainly focus on the cycloaddition reaction of aniline derivatives with α,βunsaturated carbonyl compounds,2 as well as an imino-Diels− Alder reaction of N-arylimine with olefins (Scheme 1a).3 However, most of these methods suffer from the requirement of a stoichiometric amount of expensive Lewis acids with limited substrate scope. As a result, the development of stepeconomical and cost-effective methods for the synthesis of furo[3,4-c]quinolin-4(5H)-one derivatives is highly appealing. Transition-metal-catalyzed inter- and intramolecular tandem annualtion of diynes has been recognized as a versatile synthetic approach for substituted polycyclic molecules with excellent atom-economy.4−10 To date, Rh,4 Ru,5 Au,6 Pt,7 Ni,8 Pd,9 and other transition metal catalysts10 have been proven to © 2018 American Chemical Society

Received: September 10, 2018 Published: October 11, 2018 6765

DOI: 10.1021/acs.orglett.8b02883 Org. Lett. 2018, 20, 6765−6768

Letter

Organic Letters Table 1. Screening of Optimal Reaction Conditionsa

With the optimized conditions in hand, we investigated the substrate scope of this tandem cyclization reaction with respect to N-(o-ethynlaryl)propiolamides 1 (Scheme 2). A wide range Scheme 2. Variation of the N-(o-Ethynlaryl)propiolamides (1)a

entry

variation from the standard conditions

yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24b

none without PdCl2 Pd(OAc)2 instead of PdCl2 PdBr2 instead of PdCl2 Pd(PPh3)4Cl2 instead of PdCl2 Pd(PPh3)4 instead of PdCl2 PdCl2 (10 mol %) PdCl2 (2 mol %) without CuBr2 Cu(OAc)2 instead of CuBr2 Cu(OTf)2 instead of CuBr2 CuBr instead of CuBr2 CuBr2 (20 mol %) CuBr2 (5 mol %) DMSO instead of DMF MeCN instead of DMF toluene instead of DMF at 120 °C at 80 °C air instead of O2 argon instead of O2 H2O (6 equiv) H2O (2 equiv) none

84 0 59 72 10 5 83 72 23 61 62 58 83 48 61 38 15 67 42 78 11 72 63 77

a

Reaction conditions: 1a (0.2 mmol), PdCl2 (5 mol %), CuBr2 (10 mol %), H2O (4 equiv), DMF (2 mL), O2 (1 atm), 100 °C and 24 h. b 1a (1 mmol) for 36 h.

84% yield of desired product 5-methyl-1,3-diphenylfuro[3,4c]quinolin-4(5H)-one 2a was obtained when 5 mol % of PdCl2 was used as the catalyst in combination with 10 mol % of CuBr2 in the presence of H2O and DMF at 100 °C (entry 1). However, no desired product 2a was observed in the absence of PdCl2 catalyst (entry 2). Other palladium salts, such as Pd(OAc)2, PdBr2, Pd(PPh3)4Cl2, and Pd(PPh3)4, were also reactive, but they were all less reactive than PdCl2 (entry 1 vs entries 3−6). Identical yield of 2a was generated when the loading of PdCl2 was increased to 10 mol %, whereas 72% yield of 2a was obtained when 2 mol % of PdCl2 was used (entries 7 and 8). Copper salts played an important role in this reaction, as only 23% of 2a was observed when the reaction was performed without CuBr2 (entry 9). Next, different copper salts and the amount of copper salts were investigated, and the best result was obtained when 10 mol % of CuBr2 was used as an additive (entry 1 vs entries 10−14). Subsequently, a series of solvents, such as DMSO, MeCN, and toluene, were evaluated, and they showed reactivity lower than that of DMF (entry 1 vs entries 15−17). Further screening of other parameters such as temperature and atmosphere environment did not improve the reaction yield (entries 18−21). Finally, the amount of H2O affected the yield, and 4 equiv of H2O was suggested to be the best choice (entry 1 vs entries 22 and 23). We found that the reaction on a 1 mmol scale of substrate 1a was successfully performed in 77% yield (entry 24).

a

Reaction conditions: 1 (0.2 mmol), PdCl2 (5 mol %), CuBr2 (10 mol %), H2O (4 equiv), DMF (2 mL), O2 (1 atm), 100 °C and 24 h.

of substrates were successfully transformed under the standard reaction conditions and provided the corresponding furo[3,4c]quinolin-4(5H)-ones in moderate to good yields. The N-Bnsubstituted 1,7-diyne 1b could convert to the desired product 2b in 64% yield under a PdCl2/CuBr2 system. Unfortunately, substrate 1c with a free N-H bond was not a viable substrate under these conditions. Alternatively, phenol-linked 1,7-diyne 1d was proven to be a suitable reaction partner toward synthesizing 1,3-diphenyl-4H-furo[3,4-c]chromen-4-one 2d with 68% yield. A variety of N-(o-ethynlaryl)propiolamides with different functional groups, such as Me, OMe, Br, and CN groups, at different positions of the arylalkynyl (R2) moiety, could be successfully converted to the corresponding products 2e−j with 59−91% yields. For instance, 1,7-diynes having either electron-donating (OMe, 1f) or electron-withdrawing (CN, 1h) substituents readily participated in this tandem cyclization, and gave the furo[3,4-c]quinolin-4(5H)-ones 2f and 2h in 91% and 59% yields, respectively. Moreover, 1,76766

DOI: 10.1021/acs.orglett.8b02883 Org. Lett. 2018, 20, 6765−6768

Organic Letters

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diyne 1k possessing a heteroaryl group, such as 2-thienyl, worked well (product 2k). Other 1,7-diynes 1l and 1m by installing a n-butyl or cyclopropyl groups at the end of terminal alkyne (R2) proceeded smoothly to afford the corresponding products 2l and 2m in acceptable yields. The reaction was also tolerant of various electron-deficient and electron-rich aromatic groups, including Me, OMe, Cl, and CN at substituted aryl groups and aliphatic groups to afford the desired products 2n−s in moderate to good yield. Notably, substrate N-methyl-N-(4-methyl-2-(phenylethynyl)phenyl)-3phenylpropiolamide 1t and N-(4-chloro-2-(phenylethynyl)phenyl)-N-methyl-3-phenylpropiolamide 1u still showed high reactivity, enabling their conversion into products 2t and 2u in high yields. As shown in Scheme 3, the 18O-labeled control experiment was performed to understand the mechanism for this tandem

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.8b02883. Descriptions of experimental procedures and analytical characterization (PDF) Accession Codes

CCDC 1865953 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 [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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Scheme 3. Control Experiments and Possible Mechanisms

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected] ORCID

Ren-Jie Song: 0000-0001-8708-7433 Jin-Heng Li: 0000-0001-7215-7152 Author Contributions §

X.-H.O. and F.-L.T. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Natural Science Foundation of China (Nos. 21402046, 21625203, and 21472039), the Jiangxi Province Science and Technology Project (Nos. 20171BCB23055, 20171ACB21032, 20171ACB20015, and 20165BCB18007), and the Jiangxi Educational Committee Foundation of China (No. GJJ160725) for financial support.

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cyclization. When 4 equiv of H218O was added to the reaction medium, the amount of 18O in the product 2a-18O was obviously increased, which suggested that the newly formed oxygen atom came from water. Based on the above results and previous reports,4−11 the possible mechanism for the palladium-catalyzed tandem cyclization is proposed in Scheme 3. The possible mechanism pathway was initiated by halopalladation/hydrolysis of the acetylenic link of alkynamide in the presence of water to form intermediate B. Then cyclization of intermediate B to afford the vinylpalladium intermediate C, followed by the further cyclization and the β-H elimination of intermediate C, occurred to generate the product and PdII species with the aid of CuBr2 and O2. In summary, we have developed straightforward access toward furo[3,4-c]quinolin-4(5H)-ones via palladium-catalyzed tandem annulation of 1,7-diynes with water in the presence of copper salts. The reaction tolerates a broad substrate scope, and three new chemical bonds were formed in one step. The source of the newly generated oxygen atoms has been determined. Further efforts to develop new methods to construct complex polycyclic ring skeletons are currently underway in our laboratory.

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DOI: 10.1021/acs.orglett.8b02883 Org. Lett. 2018, 20, 6765−6768