Enantioselective Epoxypyrrolidines via a Tandem Cycloaddition

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Enantioselective Epoxypyrrolidines via a Tandem Cycloaddition/ Autoxidation in Air and Mechanistic Studies Kaixiu Luo, Yongqiang Zhao, Jiawei Zhang, Jia He, Rong Huang, Shengjiao Yan,* Jun Lin,* and Yi Jin* Key Laboratory of Medicinal Chemistry for Natural Resource, Ministry of Education and Yunnan Province, School of Chemical Science and Technology, Yunnan University, Kunming 650091, P. R. China

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ABSTRACT: A tandem cycloaddition/autoxidation reaction between heterocyclic ketene aminals and diazoester in air is described for the enantioselective preparation of epoxypyrrolidines. Notably, the results of mechanistic studies suggest that epoxide was oxidized from an sp3 C−C single bond, which is of mechanistic and practical interest as this protocol may be suitable for constructing other bioactive heterocyclic epoxides.

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(Scheme 1a),4,14 (b) tandem aza-Payne/hydroamination of aziridinols (Scheme 1b),15 (c) sulfur ylide addition to

evelopment of a highly atom-efficient and environmentally friendly epoxidation reaction of alkanes or alkenes under mild conditions is a challenging task that is fundamentally important to synthetic chemists and chemical science.1 Several enantioselective epoxidations of alkenes using biomimetic metal catalysts have attracted much attention.2 Mechanistic studies have revealed two different approaches to prepare alkene epoxides by oxygenase-like and cosubstrateassisted activation of O species: nucleophilic or electrophilic oxygen mechanism.3 However, directly synthesizing an epoxide from an alkane is rarely practical,4 as activation of the alkane requires high temperatures, which decompose partial oxidation products, or promote total oxidation. Recently, Tcyrulnikov et al. serendipitously discovered a spontaneous aerobic oxidation of 1,1,2,2-tetrakis(N-methylpyridin-4-ium)ethane iodide without catalysts, by which an acceptor-substituted alkane led to both alkene and epoxide.5 Previously, a few similarly noncatalyzed autoepoxidations were found to occur in electron-deficient substituted ethane substrates, such as plumbagin dimers,6 and α-chloroacetoacetanilide dimers.7 However, the mechanism of such autoepoxidations has not been studied in detail. Here, we show that it is nevertheless possible to conceive spontaneous epoxidation of alkanes for preparing bioactive heterocyclic compounds with cheap and environmentally friendly oxidant air and without the need of a metal- or organo-catalyst and sacrificial coreductant. Epoxypyrrolidine units are found in many biologically active molecules such as hirsutellone C,8 C-3-epi-wilsonione,9 or (+)-epolactaene10 and are broadly used as intermediates in natural product synthesis, e.g., pseurotins A,11 rubrobramide,12 and (+)-broussonetine G.13 Several methods have been developed for the preparation of epoxypyrrolidine, including (a) cyclization of halohydrins and the alkene oxidation © XXXX American Chemical Society

Scheme 1. Reactions of the Preparation of Epoxypyrrolidines

aldehydes or ketones (Scheme 1c),16 and (d) [2 + 1] cycloaddition of aza-alkynals and vinyl carbenes (Scheme 1d).17 In terms of the nature of autoxidation mentioned above, the C3−C4 single bond of a polysubstituted pyrrolidine would be a potential spontaneous epoxidation site for preparing epoxypyrrolidine if the electronegativity of the substituents is sufficient. In our previous research, heterocyclic ketene aminals (HKAs), as a bis-nucleophile, have been used to synthesize various polysubstituted pyrrolidine analogues,18 but no epoxypyrrolidine was detected. This may be due to the Received: November 11, 2018

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DOI: 10.1021/acs.orglett.8b03605 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

of the equivalents of the Cs2CO3 revealed that 2.2 equiv is favorable for the formation of 5a (Table 1, entry 21). With the optimized conditions in hand, the scope of the substrates was further explored. As shown in Table 2, various R1 and R2 substituents in HKAs and diazoester, respectively, were tolerated in this reaction (Table 2, entries 1−41). Different substituted R1 or R2 groups have an influence on the reaction; usually, an electron-withdrawing group (F, Cl, or Br) improves the yield of the reaction, whereas an electron-

inadequate electronegativity of the substituent at the C3 position of pyrrolidine. In this work, for the first time, we used aryldiazoester as a bis-electrophile for cycloaddition with HKAs, thus introducing the electron-withdrawing aryl group into the C3 of pyrrolidine, and then successfully completed the preparation of epoxypyrrolidine. For oxidation reactions involving oxygen molecules and often accompanied by freeradical intermediates, we proposed a single-electron-transfer (SET) mechanism for the spontaneous epoxidation, which is experimentally verified. Lastly, the enantioselective epoxidation reaction was achieved by screening chiral ligands. To achieve the optimal conditions, HKA 1a was treated with diazoester 2a, Rh2(OAc)4 catalyst (2 mol %), and K2CO3 base in various solvents using air as the oxidant (1 atm) and stirred at room temperature for 12 h (Table 1, entries 1−7). The

Table 2. Preparation of Epoxypyrrolidine 5-8a

Table 1. Optimized Conditions for the Synthesis of 5aa

entry

no.

1/R1

2/R2

5

yieldb (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 1 3 3 3 3 2 2 2 2 2 2 2 2 2

1a/C6H5 1b/2-BrC6H4 1c/3-BrC6H4 1d/4-BrC6H4 1e/2-CH3C6H4 1f/3-CH3C6H4 1g/4-CH3C6H4 1h/2-FC6H4 1i/3-FC6H4 1g/4-FC6H4 1k/2-ClC6H4 1l/3-ClC6H4 1m/4-ClC6H4 1n/3-CH3OC6H4 1o/4-CH3OC6H4 1p/2,6-Cl,3-FC6H3 1q/3-IC6H4 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1a/C6H5 1n/4-CH3OC6H4 1d/4-BrC6H4 1d/4-BrC6H4 1g/4-FC6H4 1o/2,6-Cl,3-FC6H3 1k/2-ClC6H4 1k/2-ClC6H4 1n/3-CH3OC6H4 1a/C6H5

2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2a/C6H5 2b/2-BrC6H4 2c/2-ClC6H4 2d/2-CH3C6H4 2e/3-FC6H4 2f/3-ClC6H4 2g/3-CH3OC6H4 2h/4-CH3C6H4 2i/4-ClC6H4 2j/thiophen-3-yl 2a/C6H5 2i/4-ClC6H4 2a/C6H5 2a/4-ClC6H4 2a/3-CH3OC6H4 2a/3-ClC6H4 2j/thiophene-3-yl 2b/2-BrC6H4 2i/4-CH3C6H4 2j/4-ClC6H4 2c/2-ClC6H4 2b/2-BrC6H4 2i/4-CH3C6H4 2g/3-CH3OC6H4 2k/CF3CO

5a 5b 5c 5d 5e 5f 5g 5h 5i 5j 5k 5l 5m 5n 5o 5p 5q 6a 6b 6c 6d 6e 6f 6g 6h 6i 7a 7b 7c 7d 7e 7f 8a 8b 8c 8d 8e 8f 8g 8h 8i

89 90 81 86 83 82 81 95 91 91 92 87 86 81 83 75 85 89 91 83 89 85 79 80 85 72 55 53 67 68 64 71 64 83 71 81 72 83 78 74 43

yieldb,c (%) entry

catalyst

solvent

base (equiv)

3a

4a

5a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(oct)4 Rh2(esp)2 Rh2(OPiv)4 Rh2(R-DOSP)4 Rh2(OAc)4 Rh2(OAc)4 Rh2(OAc)4

THF CH3OH acetone toluene DCM DCE CHCl3 DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM DCM

K2CO3 (2.0) K2CO3 (2.0) K2CO3 (2.0) K2CO3 (2.0) K2CO3 (2.0) K2CO3 (2.0) K2CO3 (2.0) Cs2CO3 (2.0) KOH (2.0) Et3N (2.0) DBU (2.0) CsF (2.0) K3PO4 (2.0) KHMDS (2.0) none Cs2CO3 (2.0) Cs2CO3 (2.0) Cs2CO3 (2.0) Cs2CO3 (2.0) Cs2CO3 (1.8) Cs2CO3 (2.2) Cs2CO3 (2.4)

7 13 23 5 13 16 17 7 13 nr nr 17 13 nr nr 9 8 7 6 7 4 7

22 nr 21 4 19 15 44 9 41 nr nr 16 21 nr nr 12 10 11 9 9 7 9

33 nr 36 7 45 38 21 83 23 nr nr 49 62 nr nr 77 54 47 38 78 89 81

a Reaction conditions: In a 25 mL reaction tube, HKA 1a (1 mmol), diazoester 2a (1.2 mmol), base, Rh catalyst (0.02 mmol), and solvent (10 mL) were added under air (1 atm) with stirring for 12 h at room temperature. bIsolated yield based on 1a. cnr = no reaction.

reaction proceeded well in dichloromethane (DCM) to produce the desired epoxide 5a in 45% yield, while cycloadduct 3a and dehydroisomer 4a were obtained in 13 and 19% yields, respectively. Encouraged by these results, more bases were tested in this reaction, including Cs2CO3, KOH, Et3N, etc. (Table 1, entries 8−14). The results revealed that Cs2CO3 is the best base and produced the target epoxide with an 83% yield; instead, without base additive the reaction will not occur (Table 1, entries 15). The isolation of 5a did not significantly improve by applying various other rhodium catalysts (Table 1, entries 16−19). Moreover, the screening

a

Reaction conditions: In a 25 mL reaction tube, HKA 1 (1 mmol), diazoester 2 (1.2 mmol), Cs2CO3 (2.2 mmol), Rh2(OAc)4 (0.02 mmol), and DCM (10 mL) were added under air with stirring for 12 h at room temperature. bIsolated yield based on 1. B

DOI: 10.1021/acs.orglett.8b03605 Org. Lett. XXXX, XXX, XXX−XXX

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air, (b) by dissolving 3a with stoichiometric Cs2CO3 under an inert atmosphere, and (c) by dissolving 3a without base under an inert atmosphere. Following the time evolution of the C3 hydrogen signal of 3a (Figure S6), we recorded the disappearance of 3a by 1H NMR spectra in sample (a). In contrast, both spectra of samples (b) and (c) showed only the peaks of 3a with no formation of 4a and 5a upon extended time (Figures S7−S8). Meanwhile, sample (b) in degassed CD3OD with Cs2CO3 base showed deuterium exchange of the C3−H protons in 3a (Figure S7). Clearly, the deprotonation process on the C3 position of 3a is involved in the oxidation reaction; 4a and 5a are derived from O2 oxidation. The oxygen transfer usually occurs via single-electron oxidation to produce an alkyl radical. The reaction of 3a with O2 in the presence of TEMPO under standard conditions for 60 min afforded 10 in yields of 38% (Scheme S2). The results of this trapping experiment suggested that the substrate radical might be involved in the formation of product 4a and 5a. On the other hand, a peroxide of the 3a was detected by mass spectrometry, suggesting that the addition of an oxygen molecule to 3a occurred during the oxidation process (Figures S11−S12). Moreover, the oxidative dehydrogenation product 4a, whose structure was confirmed by X-ray crystallography (Figure 1), was found to be stable in an oxygen environment over a long period of time but can be transformed into 5a in an alkaline hydrogen peroxide environment (Scheme S3). This result suggests that, as it is typically considered, the conversion of 3a to oxygenates proceeds via the oxidative dehydrogenation of an alkane to an alkene followed by epoxidation of the alkene to provide an oxide in a separate process. Taking into consideration the above results, the possible reaction routes for this tandem cycloaddition/autoxidation of HKAs and diazoester are depicted in Scheme 3. Initially, the diazoester 2 is converted to carbene TM1 under the catalysis of Rh2(AcO)4. The Rh-carbene reacts with the HKAs 1 via vinylogy conjugate additions to form intermediate TM2. Next, the intermediate compound TM3 is obtained via 1,3-H transfer by the tautomerization of compound TM2′. Then this compound undergoes cyclization and tautomerization to produce intermediate 3. Carbanion TM4 is formed under the action of a base. Subsequently, the carbanion and oxygen molecules undergo SET to form the carbon radical TM5 and an oxygen radical negative ion TM6. Then the radical addition yields peroxide intermediate TM7, following the elimination of hydrogen the peroxide to produce 4. Ultimately, epoxide 5 is formed by peroxide hydrogen oxidation from compound 4. Next, we investigated the enantioselective epoxidation of HKA and diazoester by using a series of chiral ligands (Table S1) under the above well-established reaction conditions. L13 was ultimately selected as the most suitable ligand with an acceptable yield and good enantioselectivity for subsequent preparation of the enantioenriched epoxypyrrolidine derivatives (Scheme 4, 9a−p). Both electron-donating and -withdrawing groups at R1 or R2 were tolerated, and epoxides were obtained with 99:1−94:6 er in 45−87% yields. According to the recent discussion on the asymmetric epoxidation of α,βunsaturated ketones,20 we propose the following mechanism (Scheme 5): (i) hydrogen peroxide interacts with ligands to activate oxidants and produces tight ion pairs TMa;21c (ii) TMa regioselectively attacks the C3 of substrate 4 and forms stable hydrogen bonds between the negative charges on the benzoyl moiety and the ligand’s OH;21c (iii) therefore, the reagent prefers the less hindered Re face of the double bonds, leading

donating group (Me or OMe) decreases the yield of the reaction (e.g., Table 2, entries 2, 5, 8, 11, 18−20, and 21−23). In general, the difference in reaction efficiency is not very obvious when the R1 or R2 group bears an ortho-, meta-, or para-substituent (Table 2, entries 2−13). Multiple substituents on R1 or a heterocyclic substituent on R2 can reduce the yield of the reaction (Table 2, entries 16, 26, 32, and 36). Additionally, the HKAs with five or seven hydropyrimidine rings resulted in a much lower isolated yield of 7a−e, which might be attributed to the tension effect (Table 1, entries 27− 31). The incorporation of trifluoroacetyl as an R2 substituent can also produce the epoxide 8i (Table 2, entry 41), whereas acetyl or other aliphatic groups only resulted in cycloaddition products S8a−d (Scheme S1). To verify the structure of the epoxypyrrolidine and dehydroisomer, compounds 5b and 8g were selected as representative compounds and characterized by X-ray crystallography (Figure 1).

Figure 1. X-ray crystal structure of 5b, 8g, and 4c.

As the process for autoxidation of carbonyl compounds under air in the presence of base is well-established,19 control experiments were performed to probe the mechanistic insight of the tandem cycloaddition/autoxidation. The effects of nonoxidant and conventional oxidant were evaluated and are summarized in Scheme 2. Our results in a pure nitrogen Scheme 2. Control Experiments

environment reveal that the tandem reaction can only produce the cycloaddition product 3a, and no formation of oxide 4a and peroxide 5a was detected during the reaction time from 0 to 12 h (Scheme 2, entries 1−3). Similar results are also observed in other reaction cases of peroxide under oxygen-free conditions. The use of m-CPBA or TBHP as oxidant only generates 3a with a yield of 85 or 56%, respectively (Scheme 2, entries 5 and 6). However, H2O2 results in a low production of 4a (3% yield) and 5a (5% yield), which may be due to the decomposition of H2O2 to generate oxygen (Scheme 2, entry 4). These observations strongly suggest that the tandem reaction occurs into two stages: (a) a cycloaddition involving Rh carbene and (b) an autoxidation activated by O2. To demonstrate how the oxygen activates the cycloaddition products 3a to produce epoxide 5a, we prepared three 1H NMR samples in deuterated solvent (VCO(CD3)2/VD2O = 20:1)20 (a) by dissolving 3a with stoichiometric Cs2CO3 in C

DOI: 10.1021/acs.orglett.8b03605 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 3. Postulated Mechanism Based on Experimental Results

Scheme 4. Preparation of Enantioselective Epoxypyrrolidinea

Scheme 5. Postulated Reaction Mechanism for Enantioselective Epoxypyrrolidines 9

a

heterocyclic epoxides from simple and readily available starting materials. In particular, a direct epoxidation from an sp3 C−C single bond is of mechanistic and practical interest as this protocol is suitable for constructing other bioactive heterocyclic epoxides. Our mechanistic study suggests that pyrrolidines are epoxidized by oxygen via a SET process and an alkyl hydroperoxide intermediate. We believe that the success of this work will inspire interest in producing epoxides in noncatalyzed air oxidation reactions.

Reaction conditions: In a 25 mL reaction tube, HKA 1 (1 mmol), diazoester 2 (1.2 mmol), Cs2CO3 (2.2 mmol), Rh2(OAc)4 (0.02 mmol), chiral ligand L (0.1 mmol), and DCM (10 mL) were added under air, with stirring for 12 h at room temperature. bIsolated yield based on 1. cDetermined by chiral-phase HPLC analysis.

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to aromatic TMc intermediates; and last, (iv) the evolution of TMc may result in the epoxypyrrolidine 9 via irreversible direct ring closure. In summary, a novel tandem cycloaddition/autoxidation reactions between HKAs and diazoester in air for the preparation of enantioselective epoxypyrrolidines was described. Importantly, this reaction provides an efficient, enantioselective, and atom-economical route to prepare

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03605. Characterization data and copies of 19F, 1H, and 13C NMR spectra for all new compounds (PDF) D

DOI: 10.1021/acs.orglett.8b03605 Org. Lett. XXXX, XXX, XXX−XXX

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CCDC 1875243−1875245 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 Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shengjiao Yan: 0000-0002-7430-4096 Jun Lin: 0000-0002-2087-6013 Yi Jin: 0000-0003-1348-2486 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Program for Changjiang Scholars and Innovative Research Team in University (IRT17R94), the NSFC (Nos. 21662044, 21262043, 81760621, and U1202221), and the Foundation of “Yunling Scholar” Program of Yunnan Province (C6183005) for financial support.

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