Letter Cite This: Org. Lett. 2017, 19, 5806-5809
pubs.acs.org/OrgLett
Enantioselective 1,3-Dipolar Cycloaddition of Methyleneindolinones with α‑Diazomethylphosphonate to Access Chiral Spirophosphonylpyrazoline-oxindoles Catalyzed by Tertiary Amine Thiourea and 1,5-Diazabicyclo[4.3.0]non-5-ene Nan Huang, Liangliang Zou, and Yungui Peng* Key Laboratory of Applied Chemistry of Chongqing Municipality, School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, P. R. China S Supporting Information *
ABSTRACT: A methodology to access chiral 3,3′-spiro-phosphonylpyrazoline oxindoles via an asymmetric 1,3-dipolar cycloaddition reaction of substituted methyleneindolinones with α-diazomethylphosphonate in the catalysis of tertiary amine thiourea and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) has been established. This method exhibits high functional group compatibility, where a wide range of methyleneindolinones with various substituents and heterocyclic rings are accommodated by this reaction. The resulting chiral 3,3′-spiro-phosphonylpyrazoline oxindoles can be further transformed into spirophosphonylcyclopropane oxindoles by ring contraction.
S
Bestmann−Ohira reagent.12a For chiral spiro-phosphonylpyrazoline oxindoles, only our group has reported the enantioselective 1,3-dipolar cycloaddition of 3-alkylidene oxindoles with Seyferth−Gilbert reagents (SGR).13 However, the substrates of 3-alkylidene oxindoles were limited in the Knoevenagel condensation derivatives of isatins and malononitrile. Generally, from the structure−activity relationships’ (SARs) viewpoint, the substituents and the configuration of the stereogenic centers on the spiro-pyrazoline oxindole core may significantly influence biological activity. Thus, developing an asymmetric method to access chiral spiro-phosphonylpyrazoline oxindoles with broad substituents is necessary. Here, we report the asymmetric reaction of 3-aryl/alkylidene oxindoles with SGR to provide a wide range of chiral spirophosphonylpyrazoline oxindoles (Figure 1). The reaction of methyleneindolinone 1a with α-diazomethylphosphonate 2a was selected as the model reaction. Bifunctional tertiary amine thiourea 3a (10 mmol %) was first used to promote the reaction at 0 °C; no reaction was observed (Table 1, entry 1). After a series of trials, to our delight, the reaction proceeded smoothly when we added DBU as a cocatalyst with 3a14a and had an 86% yield with 36% ee (Table 1, entry 2). Furthermore, employing the Takemoto catalyst 3b14b in this reaction only gave the racemic product in 35% yield (Table 1,
pirooxindole is ubiquitous in natural products and biologically active molecules.1 The size of the spiro ring fused at the C3 position of the oxindole is usually varied from three to seven members.2 Spirooxindoles with a five-membered ring fused at the C3 position have drawn much attention, and great success has been made in developing an efficient enantioselective synthetic strategy to access them,3 i.e., the enantioselective 1,3-dipolar cycloaddition reactions using methyleneindolinones as olefinic dipolarophiles with various types of dipoles, such as azomethine ylides,4 azomethine imines,5 nitrones,6 nitrile imines, 7 nitrile oxides,8 and isocyanide.9 Pyrazolines are important five-membered heterocyclic compounds because of their wide range of biological activities.10 Spirooxindoles with pyrazoline fused at the C3 position have shown better cytotoxicity against MCF-7 cellular activity than that of fused isoxazoline.3b,c In a biological activity study of azaheterocyclic compounds, it was found that the activity could be enhanced significantly when a phosphonate functional group was incorporated.11 Thus, combining the oxindole, pyrazoline, and phosphonate structural motifs simultaneously into one molecule as spiro-phosphonylpyrazoline oxindole may be significant for drug candidate discovery. Consequently, the development of efficient methodologies to access variants of spiro-phosphonylpyrazoline oxindoles for biological activity screening is needed. The Mohanan group has reported the synthesis of racemic spirophosphonylpyrazoline oxindoles by reaction of 3-aryl/alkylidene oxindoles with the © 2017 American Chemical Society
Received: September 5, 2017 Published: October 18, 2017 5806
DOI: 10.1021/acs.orglett.7b02763 Org. Lett. 2017, 19, 5806−5809
Letter
Organic Letters Table 1. Optimization of Reaction Conditions
Figure 1. Asymmetric [3 + 2] cycloaddition of methyleneindolinones with α-diazomethylphosphonate.
entry 3). To enhance the stereoselectivity, considering that the methyleneindolinone could act as both an H-bond donor and H-bond acceptor, we envisioned that adding another chiral scaffold and tertiary amine group as an H-bond acceptor in the catalyst could strengthen its interaction with the substrate. We synthesized catalysts 3c−3i14c,d and screened them in the reaction. When 3c was used, only 15% ee was observed in 89% yield (Table 1, entry 4). We further investigated whether the chirality of cyclohexane-1,2-diamine with the quinidine was matched. Changing the chirality of cyclohexane-1,2-diamine from (1R, 2R)- in 3c to (1S, 2S)-3d led to excellent yield (99%) and ameliorated enantioselectivity (up to 50% ee, Table 1, entry 5). These results demonstrated that the chirality of (1S, 2S)cyclohexane-1,2-diamine was matched with that of quinidine. The performance of catalysts 3e−3i was also evaluated, and the influence of R1, R2, and R3 on the reaction was investigated (Table 1, entries 6−10). The best result was achieved when chiral thiourea 3e was used as the catalyst, affording 95% yield with 71% ee (Table 1, entry 5). With the best catalyst 3e, the other cocatalyst additive bases were investigated, including K2CO3, Et3N, DBN, and TMG (Table 1, entries 11−14). When DBN was used as cocatalyst, the enantioselectivity of the reaction increased slightly up to 74% ee (Table 1, entry 13). The influence of solvents and reaction temperature on the reaction was also examined (Table 1, entries 13 and 15−23). Toluene was the best choice for the reaction (Table 1, entry 13). Lowering the reaction temperature decreased the reactivity, but the enantioselectivity of the product increased. Good enantioselectivity (93% ee) was achieved when the reaction proceeded at −50 °C (Table 1, entry 22). Prolonging the reaction time led to satisfactory results with 95% yield and 93% ee (Table 1, entry 23). Thus, the optimal reaction conditions were established: 3e and DBN as the catalysts in toluene at a concentration of 0.1 M at −50 °C. We further evaluated the substrate scope; a wide range of methyleneindolinones were accommodated by the asymmetric 1,3-dipolar cycloaddition reaction. The results are summarized in Table 2. Substituted 3-arylidene oxindoles, regardless of whether they contained electron-donating groups (Table 2, entries 2−6) or electron-withdrawing groups (Table 2, entries 7−13) at the phenyl ring, gave the desired product in good yields (57%−99%) and enantioselectivities (88%−95% ee). The position of the substituent had some influence on the reactivity; when the substituent was at the 2-position, a slightly lower yield resulted (Table 2, entries 2−4). Only a 57% yield was observed with 86% ee for the substrate with electron-withdrawing substituent Cl at the 2-position (Table 2, entry 9). Naphthylidene oxindole and heterocyclic thienylidene oxindole were suitable for this reaction (Table 2, entries 14 and 15). The 2-naphthyl substrates only achieved a 45% yield with 87% ee
entrya
cat.
additive
solvent
temp (°C)
yieldb
eec
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23f
3a 3a 3b 3c 3d 3e 3f 3g 3h 3i 3e 3e 3e 3e 3e 3e 3e 3e 3e 3e 3e 3e 3e
− DBUd DBU DBU DBU DBU DBU DBU DBU DBU K2CO3 Et3N DBN TMGe DBN DBN DBN DBN DBN DBN DBN DBN DBN
tol tol tol tol tol tol tol tol tol tol tol tol tol tol acetone DCM THF EA tol tol tol tol tol
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 −20 −30 −40 −50 −50
− 86 35 89 99 95 99 99 78 81 trace trace 95 75 95 46 24 51 70 57 54 54 95
− 36 0 15 50 71 65 62 9 33 − − 74 57 0 8 17 37 84 88 91 93 93
a
Unless otherwise specified, the reaction was carried out with 1a/2a (0.1 mmol), additive (0.1 equiv), and toluene (1.0 mL) for 1 day at 0 °C. bIsolated yield. cDetermined by chiral HPLC, and >20:1 dr was determined by 1H NMR. d1,8-Diazabicyclo[5.4.0]undec-7-ene. e 1,1,3,3-Tetramethyl guanidine. fThe reaction time was extended to 5 days.
(Table 2, entry 14). Isopropylidene oxindole led to the product in 62% yield with 93% ee and 4:1 dr (Table 2, entry 16); the dramatically decreased diastereoselctivity maybe ascribe to part of the starting methyleneindolinone isomerized from the E- to Z-configuration in the reaction conditions when isopropylidene oxindole was used. This phenomenon was detected by TLC and demonstrated by 1H NMR. The effect of the substituent on the oxindole nucleus was also examined (Table 2, entries 17− 22). In general, when the substituent was at C5 on the aryl ring of indolinone, the enantioselectivities decreased slightly with excellent yields (Table 2, entries 17−19). When the substituent 5807
DOI: 10.1021/acs.orglett.7b02763 Org. Lett. 2017, 19, 5806−5809
Letter
Organic Letters Table 2. Substrate Scope of Methyleneindolinone in the Asymmetric 1,3-Dipolar Cycloaddition Reactiona
entry
R1
R2
yieldb (%)
eec
drd
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
H H H H H H H H H H H H H H H H 5-Me 5-OMe 5-F 7-F 6-Cl 7-Br
Ph 2-MeC6H4 3-MeC6H4 4-MeC6H4 3-OMeC6H4 4-OMeC6H4 3-FC6H4 4-FC6H4 2-ClC6H4 3-ClC6H4 4-ClC6H4 3-BrC6H4 3-CF3C6H4 2-naphthyl 2-thienyl i-Pr Ph Ph Ph Ph Ph Ph
4a, 95 4b, 88 4c, 99 4d, 99 4e, 95 4f, 95 4g, 97 4h, 99 4i, 57 4j, 88 4k, 91 4l, 99 4m, 86 4n, 45 4o, 70 4p, 62 4q, 99 4r, 88 4s, 99 4t, 59 4u, 57 4v, 46
93 95 94 91 90 92 92 92 86 90 91 91 88 87 86 93 88 86 85 90 88 91
>20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1 17:1 >20:1 >20:1 4:1 >20:1 >20:1 >20:1 >20:1 >20:1 >20:1
Scheme 1. Synthetic Utility of Phosphonylated Spiropyrazoline Oxindole 4
pound 6a was an analogue of an HIV-1 non-nucleoside reverse transcriptase inhibitor (NNRTI).16b The absolute configuration of 4a was determined by X-ray crystallographic analysis (Figure 2), and the other product
a The reaction was carried out with 1/2a (0.1 mmol), 3e (10 mol %), DBN (0.1 equiv), and toluene (1.0 mL) for 5 days at −50 °C. b Isolated yield. cDetermined by chiral HPLC. dDetermined by 1H NMR.
Figure 2. X-ray structure of compound 4a.
configurations were deduced from this. The configurations of the new stereogenic centers in 6a were confirmed by HMQC, HMBC, and NOESY 2D NMR experiments (see the Supporting Information). In the process of transformation of 4a into 6a, the configuration of chiral center C3 was untouched. NOESY experiment has shown a correlation between the benzyl proton H3 and the proton of −OCH3, and between the H4′ and H5; thus, we deduced that the configurations of the new stereogenic chiral centers in 6a as illustrated in Scheme 1. In conclusion, we developed a highly enantioselective 1,3dipolar cycloaddition reaction of substituted methyleneindolinones with α-diazomethylphosphonate catalyzed by chiral multifunctional tertiary amine thiourea and DBN. A series of chiral spiro-phosphonylpyrazoline oxindole derivatives were afforded with good yields (up to 99%) in addition to high diastereoselectivities (up to 20:1 dr) and enantioselectivities (up to 95% ee). To the best of our knowledge, those compounds were converted into spiro-phosphonylcyclopropane oxindole derivatives for the first time. Further assessment of the biological activity of these organophosphonate compounds is underway in our laboratory.
was positioned at C6 and C7, the reactivity decreased dramatically (46%−59% yield) with good enantioselectivities (Table 2, entries 20−22). All the substrates have shown different regioselectivity compared with our previous work;13 the regioselectivity was mainly controlled by the electronic effect of the substrate. In previous work, the electronwithdrawing ability of the two CN groups was over to the amide group. While in this work, the amide group was the only electron-withdrawing group. Optically active cyclopropylphosphonate skeletons are widespread in pharmacological compounds of interest,15 and spirocyclopropane oxindoles have shown HIV reverse transcriptase inhibitor activities.16 Thus, combining the two promising pivotal structural elements together as spiro-phosphonylcyclopropane oxindoles may be helpful in the discovery of lead compounds in novel medical research. To demonstrate the synthetic utility of chiral spiro-phosphonylpyrazoline oxindoles, we attempted to transform 4a into chiral spiro-phosphonylcyclopropane oxindole (Scheme 1). The documented methods to convert pyrazoline into cyclopropane via expelling nitrogen are usually catalyzed by a metal salt or under high temperature.17 After a series of efforts, we succeeded in treating spirooxindole 4a with NCS and NBS. The extrusion of nitrogen and ring contraction reactions proceeded smoothly. Simultaneously, the hydrogen atom at C5 on the aryl ring of the oxindole nucleus was replaced by Cl and Br atoms, affording the chiral spiro-cyclopropane oxindoles 5a and 6a, respectively. Com-
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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.7b02763. 5808
DOI: 10.1021/acs.orglett.7b02763 Org. Lett. 2017, 19, 5806−5809
Letter
Organic Letters
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Experimental procedures and detailed characterization data of all new compounds (PDF) X-ray crystal details for 4a (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. ORCID
Yungui Peng: 0000-0002-5815-1261 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial support from the National Science Foundation of China (21472151).
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DOI: 10.1021/acs.orglett.7b02763 Org. Lett. 2017, 19, 5806−5809
