Letter Cite This: ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/acsmedchemlett
Highly Selective and Efficient Synthesis of 7‑Aminoquinolines and Their Applications as Golgi-Localized Probes Jiahui Chen,† Huijing Liu,† Li Yang,‡ Jun Jiang,‡ Guoqiang Bi,† Guoqing Zhang,‡ Guisheng Li,*,§ and Xiaofeng Chen*,§ §
School of Environmental and Geographical Sciences, Shanghai Normal University, Shanghai 200234, China School of Life Sciences, University of Science and Technology of China, Hefei 230026, China ‡ Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China †
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ABSTRACT: Quinoline derivatives have extensively been used for both pharmaceutical agents and bioimaging. However, typical synthesis of quinoline derivatives is generally through strong acid/base-catalyzed or metal-catalyzed methods at high temperatures. Here we report a catalyst-free synthesis of 2,4-disubstituted 7-aminoquinolines with high selectivity and good yields via the introduction of a trifluoromethyl group. It is discovered that quinolines containing both amino and trifluoromethyl groups exhibit strong intramolecular charge-transfer fluorescence with large Stokes shifts. We further applied the obtained quinolines to live-cell imaging and found that some of the derivatives can target specifically Golgi apparatus in various cell lines (HeLa, U2OS, and 4T1 cells) in vitro and the colocalization with commercial Golgi marker is retained during the mitosis in HeLa cells. Moreover, the quinoline dyes can also be used for Golgi apparatus imaging with two-photon fluorescence microscopy. These results provide new insights into developing low cost Golgi-localized probes. KEYWORDS: Quinoline, selective synthesis, intramolecular change transfer, fluorescence, cell imaging
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involved in such vital intracellular activities, disruption of Golgi apparatus functions could cause many organ lesions such as eye, kidney, and liver diseases.21 Thus, it is of great significance to develop specialized probes to mark Golgi apparatus. Considering most commercial Golgi markers generally are difficult to be synthesized with complicated structures or high cost,22−24 the development of Golgi-localized small molecule probes has still remained challenging. The application of quinoline derivatives as cell organelle probes is rarely explored, even though it has been widely applied to construct medicine molecules and analyte probes. Moreover, quinoline primarily fluoresces in the near UV region, it is desirable to shift its absorption and emission signals to longer wavelengths without significantly extending the molecular size for biosensing and bioimaging. In this context, charge-transfer state could be used as an effective strategy to mediate excited states.25,26 As a strongly electron-withdrawing group, trifluoromethyl group has been used to form charge-transfer states with electron-donating groups.27−29 Meanwhile, biological and medicinal studies have
uinolines and their derivatives play a crucial role in organic chemistry due to the applications in pharmaceuticals as well as advanced functional materials.1−4 As the important “star molecules” in various research areas, quinolines continued to receive extensive research interests in the past few years, particularly as the core scaffold to construct medicine molecules5−8 and fluorescent probes for sensing.9−13 Given the importance of the quinoline scaffold in the fields of both pharmaceutical and organic chemistry, synthesis of quinoline derivatives has received tremendous attention since Skraup first reported the classical synthetic method of quinoline in 1880.14 Over the past few decades, numerous methods based on various mechanisms, including Conrad−Limpach−Knorr,15 Friedlän der, 16 Doebner−von Miller, 17 Pfitzinger, 18 or Combes,19 have been developed for the preparation of substituted quinolones. However, multiple synthetic steps and harsh reaction conditions, such as high temperatures, strong base, acid or metal catalysts, still limit the applications of these strategies. Therefore, direct catalyst-free approaches with good yields remain highly valuable to broaden the application of quinoline derivatives. As one of the common cellular organelles, Golgi apparatus is the major collecting, processing, and dispatching station of proteins to be modified and delivered for secretion.20 Since © XXXX American Chemical Society
Received: March 19, 2019 Accepted: May 28, 2019 Published: May 28, 2019 A
DOI: 10.1021/acsmedchemlett.9b00118 ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX
ACS Medicinal Chemistry Letters
Letter
demonstrated that fluorine-containing compounds also exhibit enhanced biological properties.30−32 Considering both feasible synthetic routines and biological applications of quinolines, we propose here to conduct a catalyst-free reaction of m-phenylenediamine with unsymmetric 1,3-diketones containing a trifluoromethyl group to synthesize various substituted 7-aminoquinolines as novel fluorophors in good yields. Introduction of such a trifluoromethyl group avoids the use of concentrated acid to promote the condensation as well as the subsequent treatment with excess amount of base to liberate the quinoline as previously reported.33 And the strongly electron-withdrawing trifluoromethyl group also potentially enhances intramolecular change transfer (ICT) state of the 7-aminoquolines between the strongly electron-donating amine group and the trifluoromethyl group, thus shifting the absorption and emission of the compounds to the longer wavelengths in polar media. We first tested the reaction of m-phenylenediamine with 4,4,4-trifluoro-1-phenylbutane-1,3-dione. As shown in Scheme 1, after a chloroform solution of the two compounds were
Scheme 2. Proposed Formation Mechanism of 7Aminoquinolines
In a similar way, we also conducted the reaction with various substituents on the unsymmetric trifluoromethylated 1,3diketones to test the versatility of the method to introduce a trifluoromethyl group. Compounds 1b−1d were successfully obtained from the corresponding unsymmetric 1,3-diketones in good yields, demonstrating the strategy could be widely applied. Moreover, when m-phenylenediamine was reacted with hexafluoroacetylacetone, the reaction condition can be so mild that the product 1d could even be produced in water at room temperature (Figure S1), which indicates that the two trifluoromethyl groups further accelerate the reaction. We then studied the fluorescence ICT properties of the 2,4disubstituted 7-aminoquinolines in various solvents. For comparison, we prepared 7-methoxyquinoline 2 with a methoxy group that is less electron-donating than the amine group and 7-amino-2,4-dimethylquinoline ADMQ with two methyl groups that are less electron-withdrawing than the trifluoromethyl group, and compared their optical properties with the new quinolines. A summary of the optical characterizations of compounds 1a−d, 2, and ADMQ is presented in Table 1. The absorption and emission spectra are also provided (Figures 1 and S2−S7), along with photos showing the solvatochromic fluorescent emissions (Figures S8−S13). In general, the trifluoromethyl-substituted 7-aminoquinolines (1a−1d) show rather strong absorption in the near UV to blue light region. Absorption maxima exhibit gradual bathochromic shifts as the solvent polarity increases, e.g., λmax = 365−368 nm in n-hexane and λmax = 389−399 nm in methanol. The visual emission color changes from violet in n-hexane (λem = 407−435 nm) to greenish yellow (λem = 507− 537 nm) in MeOH, indicating increased excited-state dipole moments of these molecules. Presumably this is due to an ICT process from the −NH2 lone pair to the vicinity of the electron-withdrawing −CF3 group. As a result, when −NH2 is replaced with a less powerful electron-donating −OCH3 group, or the strong electron-withdrawing group −CF3 with an electron-donating −CH3 group, the quinoline compounds 2 and ADMQ emit in the violet region in all the solvents (
