Construction of the N1-C3 linkage stereogenic centers by catalytic asymmetric amination reaction of 3-bromooxindoles with indolines.

Letter pubs.acs.org/OrgLett

Construction of the N1−C3 Linkage Stereogenic Centers by Catalytic Asymmetric Amination Reaction of 3‑Bromooxindoles with Indolines Hailong Zhang,† Hong Kang,† Liang Hong, Weiping Dong, Guolin Li, Xin Zheng, and Rui Wang* School of Life Sciences, Key Laboratory of Preclinical Study for New Drug of Gansu Province, Lanzhou University Lanzhou 730000, P.R. China S Supporting Information *

ABSTRACT: The catalytic asymmetric amination reaction of 3-bromooxindoles with indolines for the construction of the N1− C3 linkage stereogenic centers has been realized for the ﬁrst time. Moreover, the racemic substrates (3-substituted indolines) were also applicable under the same chiral conditions. The newly developed method conveniently led to a formal synthesis of (+)-psychotrimine.

interesting bioactivity proﬁles. For example, psychotrimine, a trimeric-tryptamine-related alkaloid, was isolated from the leaves of the plant Psychotria rostrata by Takayama and coworkers in 20045a and exhibits potent antitumor activity against colon and lung cancers5e and antibacterial activity against Gram-positive bacteria.6 The unique structure and interesting biological activity of psychotrimine have attracted considerable attention from synthetic chemists worldwide.5b−f Takayama et al. employed an asymmetric Ireland−Claisen rearrangement as a key step for the ﬁrst enantioselective total synthesis of (+)-psychotrimine.5e An elegant total synthesis of (+)-psychotrimine was achieved by Baran and co-workers, through the direct aniline−indole coupling of o-iodoaniline with 7-bromo-Dtryptophan derivative.5f The N1−C3 linkage stereogenic center is the pronounced feature in these natural products. From a synthetic point of view, several members of the indole alkaloids contain the N1− C3 linkage stereogenic center that could potentially be accessed via 3-aminooxindole8 4 (Scheme 1). Therefore, the catalytic asymmetric construction of the N1−C3 linkage stereogenic center in 4 would be the critical element in the divergent synthesis of these indole alkaloids. Unsurprisingly, the methods to construct this type of C−N bond directly in an asymmetric and catalytic fashion are rare.9−11 Recently, we disclosed an asymmetric alkylation reaction of 3-bromooxindoles with 3substituted indoles with high diatereoselectivity and excellent enantioselectivity by using a nickel(II) catalyst. We found that the enantioselectivity was controlled by the chiral electrophile

he indole-derived pyrroloindolines widely exist in a large number of natural products and pharmacologically important compounds.1,2 However, a small collection of molecules, including chaetomin,3 kapakahines,4 psychotrimine,5 and psychotetramine,5f are linked through N1−C3 bonds7 (Figure 1). These indole alkaloids show unique structures and

Received: March 10, 2014 Published: April 11, 2014 2394

dx.doi.org/10.1021/ol5007423 | Org. Lett. 2014, 16, 2394−2397

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Table 1. Optimization of the Reactiona

Scheme 1. Strategically Diversity-Oriented Retrosynthetic Analysis

(indol-2-one) complex induced by a chiral nickel(II) catalyst. Herein, we developed an eﬃcient catalytic asymmetric amination reaction of 3-bromooxindoles13 with indolines for the construction of the N1−C3 linkage stereogenic centers. This method also could be convenient for the synthesis of (+)-psychotrimine. In our initial experiments, we investigated an examination of the reaction between 3-bromooxindole 2a and indoline 1a in the presence of chiral nickel(II) catalysts. As shown in Table 1, the 1:1 Ni(OAc)2−3a complex (20 mol %, THF, rt, 5 h) combined with K3PO4, which we have been developing in a previous study,12 aﬀorded the desired product 4a in excellent yield, but with low enantioselectivity (Table 1, entry 1). However, the replacement of 3a with other ligands, such as 3b, 3c, 3d, or 3e, gave no signiﬁcant improvement to the enantioselectivity (Table 1, entries 2−5). Then, several bases were surveyed. To our great delight, when DABCO was used as a base, the enantioselectivity was enhanced (75% ee, Table 1, entry 9). Subsequent screening of solvents revealed that MTBE appeared to be the most suitable reaction media for high enantioselectivity (up to 87% ee; Table 1, entries 14). Finally, we found that lowering the amount of base results in a slightly enhanced enantioselectivity at 90% ee (Table 1, entry 15). Having established the optimal reaction conditions, the scope of substrates for the catalytic asymmetric amination reaction of 3-bromooxindoles with indolines was then studied (Scheme 2). On the whole, it was found that a broad range of 3bromooxindoles and indolines could readily participate in this reaction. We ﬁrst investigated a variety of indolines under the optimal reaction conditions. Indolines with electron-withdrawing groups gave adducts with high enantioselectivity (88−96% ee) and high yield (71−88%), but indolines with electron-donating groups experienced lower enantioselectivity (4c, 74% ee; 4d, 61% ee). Furthermore, the scope of 3bromooxindoles was surveyed. Substitution of the bromooxindole with a methoxy group at C5 produced the indoline amination product with good results (4k). In contrast, only moderate selectivity and yield were observed with substrate 2l, which contains an electron-withdrawing C5−Cl on the bromooxindole core. In addition, bromooxindoles bearing diﬀerent 3-substitution/alkyl chains could participate in this

entry

catalyst

base

solvent

yieldb (%)

eec (%)

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

Ni(OAc)2-3a Ni(OAc)2-3b Ni(OAc)2-3c Ni(OAc)2-3d Ni(OAc)2-3e Ni(OAc)2-3e Ni(OAc)2-3e Ni(OAc)2-3e Ni(OAc)2-3e Ni(OAc)2-3e Ni(OAc)2-3e Ni(OAc)2-3e Ni(OAc)2-3e Ni(OAc)2-3e Ni(OAc)2-3e

K3PO4 K3PO4 K3PO4 K3PO4 K3PO4 DBU Et3N N,N-DIPEA DABCO DABCO DABCO DABCO DABCO DABCO DABCO

THF THF THF THF THF THF THF THF THF Et2O DME CPME dioxane t-MTBE t-MTBE

94 92 95 91 90 83 84 87 88 64 83 87 84 88 87

3 17 13 15 21 33 55 70 75 65 67 79 75 87 90

a Unless otherwise speciﬁed, the reaction was carried out with 1a (0.12 mmol), 2a (0.1 mmol), and base (0.2 mmol) in the presence of catalyst (0.02 mmol) and solvent (1.5 mL). bIsolated yield. c Determined by chiral HPLC on a Chiralpak OD column. dDABCO (0.15 mmol) was added. DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene, DIPEA = N,N-diisopropylethylamine, DABCO = triethylenediamine, DME = 1,2-dimethoxyethane, CPME = cyclopentyl methyl ether, MTB E= methyl tert-butyl ether.

reaction (4m−o). To our delight, in addition to indolines, the tetrahydroquinoline was also suitable for the reaction to give the respective adduct with excellent results (4p). On the basis of our recent research,12 we surmised that racemic 3-substituted indolines could aminate 3-bromooxindoles stereoselectively. As shown in Scheme 3, several 3substituted indolines can participate in the process to give the desired amination adducts 4 with moderate yield. The ee value was determined by the conversion of indoline 4 into indole 5 with DDQ oxidation. Notably, the product 5d was obtained in moderate yield with excellent stereoselectivity when the tryptophan-derived indoline was employed. The potential utility of this reaction was supported by the relationship between Chaetomin’s skeleton and 5d. The synthetic utility of this strategy was demonstrated by the aminated product 4a,5e which has been achieved in 12 chemical steps by Takayama et al., as a key intermediate in the enantioselective synthesis of (+)-psychotrimine (Scheme 4). 2395

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Scheme 2. Scope of the Reactiona

Scheme 3. Scope of the Racemic 3-Substituted Indolinesa

The reaction was performed on 0.1 mmol scale with 1 (1.2 equiv) and 2a (1.0 equiv). DDQ = 2,3-dichloro-5,6-dicyanobenzoquinone.

Scheme 4. Formal Synthesis of Psychotrimine

with indolines for the construction of the N1−C3 linkage quaternary stereogenic centers. Notably, the racemic substrates (3-substituted indolines) could also participate in the reaction with high stereoselectivity. Furthermore, this method facilitated access to cytotoxic (+)-psychotrimine. Further application of this method to this class of alkaloids is underway.

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ASSOCIATED CONTENT

S Supporting Information *

General methods, experimental and HPLC data, and 1H and 13 C NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [emailprotected]. Author Contributions †

These authors contributed equally.

Notes

The authors declare no competing ﬁnancial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge ﬁnancial support from NSFC (91213302, 21272102, 21202071) and the National S&T Major Project of China (2012ZX09504001-003).

Unless otherwise speciﬁed, the reaction was carried out with 1 (0.12 mmol), 2 (0.1 mmol), and base (0.15 mmol) in the presence of a catalyst (0.02 mmol) and solvent (1.5 mL). Isolated yield. The ee values were determined by HPLC on a chiral phase (Chiralcel column).

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REFERENCES

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Here, we could expediently obtain a series of 4a analogues by the direct catalytic asymmetric amination reaction of 3bromooxindoles with indolines, and the chemical diversity of (+)-psychotrimine may be accessed via this method. In conclusion, we have developed a mild enantioselective nickel(II)-catalyzed amination reaction of 3-bromooxindoles 2396

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NOTE ADDED AFTER ASAP PUBLICATION References 5e−g contained errors in the version published ASAP April 11, 2014; the correct version reposted April 18, 2014.

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dx.doi.org/10.1021/ol5007423 | Org. Lett. 2014, 16, 2394−2397

Construction of the N1-C3 linkage stereogenic centers by catalytic asymmetric amination reaction of 3-bromooxindoles with indolines. - PDF Download Free (2024)