Cyclopropanation of Cyclic Sulfur

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

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Catalytic Asymmetric Ring-Opening/Cyclopropanation of Cyclic Sulfur Ylides: Construction of Sulfur-Containing Spirocyclopropyloxindoles with Three Vicinal Stereocenters Hongjiang Mei, Guihua Pan, Xiying Zhang, Lili Lin, Xiaohua Liu, and Xiaoming Feng* Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, P. R. China

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S Supporting Information *

ABSTRACT: A highly efficient asymmetric ring-opening/ cyclopropanation reaction of (E)-3-(oxyethylidene)-2-oxoindolines with 1-alkyl-3-oxotetrahydro-1H-thiophen-1-ium salts as cyclic sulfur ylides was realized by using a chiral N,N′dioxide/Mg(OTf)2 complex as catalyst. A range of sulfurcontaining syn,anti spirocyclopropyloxindoles with three contiguous stereocenters were obtained in excellent yields with excellent dr and good ee values under mild reaction conditions. The origin of stereoselectivity was discussed.

C

ompared to acyclic sulfur ylides,1 cyclosulfur ylides are less developed in organic chemistry but are versatile. In the cyclopropanation reaction, when acyclic sulfur ylides act as substrates, the sulfur moiety generally leaves and no sulfur group remains in the resulting cyclopropanes, which reduces the atomic economy of the reaction. However, when cyclic sulfur ylides act as substrates, the sulfur group exists in the products after the sulfur-ring-opening process, offering sulfurcontaining cyclopropanes with high atomic economy. For stabilized cyclic sulfur ylides or salts, 1- or 2-thianaphthalenes,2,3 10-thiaanthracene,4 and 9-thaphenanthrenes5 1−3 can undergo not only thermal [1,2]- or [1,4]-rearrangement but also novel ring-opening reactions involving a rearrangement to construct organosulfur compounds (Scheme 1a).2−6 The five-

reaction of chiral cyclic sulfur salts 8 with aldehydes to synthesize chiral epoxidations (Scheme 1d).9 However, to the best of our knowledge, there has been no report on the catalytic asymmetric ring-opening reaction of cyclic sulfur ylides or salts until now. 1-Alkyl-3-oxotetrahydro-1H-thiophen-1-ium salts 9 (Scheme 1e) were reported as positive resist compositions in polymaterials10 but have not been applied in organic synthesis. We noticed that the reaction of 9 with 3-alkenylindolines can construct sulfur-containing spiro-cyclopropyloxindoles, which combine the beneficial properties of the sulfur moiety11 and the spirocyclic oxindole scaffold.12 Of course, it cannot be ignored that the resulting products bear three vicinal stereocenters, and eight diastereoisomers exist in principle, making a great challenge to control the stereoselectivity. In addition, kinetic resolution or dynamic kinetic resolution of the cyclic sulfur ylides might exist.8,13 Since the spirocyclopropanes have been reported to be capable of promoting the generation of the iPS cell14 and some sulfur-containing spirocyclic oxindoles have proven to be potential anticancer and antiproliferative agents,15 it is meaningful to investigate the reaction. Herein, we report our efforts in developing a chiral N,N′-dioxide/Mg(II) complex16 catalytic system to realize the ring-opening/cyclopropanation reaction of 1-alkyl-3-oxotetrahydro-1H-thiophen-1-ium salts with (E)-3-(oxyethylidene)-2oxoindolines, constructing organosulfur spiro cyclopropyloxindoles with vicinal tertiary or quaternary carbon chiral centers in high efficiency. We began our investigation by using the reaction of cyclic sulfur ylide 9a with (E)-3-(2-methoxy-2-oxyethylidene)-2oxoindoline 10a to optimize the reaction conditions (Table 1). First, different backbones of chiral N,N′-dioxides were

Scheme 1. Examples of Stabilized Cyclic Sulfur Ylides or Salts

or six-membered cyclic sulfonium salts 4 are also employed as one-carbon synthons and sulfur sources to afford the sulfurcontaining oxirane derivatives (Scheme 1b).7 Recently, Maas has reported the conversion of methionine-derived diazoketones into the cyclic sulfonium ylides 5−7 in the presence of Rh2(OAc)4.8 The thiochiral center can be stabilized when the α-position was substituted by an ether group (Scheme 1c). In 2009, Sarabia reported the asymmetric sulfur ring-opening © XXXX American Chemical Society

Received: October 8, 2018

A

DOI: 10.1021/acs.orglett.8b03187 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 1. Optimization of the Reaction Conditionsa

entry

ligand

solvent

yieldb (%)

1 2 3 4 5 6 7d 8d,e 9d,e,f

L-PiPr2 L-RaPr2 L-PrPr2 L-PiPr3 L2-PiPr3 L2-PiPr3 L2-PiPr3 L2-PiPr3 L2-PiPr3

DCE DCE DCE DCE DCE CHCl3 CHCl3 CHCl3 CHCl3

87 97 85 85 79 90 85 96 96

Table 2. Substrate Scope of (E)-3-(Oxyethylidene)-2oxoindolinesa

ratio of syn,anti isomerc

eec (%)

71/29 50/50 76/24 78/22 93/7 90/10 91/9 93/7 93/7

90 70 75 95 88 90 91 92 92

a

Unless otherwise noted, all reactions were performed with ligand/ Mg(OTf)2 (1:1, 10 mol %), 10a (0.10 mmol), 9a (0.10 mmol), and 1.1 equiv of K2CO3 in DCE (0.5 mL) under N2 at 35 °C for 14 h. b Isolated yield. cDetermined by HPLC. d10a/9a = 1/1.1, for 4 h. e2 mol % catalyst loading, 20 mg of 3 Å MS. fUnder air atmosphere.

tested by complexing them with Mg(OTf)2 in the presence of K2CO3. It was found that syn,anti-11aa was the major diastreoisomer. The S-pipecolic acid derived L-PiPr2 gave the syn,anti-11aa in 87% yield with 90% ee, which was better than the L-proline-derived L-PrPr2 and L-ramipril-derived LRaPr2 did (entries 1−3). When L-PiPr3 with an isopropyl group on the para-position of the phenyl ring was used, the enantioselectivity could be improved to 95% ee (entry 4). Then the length of the linker on chiral ligand was investigated. It was found that the ligand with a shorter linker was beneficial for diastereoselectivity, and the L2-PiPr3 with a two-carbon linker improved the ratio of the syn,anti isomer to 93/7, albeit with lower reactivity and enantioselectivity (entry 5). When the reaction was performed in CHCl3, 11aa was obtained in 90% yield with a 90/10 ratio of syn,anti isomer and 90% ee. The reaction time could be shortened to 4 h by increasing the loading of 9a to 1.1 equiv (entries 6 and 7). Remarkably, the catalyst loading could be decreased to 2 mol % in the presence of 3 Å molecular sieves (entry 8). More importantly, the reaction could be performed in air atmosphere (entry 11), which simplified the operation. Thus, the optimal reaction condition was established as 10a reacting with 1.1 equiv of 9a in the presence of 2 mol % of L2-PiPr3/Mg(OTf)2 complex, 1.1 equiv of K2CO3, and 20 mg of 3 Å molecular sieves in CHCl3 at 35 °C under air atmosphere. With the optimized reaction conditions in hand, the substrate scope was then explored. When (E)-3-(2-oxyethylidene)-2-oxoindolines were varied by reaction with 9a (Table 2), the electron-donating or electron-withdrawing group on the oxindole backbone had no obvious effect on the reaction. The corresponding products 11ba−na were obtained in good to excellent yields with excellent ratios of the syn,anti isomer and good to excellent ee values (entries 1−14). Besides the ester group, 11oa with amide substitution delivered the products in an excellent ratio of the syn,anti

entry

R1

1 2e 3 4 5 6 7e 8 9 10 11 12 13 14

H 5-F 5-Cl 5-Br 5-I 5-MeO 6-F 6-Cl 6-Br 6-Me 6-MeO 5,6-F2 7-F 7-Me

yieldb (%) 96 80 94 91 95 96 91 99 97 97 89 92 95 87

(11aa) (11ba) (11ca) (11da) (11ea) (11fa) (11ga) (11ha) (11ia) (11ja) (11ka) (11la) (11ma) (11na)

ratio of syn,anti isomerc

eed (%)

92/8 98/2 95/5 94/6 99/1 96/4 93/7 95/5 96/4 96/4 97/3 97/3 90/10 97/3

92 92 88 91 90 92 93 88 92 88 97 90 89 94

a Unless otherwise noted, all reactions were performed with L2PiPr3/Mg(OTf)2 (prep, 1:1, 2 mol %), 10 (0.10 mmol), 9 (0.11 mmol), and 1.1 equiv of K2CO3 in CHCl3 (0.5 mL) under air atmosphere at 35 °C for 4 h. bIsolated yield. cDetermined by 1H NMR. dDetermined by HPLC. e1.5 equiv of 9a. f2 mol % of L-PiPr3/ Mg(OTf)2 was used.

isomer and excellent ee, but much lower yield, which might be caused by the lower reactivity and higher steric hindrance. Moreover, in the presence of L-PiPr3/Mg(OTf)2 complex, 3acetylindoline 10p reacted with 9a smoothly, giving 11pa in 94% yield with a 99/1 ratio of syn,anti isomer and 92% ee. The vinyloxy carbonate protected 3-alkylindoline 10q gave the sulfur-containing spirocyclic oxindole 11qa in 85% yield with a 91/9 ratio of syn,anti isomer and 88% ee. The scope of 1-alkyl3-oxotetrahydro-1H-thiophen-1-ium salts was next investigated. When sulfur salts 1-ethyl-3-oxotetrahydro-1H-thiophen-1-ium trifluoromethane-sulfonate 9b and 3-oxo-1-phenethyltetrahydro-1H-thiophen-1-ium trifluoromethane-sulfonate 9c were applied, the corresponding 11ab and 11ac with longer sulfur chains were obtained in good yields, good ratios of syn-anti isomer, and excellent ee values. α-Methyl sulfur ylide 9d was also tested, and the desired product 11ad bearing two adjacent quaternary chiral centers was isolated in lower yield and ee value but still excellent ratio of syn,anti isomer. The lower yield and ee value might be caused by the larger steric hindrance between the two substrates during the B

DOI: 10.1021/acs.orglett.8b03187 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters reaction. Moreover, 11ia could be fully transformed to sulfone 12 with mantained stereoselectivity, and the absolute configuration of 12 was determined to be (1S,2R,3R) by Xray crystallographic analysis (Scheme 2). Hence, the absolute configuration of 11ia was inferred to be (1S,2R,3R).

Table 3. Tunable Diastereoselectivity of the Reaction about (E)-3-Arylidene-2-oxoindolinesa

Scheme 2. Transformation and Absolute Configuration Determination of 11ia

[(anti,syn) + (anti,anti)]-14

According to our previous work on N,N′-dioxide/Mg(II) complex-catalyzed asymmetric reactions,17 as well as the absolute configuration determination of product 11ia, possible transition-state models were proposed to explain the origin of stereoselectivity (Scheme 3). The four oxygens of the chiral

entry

14

conditions

yieldb (%)

drc (%)

ee of major isomerd (%)

1 2 3 4 5

14aa

A B C A C

47 67 77 47 70

83/17 9/91 90/10 49/51 86/14

98 67 99 98 82

Scheme 3. Proposed Transition-State Models

14ba

(syn,anti)-14

yieldb (%)

eed (%)

48 19 15 41 15

99 90 99 98 98

a

Unless otherwise noted, all reactions were performed with catalyst L/Mg(OTf)2 (prep. 1:1, 2 mol %), 13 (0.1 mmol), 9a, 3 Å MS 20.0 mg, K2CO3 and CHCl3 (0.5 mL), the reaction mixture was stirred at 35 °C for 17 h under air atmosphere. bIsolated yield. cThe ratio of anti,syn/anti,anti, determined by 1H NMR. dDetermined by HPLC.

with 9a using L4-PiPr3/Mg(OTf)2 complex as catalyst (conditions A), the corresponding products anti-syn-14aa and syn-anti-14aa were mainly obtained with 98% ee and 99% ee, respectively (entry 1). The L2-PiEt2/Mg(OTf)2 complex also promoted this reaction smoothly (conditions B), and anti,syn-14aa was mainly obtained in 67% yield with 91/9 anti,anti/anti,syn and moderate enantioselectivity (67% ee) (entry 2). anti,syn-14aa was given in 77% yield with 90/10 anti,syn/anti,anti and 99% ee in the presence of L4-RaPr3/ Mg(OTf)2 complex (conditions C, entry 3). tert-Butyl (E)-3(3-bromobenzylidene)-2-oxoindoline-1-carboxylate 13b was also tolerated under conditions A and C, and the sulfurcontaining spirocyclopropyloxindole 14ba was isolated in moderate diastereoselectivity with perfect enantioselectivity (entries 4 and 5). Moreover, the absolute configuration or relative configuration of the products was determined by X-ray crystallographic analysis. The absolute configuration of stereoisomer syn,anti-14aa under conditions A was determined to be (1S,2R,3S). The anti,syn-14ba, which was obtained under conditions C, was determined to be (1S,2S,3S). The relative configuration of major 14aa under conditions B was determined to be anti,anti. In summary, we have realized the first catalytic asymmetric sulfur ring-opening/cyclopropanation reaction of 1-alkyl-3oxotetrahydro-1H-thiophen-1-ium salts with (E)-3-(oxyethylidene)-2-oxoindolines using a chiral N,N′-dioxide/Mg(OTf)2 complex. The reaction offered an efficient method to construct various chiral sulfur-containing compounds bearing vicinal tertiary or quaternary carbon chiral centers under very mild

N,N′-dioxide ligand coordinates with Mg(II), forming an octahedral geometry. Then the two oxygens of 3-(alkylmethylene)-2-oxoindoline coordinate to Mg(II) in a bidentate manner. The Si face of the methyleneindolinone 10i is shielded by the neighboring amide group of the ligand. Thus, the Re face of nucleophile 9a approaches from the Re face of 10i. During the ring-closing step via an SN2 process, the enolate anion should attack from the opposite position of the releasing sulfur salt. The repulsion effect will increase if enolate approaches the thianone with its Si face (intermediate II). Consequentially, a C−C single bond linked with enolate rotates and the Re face of enolate anion attacks the thianone to deliver the (1S,2R,3R)-product 11ia via an SN2 process. Then tert-butyl (E)-3-arylidene-2-oxoindoline-1-carboxylates 13a and 13b were investigated (Table 3). It was found that the anti,syn, anti,anti, and syn,anti diastereoisomers of the corresponding sulfur-containing spirocyclopropyloxindoles could be regulated by adjusting the ligand. When 13a reacted C

DOI: 10.1021/acs.orglett.8b03187 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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conditions. Other asymmetric reactions focusing on cyclic sulfur ylides are underway.

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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.8b03187. Experimental details and analytical data (NMR, HPLC, and ESI-HRMS) (PDF) Accession Codes

CCDC 1841132−1841135 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaohua Liu: 0000-0001-9555-0555 Xiaoming Feng: 0000-0003-4507-0478 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the National Natural Science Foundation of China (Nos. 21432006 and 21772127) for their financial support of this work.

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REFERENCES

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DOI: 10.1021/acs.orglett.8b03187 Org. Lett. XXXX, XXX, XXX−XXX