Letter Cite This: Org. Lett. 2018, 20, 5380−5383
pubs.acs.org/OrgLett
Umpolung of o‑Hydroxyaryl Azomethine Ylides: Entry to Functionalized γ‑Aminobutyric Acid under Phosphine Catalysis Qingqing Chen, Yishu Bao, Xiuqin Yang, Zonghao Dai, Fulai Yang, and Qingfa Zhou* State Key Laboratory of Natural Medicines, Department of Organic Chemistry, China Pharmaceutical University, Nanjing, 210009, China
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S Supporting Information *
ABSTRACT: A phosphine-catalyzed reaction between ohydroxyaryl azomethine ylides and MBH carbonates provides access to highly functionalized γ-aminobutyric acid derivatives in moderate to good yields. Mechanistically, the reaction involves a phosphine-catalyzed tandem SN2′/2-aza-Cope rearrangement/intramolecular addition process. ylides derived from α-amino acids as a nucleophile is very scarce, although they have proven to be versatile synthetic intermediates to synthesize diverse pyrrolidines11 by coupling with various dipolarophiles under metal catalysis or organcatalysis conditions and to synthesize α,α-disubstituted α-amino acids12 under various conditions (see Scheme 1). Umpolung of
γ-Aminobutyric acid (GABA) is a unique substructure present in the central nervous system (CNS) of mammals exhibiting many important bioactivities.1 For example, GABA in humans plays a central part in the regulation of muscle tone, and it has been one important target for drug discovery.2 Compounds containing this structural motif are also widely present in natural products, and many of them exhibit a wide range of biological activities:3 γamino-β-hydroxybutyric acid (I), which has been identified as a key fragment of the microsclerodermins, exhibits antitumor and antifungal activity;4 gabapentin (II), which was developed by Pfizer, has been used as a drug for the treatment of neuropathic pain;5 Hemiasterlin (III), which is extracted from the marine sponge Hemiasterella minor, exhibits a potent in vitro cytotoxin and antimitotic agent (see Figure 1).6 Because of the prevalence
Scheme 1. Reaction Patterns of Azomethine Ylide Derived from α-Amino Acids
Figure 1. Related compounds containing the γ-aminobutyric acid scaffold.
of γ-aminobutyric acids in medicinal chemistry, it is highly demanded to develop novel synthetic methodologies that enable an avenue to new molecular architectures that contain this important pharmacophore. Recently, imine umpolung reactions have emerged as a powerful synthetic strategy for constructing structural amino compounds.7 For example, many γ-amino aldehydes/ketones have been constructed via umpolung β-addition of ketimines to enals or enones, using the phase-transfer catalysts reported by Deng’s group.8 A series of functionalized α-quaternary amines could be effectively synthesized through phosphine-catalyzed umpolung γ-addition of ketimines to allenoates by Zhang’s group9 and diverse γ-aminobutyric acids containing a trifluoromethyl group could also be obtained by phosphine-catalyzed umpolung addition of trifluoromethyl ketimines to Morita− Baylis−Hillman (MBH) carbonates. 10 In contrast with umpolung of imines, inverting the polarization of azomethine © 2018 American Chemical Society
azomethine ylides may provide alternative addition/cycloaddition patterns to synthesize chemical entry with structural diversity, which has neither been previously accessible nor required. Very recently, an intramolecular hydrogen bond in the azomethine ylide has shown a huge impact on both reactivity and reaction site of the substrates.13 Inspired by the above work and our continual interest in phosphine catalysis,14 we attempted the reaction of o-hydroxyaryl azomethine ylides with MBH carbonates in the presence of phosphine catalyst. Unexpectedly, a functionalized γ-aminobutyric acid was Received: July 21, 2018 Published: August 21, 2018 5380
DOI: 10.1021/acs.orglett.8b02297 Org. Lett. 2018, 20, 5380−5383
Letter
Organic Letters
After establishing the optimized reaction conditions for the synthesis of functional γ-aminobutyric acid derivatives, several ohydroxyaryl azomethine ylides were first synthesized and applied to couple with MBH carbonate 2a, and the results are outlined in Scheme 2. As can be seen from Scheme 2, o-hydroxyaryl
obtained, which not only shows a new reaction pattern but also exploits the potential of azomethine ylides derived from αamino acids. Herein, we report a novel phosphine-catalyzed and highly regioselective umpolung addition of o-hydroxyaryl azomethine ylides to MBH carbonates.15 In the beginning, o-hydroxyaryl azomethine 1a and MBH carbonate 2a were selected for the initial reaction using 20 mol % triphenylphosphine as a catalyst in CH2Cl2 at room temperature (see Table 1, entry 1). A regioselective SN2′
Scheme 2. Substrate Scope for Umpolung Reactions of oHydroxyaryl Azomethine with MBH Carbonatesa,b
Table 1. Survey on Conditions for Formation of 3aa
Yieldb (%) entry
solvent
catalyst (mol %)
oil bath temperature (°C)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
CH2Cl2 CH2Cl2 DCE CHCl3 CH3CN DMF THF toluene toluene toluene toluene toluene toluene toluene toluene
Ph3P (20) Ph3P (20) Ph3P (20) Ph3P (20) Ph3P (20) Ph3P (20) Ph3P (20) Ph3P (20) Ph3P (20) Ph3P (20) Ph3P (20) Ph3P (20) Ph2PEt (20) PhPEt2 (20) Bu3P (20)
25 40 80 60 80 150 70 110 0 60 150 200 150 150 150
3a
4a 46 76 61 83 75 69
5a 75 40
20
57
65 67 58 27 30
80 20 16 29 57 52
a
The reaction was performed with o-hydroxyaryl azomethine (1a) (0.1 mmol) and MBH carbonate (2a) (0.12 mmol) in a sealed tube for 6 h. bIsolated yields based on 1.
product 5a with a terminal CC bond was obtained. We conjectured that product 5a could undergo a 2-aza-Cope rearrangement under higher-temperature conditions,16 so we performed the reaction at 40 °C in CH2Cl2. To our disappointment, product 5a and 4a were obtained with comparative yields and no 2-aza-Cope rearrangement product was found. Various solvents were examined carefully (see Table 1, entries 3−8). Product 4a was selectively generated in dichloroethane (DCE), CHCl3, acetonitrile, dimethylformamide (DMF), and tetrahydrofuran (THF). Remarkably, when toluene was used, a novel product 3a was isolated, despite the fact that the yield remained to be raised to a higher level. The structure of the product 3a was confirmed through X-ray crystallographic analysis. The temperature was then examined using toluene as a solvent. No reaction was found when the temperature was reduced to 0 °C. However, the product 5a was obtained with a yield of 80% when the reaction was performed at 60 °C. Pleasingly, a good yield of product 3a was obtained when the reaction was performed at 150 °C in a sealed tube. A higher temperature did not give a better yield of 3a. No better result was obtained when a more nucleophilic phosphine instead of PPh3 was employed, such as Ph2PEt, PhPEt2, or Bu3P. Thus, the optimal conditions for the formation of γ-aminobutyric acid derivatives were established.
a
The reaction was performed with o-hydroxyaryl azomethine 1 (0.1 mmol) and MBH carbonates 2 (0.12 mmol) in a sealed tube under 150 °C for 6 h. bIsolated yields based on 1.
azomethine ylides containing different electrical groups on the benzene ring worked well with MBH carbonate 2a, giving the target products γ-aminobutyric acid derivatives in moderate to good yields. For example, for the substrates with a methyl or methoxyl group attached on the benzene ring, the corresponding γ-aminobutyric acid derivatives 3b and 3d were obtained in yields of 64% and 73%, respectively. Various halogen substituents (F, Cl, Br) could still give the corresponding γaminobutyric acid derivatives in moderate to good yields under the reaction conditions, which offers a potential handle for the subsequent coupling operation. In addition, dihalogen substituents also afforded the corresponding products. Note that naphthyl-containing substrates also are compatible, efficiently giving γ-aminobutyric acid 3i in a yield of 63%. We next 5381
DOI: 10.1021/acs.orglett.8b02297 Org. Lett. 2018, 20, 5380−5383
Letter
Organic Letters investigated the scope of MBH carbonates. As shown in Scheme 2, both electron-withdrawing and electron-donating groups at the para position of the benzene ring were tolerated and gave the corresponding γ-aminobutyric acid derivatives in good yields. For example, for substrates with a methyl or methoxyl group attached on the benzene ring, the corresponding γ-aminobutyric acid derivatives 3k and 3l were obtained in yields of 57% and 56%, respectively. Substrates with an electron-withdrawing group, such as a nitrile group, on the benzene ring also gave the target product 3p in a yield of 55%. However, for the substrate with a stronger electron-withdrawing group, such as methyl 2[((tert-butoxycarbonyl)oxy) (4-nitrophenyl)methyl] acrylate, a novel azetidine derivative 7 was found (see Scheme 3, eq 1).
Scheme 5. Plausible Mechanisms
Scheme 3. Diverse Reaction of 1a with MBH Carbonates
MBH carbonate to produce the phosphonium intermediate A with concurrent release of CO2 and tert-butoxide. The tertbutoxide then deprotonates o-hydroxy aromatic aldimine 1a and generates intermediates B. The hydroxy group located at the 2position in 1a might stabilize azomethine ylide B via an intramolecular hydrogen bond. The azomethine ylide B might happen a nucleophilic attack on the β-carbon of MBH carbonate, which led to the formation of allylation product 4a (Scheme 5, path a). On the other hand, azomethine ylide B undergoes a SN2′ with phosphonium intermediate A to give γallylation product 5. Product 5 follows a facile 2-aza-Cope rearrangement to deliver the γ-aminobutyric acid C, which might then undergo an intramolecular addition pathway to form product 3 ultimately. Because of the effect of nitro group at the para position of the benzene ring, compound 7 could be formed via an intramolecular anti-Michael process (Scheme 5, path b). In summary, we have developed an unprecedented phosphine-catalyzed and highly regioselective domino process of o-hydroxyaryl azomethine with MBH carbonates to highly functionalized γ-aminobutyric acid derivatives. This catalytic process provides access to γ-aminobutyric acid derivatives in moderate to good yields. This transformation is a rare example of umpolung of azomethine ylides derived from α-amino acids.
Pleasingly, the benzene ring can be replaced by a naphthalene ring or a heteroaromatic ring to provide corresponding γaminobutyric acid derivatives 3q and 3r in yields of 66% and 64%, respectively. We also tested the feasibility of this reaction using alkyl MBH carbonate; for example, methyl 2-[((tertbutoxycarbonyl)oxy)methyl] acrylate, under the same reaction conditions, gave the SN2′ product 9 (Scheme 3, eq 2). To understand these novel processes, diethyl (E)-2(benzylideneamino) malonate was synthesized and treated with MBH carbonate 2a under the aforementioned conditions (see Scheme 4). The reaction was complicated and no major Scheme 4. Control Experiments
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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.8b02297. Full experimental procedures and copies of NMR spectra (PDF) Accession Codes
CCDC 1857200 contains 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 data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.
product was formed. This implied that the hydroxy group located at the 2-position of azomethine ylide might play an important role for the present process.13 Product 5a could be effectively transformed to 3a when 5a was treated in toluene at 150 °C in a sealed tube. However, no product was formed when 4a treated in toluene at 150 °C in a sealed tube. These suggested that steric effect of phenyl group in 4a was responsible for the failure of 2-aza-Cope rearrangement (see Scheme 4). On the basis of these results and the previous work,17 we proposed a reasonable mechanism for present domino processes18 (see Scheme 5). Initially, phosphine reacts with
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AUTHOR INFORMATION
Corresponding Author
*E-mail addresses:
[email protected],
[email protected]. cn. 5382
DOI: 10.1021/acs.orglett.8b02297 Org. Lett. 2018, 20, 5380−5383
Letter
Organic Letters ORCID
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Fulai Yang: 0000-0002-1136-0867 Qingfa Zhou: 0000-0001-6360-2285 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21102179 and 21572271), the Qing Lan Project of Jiangsu Province, the National Found for Fostering Talents of Basic Science (Grant No. J1030830).
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DOI: 10.1021/acs.orglett.8b02297 Org. Lett. 2018, 20, 5380−5383
