Organocatalytic, Enantioselective, Polarity-Matched Ring

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

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Organocatalytic, Enantioselective, Polarity-Matched RingReorganization Domino Sequence Based on the 3‑Oxindole Scaffold Ji-Wei Ren,† Lan Zheng,† Zhi-Peng Ye,† Zhi-Xiong Deng,† Zhen-Zhen Xie,† Jun-An Xiao,‡ Fa-Wei Zhu,† Hao-Yue Xiang,*,† Xiao-Qing Chen,*,† and Hua Yang*,† †

College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, P. R. China College of Chemistry and Materials Science, Nanning Normal University, Nanning, Guangxi 530001, P. R. China

‡

Org. Lett. Downloaded from pubs.acs.org by IDAHO STATE UNIV on 03/25/19. For personal use only.

S Supporting Information *

ABSTRACT: A one-pot squaramide-catalyzed enantioselective ring-reorganization domino sequence (Michael addition/ intramolecular ring-opening/lactamization) of 3-hydroxyoxindole and methyleneindolinone, which can be readily derived from 3-oxindole, has been established in this work. As a result, novel polycyclic quinolinone-spirooxindoles bearing three contiguous chiral centers were efficiently and step-economically assembled under mild conditions in high yields (up to 97%) with excellent enantioselectivities (up to >99% ee) and moderate to good diastereoselectivities (up to >95:5 dr).

P

Given its chemical and electronic features, 3-hydroxyoxindole bearing two active nucleophilic sites could serve as a valuable synthon for designing organocatalytic processes to access a wide range of optically enriched 3-substituted 3-hydroxyoxindoles (Scheme 1, top). To date, this unique synthon has

olycyclic heterocycles have always attracted considerable attention because of their structural complexity and remarkable potentials in biological and pharmacological studies.1 Many effective synthetic strategies have been documented for the construction of multicyclic heterocycles ever since.2 However, how to enantioselectively and rapidly install multicyclic heterocycles in atom- and step-economical manners is always an attractive, but challenging, topic to synthetic and medicinal chemists. Accordingly, more intensive efforts are in demand to develop elegant synthetic methods and expand the structural diversity of valuable multicyclic heterocyclic architectures.3 It can be conceived that a rational design of the domino process would offer an effective solution and markedly improve the bond-forming and cyclization efficiency.4 Well-defined, polarity-matched substrates would be crucial for the success in designing highly efficient domino processes. Though challenging, starting from a single source through a facile reactivity umpolung would further streamline the whole synthetic sequence and enable diversified reactivity modes. Herein, we design a novel, organocatalytic enantioselective ring-reorganization domino sequence based on the 3oxindole scaffold. Noticeably, in this process, 3-hydroxyoxindole and methyleneindolinone, readily derived from 3oxindole, act as nucleophilic and electrophilic counterparts, respectively, enabling a rapid and step-economical assembly of polycyclic quinolinone-spirooxindoles. 3-Oxindole, a valuable and versatile synthon, has found comprehensive utilizations in synthetic chemistry.5 A wide array of transformations based on 3-oxindole have been established, facilitating diversely target-directed structural modification. In particular, 3-hydroxyoxindole, a 3-oxindolederived synthetic building block, is frequently utilized in the syntheses of biologically active products and pharmaceuticals.6 © XXXX American Chemical Society

Scheme 1. 3-Hydroxyoxindole and the Polarity-Matched Design in the Construction of oxo-Spirooxindoles

Received: February 5, 2019

A

DOI: 10.1021/acs.orglett.9b00477 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Table 1. Catalyst Screeninga

been successfully applied in asymmetric Michael additions,7 [3 + 2] cycloadditions,8 and domino reactions9 to afford various 3,3-disubstituted oxindoles and spirocyclic oxindole-γ-lactones by using nitroalkenes, Morita−Baylis−Hillman carbonates, α,β-unsaturated aldehydes, α,β-unsaturated esters, α,β-unsaturated acyl phosphonates, or α,β-unsaturated N-acylated succinimides as the Michael acceptors. Frequently, the following cyclization generated various oxo-spirooxindoles, which are a class of biologically important polycyclic scaffolds. For instance, Melchiorre and co-workers reported an efficient method for the construction of spirocyclic oxindole-γ-lactones via a novel aminocatalytic cascade reaction of 3-hydroxyoxindole with α,β-unsaturated aldehyde.9a Despite these advances, it is highly desirable to find the application of 3hydroxyoxindole in facilitating and simplifying the assembly of difficultly accessible polycyclic scaffolds. On the other hand, methyleneindolinones, which also can be facilely prepared from 3-oxindole, are inherently endowed with multiple electrophilic sites to offer broader synthetic opportunities.10 Specifically, a ring opening of the chemically inert amide moiety in methyleneindolinone would enable versatile reorganization of cyclic frameworks,11 providing an underexplored but interesting strategy for constructing novel heterocycles with high synthetic difficulty. To date, the ringopening domino process using methyleneindolinones to assemble polycyclic spirooxindole, not mentioning enantioselectively, has remained surprisingly underdeveloped and challenging. We envisioned that a polarity-matched, rationally designed domino sequence involving methyleneindolinones and 3-hydroxyoxindoles would give birth to innovative and efficient pathways to construct novel polycyclic spirooxindole with diversified structural features. In this work, we disclose a novel and one-pot squaramide-catalyzed enantioselective Michael addition/intramolecular ring-opening/lactamization domino sequence of 3-hydroxyoxindole with methyleneindolinone to rapidly construct polycyclic quinolinone-spirooxindoles bearing three contiguous chiral centers in high yields (up to 97%) with excellent enantioselectivities (up to >99% ee) and moderate to good diastereoselectivities (up to >95:5 dr) (Scheme 1, bottom). To execute our idea, we designed a one-pot process by initially treating methyleneindolinone 1a with 3-hydroxyoxindole 2a and catalyst 4 in toluene under argon atmosphere at room temperature for 2 h and then adding 1.0 equiv of TsOH· H2O followed by heating the resulting mixture at 80 °C for 1 h (please see Supporting Information for details). The model reaction was carried out for screening various chiral bifunctional cinchona alkaloid derived thioureas and squaramides 4a−4f (as shown in Table 1). The results demonstrate that the chiral thiourea and squaramide catalysts displayed varied catalytic efficiencies in this reaction, and squaramide 4e was proven to be the most efficient catalyst, giving polycyclic quinolinone-spirooxindole 3aa in an excellent yield (90%) with a good enantioselectivity (81% ee) and moderate diastereoselectivity (74:26 dr). Afterward, a series of solvents including toluene, 1,2-dichloroethane, N,N-dimethylformamide, ethanol, 1,4-dioxane, and tetrahydrofuran were evaluated (as shown in Table 2). Pleasingly, it was found that 1,4dioxane was the suitable solvent and gave the product 3aa in 92% yield with >99% ee and 89:11 dr. Further reduction of the catalyst loading to 10 mol % obviously elongated the reaction time (Table 2, entry 7). Accordingly, in light of the above surveys, the optimal reaction conditions were established by

entry

catalyst

yield (%)b

ee (%)c

drd

1 2 3 4 5 6

4a 4b 4c 4d 4e 4f

81 87 87 92 90 91

−45 −75 −69 61 81 −69

51:49 42:58 77:23 76:24 74:26 74:26

a

Unless otherwise noted, all reactions were carried out using methyleneindolinone 1a (0.20 mmol, 1.0 equiv), 3-hydroxyoxindole 2a (0.24 mmol, 1.2 equiv), and catalyst (0.04 mmol, 20 mol %) in toluene (1.0 mL) under argon atmosphere. After 2 h, 1.0 equiv of TsOH·H2O was added, and the reaction mixture was heated at 80 °C for 1 h. bIsolated yield. cDetermined by HPLC on a chiral stationary phase. dDetermined by 1H NMR.

Table 2. Optimization of Conditionsa

entry

solvent

time (h)b

yield (%)c

ee (%)d

dre

1 2 3 4 5 6 7f

PhMe DCE DMF EtOH 1,4-dioxane THF 1,4-dioxane

2 1 2 2 1 1 4

90 88 83 85 92 89 92

81 78 51 68 >99 92 97

74:26 65:35 51:49 59:41 89:11 73:27 89:11

a

Unless otherwise noted, all reactions were carried out using methyleneindolinone 1a (0.20 mmol, 1.0 equiv), 3-hydroxyoxindole 2a (0.24 mmol, 1.2 equiv), and catalyst 4e (0.04 mmol, 20 mol %) in solvent (1.0 mL) under argon atmosphere. After 1a was completely consumed, 1.0 equiv of TsOH·H2O was added, and the reaction mixture was heated at 80 °C for 1 h. bThe reaction time for step 1. c Isolated yield. dDetermined by HPLC on a chiral stationary phase. e Determined by 1H NMR. f10 mol % of catalyst 4e was added. B

DOI: 10.1021/acs.orglett.9b00477 Org. Lett. XXXX, XXX, XXX−XXX

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94% ee), and diastereoselectivity (90:10 and >95:5 dr) for 3an and 3ao, respectively (entries 14 and 15). In the presence of an allyl group, 3ap was achieved in a good yield (86%) with an excellent enantioselectivity (>99% ee) and a good diastereoselectivity (86:14 dr) (entry 16). Interestingly, the substrate without any single substituent furnished the corresponding product 3aq with excellent enantioselectivity (91% ee) and diastereoselectivity (>95:5 dr) but in a slightly lower yield (75%) (entry 17). Moreover, the absolute configuration of polycyclic spirooxindole 3ai and the relative configuration of the minor diastereomer of 3aa (3aa′) were established by Xray crystallographic analysis (see Supporting Information for details) (Figure 1).

initially treating methyleneindolinone with 1.2 equiv of 3hydroxyoxindole and catalyst 4e in 1,4-dioxane under argon atmosphere at room temperature for 1 h followed by adding 1.0 equiv of TsOH·H2O and heating the mixture at 80 °C for 1 h. Having the optimal conditions in hand, we sought to examine the substrate tolerance for this enantioselective ringopening domino reaction. First, diversely substituted 3hydroxyoxindoles 2 were evaluated under the standard conditions (as shown in Table 3). Gratifyingly, a variety of Table 3. Substrate Scope of 3-Hydroxyoxindoles 2a

entry

R1

R2

yield (%)b

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

H 4-Cl 5-F 5-Cl 5-Br 5-Me 5-MeO 6-Cl 6-Br 7-F 7-Br 5,7-dimethyl 5,6-difluoro H H H H

Me Me Me Me Me Me Me Me Me Me Me Me Me Bn Ph allyl H

92 95 96 97 91 96 85 96 97 93 96 88 89 86 84 86 75

(3aa) (3ab) (3ac) (3ad) (3ae) (3af) (3ag) (3ah) (3ai) (3aj) (3ak) (3al) (3am) (3an) (3ao) (3ap) (3aq)

ee (%)c

drd

>99 97 99 >99 >99 >99 95 99 92 91 >99 92 90 >99 94 >99 91

89:11 >95:5 86:14 71:29 >95:5 >95:5 >95:5 >95:5 87:13 89:11 94:6 >95:5 85:15 90:10 >95:5 86:14 >95:5

Figure 1. X-ray crystal structures of compounds 3ai and 3aa′.

Subsequently, various substituted methyleneindolinones 1 were also extensively investigated under the standard conditions, and the obtained results are illustrated in Table 4. Gratifyingly, in the presence of 5-bromo-3-hydroxyoxindole Table 4. Substrate Scope of Methyleneindolinones 1a

a

Unless otherwise noted, all reactions were carried out using methyleneindolinone 1a (0.20 mmol, 1.0 equiv), 3-hydroxyoxindole 2 (0.24 mmol, 1.2 equiv), and catalyst 4e (0.04 mmol, 20 mol %) in 1,4-dioxane (1.0 mL) under argon atmosphere. After 1 h, 1.0 equiv of TsOH·H2O was added, and the reaction mixture was heated at 80 °C for 1 h. bIsolated yield. cDetermined by HPLC on a chiral stationary phase. dDetermined by 1H NMR.

substituents (R1) on the phenyl moiety were well tolerated to yield the corresponding products 3aa−3ak in excellent yields (85%−97%) with excellent enantioselectivities (91% → 99% ee) (entries 1−11). However, the diastereoselectivity varied obviously (71:29 → 95:5 dr), and the substituent effect of the phenyl moiety has a sizable impact on the diastereoselectivity. It can be concluded that the substrates with electron-donating substituents gave better diastereoselectivities than those with electron-withdrawing substituents. In the presence of 5-F, 5-Cl, and 6-Br groups on the phenyl ring, only moderate diastereoselectivities were obtained, and for those substrates with electron-withdrawing substituents at the 5-position, the bulkier group Br led to a superior diastereoselectivity. Not surprisingly, 3al bearing two methyl substituents was obtained in 88% yield with 92% ee and >95:5 dr (entry 12). However, 3am substituted by two fluoro groups was obtained in 89% yield with 90% ee and 85:15 dr (entry 13). In addition, the replacement of methyl with benzyl and phenyl in 2 also gave good yields (86% and 84%), enantioselectivities (>99% and

entry

R

1 2 3 4 5 6 7 8 9 10 11

4-Cl 5-F 5-Cl 5-Br 5-Me 5-MeO 6-Cl 6-Br 7-F 5,7-dimethyl 5,6-difluoro

yield (%)b 84 85 91 95 86 81 95 96 85 88 86

(3bb) (3cb) (3db) (3eb) (3fb) (3gb) (3hb) (3ib) (3jb) (3kb) (3 lb)

ee (%)c

drd

>99 >99 91 99 >99 97 >99 >99 91 94 98

>95:5 >95:5 78:22 81:19 >95:5 >95:5 92:8 74:26 >95:5 >95:5 69:31

a

Unless otherwise noted, all reactions were carried out using methyleneindolinones 1 (0.20 mmol, 1.0 equiv), 3-hydroxyoxindole 2b (0.24 mmol, 1.2 equiv), and catalyst 4e (0.04 mmol, 20 mol %) in 1,4-dioxane (1.0 mL) under argon atmosphere. After 1 h, 1.0 equiv of TsOH·H2O was added, and the reaction mixture was carried out at 80 °C for 1 h. bIsolated yield. cDetermined by HPLC on a chiral stationary phase. dDetermined by 1H NMR.

(2b), methyleneindolinones 1 with a variety of substituents on the phenyl moiety proceeded the title reaction in high yields (84%−95%) with excellent enantioselectivities (91% → 99% ee) (entries 1−9). Similarly, the corresponding diastereoselectivities (74:26 → 95:5 dr) were also affected by the substitution pattern to some degree. With 5-Cl, 5-Br, and 6-Br substituents on the phenyl ring, slightly eroded diastereoselectivities were obtained. Moreover, disubstituted 3kb and 3lb C

DOI: 10.1021/acs.orglett.9b00477 Org. Lett. XXXX, XXX, XXX−XXX

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2a was facilely deprotonated and enolized by the tertiary amine scaffold in 4a, generating an effective Michael donor. Synchronously, methyleneindolinone 1a as a Michael acceptor was guided by the squaramide motif through the hydrogenbonding interaction. Accordingly, the enolized 3-hydroxyoxindole proceeded a Re-attack to methyleneindolinone, enantioselectively generating the intermediate A. Subsequently, an intramolecular ring opening of lactam occurred to give the key intermediatelactone B. Finally, promoted by TsOH· H2O, an intramolecular lactamization of intermediate B directly delivered polycyclic spirooxindole 3aa. In conclusion, we have developed a one-pot squaramidecatalyzed enantioselective Michael addition/intramolecular ring-opening/lactamization domino sequence of 3-hydroxyoxindole with methyleneindolinone by means of a polaritymatched design based on a single source3-oxdindole. Novel polycyclic quinolinone-spirooxindoles bearing three contiguous stereocenters were efficiently and step-economically assembled under mild conditions in high yields (up to 97%) with excellent enantioselectivities (up to >99% ee) and moderate to good diastereoselectivities (up to >95:5 dr). We believe that this work would inspire extensive exploitation of the polarity-matched strategy from a single source, enabling designing efficient pathways to access synthetically challenging chiral heterocycles.

were achieved in good yields (88% and 86%) and enantioselectivities (94% and 98% ee) (entries 10 and 11). Notably, 3kb was obtained with an excellent diastereoselectivity (>95:5 dr), while only a moderate diastereoselectivity (69:31 dr) was achieved for 3 lb. Unfortunately, the corresponding products were not obtained in the optimal conditions when the carbobenzoxy or benzyl motif was used on the nitrogen atom of methyleneindolinone. This attractive reactivity between 3-hydroxyoxindole and methyleneindolinone prompted us to further explore the performance of 3-aminooxindole 5 in this domino reaction, which would generate the aza-spirooxindole analogue (Scheme 2, eq a). Gratifyingly, the enantioselective ring-opening Scheme 2. Domino Sequence of 3-Aminooxindole with Methyleneindolinone and 1 mmol Scale Model Reaction

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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.9b00477. Complete experimental procedures and characterization of new products; NMR spectra and HPLC chromatograms (PDF)

domino reaction of 3-aminooxindole 5 with methyleneindolinone 1a proceeded well to give a new polycyclic spirooxindole 6 in 84% yield with 71% ee and 93:7 dr. Gratifyingly, the practicality and scalability of this protocol were successfully demonstrated by performing the reaction for 3aa at 1 mmol scale under the standard reaction conditions to give a similar result without any loss of efficiency (Scheme 2, eq b). On the basis of experimental results and the absolute configuration of the major isomer, a possible reaction mechanism involving a dual activation of the catalyst is tentatively proposed in Scheme 3. Initially, 3-hydroxyoxindole

Accession Codes

CCDC 1888759−1888760 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], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Scheme 3. Proposed Reaction Mechanism

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H. Yang). *E-mail: [email protected] (X.-Q. Chen). *E-mail: [email protected] (H.-Y. Xiang). ORCID

Jun-An Xiao: 0000-0002-5518-5255 Hao-Yue Xiang: 0000-0002-7404-4247 Xiao-Qing Chen: 0000-0002-8768-8965 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21576296, 21676302, 21776318, and 81703365), Natural Science Foundation of Hunan Province (2017JJ3401), and Central South University. D

DOI: 10.1021/acs.orglett.9b00477 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.9b00477 Org. Lett. XXXX, XXX, XXX−XXX