Copper-Catalyzed Cascade Cyclization of 1,7-Enynes toward

Letter pubs.acs.org/OrgLett

Copper-Catalyzed Cascade Cyclization of 1,7-Enynes toward Trifluoromethyl-Substituted 1′H‑Spiro[azirine-2,4′-quinolin]2′(3′H)‑ones Qiang Meng, Fei Chen, Wei Yu,* and Bing Han* State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, P. R. China S Supporting Information *

ABSTRACT: A novel method for the synthesis of trifluoromethyl-containing 1′Hspiro[azirine-2,4′-quinolin]-2′(3′H)-ones by a CF3-radical-triggered tandem reaction of benzene-linked 1,7-enynes is described. This protocol utilizes 1trifluoromethyl-1,2-benziodoxole as the trifluoromethylating reagent and TMSN3 as the aminating reagent. By this method, various potentially bioactive trifluoromethylated 1′H-spiro[azirine-2,4′-quinolin]-2′(3′H)-ones were facilely synthesized via a radical cascade process.

A

Scheme 1. Radical-Triggered 1,7-Enyne Cyclization and Trifluoromethylazidation of Alkynes

s prevalent scaffolds, 3,4-dihydroquinolin-2(1H)-ones and 2H-azirines widely exist in many bioactive natural products and drugs.1 For example, aripiprazole is a new atypical antipsychotic.2 Vesnarinone is used to treat congestive heart failure.3 Thiotepa can prevent the recurrence of bladder cancer.4 Tretamine can inhibit the proliferation of gastric cancer cells (Figure 1).5 Because of their structural importance, efficient syntheses of two such heterocyclic skeletons have drawn much attention from chemists and pharmacologists.6

Figure 1. Drugs featuring 3,4-dihydroquinolin-2(1H)-ones and 2Hazirines.

by a sequential process including copper-catalyzed trifluoromethylazidation of alkynes and photocatalyzed rearrangement of CF3 vinyl azides (Scheme 1c).8e Afterward, Liang’s group exploited a similar copper-catalyzed method to construct βtrifluoromethylated 2H-azirines from alkynes in one pot.8f Despite these elegant achievements for this purpose,8 the synthesis of trifluoromethyl-substituted 1′H-spiro[azirine-2,4′quinolin]-2′(3′H)-ones via a tandem reaction of benzenelinked 1,7-enynes has not received enough attention, and its synthetic potential has remained largely unappreciated. In continuation of our research on the construction of structurally important heterocyclic scaffolds via radical cyclizations and annulations,9 we recently developed a novel and facile CF3-radical-triggered tandem approach10 toward the structurally important trifluoromethylated 1′H-spiro[azirine-2,4′-qui-

Radical cascade reactions constitute highly efficient strategies for the preparation of small molecules with structural diversity and complexity.7 As a kind of easily accessible precursors, 1,7enynes can be used to synthesize 3,4-dihydroquinolin-2(1H)ones conveniently via a radical cascade reaction. For instance, in 2014, Li’s group reported a simple and efficient radical-based one-pot approach to produce pyrrolo[4,3,2-de]quinolinones via cascade nitration/cyclization of 1,7-enynes (Scheme 1a).8a Later on, Li’s group and Tu’s group independently developed new radical [2 + 2 + 1] carbocyclization reactions between benzene-linked 1,7-enynes and a cyclic methylene group to incorporate a fused five-membered carbocyclic ring into the 3,4-dihydroquinolin-2(1H)-one moiety (Scheme 1b).8b,c On the other hand, trifluoromethylazidation of alkynes has become one of the most effective means for the synthesis of 2H-azirines in recent years. For example, in 2015, Liu’s group reported an approach for the selective synthesis of trifluoromethylsubstituted azirines from simple alkynes, which was enabled © 2017 American Chemical Society

Received: August 8, 2017 Published: September 21, 2017 5186

DOI: 10.1021/acs.orglett.7b02453 Org. Lett. 2017, 19, 5186−5189

Letter

Organic Letters

several other solvents such as DMSO, CH3CN, and toluene, but the results obtained under these conditions were less satisfactory (Table 1, entries 17−19). When the reaction was carried out under the conditions of TMSN3 without Togni’s reagent, no separable product was obtained, and the substrate 1a was recovered in 85% yield (Table 1, entry 20). With the optimized reaction conditions in hand (Table 1, entry 9), a variety of 1,7-enynes were reacted with TMSN3 and Togni’s reagent to investigate the scope of this cyclization protocol. As shown in Scheme 2, substituents on the phenyl

nolin]-2′(3′H)-ones (Scheme 1d) from benzene-linked 1,7enynes.11 The reaction, which was effected using readily accessible Togni’s reagent as the trifluoromethylating reagent and commercially available TMSN3 as the aminating reagent, proceeded highly efficiently in a cascade inter/intramolecular C−C/C−C/CN/C−N bond-forming pattern to construct simultaneously both the quinolinone and azirine scaffolds. Herein we present the results. We initiated our investigation by examining the reaction of 1a (0.2 mmol) with TMSN3 (0.5 mmol) and Togni’s reagent (0.5 mmol) in DMF. To our delight, when 20 mol % CuCl was used as the catalyst, the desired product 2a was obtained in 47% yield after the reaction was stirred for 5 h at 60 °C (Table 1, entry 1). Significantly, when TMSN3 and Togni’s reagent

Scheme 2. Scope of 1,7-Enynesa,b

Table 1. Optimization of the Reaction Conditionsa

entry c

1 2 3 4 5 6 7 8 9 10d 11e 12 13 14 15 16 17 18 19 20f

catalyst (mol %)

t (°C)

solvent

yield (%)b

CuCl (20) CuCl (20) CuBr (20) CuI (20) CuCN (20) Cu(OAc)2 (20) Cu(OTf)2 (20) CuCl2 (20) CuSO4 (20) CuSO4 (20) CuSO4 (20) CuSO4 (20) CuSO4 (20) CuSO4 (5) CuSO4 (10) CuSO4 (30) CuSO4 (20) CuSO4 (20) CuSO4 (20) CuSO4 (20)

60 60 60 60 60 60 60 60 60 60 60 70 50 60 60 60 60 60 60 60

DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMSO CH3CN toluene DMF

47 61 52 55 56 50 31 29 88 67 53 59 60 68 74 80 70 32 15 0

a

Reaction conditions: 1a (0.2 mmol), TMSN3 (0.5 mmol), Togni’s reagent (0.5 mmol), and DMF (1 mL) under argon for 5 h. TMSN3 and Togni’s reagent were divided into two equal parts and added per 2.5 h, except as noted. bIsolated yields. cTMSN3 (0.5 mmol) and Togni’s reagent (0.5 mmol) were added to the system once. dTMSN3 (0.3 mmol) and Togni’s reagent (0.3 mmol) were used. eTMSN3 (0.2 mmol) and Togni’s reagent (0.2 mmol) were used. fOnly TMSN3 (0.5 mmol) was used.

a All of the reactions were run in DMF (1 mL) using 1 (0.2 mmol), TMSN3 (0.5 mmol), Togni’s reagent (0.5 mmol), and CuSO4 (20 mol %) at 60 °C under Ar for 5 h. bIsolated yields are shown. cThe reaction was conducted on a 4 mmol scale of 1a. d The diastereoisomeric ratios were determined by 1H NMR spectroscopy. e The diastereoisomers were separated by flash column chromatography. fTogni’s reagent (0.5 mmol) was added once, and TMSN3 was not used.

were divided into two equal parts and added separately (at the interval of 2.5 h), the yield of 2a increased to 61% (Table 1, entry 2). To further improve the yield, other copper salts such as CuBr, CuI, CuCN, Cu(OAc)2, Cu(OTf)2, CuCl2, and CuSO4 were investigated, among which CuSO4 was found to be the most efficient, with the yield of 2a being improved to 88% (Table 1, entries 3−9). The yield of 2a decreased when lower amounts of TMSN3 and Togni’s reagent were used (Table 1, entries 10 and 11). The optimal reaction temperature was found to be 60 °C; the yield of 2a was lower at higher or lower temperature (Table 1, entries 12 and 13). Reducing or increasing the amount of CuSO4 did not give better results (Table 1, entries 14−16). The reaction was also carried out in

ring of the aniline moiety, such as Me, F, Cl, and Br, were welltolerated under the standard conditions, and the corresponding products 2b−e were obtained in good to excellent yields. When the phenyl group had multiple substituents such as 2,4dimethyl, the reaction also proceeded smoothly to give 2f in 61% yield. Various N-protecting groups, such as N-isopropyl, N-Bn, N-Ms, N-SO2Ph, N-SO2C6H4Cl-m, N-SO2C6H4Cl-p, NSO2C6H4Me-o, N-Ts, and N-2-naphthalenylsulfonyl, were 5187

DOI: 10.1021/acs.orglett.7b02453 Org. Lett. 2017, 19, 5186−5189

Letter

Organic Letters tolerated as well to afford the desired products 2g−o in moderate to good yields. The structure of 2k was confirmed by single-crystal X-ray analysis (see the Supporting Information). However, the desired reaction failed to occur for Nunprotected amide 1p, which was converted into 3 in 69% yield. The reason for this result might be that existence of Nprotecting groups engenders a larger population of the reactive rotamer in which the radical center is closer to the triple bond, thereby facilitating the intramolecular annulation.12 The present protocol was also ineffective for the phenyl-substituted substrate 1q and unsubstituted allylic substrate 1r. In the case of 1q, compound 4 was generated in a yield of 30%. The yield of 4 could be raised to 72% by removing TMSN3 from the reaction system. The effect of substituents on the phenyl ring in the terminal alkyne was also studied, and it can be seen that substituents such as p-MeO, p-Me, p-tBu, p-Cl, and p-phenyl groups were all well-tolerated under the optimal conditions (2s−w). Thiophene-substituted alkyne 1x was also a suitable candidate, which was converted into the corresponding product 2x in moderate yield. Unfortunately, alkyl-substituted alkyne 1y had no reactivity in the method. It is noteworthy that this method would be useful for practical synthesis. As such, the reaction of 1a (4 mmol) proceeded well on a gram scale to afford 2a in 77% yield (1.10 g). Having successfully achieved the tandem reaction with various benzene-linked 1,7-enynes, we then shifted our attention to investigate the benzene-linked 1,6-enyne 1z. However, the reaction of 1z under the standard conditions did not yield the expected product 5; instead, compound 6 was obtained in a yield of 41%, probably because of the lower stability of product 5 (Scheme 3).

Scheme 5. Control Experiments

2). These results suggest that a radical process might be involved in the present reaction. On the basis of the present results and literature studies,14 a plausible mechanism is proposed in Figure 2 to rationalize this

Figure 2. Proposed mechanism.

Scheme 3. Reaction of 1,6-Enynes

1,7-enyne-cascade process. The reaction is believed to be initiated by the formation of CF3 radical, which is derived from Togni’s reagent through the action of Cu(I) species. Although CuSO4 was initially used in the current reaction system, it can be reduced in situ to Cu(I) in small amounts by reaction with certain reducing species such as azide anion. Trapping of CF3 radical by 1 via radical addition results in the formation of radical intermediate A. Two mechanisms might work for the subsequent reaction of A. First, A might cyclize to give radical B (path a); B then would undergo azidation by the action of Cu(II) to afford compound E. 2 would finally be formed from E via denitrogenative cyclization. Besides this path, it is also possible that the formation of E follows an alternative pathway where trapping of radical A by Cu(II) takes place before the cyclization step (path b). In this case, intermediate C would be generated first from A and then would be transformed into E via cyclization. In conclusion, we have developed an efficient Cu-catalyzed method for the synthesis of trifluoromethylated 1′H-spiro[azirine-2,4′-quinolin]-2′(3′H)-ones from benzene-linked 1,7enynes. The present reactions take place via a CF3-radicaltriggered cascade that comprises a sequential inter/intramolecular C−C/C−C/CN/C−N bond-forming process. Since a wide range of substrates can be utilized for the cascade annulation under mild conditions, this protocol may provide a general approach toward structurally important 1′H-spiro[azirine-2,4′-quinolin]-2′(3′H)-ones. Further studies on applying radical cascade reactions to the construction of heterocyclic scaffolds are in progress in our laboratory.

Organic azirines are useful nitrogen-containing building blocks in synthetic chemistry.13 The azirine ring in compound 2 is convertible by subsequent operations. For instance, 2a can react with methyl 2-diazo-2-phenylacetate in DCM under the catalysis of 3.3 mol % Rh2(OAc)4 to generate 7 in 70% yield (Scheme 4). Scheme 4. Further Synthetic Transformation of 2a

To gain insights into the mechanistic pathway, radical trapping experiments were conducted. As shown in Scheme 5, when 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) was added to this reaction system under the standard reaction conditions, only a trace amount of 2a was detected; most of the 1a was recovered along with a tiny amount of TEMPO−CF3 adduct 8 (Scheme 5, eq 1). When 2,6-di-tert-butyl-4-methylphenol (BHT) was added to this reaction system, the yield of 2a dropped to 45%, with 22% of 1a being recovered (Scheme 5, eq 5188

DOI: 10.1021/acs.orglett.7b02453 Org. Lett. 2017, 19, 5186−5189

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Organic Letters

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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.7b02453. Detailed experimental procedures and spectral data for all products (PDF) Crystallographic data for 2k (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Wei Yu: 0000-0002-3131-3080 Bing Han: 0000-0003-0507-9742 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21422205, 21272106, and 21632001), the Changjiang Scholars and Innovative Research Team in University (IRT15R28), the “111” Project, and the Fundamental Research Funds for the Central Universities (lzujbky-2016-ct02 and lzujbky-2016-ct08) for financial support.

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DOI: 10.1021/acs.orglett.7b02453 Org. Lett. 2017, 19, 5186−5189