Synthesis of Fused or Spiro Polyheterocyclic Compounds via

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

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Synthesis of Fused or Spiro Polyheterocyclic Compounds via the Dehydrogenative Annulation Reactions of 2‑Arylindazoles with Maleimides Chenhao Guo, Bin Li, Huilai Liu, Xiaopeng Zhang, Xinying Zhang,* and Xuesen Fan*

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Henan Key Laboratory of Organic Functional Molecules and Drug Innovation, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, School of Chemistry and Chemical Engineering, School of Environment, Henan Normal University, Xinxiang, Henan 453007, China S Supporting Information *

ABSTRACT: A dehydrogenative annulation of 2-arylindazoles with maleimides for the switchable synthesis of indazolo[2,3-a]pyrrolo[3,4-c]quinolinones or spiroindolo[1,2-b]indazole-11,3′-pyrrolidinones is presented. Mechanistically, the formation of the title compounds involves a Rh(III)catalyzed C−H metalation of 2-arylindazole, followed by maleimide insertion and intramolecular cyclization. Interestingly, the selectivity to form the fused or spiro compounds could be switched by resorting to different additives. The notable features of this protocol include simple substrates and excellent atom economy and regioselectivity.

I

The construction of PHCs via transition-metal-catalyzed dehydrogenative annulation reactions (DHARs) of heteroaromatics with different coupling partners is rapidly flourishing. Using this strategy, PHCs could be prepared in an atomeconomy and environmentally friendly manner by using simple and affordable starting materials without prefunctionalization of the substrates.7 In continuation of our interest in this aspect,8 we have explored the synthesis of indazolo[2,3a]pyrrolo[3,4-c]quinolinones as a novel class of PHCs through our designed dehydrogenative annulation between 2-arylindazole9,10 and maleimide11−13 by using the N1 atom of indazole as a possible coordinating and activating site. During this study, we serendipitously found that the reaction of 2arylindazoles with maleimides could not only afford the initially designed indazolo[2,3-a]pyrrolo[3,4-c]quinolinones but also lead to the formation of spiroindolo[1,2-b]indazole11,3′-pyrrolidinones. Promisingly, the formation of different compounds could be easily switched by resorting to different additives (Scheme 1, (3)). It is noted that Punniyamurthy and coworkers described a pioneering annulation of 2-arylindazoles with alkynes to give indazolo[2,3-a]quinoline derivatives with good efficiency (Scheme 1, (1)).10 In addition, Prabhu et al. reported an elegant switchable synthesis of indole-based maleimides in which the additive plays an important role (Scheme 1, (2)).11g Initially, 2-phenyl-2H-indazole (1a) was allowed to react with N-methylmaleimide (2a) in the presence of [RhCp*Cl2]2

ndazole is not only a key scaffold of many natural products but also an essential unit of a number of synthetic compounds possessing a wide spectrum of pharmaceutical activities including anticancer, antiviral, antidepressant, and anti-inflammatory (Figure 1).1,2 In addition, indazole-related

Figure 1. Important indazole and maleimide derivatives.

conjugated systems are of great interest in biological, chemical, and material sciences.1,3 Despite their importance, reliable methods for the synthesis of indazole-related polyheterocycles (PHCs) are rather limited,4 and some of them still suffer from the use of highly functionalized substrates, the production of a large amount of waste, and low atom economy. On the other hand, maleimide derivatives are well known for their unique photophysical properties and potent pharmaceutical activities (Figure 1).5,6 Given the significance of both indazole- and maleimide-containing compounds, it is reasonable to expect that hybrid compounds combining these two pharmacophores may be bestowed with enhanced or synergetic biological and physical characteristics compared with their parent structures. © XXXX American Chemical Society

Received: June 2, 2019

A

DOI: 10.1021/acs.orglett.9b01889 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 1. C(sp2)−H Functionalizations with Maleimide as the Coupling Partner

carboxylic acid (ADA), 3a was obtained in an excellent yield of 90%, and the formation of 4a was not observed (entry 14). After establishing the optimum reaction conditions for the formation of 3a, we investigated the appropriate conditions for the formation of 4a. The rationale behind this endeavor is that (1) spiroheterocycles are of great interest in synthetic and medicinal chemistry and (2) in previous reports, spirosuccinimide derivatives were formed through the kinetically more favorable N-nucleophilic attack,12 and the formation of spirosuccinimides via C-nucleophilic attack in the generation of 4a has not been reported. Considering that acid as an additive was beneficial for the formation of 3a, we postulated that a base as an additive might be advantageous for the formation of 4a. Thus several tertiary amines including triethylamine (Et 3 N), N,N-dicyclohexyl methylamine (Cy2NMe), and N,N-diisopropyl ethylamine (DIPEA) were employed. It was observed that in the presence of DIPEA, 4a was obtained in 51% yield, and the yield of 2a dropped to 26% (entry 17).14 After optimizing the reaction conditions, the substrate scope for the synthesis of indazolo[2,3-a]pyrrolo[3,4-c]quinolinone 3 was explored. First, with 2a as a model substrate, the reactions of several 2-aryl-2H-indazoles 1 were studied. The results listed in Scheme 2 showed that 1 with an electron-donating group (EDG) such as methyl or methoxy or an electron-withdrawing group (EWG) such as fluoro-, chloro-, or trifluoromethyl attached on the para position of the 2-phenyl unit participated in the reaction efficiently to afford 3b−f in 72−93% yield. For 1 with a meta-methyl-, methoxy-, or chloro-substituted 2-

and Cu(OAc)2 in 1,2-dichloroethane (DCE) at 120 °C under air for 12 h. To our delight, the product, 2-methyl-1Hindazolo[2,3-a]pyrrolo[3,4-c]quinoline-1,3(2H)-dione (3a), was formed in 40% yield (Table 1, entry 1). Meanwhile, the Table 1. Optimization Study for the Synthesis of 3a and 4aa

yield (%)b entry

oxidant

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

Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2·H2O Cu(OTf)2 AgOAc Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2

additive

solvent

3a

4a

HOAc TFA TsOH ADA Et3N Cy2NMe DIPEA

DCE CH3CN THF dioxane HFIP toluene PhCl toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene

40 32 35 41 20 49 45 38 trace trace 40 58 78 90 24 30 26

35 25 23 30 trace 38 36 30 trace trace 26 20 8

Scheme 2. Substrate Scope for the Synthesis of 3a

40 38 51

a

Conditions: 1a (0.3 mmol), 2a (0.45 mmol), [RhCp*Cl2]2 (0.015 mmol), oxidant (0.6 mmol), additive (0.3 mmol), solvent (3 mL), 120 °C, 16 h. bIsolated yield.

spirosuccinimide derivative, 1′-methylspiro[indolo[1,2-b]indazole-11,3′-pyrrolidine]-2′,5′-dione (4a), was also obtained in 35% yield. To improve the efficiency and selectivity, CH3CN, tetrahydrofuran (THF), dioxane, hexafluoroisopropanol (HFIP), toluene, and chlorobenzene (PhCl) were tried as the solvent (entries 2−7), and toluene was found to be more efficient than others to give 3a and 4a in 49 and 38% yields, respectively (entry 6). Cu(OAc)2·H2O, Cu(OTf)2, and AgOAc were found to be less effective than Cu(OAc)2 as an oxidant in the reaction (entries 8−10). We were pleased to find that using acid additives enhanced the selectivity toward the formation of 3a (entries 11−14). In the presence of 1-adamantane

a

Conditions: 1 (0.3 mmol), 2 (0.45 mmol), [RhCp*Cl2]2 (0.015 mmol), Cu(OAc)2 (0.6 mmol), ADA (0.3 mmol), toluene (3 mL), 120 °C, 16 h. bIsolated yield. B

DOI: 10.1021/acs.orglett.9b01889 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

The structure of the formed spirocyclic compound was established by single-crystal X-ray diffraction analysis of 4g. It is also worth mentioning that the preparation of 3a was performed on a 5 mmol scale, affording the product in 75% yield. To understand the reaction pathway of the cascade process, a set of experiments was conducted. First, 1a was subjected to CD3OD under standard conditions for the formation of 3a, resulting in deuterium incorporations at the ortho site of the 2phenyl unit and the C3 position of the indazolyl scaffold (Scheme 4). These results indicate that the C−H bond cleavage should be reversible.

phenyl unit, the reaction was regioselective and gave 3g, 3h, and 3i in 85, 90, and 77% yield, respectively. Notably, 1 with an ortho-substituted 2-phenyl moiety gave 3j in 89% yield. Next, it was noted that 1 bearing a methoxy, fluoro, or chloro unit on the 5-position, a methoxy unit on the 6-position, or a methylenedioxy group on the 5,6-positions of the indazoyl scaffold participated in the reaction smoothly to give 3k−o in good yield. Moreover, halogenated aromatic moieties tolerated the reaction conditions well, making subsequent structural elaboration possible. The versatility for the preparation of 3 was further explored by the reaction of different maleimide derivatives 2. It was thus found that N-ethyl-, N-benzyl-, Ncyclohexyl-, or N-tert-butyl-substituted maleimides reacted with 1a efficiently to furnish the corresponding products 3p, 3q, 3r, and 3s in 91, 92, 91, and 88% yields, respectively. In addition to N-alkyl maleimides, N-phenyl maleimide could also take part in the reaction to give 3t and 3u efficiently. Next, the substrate scope for the preparation of spiroindolo[1,2-b]indazole-11,3′-pyrrolidinone derivatives 4 was studied (Scheme 3). 1 with various substituents attached on

Scheme 4. Reversibility of C−H Bond Cleavage

An intermolecular kinetic isotopic effect (KIE) study was carried out with the competition reactions between 1a with 2a and 1a-d3 with 2a. As a result, a KIE value (KH/KD) of 4.56 was observed (Scheme 5). This result indicates that the cleavage of the ortho-C−H bond of the 2-phenyl unit is probably involved in the rate-limiting step.

Scheme 3. Substrate Scope for the Synthesis of 4a,b

Scheme 5. Intermolecular Kinetic Isotopic Effect Study

Competition experiments were carried out with 2-phenyl indoles 1 bearing a methoxy or a trifluoromethyl group on the para position of the 2-phenyl moiety (1c vs 1f, Scheme 6). It Scheme 6. Competition Experiments of 1 with Different Electronic Features

a Conditions: 1 (0.3 mmol), 2 (0.45 mmol), [RhCp*Cl2]2 (0.015 mmol), Cu(OAc)2 (0.6 mmol), DIPEA (0.3 mmol), toluene (3 mL), 120 °C, 16 h. bIsolated yield. cIsolated yield of 3.

was thus observed that the reaction of 1c with 2a is more favorable than that of 1f with 2a. These results demonstrated a preference for the more electron-rich 2-phenyl moiety in the C−H bond functionalization. Another set of competition experiments were performed by reacting N-methyl maleimide (2a) and N-phenyl maleimide (2f) with 2-(4-methyl)phenylindazole (1b) (Scheme 7). It turned out that 2a is more favorable than 2f for this cascade reaction. On the basis of the experimental results shown above, a plausible reaction pathway accounting for the formation of 3a from the reaction of 1a with 2a is proposed in Scheme 8. First, 1a coordinates with the in situ formed [RhCp*(OAc)2] to afford intermediate I. Next, C−H metalation of I occurs to

the para, meta, or even ortho site of the 2-phenyl ring participated in the reaction to give 4a−g in 41−54% yield. Various functional groups, from EDGs such as methyl and methoxy to EWGs such as chloro and trifluoromethyl, remained intact under the reaction conditions. 1 bearing a chloro or methoxy group on different sites of the indazoyl scaffold also reacted with 2a smoothly to give 4h and 4i in moderate yield. Notably, 2-phenyl-2H-[1,3]dioxolo[4,5-f ]indazole was found to be a suitable substrate to give 4j in a yield of 46%. As for the generality of maleimide substrates 2, we were pleased to find that either N-alkyl- or N-phenylsubstituted maleimides reacted with 1a smoothly to give 4k−o. C

DOI: 10.1021/acs.orglett.9b01889 Org. Lett. XXXX, XXX, XXX−XXX

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corresponding Heck-type product. (The analogue of intermediate VI as shown in Scheme 10.) As expected, 1-methyl-3-

Scheme 7. Competition Experiments of 2 with Different Structural Features

Scheme 10. Reaction of 2,3-Diphenyl-2H-indazole (5) with 2a Affording Heck-Type Product 6

Scheme 8. Proposed Reaction Pathway Accounting for the Formation of 3a

(2-(3-phenyl-2H-indazol-2-yl)phenyl)-1H-pyrrole-2,5-dione (6) was obtained in 43% yield from this reaction. This is convincing evidence of the proposed reaction pathway for the formation of 4. In conclusion, a novel and selective synthesis of fused or spiro polyheterocyclic compounds through DHARs of 2arylindazoles with maleimides is presented. In these reactions, additives play an important part in switching the selectivity toward fused or polyheterocyclic compounds. The notable features of this method include easily obtainable substrates and controllable selectivity in the formation of the products.

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provide a rhodacyclic intermediate II, which then undergoes coordination and migratory insertion with 2a to afford rhodacyclic intermediate III. The following rollover of cyclometalation of III under acidic condition affords intermediate IV.10 Reductive elimination of IV affords intermediate V and the Rh(I) species, which is oxidized by Cu(II) to regenerate the active Rh(III) catalyst. Next, an oxidative dehydrogenation occurs with V to give product 3a. As for the reaction pathway leading to the formation of 4a, it should also go through an initial C−H metalation and maleimide insertion to give intermediates I, II, and III. Under the influence of the basic additive, β-hydride elimination of III is facilitated to form the corresponding Heck-type product VI and the release of Rh(I) species.15 Next, the in situ generated VI undergoes an intramolecular Michaeltype addition to afford 4a. Meanwhile, Rh(I) is oxidized by Cu(II) to regenerate the active Rh(III) catalyst (Scheme 9).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01889. 1

H and 13C NMR spectra of all products and the X-ray crystal structure and data of 4g (PDF) Accession Codes

CCDC 1920160 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 9. Proposed Reaction Pathway Accounting for the Formation of 4a

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Z.). *E-mail: [email protected] (X.F.). ORCID

Xinying Zhang: 0000-0002-3416-4623 Xuesen Fan: 0000-0002-2040-6919 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (NSFC) (21572047, 21772033), Plan for Scientific Innovation Talents of Henan Province (184200510012), Program for Innovative Research Team in Science and Technology in Un iversities of Henan P rovince (20IRTSTHN005), and 111 Project (D17007) for financial support.

To verify the proposed reaction pathway, a mixture of 2,3diphenyl-2H-indazole (5) and 2a was subjected to standard reaction conditions used for the formation of 4a. Because the C-3 site of the indazole scaffold in 5 is occupied by a phenyl group, the formation of a spiro compound like 4a is not possible, and the reaction is expected to afford the D

DOI: 10.1021/acs.orglett.9b01889 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.9b01889 Org. Lett. XXXX, XXX, XXX−XXX