Copper-Catalyzed Intermolecular Cyclization between Oximes and

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Copper-Catalyzed Intermolecular Cyclization between Oximes and Alkenes: A Facile Access to Spiropyrrolines Bo Zhao, Hong-Wen Liang, Jie Yang, Zhen Yang, and Ye Wei ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01876 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 14, 2017

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Copper-Catalyzed

Intermolecular

Cyclization

between Oximes and Alkenes: A Facile Access to Spiropyrrolines Bo Zhao,‡ Hong-Wen Liang,‡ Jie Yang, Zhen Yang, and Ye Wei* College of Pharmacy, Third Military Medical University, Chongqing, 400038, China

ABSTRACT: A Cu-catalyzed protocol has been developed for the rapid construction of a wide spectrum of structurally interesting spiropyrroline skeletons. This method utilizes readily accessible ketoximes and alkenes as the starting materials, and exhibits broad substrate scope and good functional group compatibility. Furthermore, the reaction can be applied for the late-stage modification of bioactive pregnenolone derivatives. The mechanistic investigation suggests that the reactions proceed through a radical process.

KEYWORDS: copper catalysis, oximes, alkenes, heterocycles, cyclization

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INTRODUCTION Heterocyclic spiro compounds are prevalent in natural products and biologically active molecules.1 One important class of such heterocycles is spiropyrrolines, which display bioactivities in some aspects,2 such as antiviral and antimicrobial activities, and alleviation of

Scheme 1. Representative Biologically Active Molecules Containing Spiropyrroline Fragment

neuropathic pain (Scheme 1). Therefore, various synthetic protocols have been developed for their synthesis.2b,3-5 For instance, reduction of amide3 (Scheme 2a) and intramolecular condensation of amine with aldehyde4 (Scheme 2b) could afford the spiropyrrolines. Nevertheless, the reactions require the use of air- or moisture sensitive reagents. Furthermore, the starting materials are not easily accessible and their preparation involves multi-steps. Considering the interesting molecular architecture of the spiropyrrolines, it is highly desirable to develop economic and efficient synthetic methods for the construction of such a significant scaffold from easily accessible reagents.


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Scheme 2. Synthetic Methods for Spiropyrroline Synthesis

Oximes can be readily prepared and these compounds are generally nontoxic, nonexplosive, stable to air and moisture, easy to store, and simple to handle. Due to the relatively low bond energy of N−O bond,6 the oximes can participate in various transformations involving N−O bond cleavage,7 such as Beckmann rearrangement,8 Neber rearrangemennt,9 and Semmler–Wolff reaction.10 Much recent attention has been paid to transition metal catalysis since many transition metals can efficiently trigger the oxime N−O bond cleavage to generate iminyl radicals or iminometal intermediates by means of oxidative addition or single electron transfer (SET) pathways.11,12 For ketoximes bearing α-hydrogens, the generated iminyl radicals and imino-metal intermediates may convert into the corresponding α-carbon radical and α-carbon-metal species, respectively. Under suitable reaction conditions, these intermediates can be used to construct


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new C−C and C−heteroatom bonds.13 For example, Cu-catalyzed approaches for the preparation of sulfonylvinylamines and ketophosphonates were studied by the groups of Jiang13b and Lei,13c respectively. Another interesting reaction mode regarding this type of ketoximes is the intermolecular cyclization, in which both the nitrogen and α-carbon atoms are incorporated into the final N-heterocycles.14 For example, Yoshikai recently developed a novel synergistic copper/iminium protocol for the construction of pyridine skeleton from oximes and enals. 14b Jiang reported Cu-catalyzed cyclization of oximes with dimethyl acetylenedicarboxylate for the synthesis of pyrroles.14c Despite of these progress, the use of readily accessible ketoximes as starting materials for the assembly of spiropyrrolines remains a significant challenge.15 Inspired by the above-mentioned work and in continuation of our research interests in the synthesis of spiro compounds,16 we envisaged that it would be possible to achieve the construction of the spiropyrroline skeleton through transition metal-catalyzed cyclization between ketoximes and alkenes. Herein, we represent a novel protocol for the rapid assembly of spiropyrrolines employing inexpensive CuBr as the catalyst (Scheme 2c). The reaction is operationally simple, scalable, and proceeds under mild reaction conditions. Our method exhibits broad substrate scope, good functional group compatibility, and can be applied for the synthesis of various spiropyrrolines. More importantly, the reaction is suitable for the late-stage modification of bioactive pregnenolone derivatives. Electron paramagnetic resonance (EPR) and radical clock experiments indicated that the reactions proceed through a radical process.


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RESULTS AND DISCUSSION Table 1. Screening of Reaction Conditions for Cu-Catalyzed Spiropyrroline Synthesisa

entry

change from the standard conditions

yield %b

1

no change

85 (89)c

2

CuOAc instead of CuBr

26

3

CuI instead of CuBr

54

4

Cu(OAc)2 instead of CuBr

19

5

Pd2(dba)3 or Pd(PPh3)4 instead of CuBr

0

6

toluene instead of THF

82

7

PhCl instead of THF

40

8

C6F6 instead of THF

29

9

DMSO instead of THF

51

10

40 °C instead of 60 °C

68

11

CuBr (2.5 mol%) instead of CuBr (5 mol%)

67

12

4 instead of 2

71

13

5 instead of 2

72

a

Reactions were performed on a 0.2 mmol scale in 2 mL of solvent under Ar. b Determined by

1

H NMR spectroscopy using 1,1,2,2-tetrachloroethane as an internal standard. c Isolated yield.

We commenced our study by choosing 2-benzylidene indane-1,3-dione 1 and acetophenone Oacetyl oxime 2 as model substrates to optimize the reaction conditions. After considerable experimentation, we found that that the spiropyrroline product 3 was produced in 89% isolated yield with CuBr (5 mol%) as a catalyst at 60 °C in THF within 18 h (entry 1, Table 1). The


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structure of 3 was unambiguously confirmed by single crystal X-ray diffraction.17 Some important results obtained during the optimization process are shown in Table 1. Both CuOAc and Cu(OAc)2 displayed low catalytic activity toward the formation of 3 (entries 2 and 4). The reaction delivered 3 in 54% yield in the presence of CuI as the catalyst (entry 3). Although Pd(0) complex is competent in the N–O bond cleavage, such as amino-Heck reaction,12a 3 could not be generated at all with Pd2(dba)3 or Pd(PPh3)4 as the catalyst (entry 5). Among the various solvents, toluene showed good yield (entry 6), while PhCl (chlorobenzene), C6F6 (hexafluorobenzene), and DMSO (dimethyl sulfoxide) exhibited low to moderate yields (entries 7–9). Decreasing the reaction temperature from 60 to 40 °C resulted in 68% yield (entry 10). The reaction afforded 3 in 67% yield when reducing the catalyst loading to 2.5 mol% (entry 11). Besides the acetophenone O-acetyl oxime 2, O-tert-butyl acetyl oxime 4 and O-pentafluorobenzoyl oxime 5 were also investigated, and the reactions delivered 3 in 71 and 72% yields, respectively (entries 12 and 13). With the optimal reaction conditions in hand, we subsequently investigated the reactions between a series of oximes and 2-arylideneindane-1,3-diones (Table 2). Various ketoximes derived from substituted acetophenones reacted well with 2-benzylidene indane-1,3-dione, producing the corresponding spiropyrrolines in 64−89% yields (6−17), with tolerance of many functional groups, including iodo (8), chloro (9), trifluoromethyl (10), nitro (11), cyano (12), and bromo (14) groups. Importantly, methylthio group that usually deactivates the transition metal


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Table 2. Reactions of Various Oximes with 2-Arylideneindane-1,3-dionesa

a

Reactions conditions: oximes (0.2 mmol), 2-arylideneindane-1,3-diones (1.5 equiv), CuBr (5

mol%), THF (2 mL), 60 °C, 18 h, under Ar.

b

>19:1 dr value was determined by 1H NMR

spectroscopy. c 20 mol% of CuBr was used.

catalyst was also well compatible in this transformation (13). In addition, the o-methyl group on the aryl ring of the oxime did not affect the reaction seriously, because the desired product was


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generated in 78% yield (16). Besides the acetophenone-derived oximes, oximes derived from acyclic aliphatic ketones and ethyl pyruvate could also participate in the reactions, which afforded the products 18−20 in moderate to good yields. For the ketoximes bearing substituents at the α position of the C=N group (21, 22, and 25), the target products were obtained in moderate yields. This method was also amenable to tetralone and cyclododecanone-derived oximes, and the corresponding products were generated in synthetically useful yields (23 and 24). Unfortunately, the substrate derived from the condensation of 1,3-indandione with 3-pentanone showed no reactivity toward the spiropyrroline formation. Note that our method is suitable for gram scale, which was demonstrated by a 10 mmol scale reaction between 1 and 2.

Table 3. Reactions of Various Oximes Bearing Heterocyclic Moiety with 2Arylideneindane-1,3-dionesa

a

Reactions conditions: oximes (0.2 mmol), 2-arylideneindane-1,3-diones (1.5 equiv), CuBr (5

mol%), THF (2 mL), 60 °C, 18 h, under Ar. b 10 mol% of CuBr was used.


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In order to demonstrate the good compatibility of our method with heterocyclic substrates, we further performed the reactions between 2-arylideneindane-1,3-diones and a range of oximes containing heterocyclic skeleton. As shown in Table 3, a series of heterocycles-derived ketoximes, including pyridine- (26), furan- (27), thiophene- (28), indole- (29), pyrrole- (30), pyrazole- (31), thiazole- (32), benzodioxole- (33), and benzothiophene-derived ones (34), could take part in the spiropyrroline synthesis, producing the products in moderate to excellent yields.

Table 4. Reactions of Various 1,3-Indandione-derived Alkenes with Oximesa

a

Reactions conditions: oximes (0.2 mmol), 1,3-indandione-derived alkenes (1.5 equiv), CuBr (5

mol%), THF (2 mL), 60 °C, 18 h, under Ar.


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Subsequently, the substrate scope with regard to 1,3-indandione-derived alkenes was evaluated (Table 4). A large number of 2-arylideneindane-1,3-diones displayed moderate to good reactivity in the reactions (35−43, 49, and 50), and numerous functional groups were well-tolerated, such as trifluoromethyl (35), iodo (36), nitro (37), ester (38), formyl (39), cyano (40), fluoro (41), olefinic (42), and alkynyl groups (43). Additionally, furan- (44) and thiophene-derived substrates (45) also reacted with ketoxime 2, affording the products in 65 and 86% yields, respectively. More importantly, this method is not only suitable for 2-arylideneindane-1,3-diones, but also for 2-alkenylideneindane-1,3-dione

(46),

2-alkynylideneindane-1,3-dione

(47),

and

2-

alkylideneindane-1,3-dione (48).

Table 5. Reactions of Heterocycles with Oximes

a

Reactions conditions: oxime (0.2 mmol), oxindole (1.5 equiv), CuBr (10 mol%), THF (2 mL),

60 °C, 18 h, under Ar. b Reactions conditions: 1,3-thiazol-4-one derivative (0.2 mmol), oxime (2 equiv), CuBr (10 mol%), THF (2 mL), 60 °C, 18 h, under Ar. c Reactions conditions: oxime (0.2 mmol), pyrimidine-2,4,6-trione derivative (1.5 equiv), CuBr (10 mol%), THF (2 mL), 80 °C, 18 h, under Ar.


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Encouraged by the results of the reactions between 1,3-indandione derived alkenes and ketoximes, we then evaluated the generality of our method for the construction of other spiropyrrolines (Table 5). To our delight, we found that oxindole-derived alkenes reacted with acetophenone O-acetyl oxime 2 to give rise to the corresponding spiropyrrolines in moderate yields (51−54). In addition, the reactions of 1,3-thiazol-4-one- or barbituric acid-derived alkenes with 2 also proceeded to deliver the desired products 55 and 56 in 38 and 50% yields, respectively.

Scheme 3. Reactions between 2-Arylideneindane-1,3-diones and Pregnenolone Derivative

Subsequently, the newly developed method was utilized to the late-stage functionalization18 of biologically active compound, which is receiving increasing attention in medical chemistry (Scheme 3). When the pregnenolone-derived oxime was subjected to the reaction conditions, the corresponding spiropyrrolines 57 and 58 were obtained in 86 and 83 % yields, respectively.


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Scheme 4. Mechanistic Studies

To shed light on the reaction mechanism, several experiments were carried out (Scheme 4). First, a reaction of 2-benzylidene indane-1,3-dione 1 with 3-phenyl-2H-azirine 59 did not afford the spiropyrroline at all in the presence or in the absence of CuBr, which suggested that the azirine intermediate was not involved in the process (Scheme 4a). Second, a radical clock experiment was conducted using a ketoxime 60 as the substrate. When 60 was treated with 5 mol% of CuBr in THF at 60 °C within 18 h, 2-phenylpyridine 61 was obtained in 26% yield (Scheme 4b). Obviously, the formation of 2-phenylpyridine involves a ring-opening process that might be triggered by the α-carbon radical.19 Third, an EPR study of the reaction containing 1, 2, and the spin-trapping agent phenyl tert-butyl nitrone (PBN) was investigated to probe the radical 12 Environment ACS Paragon Plus

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intermediate (Scheme 4c). Data analysis suggests that an iminyl radical13c is generated, which is quickly trapped by PBN to form the relatively stable radical 62. Lastly, the cyclization reaction between 1 and 2 resulted in moderate but noticeably lower yield of 3 when 1 equiv of radical scavenger was added (Scheme 4d). The partial inhibitory effect of the radical scavenger suggests that while radical intermediates are involved, the radical chain pathway seems unlikely.20

Scheme 5. Putative Mechanism

On the basis of the mechanistic results and the previous studies on Cu-mediated N−O bond cleavage,12h,13c,14b the plausible reaction pathways of this Cu-catalyzed spiropyrroline synthesis are outlined in Scheme 5, although the detailed mechanism remain unclear at this stage. Upon the reaction of oxime with Cu(I) salt, an iminyl radical A and a Cu(II) species would form through single-electron reduction of the N−O bond. The former may quickly tautomerize to an α-


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carbon radical B, which then undergoes nucleophilic addition to activated alkenes to generate an intermediate C. The intermediate C then react with the Cu(II) species to give a Cu(III) species D, followed by its reductive elimination to furnish the desired spiropyrrolines. 21

CONCLUSIONS In conclusion, we have developed an efficient, operationally simple, and scalable Cu-catalyzed approach for the construction of spiropyrrolines from easily accessible ketoximes and activated alkenes. Our protocol is amenable to a variety of acyclic and cyclic ketoximes as well as various alkenes, thus a vast array of structurally interesting spiropyrrolines are prepared. Mechanistic studies reveal that the reactions would involve radical processes. Our further work will focus on the synthesis of more heterocycles through transition metal-catalyzed oxime transformations.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Author Contributions ‡These authors contributed equally to this work. Notes The authors declare no competing financial interest. ASSOCIATED CONTENT Supporting Information


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Detailed experimental procedures, characterization of products, and copies of NMR spectra. This Supporting Information is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT Financial support from the NSFC (No. 21302220), the State Key Laboratory of Structural Chemistry (No. 20150009), Natural Science Foundation of Chongqing (No. cstc2016jcyjA0008), and the Third Military Medical University is greatly appreciated. REFERENCES (1) Zheng, Y.; Tice, C. M.; Singh, S. B. Bioorg. Med. Chem. Lett. 2014, 24, 3673–3682. (2) (a) Morris, B. D.; Prinsep, M. R. J. Nat. Prod. 1999, 62, 688–693. (b) Pearson, M. S. M.; Floquet, N.; Bello, C.; Voge, P.; Plantier-Royon, R.; Szymoniak, J.; Bertus, P.; Behr, J.-B. Bioorg. Med. Chem. 2009, 17, 8020–8026; (c) Khan, A.; Wood, P.; Moskal, J. U.S. Patent 20110306586, December 15, 2011. (3) Guéret, S. M.; Brimble, M. A. Pure Appl. Chem. 2011, 83, 425–433. (4) (a) Smolanoff, J.; Kluge, A. F.; Meinwald, J.; McPhail, A.; Miller, R. W.; Hicks, K.; Eisner, T. Science 1975, 188, 734–736. (b) Takagi, Y.; Mori, K. J. Braz. Chem. Soc. 2010, 11, 578–583. (5) Badillo, J. J.; Ribeiro, C. J. A.; Olmstead, M. M.; Franz, A. K. Org. Lett. 2014, 16, 6270– 6273. (6) Pereira, M. M. A.; Santos, P. P. Rearrangement of Hydroxylamines, Oximes, and Hydroxamic Acids. In The Chemistry of Hydroxylamines, Oximes, and Hydroxamic Acids; Rappoport, Z., Liebman, J. F., Eds.; Wiely-VCH: West Sussex, 2009; Chapter 9, pp 345–346. (7) (a) Zard, S. Z. Chem. Soc. Rev. 2008, 37, 1603–1618. (b) Walton, J. C. Acc. Chem. Res. 2014, 47, 1406–1416. (8) Gawly, R. E Org. React. 1988, 35, 1–420. 15 Environment ACS Paragon Plus

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Copper-Catalyzed Intermolecular Cyclization between Oximes and

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