Combining Visible-Light-Photoredox and Lewis Acid Catalysis for the

Dec 7, 2017 - One-step construction of substituted indolizino[1,2-b]quinolin-9(11H)-ones was achieved by combining visible-light-photoredox and Lewis ...
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Letter Cite This: Org. Lett. 2018, 20, 80−83

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Combining Visible-Light-Photoredox and Lewis Acid Catalysis for the Synthesis of Indolizino[1,2‑b]quinolin-9(11H)‑ones and Irinotecan Precursor Wuheng Dong, Yao Yuan, Bei Hu, Xiaoshuang Gao, Huang Gao, Xiaomin Xie, and Zhaoguo Zhang* School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China S Supporting Information *

ABSTRACT: One-step construction of substituted indolizino[1,2-b]quinolin-9(11H)-ones was achieved by combining visible-light-photoredox and Lewis acid catalysis for an intramolecular Povarov cycloaddition reaction under mild conditions. In this catalytic process, the visible-light-promoted dehydrogenation protocol of tetrahydroquinolines constitutes the key procedure. Moreover, this method can be applied to the formal synthesis of the precursor of irinotecan, which exhibited good anticancer activities.

I

Yao7f,g developed cascade reactions triggered by Me3OBF4 or bis(triphenyl)oxodiphosphonium trifluoromethanesulfonate, affording a variety of indolizino[1,2-b]quinolin-9(11H)-ones compounds (Scheme 1a and 1b). Moreover, a Dy(OTf)3-

ndolizino[1,2-b]quinolin-9(11H)-ones represent an important class of N-containing heterocycle skeletons that are present in a variety of natural products and biologically active molecules. For instance, camptothecin was isolated from camptotheca acuminate by Wani and Wall,1 which has been proven to be a potent tumor inhibitor. Subsequent belotecan2 and topotecan3 are two commercially available anticancer drugs that are the most frequently used anticancer drugs in clinical settings. In addition, irinotecan4 and camptothecin analogues are commonly used anticancer drugs (Figure 1). Therefore, a

Scheme 1. Povarov Cycloaddition Reaction for Synthesis of Quinoline Derivatives

catalyzed Povarov cycloaddition/aromatization cascade reaction between an N-arylimine and an unactivated alkyne was also found in the synthesis of this structure by Batey et al. in 2004 (Scheme 1c).7d In the above cases, an alkyne, which serves as a dienophile, is often required. Despite these advances, more efficient and straightforward protocols for synthesizing diverse indolizino[1,2-b]quinolin9(11H)-ones using olefins as a dienophile under mild reaction conditions remains a major challenge.

Figure 1. Representative drugs with indolizino[1,2-b]quinolin-9(11H)one frameworks.

number of approaches for their synthesis have been developed, which involve transition-metal-catalyzed cross-coupling,5 ringclosing metathesis,6 Povarov cycloaddition,7 and radical reactions.8 Among them, the Povarov cycloaddition reaction of N-aryl imines derived from aldehydes and anilines with electronrich olefins was thought to constitute the most efficient method for tetrahydroquinolines synthesis.9 To furnish the quinolone structure, however, the dehydrogenation protocol requires a stoichiometric oxidant and harsh conditions.10 Fortunak7a,b and © 2017 American Chemical Society

Received: November 1, 2017 Published: December 7, 2017 80

DOI: 10.1021/acs.orglett.7b03395 Org. Lett. 2018, 20, 80−83

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Organic Letters Table 1. Optimization of Reaction Conditionsa

In the past few years, visible-light-induced chemical transformations have attracted increased interest from organic chemists due to their green and sustainable features.11 Recently, this strategy has been successfully applied to catalytic dehydrogenation of tetrahydroquinolines. For example, in 2016, Badu-Tawiah and co-workers presented a concise synthesis of quinolines through visible-light-promoted dehydrogenation of tetrahydroquinolines under ambient conditions (Scheme 2a).12 Soon afterward, the Li group reported a dual Scheme 2. Visible-Light-Promoted Dehydrogenation of Tetrahydroquinolines

entry

photocatalyst

catalyst

solvent

yieldb(%)

1 2 3 4 5 6 7 8 9 10c 11d 12f 13g 14h 15i 16j

Eosin Y Ru(bpy)3Cl2·6H2O Ru(bpy)3(PF6)2 Ru(bpy)3Cl2·6H2O Ru(bpy)3Cl2·6H2O Ru(bpy)3Cl2·6H2O Ru(bpy)3Cl2·6H2O Ru(bpy)3Cl2·6H2O Ru(bpy)3Cl2·6H2O Ru(bpy)3Cl2·6H2O Ru(bpy)3Cl2·6H2O Ru(bpy)3Cl2·6H2O

BF3·Et2O BF3·Et2O BF3·Et2O TsOH·H2O Zn(OTf)2 TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O TsOH·H2O

CH3CN CH3CN CH3CN CH3CN CH3CN MeOH DMF DCM toluene MeOH MeOH MeOH MeOH MeOH MeOH MeOH

20 >95 >95 >95 95 >95 83 64 28 >95 >95 (96)e 30 25 0 0 0

TsOH·H2O Ru(bpy)3Cl2·6H2O Ru(bpy)3Cl2·6H2O

TsOH·H2O TsOH·H2O

a

Conditions: 1a (61.0 mg, 0.3 mmol), 2a (32.7 mg, 0.3 mmol), photocatalyst (2 mol %), catalyst (10 mol %), solvent (3 mL), irradiation with 23 W household light bulb at rt for 24 h. b1HNMR yield was reported using 4,4′-ditertbutylbiphenyl as an internal standard. cPhotocatalyst (0.5 mol %). dCatalyst (5 mol %). eIsolated yield. fWithout TsOH·H2O. gWithout photocatalyst. hWithout photocatalyst and TsOH·H2O. iReaction was run in the dark. jUnder N2 atmosphere.

photo- and cobalt-catalyzed dehydrogenation of N-heterocycles (Scheme 2b).13 Inspired by these two reports, we envisaged an intramolecular Povarov cycloaddition reaction between in situ generated benzylidene anilines derived from arylamines and salicylaldehyde bearing a tethered alkene partner, followed by a visible-light-induced photocatalytic dehydrogenation to yield the target products (Scheme 2c). As part of our ongoing research into the synthesis of heterocycles under visible light, we herein report an efficient and alternative route to indolizino[1,2b]quinolin-9(11H)-ones using a photoredox strategy. Considering that the alkene could serve as synthons for introducing other functional groups,14 we chose 1-((2E,4E)hexa-2,4-dien-1-yl)-6-oxo-1,6-dihydropyridine-2-carbaldehyde (1a) as the model substrates to test the feasibility of visible-light photoredox catalysis (Table 1). Initially, we investigated the reaction of 1-((2E,4E)-hexa-2,4-dien-1-yl)-6-oxo-1,6-dihydropyridine-2-carbaldehyde (1a) and 4-aminophenol (2a) using Eosin Y (2 mol %) as a photocatalyst with BF3·Et2O (10 mol %) as a catalyst in acetonitrile under the irradiation of a 23 W household compact fluorescent lamp at room temperature for 24 h. Fortunately, the starting material was completely converted to the corresponding tetrahydroquinolines (II), along with the target product 3a in 20% yield (entry 1). Since we thought that the photocatalyst might play an important role in the dehydrogenation of tetrahydroquinolines, other commonly used transitional metal photocatalysts were examined to improve reaction efficiency. To our satisfaction, when both Ru(bpy)3Cl2· 6H2O and Ru(bpy)3(PF6)2 were used as the photocatalyst, the desired product could be obtained at more than a 95% yield. Other acids, such as p-methylbenzene sulfonic acid and zinc trifluoromethanesulfonate, exhibited a similarly high catalytic efficiency (Table 1, entries 4 and 5). In addition, solvent screening proved that acetonitrile and methanol were superior to other solvents (Table 1, entries 6−9). Further optimization

showed that the loading of acid and photocatalyst could be decreased to 5 mol % and 0.5 mol %, respectively, without diminishing product yield (Table 1, entries 10 and 11). Finally, some control experiments revealed that both photocatalyst and visible light were necessary. Specifically, only 25% of the desired product was obtained when the photocatalyst was absent, and no desired product was detected when the reaction was conducted either in the dark or under a N2 atmosphere (Table 1, entries 12− 16). With the optimized reaction conditions elucidated, we next investigated the scope of this cascade process by utilizing various substituted anilines in combination with 1-((2E,4E)-hexa-2,4dien-1-yl)-6-oxo-1,6-dihydropyridine-2-carbaldehyde (1a) as the reaction partner (Scheme 3; for the crystal structures of 3a, 3d, 3j, 3k, and 3m; see Supporting Information). The substituent on the aniline plays a critical role in the success of the dehydrogenation of tetrahydroquinolines. Both weakly electron-donating groups (Scheme 3, 3b) and strongly electrondonating groups (Scheme 3, 3c, 3d) located para to the aniline resulted in excellent yields. However, aniline with electronwithdrawing groups at this position exhibited an obvious negative effect on the yield (Scheme 3, 3e, 3f). Increasing steric demand ortho to the amino group did not compromise reaction efficiency, and a high yield of target product was obtained (Scheme 3, 3h, 3i). The scope of the reaction was next examined with respect to the substituent effect (R1) on the alkene terminus. With substrates bearing no substituent or more than one substituent at the C−C double bond, corresponding products were obtained in excellent yield (Scheme 3, 3j, 3k). Moreover, the R1-alkyl substituent can be replaced by an aryl substituent, as documented for the phenyl (Scheme 3, 3l). A good yield was also 81

DOI: 10.1021/acs.orglett.7b03395 Org. Lett. 2018, 20, 80−83

Letter

Organic Letters Scheme 3. Substrate Scopea

chloride and DMSO. Subsequently, under the optimized conditions, the Povarov cycloaddition/dehydrogenation aromatization cascade reaction of aldehyde 7 and 4-aminophenol gave a good yield (92%) of the product 8, which constitutes the key synthetic intermediate to 10-hydroxycamptothecin6l,15 and irinotecan 9.16 Based on the above results and related reports,6m,13 we propose a plausible mechanism to account for the reaction in Scheme 5. The intermediate II is formed via an intramolecular Scheme 5. Proposed Mechanism for the Catalysis

a

Conditions: 1 (0.3 mmol), 2 (0.3 mmol), photocatalyst (0.5 mol %), catalyst (5 mol %), solvent (3 mL), irradiation with 23 W household light bulb at rt for 24 h. b96 h irradiation.

attained when a conjugated diene was replaced by styrene functionality (Scheme 3, 3m). However, no desired product was obtained, except for a mixture of imine and corresponding tetrahydroquinolines in the substrate of ethenyl or crotyl functionality (Scheme 3, 3n and 3o). Furthermore, we utilized this mild and efficient cascade reaction to synthesize the irinotecan precursor from the known methyl 4-ethyl-8-oxo-7,8-dihydro-1H-pyrano[3,4-c]pyridine-6carboxylate 47f (Scheme 4).

Povarov cycloaddition reaction between the in situ generated imine I derived from 1a and 2a. Then, the intermediate II was oxidized by the excited RuII* to give the radical amine cation III and the reduced species RuI. The intermediate III was converted to intermediate IV or V. The desired product 3a was then obtained after a further oxidative dehydrogenation reaction. In summary, we have developed an intramolecular Povarov cycloaddition reaction to construct substituted indolizino[1,2b]quinolin-9(11H)-one, and visible-light-promoted dehydrogenation of tetrahydroquinolines as the key procedure. Furthermore, this reaction has been successfully applied to the formal synthesis of the precursor of irinotecan 9, which possesses anticancer activities. The applicability of readily available starting materials and the use of air as the sole oxidant under mild reaction conditions make this process very attractive. Further applications of this transformation toward other heterocyclic products are currently underway.

Scheme 4. Synthetic Route to the Irinotecan Precursor

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ASSOCIATED CONTENT

S Supporting Information *

The corresponding pyridone 5 was obtained by the Nallylation of 4 with (E)-5-bromopenta-1,3-diene in 50% yield, and then the reduction of pyridone 5 selectively achieved the corresponding alcohol 6 (92%). The important intermediate, aldehyde 7, has been previously synthesized in 85% yield by the Swern oxidation of the corresponding alcohol 6 with oxalyl

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03395. Preparation of substrates; general procedure; X-ray crystal structure of 3a, 3d, 3j, 3k, and 3m; characterization data; 1 H and 13C NMR spectra (PDF) 82

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Organic Letters Accession Codes

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CCDC 1573982, 1573984−1573985, and 1573987−1573988 contain 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]. ac.uk, 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

Xiaomin Xie: 0000-0002-5798-291X Zhaoguo Zhang: 0000-0003-3270-6617 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support provided by the National Natural Science Foundation of China (No. 21232003). We also express gratitude for the support and valuable suggestions regarding SC-XRD measurements from Ms. Xiao-li Bao and Ms. Ling-ling Li of the Instrumental Analysis Center of Shanghai Jiao Tong University.

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REFERENCES

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DOI: 10.1021/acs.orglett.7b03395 Org. Lett. 2018, 20, 80−83