Access to Polycyclic Sulfonyl Indolines via Fe(II)-Catalyzed or UV

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

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Access to Polycyclic Sulfonyl Indolines via Fe(II)-Catalyzed or UVDriven Formal [2 + 2 + 1] Cyclization Reactions of N‑((1H-indol-3yl)methyl)propiolamides with NaHSO3 Lin Lu,† Chenguang Luo,‡ Hui Peng,† Huanfeng Jiang,† Ming Lei,‡,* and Biaolin Yin*,† †

Org. Lett. Downloaded from pubs.acs.org by ALBRIGHT COLG on 04/08/19. For personal use only.

Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China ‡ State Key Laboratory of Chemical Resource Engineering, College of Science, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: A variety of structurally novel polycyclic sulfonyl indolines have been synthesized via FeCl2-catalyzed or UV-driven intramolecular formal [2 + 2 + 1] dearomatizing cyclization reactions of N-(1H-indol-3-yl)methyl)propiolamides with NaHSO3 in an aqueous medium. The reactions involve the formation of one C−C bond and two C−S bonds in a single step.

P

carbon monoxide or a variety of other one-atom units.3 Such cyclizations have recently been exploited as key steps in the syntheses of many natural products.4 However, although the use of cyclizations involving masked alkenes, such as that of an indole, to access biologically interesting bicyclic indolines via dearomatization 5 would be highly desirable, no such cyclizations have yet been reported, to our knowledge. In addition, despite the electronic similarity between carbon monoxide and sulfur dioxide,6 a sulfonylative variant, which would provide biologically7 and photoelectrically valuable8 cyclic sulfone products, has also not been achieved. Herein, as part of our ongoing work on the synthesis of polyheterocycles via dearomatization of heteroaromatic rings,9 we herein wish to report a formal intramolecular sulfonyl [2 + 2 + 1] cyclization accompanying the dearomatization of the indole ring to access polycyclic sulfonyl Indolines 1 (Scheme 1). To access sulfonyl indolines 1, we decided to introduce SO2, in the form of a surrogate (designated [SO2]), into N-(1Hindol-3-yl)methyl)propiolamides 2. This approach has the advantages of atom economy and step economy, and it avoids the need to use SO2, which is hazardous. Introduction of SO2 by means of a surrogate to form sulfones has been explored by Willis, Wu, Jiang, and others.10 However, most of the reported protocols involve three-component reactions that form acyclic sulfones and require preactivation of the substrates to produce free radicals or organic metal species prior to insertion of SO2 (method a in Scheme 2). To our knowledge, two-component insertion of SO2 into enynes (or masked enynes) without substrate preactivation to form cyclic sulfones is still unknown by far.

olycyclic indoline moieties are found in numerous pharmaceuticals and biologically active natural products, such as perophoramidine, (−)-aspidospermidine, and (−)-kopsinine (Scheme 1).1 Thus, the development of Scheme 1. Bioactive Cyclic Sulfones and Polycyclic Sulfonyl Indolines

operationally simple, step-economical reactions that generate structurally novel polycyclic indolines from readily available starting materials (such as indoles) via dearomatization would be highly desirable.2 One of the most common methods for the construction of complex polycyclic molecules involves [2 + 2 + 1] cyclization reactions of unfunctionalized or functionalized 1,n-enynes with © XXXX American Chemical Society

Received: February 14, 2019

A

DOI: 10.1021/acs.orglett.9b00573 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

was optimal. When (NH4)2S2O8 was used as the persulfate, the yield of 1a decreased to 60% (entry 8 in Table 1), whereas the use of Na2S2O8 increased the yield to 86% (entry 9 in Table 1). Lowering the amount of Na2S2O8 to 1 equiv, the yields decreased dramatically (entry 10 in Table 1). Screening of a variety of additives (KBr, LiBr, KCl, NaCl, and LiClO4, corresponding to entries 11−15 in Table 1) revealed that KCl, NaCl, and LiClO4 gave only slightly lower yields than LiCl, but KBr and LiBr gave very low yields, or only trace amounts, of 1a. In the absence of FeCl2, the yield of 1a sharply decreased (to 15%; see entry 16 in Table 1). In addition, when NaHSO3 or Na2S2O8 was absent, none of the desired product formed and most of the starting 2a was recovered (see entries 17 and 18 in Table 1). The addition of LiCl accelerated the reaction; in the absence of LiCl, decomposition of either the starting material or the product was observed (entry 19 in Table 1).13 To explore the substrate scope of the reaction, we subjected N-(1H-indol-3-yl)methyl)propiolamides 2 bearing a variety of R1−R4 groups to the optimized reaction conditions and found that the nature of these R groups clearly influenced the reaction outcome (Scheme 3). Specifically, for R2 = R3 = H and R4 = n-Pr, when R1 was an unsubstituted phenyl group or a

Scheme 2. Formation of Sulfones by Introduction of SO2 Surrogates

At the outset, we chose N-((1-methyl-1H-indol-3-yl)methyl)-3-phenyl-N-propylpropiolamide (2a) as the model substrate and K2S2O8, which generates sulfate radicals in the presence of an iron catalyst11 or light,12 which serves as the initiator (Table 1). Fortunately, reaction of 2a in the presence Table 1. Optimization of Reaction Conditionsa

Scheme 3. Substrates Scope (Reaction Conditions Are Described in Footnotes)

entry

[Fe]

[SO2]

persulfate

additive

1ab (%)

1 2 3 4 5 6 7 8 9 10c 11 12 13 14 15 16 17 18 19

FeCl2 FeCl2 FeCl2 Fe(acac)3 Fe(acac)2 Fe(OAc)2 Fe(OTf)2 FeCl2 FeCl2 FeCl2 FeCl2 FeCl2 FeCl2 FeCl2 FeCl2 − FeCl2 FeCl2 FeCl2

NaHSO3 Na2SO3 Na2S2O5 NaHSO3 NaHSO3 NaHSO3 NaHSO3 NaHSO3 NaHSO3 NaHSO3 NaHSO3 NaHSO3 NaHSO3 NaHSO3 NaHSO3 NaHSO3 − NaHSO3 NaHSO3

K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 (NH4)2S2O8 Na2S2O8 Na2S2O8 Na2S2O8 Na2S2O8 Na2S2O8 Na2S2O8 Na2S2O8 Na2S2O8 Na2S2O8 − Na2S2O8

LiCl LiCl LiCl LiCl LiCl LiCl LiCl LiCl LiCl LiCl KBr LiBr KCl NaCl LiClO4 LiCl LiCl LiCl −

81 trace 50 51 39 65 76 60 86 trace 30 trace 67 81 78 15 NDd NDd 51

a

Reaction conditions: unless otherwise noted, 2a (0.2 mmol), [SO2] (2.5 equiv), catalyst (0.1 equiv), persulfate (2.0 equiv), additive (2.0 equiv), CH3CN/H2O = 3:1 (v/v, 2 mL), room temperature, N2 atmosphere, 6 h. bIsolated yields are provided. cNa2S2O8 (1 equiv). d Not detected.

of FeCl2 (0.1 equiv), NaHSO3 (2.5 equiv), K2S2O8 (2.0 equiv), and LiCl (2.0 equiv) in CH3CN/H2O at room temperature for 6 h gave desired product 1a in 81% yield (entry 1 in Table 1). The structure of 1a was verified by single-crystal X-ray analysis (see the Supporting Information (SI)). Screening of other SO2 surrogates, Na2SO3 and Na2S2O5, did not improve the yield (see entries 2 and 3 in Table 1). We examined several iron catalysts, specifically, Fe(acac)3, Fe(acac)2, Fe(OAc)2, and Fe(OTf)2 (entries 4−7 in Table 1), and we found that FeCl2

a

Condition A: 2 (0.2 mmol), NaHSO3 (2.5 equiv), FeCl2 (0.1 equiv), Na2S2O8 (2.0 equiv), LiCl (2.0 equiv), CH3CN/H2O = 3/1 (v/v, 2 mL), room temperature, N2 atmosphere, 6 h. Isolated yields are provided. bLiClO4 (2.0 equiv) instead of LiCl (2.0 equiv). cCondition B: 2 (0.2 mmol), NaHSO3 (2.5 equiv), Na2S2O8 (2.0 equiv), LiCl (2.0 equiv), CH3CN/H2O = 3/1 (v/v, 2 mL), irradiation with 254 nm UV light at 16 W, room temperature, N2 atmosphere, 6 h. Isolated yields are provided. B

DOI: 10.1021/acs.orglett.9b00573 Org. Lett. XXXX, XXX, XXX−XXX

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driven by UV,11,12,14 the exact radical species in this organic reaction are unclear at the moment. On the other hand, electron-withdrawing of R2 (2o and 2p, Scheme 3) and Nprotective group of Boc or Ac of the indole ring (2ag and 2ah, Scheme 4b) did not lead to the corresponding products, indicating that the reactivity was strongly dependent on the electron density of indole rings. Thus, the formation of cation radicals cannot be ruled out. Two possible mechanisms are proposed for the formation of 1a shown in the Supporting Information (SI) for preliminary consideration, and the further mechanism study is underway. In summary, we have developed a protocol for access to polycyclic indoles via formal [2 + 2 + 1] cyclization reactions of N-(1H-indol-3-yl)methyl)propiolamides with NaHSO3 driven by FeCl2 catalysis or UV irradiation in an aqueous medium at room temperature. These mild reactions use readily available starting materials, show high step-economy, and generate structurally complex products. Further studies of this protocol, including the use of variants of the heteroaromatic ring and its replacement by other unsaturated moieties, and exploration of the bioactivities of the structurally novel products are underway in our laboratory.

phenyl group substituted with an electron-donating group (Me, MeO, or t-Bu), the desired products were obtained in good to excellent yields (86%−94%, 1a−1g); moderately electron-withdrawing groups (F and Cl) also gave good yields (75%−88%, 1h−1j). However, a strongly electron-withdrawing group (4-CF3) gave desired product 1k in only 65% yield. When R1 was Me or H, corresponding products 1l and 1m were not detected, and a small amount of the substrate decomposed. Various other substituents (R2) on the phenyl moiety of the indole ring were also screened. For R1 = Ph, R3 = H, and R4 = n-Pr, the R2 group could be electron-donating (Me or MeO) or moderately electron-withdrawing (F, Cl, or Br), and the yields of the products ranged from 56%−96% (1n, 1q−1x). Unfortunately, when R2 was strongly electronwithdrawing (CN or NO2), desired products 1o and 1p were not detected, and most of the substrate was recovered. For R1 = Ph, R2 = H, and R4 = n-Pr, when R3 was Me, Ph, or a substituted Ph group, the yields of the corresponding products (1y−1ac) were lower than the yield of 1a (R3 = H). For R1 = Ph and R2 = R3 = H, when R4 was Me or Et, the yields of 1ad and 1ae were good. However, when R4 was Bn, the yield of 1af was only 17%, indicating that the Bn group was not welltolerated, possibly because of overoxidation of the methylene group. Encouraged by the above-described results, we explored the possibility of accomplishing this transformation with UV irradiation, in the absence of FeCl2. Using 2a as the substrate, we tested the following irradiation sources: 254 nm UV light (16 W), 380−385 nm UV light (18 W), blue light (450−470 nm, 18 W), green light (520−530 nm, 18 W), yellow light (580−585 nm,18 W), reddish orange light (600−610 nm, 18W), and red light (620−630 nm, 18 W), and we obtained 1a in yields of 72%, 33%, 40%, 38%, 13%, 63%, and 46%, respectively. That is, UV light with a wavelength of 254 nm was optimal for this reaction. With irradiation at 254 nm, the reaction showed a broad substrate scope; most of the tested substrates 2 gave the desired products in moderate to good yields, except 2l, 2m, 2o, and 2p (Scheme 3). Notably, the yields of the UV-driven reactions were not higher than the yields of the corresponding FeCl2-catalyzed reactions, except in the cases of products 1y, 1z, 1aa, 1ab, 1ac, 1ae, and 1af. To gain insight into the reaction mechanism, some control experiments were conducted. On one hand, when the radical scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO, 2 equiv) was added under the standard conditions for the FeCl2-catalyzed reaction, the formation of 1a was not detected, indicating that radical step(s) might be involved in this transformation (Scheme 4a). Furthermore, an aryl group at R1 is essential, probably because it stabilizes the intermediary radical. Although formation of the HSO3 radical from K2S2O8 is known in the references under the catalysis of Fe salt or

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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.9b00573. Typical experimental procedures, characterization for all products, computational results and crystallographic data for 1a (PDF) Accession Codes

CCDC 1889285 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Huanfeng Jiang: 0000-0002-4355-0294 Ming Lei: 0000-0001-5765-9664 Biaolin Yin: 0000-0002-2547-0231 Notes

The authors declare no competing financial interest.

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Scheme 4. Control Experiment

ACKNOWLEDGMENTS This work was supported by grants from the National Program on Key Research Project (No. 2016YFA0602900), the National Natural Science Foundation of China (Nos. 21572068 and 21871094), the Science and Technology Program of Guangzhou, China (No. 201707010057), Guangdong Natural Science Foundation (No. 2017A030312005), and the Science and Technology Planning Project of Guangdong Province, China (No. 2017A020216021). C

DOI: 10.1021/acs.orglett.9b00573 Org. Lett. XXXX, XXX, XXX−XXX

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