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Abstract

A new method for the synthesis of indolin-2-ones has been realized by an I₂-promoted oxidative reaction from 1,2,3,3-tetramethyl-3H-indolium iodides. This transformation proceeded smoothly under metal-free and peroxide-free conditions in a cascade manner.

Keywords

indolin-2-one metal-free one-pot peroxide-free

Introduction

Indolin-2-one rings have been recognized as crucial motifs in diverse natural products and pharmaceutically relevant entities.^1–3 Several drug discovery programs in industry and academia are anchored around the indolin-2-one scaffold, and drugs such as Sunitinib and Nintedanib are in clinical use for targeted anticancer therapies.^4,5 A number of methods for the synthesis of indolin-2-one have been reported. Classical synthetic methods include the derivatization of other heterocycles (such as Wolff–Kishner reduction of isatin and oxidation of indole), Friedel–Crafts cyclizations of α-haloacetanilides, and variations in the Fischer indole synthesis.^6–8 Cyclizations of 2-haloacryloylanilide derivatives by a variety of radical initiators have also been used to prepare indolin-2-ones.^9–12 In recent years, metal-mediated activations/cyclizations on precrafted advanced precursors have been a commonly pursued approach (Scheme 1). Notable examples are Cu-catalyzed intermolecular C–H cyclization (Scheme 1(a)),¹³ Pd-catalyzed insertion of isocyanide (Scheme 1(b)),¹⁴ ruthenium-catalyzed intramolecular alkene hydroarylation (Scheme 1(c)),¹⁵ and Fe-mediated hydrometallation-cyclization (Scheme 1(d)).¹⁶ In addition, Ni-catalyzed aromatic C–H alkylation (Scheme 1(e)) and Ir-catalyzed radical cyclization under visible light (Scheme 1(f)) have also been used to construct indolin-2-one rings.^17,18

Scheme 1.

Representative recent approaches for accessing indolin-2-ones.

Recently, due to the need for “greener chemistry,” chemists are beginning to develop transformations under metal-free conditions. Iodine, a cheap, nontoxic, and abundant halogen, is attracting more and more attention, especially in oxidative coupling reactions to form new carbon–carbon (C–C) or carbon–heteroatom (C–X) bonds.^19,20 An in situ iodination usually occurs in iodine-mediated oxidative couplings, followed by in situ oxidation to generate an electrophilic aldehyde group.^21–23 1,2,3,3-Tetramethyl-3H-indolium iodide, a well-known synthetic agent of many fluorescent dyes, is considered to contain an activated methyl group which can react with an aldehyde or carbonyl group.^24,25 In our previous research, the activated C-2 methyl group reacted with molecular iodine to form an aldehyde via Kornblum oxidation in the presence of dimethyl sulfoxide (DMSO).²⁶ Based on these results, we report an I₂-promoted one-pot method for the synthesis of indolin-2-ones under metal-free conditions.

Results and discussion

The reaction is demonstrated by using 1,2,3,3-tetramethyl-3H-indol-1-ium iodide (1a) in the presence of molecular iodine (3.0 equiv.) in DMSO at 100°C for 2 h (Table 1, entry 1). We next studied the effect of the temperature on reaction (Table 1, entries 2 and 3), and only at a higher temperature was the product obtained. An acid, methanesulfonic acid, could effectively promote the reaction by lowering the reaction temperature (Table 1, entries 4 and 5). An additive, N-methyl-1,2-benzenediamine dihydrochloride (2a), was added to the reaction system and a higher yield was observed (Table 1, entries 6 and 7). Subsequently, the addition of water or 3-Å molecular sieves (Table 1, entries 8 and 9) led to a drastic reduction in the yield, suggesting the unusual role of water in the reaction system. We then screened the amount of iodine at 70°C (Table 1, entries 12–15) and found that 2.0 equiv. was optimum. Further increasing the amount of I₂ had no effect on the yield, and no reaction occurred in the absence of I₂ (Table 1, entry 15), indicating that iodine was essential. When additive was reduced to catalytic amounts (Table 1, entries 16 and 17), only small amounts of products were obtained. The addition of pyridine greatly decreased the yield, suggesting the importance of acidic conditions (Table 1, entry 18). In addition, several reactions were performed in different solvents so as to ascertain the unusual role of DMSO in this process (see Table S1 in Supplementary material). The amount of I₂ was increased to 7.0 equiv., and the reaction time was prolonged to 3.0 h (Table 1, entries 19 and 20). However, the indolin-2-one was not completely converted into the iodide product. To further optimize this reaction, we explored the influence of various additives (see Table S2 in Supplementary material). After several experimental iterations, optimum reaction conditions were found: 1a (1.0 mmol), 2a (1.2 mmol), I₂ (2.0 mmol), DMSO (3 mL), and 70°C (Table 1, entry 6).

Table 1.

Optimization of the reaction conditions^a.


Entry	2a (equiv.)	I₂ (equiv.)	Temperature (°C)	Time (h)	Ratio a′/b′^b	Yield (%)^c
1	−	3	100	2	63:37	25
2	−	2	70	1.5	−	0
3	−	2	70	2	n.d.	Trace
4	−	3	100	2	50:50	35^d
5	−	2	70	0.5	80:20	34^d
6	1.2	2	70	0.5	61:38	50
7	1.2	2	70	2	40:60	45
8	1.2	2	70	0.5	n.d.	Trace^e
9	1.2	2	70	0.5	n.d.	Trace^f
10	1.2	2	100	0.5	25:75	42
11	1.2	2	50	1	88:12	18
12	1.2	1	70	0.5	77:23	35
13	1.2	1.5	70	0.5	65:35	36
14	1.2	3	70	0.5	60:40	44
15	1.2	−	70	0.5	−	0
16	0.1	2	70	0.5	50:50	3
17	0.2	2	70	1.5	56:44	9
18	1.2	2	70	0.5	n.d.	Trace^g
19	1.2	5	70	1	14:68^h	−
20	1.2	7	70	3	12:69^h	−

DMSO: dimethyl sulfoxide; HPLC: high-performance liquid chromatography.

Reaction conditions: 1a (1 mmol), 2a, and DMSO (3 mL).

Ratio of a′ to b′ in the isolated product is determined by HPLC.

Isolated yield of 3a. n.d. = not determined.

Methanesulfonic acid (2.0 mmol) was added.

H₂O (1 mL) was added.

3-Å molecular sieves were added.

Pyridine (0.1 mL) was added.

Ratio of a′ to b′ in the reaction solution was determined by HPLC.

With optimized conditions in hand, the generality and scope of the reaction were investigated (Table 2). 1,2,3,3-Tetramethyl-3H-indol-1-ium iodides 1 with electron-donating or electron-withdrawing groups on the phenyl ring participated in the reaction to afford the corresponding products 3b–m. Compared to electron-donating groups (e.g. 3b, 3d, and 3h) on the phenyl ring or on the fused ring (3i), electron-withdrawing groups (e.g. 3c, 3e, 3f, and 3g) tended to offer the products with higher yields. Furthermore, 7-substituted substrates (e.g. 3h, 3j, and 3k) participated in the transformation as well, but the yields were slightly lower than those obtained with 5-substituted substrates. When the 7-position of the substrate was substituted by a bromine atom, no iodide product could be obtained (3j). Finally, a substituent on the nitrogen and a bulky group at the 3-position are well tolerated, but their reactions afforded the target products in lower yields (3l and 3m).

Table 2.

Substrate scope^a.

Entry	Substrate 1 (R¹)	Product 3 (R¹)	Yield (%)^b
1	1b (5-Me)	3b (5-Me)	43
2	1c (5-Cl)	3c (5-Cl)	55
3	1d (5-OMe)	3d (5-OMe)	40
4	1e (5-Br)	3e (5-Br)	56
5	1f (5-CF₃)	3f (5-CF₃)	52
6	1g (5,7-diCl)	3g (5,7-diCl)	54
7	1h (5,7-diMe)	3h (5,7-diMe)	39
8			42
9	1j (7-Br)	3j (7-Br)	45
10	1k (7-Me)		41^c
11			40^c
12			41^c

DMSO: dimethyl sulfoxide; NMR: nuclear magnetic resonance.

Reaction was performed with 1 (1.0 mmol), 2a (1.2 mmol), and I₂ (2 mmol) in DMSO (3 mL) at 70°C for 0.5 h.

Yields of isolated products.

The ratio was determined by ¹H NMR spectroscopy.

To gain more information about the reaction mechanism, a model reaction of 1a (0.2 mmol) with I₂ (0.4 mmol) in DMSO-d₆ was tracked by ¹H nuclear magnetic resonance (NMR; see Figure S1 in Supplementary material). As the reaction proceeded, a signal around 10 ppm rapidly appeared, indicating the existence of an aldehyde group due to the release of formaldehyde in the reaction system.

With the aforementioned results and previous reports in mind,²⁶ a possible mechanism is proposed in Scheme 2 using 1,2,3,3,5-pentamethyl-3H-indol-1-ium iodide 1b as an example. Initially, the substrate indolium iodide 1b reacted with I₂ to afford the intermediate 2-iodomethyl indolium iodide A. Subsequently, the intermediate A underwent a nucleophilic substitution reaction with the nucleophilic oxygen atom of DMSO to form the alkoxysulfonium salt B, which was then attacked by H₂O to give the intermediate C. Under the reaction conditions, intermediate C was unstable and underwent C–C bond cleavage to give the final indolin-2-one 3b, along with the release of formaldehyde and dimethyl sulfide.

Scheme 2.

A plausible reaction mechanism.

In conclusion, a molecular I₂ promoted C–C bond cleavage has been developed to construct indolin-2-ones from simple and readily available indolinium iodides. In addition, the reaction performs well in the absence of any metal or peroxide. Further studies to elucidate a detailed mechanism and further explorations of this I₂/DMSO-promoted oxidative reaction are currently underway in our laboratory.

Experimental analysis

All reagents and solvents were obtained from commercial suppliers and were used without further purification. The method of preparing starting compounds 1 is given in Supplementary material. NMR spectra were recorded on a Bruker FT-NMR 400 spectrometer with chemical shifts reported in ppm relative to the internal standard tetramethylsilane (TMS). HRMS was determined on a Waters GCT Premier spectrometer.

General procedure for the synthesis of indolin-2-ones 3

A mixture of quaternary salt 1 (1.0 mmol), 2a (1.2 mmol), and iodine (2 mmol) in DMSO (3 mL) was stirred at 70°C. After the reaction was complete (monitored by thin-layer chromatography (TLC)), the mixture was cooled to room temperature and quenched with saturated aqueous Na₂S₂O₃, then extracted with ethyl acetate (3 × 20 mL). The combined organic layers were washed with brine, dried over anhydrous Na₂SO₄, and evaporated. The residue was purified by column chromatography using ethyl acetate and petroleum ether to yield the desired pure product.

1,3,3-Trimethylindolin-2-one and 7-iodo-1,3,3-trimethylindolin-2-one (mixture obtained from entry 7 in Table 1, 3a): Brown oil, 50% yield; ¹H NMR (400 MHz, DMSO-d₆): δ_H 7.71 (d, J = 1.7 Hz, 1H), 7.61 (dd, J = 8.2, 1.8 Hz, 1H), 7.35–7.31 (m, 0.67H), 7.26 (td, J = 7.7, 1.3 Hz, 0.67H), 7.04 (td, J = 7.6, 1.3 Hz, 0.67H), 7.00 (d, J = 7.8 Hz, 0.67H), 6.87 (d, J = 8.2 Hz, 1H), 3.13 (s, 2H), 3.11 (s, 3H), 1.26 (s, 10H); ¹³C NMR (100 MHz, DMSO-d₆): δ_C 180.6, 180.1, 142.9, 142.8, 138.4, 136.6, 135.7, 131.4, 128.1, 122.8, 122.6, 111.4, 108.8, 85.8, 44.0, 26.5, 26.4, 24.6, 24.3; LC-MS: t_R = 3.94 min, m/z = 176.1 [M + H]⁺; t_R = 4.48 min, m/z = 302.0 [M + H]⁺; HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₁H₁₄NO⁺ and C₁₁H₁₃INO⁺: 176.1075 and 302.0042, found: 176.1074 and 302.0040.

1,3,3,5-Tetramethylindolin-2-one (3b):¹⁶ Brown oil, 43% yield; ¹H NMR (400 MHz, DMSO-d₆): δ_H 7.18 (s, 1H), 7.09 (d, J = 7.9 Hz, 1H), 6.91 (d, J = 7.9 Hz, 1H), 3.13 (s, 3H), 2.31 (s, 3H), 1.27 (s, 6H); ¹³C NMR (100 MHz, DMSO-d₆): δ_C 180.5, 140.5, 135.7, 131.5, 128.2, 123.5, 108.5, 43.9, 26.4, 24.6, 21.2; HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₂H₁₆NO⁺: 190.1226, found: 190.1230.

5-Chloro-1,3,3-trimethylindolin-2-one (3c):¹⁶ White solid, 55% yield; m.p. 141–143°C (lit.¹⁶ 140–143°C); ¹H NMR (400 MHz, DMSO-d₆): δ_H 7.52 (d, J = 2.1 Hz, 1H), 7.35 (dd, J = 8.3, 2.2 Hz, 1H), 7.06 (d, J = 8.3 Hz, 1H), 3.15 (s, 3H), 1.30 (s, 6H); ¹³C NMR (100 MHz, DMSO-d₆): δ_C 180.3, 141.8, 137.8, 127.9, 126.8, 123.4, 110.3, 44.2, 26.5, 24.2; HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₁H₁₃ClNO⁺: 210.0680, found: 210.0683.

5-Methoxy-1,3,3-trimethylindolin-2-one (3d):¹⁶ Yellow oil, 40% yield; ¹H NMR (400 MHz, DMSO-d₆): δ_H 7.02 (d, J = 2.5 Hz, 1H), 6.90 (d, J = 8.4 Hz, 1H), 6.81 (dd, J = 8.4, 2.5 Hz, 1H), 3.72 (s, 3H), 3.09 (s, 3H), 1.24 (s, 6H); ¹³C NMR (100 MHz, DMSO-d₆): δ_C 180.3, 156.0, 137.0, 136.2, 112.3, 110.4, 109.1, 56.0, 44.3, 26.4, 24.5; HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₂H₁₆NO₂⁺: 206.1176, found: 206.1183.

5-Bromo-1,3,3-trimethylindolin-2-one (3e):¹⁶ White solid, 56% yield; m.p. 152–154°C (lit.¹⁶ 152–154°C); ¹H NMR (400 MHz, DMSO-d₆): δ_H 7.63 (s, 1H), 7.47 (d, J = 8.2 Hz, 1H), 7.01 (d, J = 8.3 Hz, 1H), 3.14 (s, 3H), 1.29 (s, 6H); ¹³C NMR (100 MHz, DMSO-d₆): δ_C 180.2, 142.3, 138.2, 130.7, 126.0, 114.6, 110.8, 44.2, 26.5, 24.2; HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₁H₁₃BrNO⁺: 254.0175, found: 254.0174.

1,3,3-Trimethyl-5-(trifluoromethyl)indolin-2-one (3f):²⁷ Light yellow solid, 52% yield; m.p. 38–40°C (lit.²⁷ 37–39°C); ¹H NMR (400 MHz, DMSO-d₆): δ_H 7.80 (s, 1H), 7.68 (d, J = 8.2 Hz, 1H), 7.22 (d, J = 8.2 Hz, 1H), 3.20 (s, 3H), 1.33 (s, 6H); ¹³C NMR (100 MHz, DMSO-d₆): δ_C 180.8, 146.5, 136.6, 126.0, 125.9, 123.3, 122.9, 120.0, 109.1, 44.0, 26.6, 24.1; HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₂H₁₃F₃NO⁺: 244.0944, found: 244.0943.

5,7-Dichloro-1,3,3-trimethylindolin-2-one (3g): Yellow oil, 54% yield; ¹H NMR (400 MHz, DMSO-d₆): δ_H 7.56 (d, J = 1.9 Hz, 1H), 7.44 (d, J = 1.9 Hz, 1H), 3.46 (s, 3H), 1.31 (s, 6H); ¹³C NMR (100 MHz, DMSO-d₆): δ_C 180.7, 140.4, 137.9, 129.1, 127.2, 122.7, 115.1, 44.3, 29.5, 24.3; HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₁H₁₂Cl₂NO⁺: 244.0290, found: 244.0290.

1,3,3,5,7-Pentamethylindolin-2-one (3h):¹⁶ Yellow oil, 39% yield; ¹H NMR (400 MHz, DMSO-d₆): δ_H 7.00 (s, 1H), 6.83 (s, 1H), 3.35 (s, 3H), 2.52 (s, 3H), 2.25 (s, 3H), 1.23 (s, 6H); ¹³C NMR (100 MHz, DMSO-d₆): δ_C 181.1, 138.1, 136.4, 131.9, 131.4, 121.4, 119.7, 43.2, 29.3, 24.9, 20.9, 18.8; HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₃H₁₈NO⁺: 204.1383, found: 204.1391.

1,1,3-Trimethyl-1,3-dihydro-2H-benzo[e]indol-2-one (3i): Yellow oil, 42% yield; ¹H NMR (400 MHz, DMSO-d₆): δ_H 8.02 (d, J = 8.5 Hz, 1H), 7.98 (s, 1H), 7.96 (s, 1H), 7.57–7.53 (m, 1H), 7.47 (d, J = 8.6 Hz, 1H), 7.39 (t, J = 7.5 Hz, 1H), 3.28 (s, 3H), 1.54 (s, 6H); ¹³C NMR (100 MHz, DMSO-d₆): δ_C 181.7, 140.5, 130.5, 130.0, 129.3, 129.1, 127.5, 126.6, 123.7, 122.4, 110.9, 45.4, 26.7, 24.1; HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₅H₁₆NO⁺: 226.1226, found: 226.1230.

7-Bromo-1,3,3-trimethylindolin-2-one (3j): Light yellow oil, 45% yield; ¹H NMR (400 MHz, DMSO-d₆): δ_H 7.44 (d, J = 8.1 Hz, 1H), 7.40 (d, J = 7.2 Hz, 1H), 7.00 (t, J = 7.7 Hz, 1H), 3.49 (s, 3H), 1.30 (s, 6H); ¹³C NMR (100 MHz, DMSO-d₆): δ_C 181.1, 140.0, 139.1, 133.4, 124.4, 122.5, 101.9, 43.7, 29.8, 24.7; HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₁H₁₃BrNO⁺: 254.0175, found: 254.0174.

1,3,3,7-Tetramethylindolin-2-one and 5-Iodo-1,3,3,7-tetramethylindolin-2-one (mixture, 3k): Yellow oil, 41% yield; ¹H NMR (400 MHz, DMSO-d₆): δ_H 7.56 (d, J = 1.3 Hz, 0.68H), 7.41 (s, 0.68H), 7.18 (d, J = 7.1 Hz, 1H), 7.02 (d, J = 7.5 Hz, 1H), 6.94 (t, J = 7.5 Hz, 1H), 3.43 (s, 3H), 3.40 (s, 2H), 2.57 (s, 3H), 2.53 (s, 2H), 1.25 (s, 10H); ¹³C NMR (100 MHz, DMSO-d₆): δ_C 181.2, 180.7, 140.6, 140.5, 139.5, 138.9, 136.3, 131.6, 129.3, 123.0, 122.5, 120.7, 120.0, 85.9, 43.3, 43.1, 29.3, 24.9, 24.6, 19.0, 18.4; HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₂H₁₆NO⁺ and C₁₂H₁₅INO⁺: 190.1226 and 316.0193, found: 190.1228 and 316.0192.

1-Ethyl-3,3-dimethylindolin-2-one and 1-Ethyl-5-iodo-3,3-dimethylindolin-2-one (mixture, 3l): Yellow oil, 40% yield; ¹H NMR (400 MHz, DMSO-d₆): δ_H 7.76 (d, J = 1.5 Hz, 0.8H), 7.63 (dd, J = 8.2, 1.5 Hz, 0.8H), 7.38 (d, J = 7.3 Hz, 1H), 7.28 (d, J = 7.5 Hz, 1H), 7.09–7.06 (m, 2H), 6.96 (d, J = 8.2 Hz, 0.8H), 3.78–3.67 (m, 3.6H), 1.29 (s, 10.8H), 1.19–1.13 (m, 5.4H); ¹³C NMR (100 MHz, DMSO-d₆): δ_C 180.3, 179.7, 141.8,141.6, 138.6, 136.6, 135.9, 131.6, 128.1, 123.0, 122.4, 111.5, 108.9, 85.7, 44.0, 43.8, 34.4, 34.3, 24.5, 24.2, 13.0, 12.9; HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₂H₁₆NO⁺ and C₁₂H₁₅INO⁺: 190.1226 and 316.0193, found: 190.1226 and 316.0190.

1′-Methylspiro[cyclohexane-1,3′-indolin]-2′-one and 5′-Iodo-1′-methylspiro [cyclohexane-1,3′-indolin]-2′-one (mixture, 3m): Yellow oil, 41% yield; ¹H NMR (400 MHz, DMSO-d₆): δ_H 7.69 (d, J = 1.6 Hz, 0.42H), 7.57 (dd, J = 8.2, 1.7 Hz, 0.42H), 7.43 (d, J = 7.3 Hz, 1H), 7.24–7.20 (m, 1H), 6.99–6.93 (m, 2H), 6.81 (d, J = 8.2 Hz, 0.42H), 3.06 (s, 3H), 3.03 (s, 1.26H), 1.88–1.43 (m, 14.2H); ¹³C NMR (100 MHz, DMSO-d₆): δ_C 179.9, 179.2, 143.0, 142.8, 137.9, 136.5, 135.2, 132.0, 127.9, 123.9, 122.2, 111.3, 108.7, 85.5, 46.9, 46.8, 33.1, 32.9, 26.3, 25.3, 21.0, 20.9; HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₄H₁₈NO⁺ and C₁₄H₁₇INO⁺: 216.1383 and 342.0349, found: 216.1385 and 342.0348.

Supplemental Material

Supporting_Information_for_An_iodine-promoted_one-pot_and_metal-free_access_to_indolin-2-ones – Supplemental material for An iodine-promoted one-pot and metal-free access to indolin-2-ones

Supplemental material, Supporting_Information_for_An_iodine-promoted_one-pot_and_metal-free_access_to_indolin-2-ones for An iodine-promoted one-pot and metal-free access to indolin-2-ones by Yong Zhang, Xi Zong and Min Ji in Journal of Chemical Research

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (Grant No. 81671745).

ORCID iD

Yong Zhang

Supplemental material

Supplemental material for this article, which contains the NMR spectra and preliminary study of reaction mechanism, is available online.

References

Dounay

Overman

LE.

Chem Rev 2003; 103: 2945.

Cerchiaro

Ferreira

AMdC

. J Brazil Chem Soc 2006; 17: 1473.

Higuchi

Kawasaki

Nat Prod Rep 2007; 24: 843.

Faivre

Niccoli

Castellano

, et al. Ann Oncol 2017; 28: 339.

Roth

Binder

Colbatzky

, et al. J Med Chem 2015; 58: 1053.

Crestini

Saladino

Synth Comm 1994; 24: 2835.

Vazquez

Payack

JF.

Tetrahedron Lett 2004; 45: 6549.

Beckett

Daisley

Walker

Tetrahedron 1968; 24: 6093.

Jones

Wilkinson

Ewin

Tetrahedron Lett 1994; 35: 7673.

10.

Clark

Davies

Jones

, et al. J Chem Soc Chem Comm 1994; 1: 41.

11.

Jones

Brunton

Gosain

Tetrahedron Lett 1999; 40: 8935.

12.

Nishio

Iseki

Araki

, et al. Helv Chim Acta 2005; 88: 35.

13.

Theunissen

Wang

Evano

Chem Sci 2017; 8: 3465.

14.

Kong

Wang

Zhu

Angew Chem Int Edit 2016; 55: 9714.

15.

Kilaru

Acharya

Zhao

Chem Commun 2018; 54: 924.

16.

Gui

Chen

, et al. Asian J Org Chem 2015; 4: 870.

17.

Liu

Zhang

, et al. Org Lett 2013; 15: 6166.

18.

Dong

Liu

, et al. Chem Commun 2015; 51: 4587.

19.

Finkbeiner

Nachtsheim

BJ.

Synthesis 2013; 45: 979.

20.

Breugst

von der Heiden

Chem Eur J 2018; 24: 9187.

21.

Geng

Zhao

, et al. Org Lett 2017; 19: 4584.

22.

Zhang

Gao

, et al. Org Lett 2017; 19: 408.

23.

Gao

Geng

, et al. Org Lett 2016; 18: 2507.

24.

Gaur

Kumar

Dey

, et al. ACS Appl Mater Inter 2016; 8: 10690.

25.

Shivashimpi

Pandey

Ogomi

, et al. J Photoch Photobio A 2014; 273: 1.

26.

Liu

Chen

, et al. Tetrahedron Lett 2015; 56: 2332.

27.

Chen

Wei

, et al. Chem Commun 2016; 52: 6455.

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