Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Rapid Synthesis of γ‑Halide/Pseudohalide-Substituted Cyanine Sensors with Programmed Generation of Singlet Oxygen Qingyang Zhang,†,∥ Shengnan Xu,†,∥ Fangfang Lai,†,∥ Yali Wang,† Na Zhang,† Marc Nazare,‡ and Hai-Yu Hu*,†
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†
State Key Laboratory of Bioactive Substances and Function of Natural Medicine, Beijing Key Laboratory of Active Substances Discovery and Drugability Evaluation, Institute of Materia Medica, Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing 100050, China ‡ Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP), Campus Berlin-Buch, Robert-Roessle-Strasse 10, 13125 Berlin, Germany S Supporting Information *
ABSTRACT: A range of γ-halide/pseudohalide-substituted cyaninebased photosensitizers with programmed levels of singlet oxygen generation capability have been developed as theranostic agents for photodynamic therapy of cancer and infectious diseases.
F
luorescent probes based on cyanine platforms1 have attracted considerable attention because these dyes incorporate a number of favorable characteristics such as a high extinction coefficient, long absorption and emission, ease of synthesis, and a high quantum yield. Among the cyanine dyes, the cyanine 5 (Cy 5) and cyanine 5.5 (Cy 5.5) derivatives are favorable for developing fluorescent sensors which are suitable for both in vitro and in vivo experiments. Due to the polyenic backbone of the Cy 5/5.5 dyes, different substituents allow control and fine-tuning of the luminescent properties of the chromophore, such as absorbance wavelength, photostability, and fluorescence.2 Thus, an efficient and selective access to γ-functionalized derivatives would greatly enhance the utility and versatility of the cyanine dyes and expand their applicability. However, so far this functionalization is hampered by low yields, very limited substituent variability, and unwanted late-stage elimination reactions as the available methods of introducing, e.g., a halide to the polyenic methine chain rely on an early introduction at the initial steps of the cyanine chromophore construction.3 This might also be an evident reason why pseudohalides, such as SCN− and SeCN−, to Cy 5/5.5 have not been reported so far. Additionally, cyanine derivatives have also been shown to be efficient singlet oxygen (1O2) generators and photosensitizers in photodynamic therapy (PDT) applications.4 Great efforts have been devoted to enhance 1O2 generation in photosensitizer development for medical applications.5 In particular, a range of photosensitizers with programmed levels of 1O2 © XXXX American Chemical Society
production capability that only differed in chemical structure by minor peripheral modifications to a single core photosensitizer are of significant clinical benefit, which would facilitate the building of an array of varying singlet oxygen producers to suit different therapeutic needs and profiles.6 Due to low 1O2 generation capability, Cy 5/5.5 shows no photosensitivity in cell culture. Previous studies showed the introduction of bromine atoms in the benzene ring of cyanine dyes could enhance their 1O2 generation capability;7 however, the introduction of heavy atoms into the methine backbone of cyanine dyes to control their 1O2 generation efficiency has not been reported. We were interested in the design and synthesis of Cy 5/5.5 analogues with a series of heavy atoms that might function as photosensitizers with programmed levels of singlet oxygen generation capability. Here, we describe a simple, direct and convenient synthetic approach for generating γ-substituted Cy 5/5.5 sensors with halides (Cl−, Br−, I−) or pseudohalides (SCN−, SeCN−) in good yields starting from Cy 5/5.5 via a two steps sequence consisting of an anion exchange with the corresponding halide ion, followed by in situ electrophilic substitutions mediated by NCS (Scheme 1). Moreover, we illustrated that Cy 5 substituted with bromine, iodine, and selenium atoms in the γ-position of the polyenic backbone are very good photosensitizers for PDT action due to the heavy Received: January 31, 2019
A
DOI: 10.1021/acs.orglett.9b00404 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 1. Synthesis of γ-Substituted Cy 5 Cyanine Sensors
Table 1. γ-Halogenation of Cy 5 Using N-Halosuccinimide
atom effect. A range of photosensitizers with programmed levels of 1O2 generation capability have been developed, and the best photosensitizer γ-I substituted Cy 5 could be used as a promising theranostic agent for photodynamic therapy of cancer and infectious diseases. Halogenation is one of the fundamental reactions in organic chemistry since halogenates are essential as starting materials and key intermediates for organic synthesis.8 Among the plethora of halogenating agents, N-chloro-, N-bromo-, and Niodosuccinimide (NCS, NBS, and NIS, respectively) are widely used as the halide sources in free-radical halogenation, electrophilic halogenation, and addition reactions due to their ease of handling as well as the generation of a relatively inert succinimide as byproduct.9 Very recently, Wang and coworkers10 reported the replacement of the hydrogen on the methine chain of Cy 3.5 with halides by using Nhalosuccinimides (NCS, NBS, and NIS) as halogen source. In order to synthesize halide-substituted Cy 5, the halogen substitution reaction of Cy 5 bromide (1a) was investigated using NCS as the halogen sources. To our surprise, the γbromo-substituted Cy 5 (2a) was obtained in 90% yield in a very clean reaction instead of the expected γ-Cl substituted Cy 5 (Table 1, entry 1). The structure of 2a was unambiguously confirmed by 1H, 13C NMR, H−H COSY (Figure S1), HRMS, and HPLC (Figure S2a). We concluded that the γ-Br atom of 2a originated from the bromine counterion of 1a since that was the only bromine source in this reaction. Based on this serendipitous experimental observation, we started to systematically screen various parameters to determine the factors influencing the formation of the product 2a. First, we investigated if this highly efficient and selective γsubstitution of Cy 5 reaction could be extended to other halogen substituents. Cy 5 with the corresponding halide salts (F−, Cl−, I−) as counterion, prepared by a simple ion exchange,11 were reacted with NCS (1.2 equiv) in dichloromethane (DCM) at room temperature for 1 h, respectively. When X = F (1b) or Cl (1c), the electrophilic chlorination product (2c) was isolated as the main product in low yields (45% and 44%, respectively) due to the formation of dichloride side products (Table 1, entries 2 and 3). In contrast, the desired γ-iodinated product 2d was obtained in good yields (83%) by using Cy 5 iodide 1d as substrate (Table 1, entry 4, and Figure S2b). Based on these interesting preliminary results, we hypothesized that the outcome of these halogenation reactions might be related to the redox potential difference12 between the chlorine from NCS and the halide
entry
X
reagent
X′
yielda (%)
1 2 3 4 5 6 7 8 9 10 11 12
Br F Cl I F Cl Br I F Cl Br I
NCS NCS NCS NCS NBS NBS NBS NBS NIS NIS NIS NIS
Br Cl Cl I Br Br Br I I I I I
90 45 44 83 52 60 63 80 70 55 66 60
a
The reaction was run with 1 (100 mg) and reagent (1.2 equiv) in DCM at rt for 1 h.
anion of Cy 5. Consequently, a series of Cy 5 halides (F−, Cl−, Br−, I−) was used for reaction with NBS and NIS, respectively, to validate our hypothesis. Table 1 indicates that, in the case of Cy 5 halides reacting with NBS, when the redox potential of the halogen anions of Cy 5 (X) were higher than or equal to that of the bromine from NBS (X = F, Cl, Br), the bromination reaction occurred and NBS served as an electrophilic bromination reagent (Table 1, entries 5−7). In contrast, when the redox potential of X is lower than that of bromine, the γ-X-substituted Cy 5 product was obtained (Table 1, entry 8, X = I). As expected, only the γ-iodinated Cy 5 product was isolated as the main product in low yields in all cases of the Cy 5 halides (X = F, Cl, Br) when using NIS, since iodine has the lowest level of redox potential of all halogens (Table 1, entires 9−12). When NFIS was used as a fluorinating reagent we mainly obtained γ-fluorinated product with low regioselectivity accompanied by significant difluorinated side product. Here, only a minor amount of the γ-chloride product (
