Letter Cite This: Org. Lett. 2019, 21, 206−209
pubs.acs.org/OrgLett
Improved Xanthone Synthesis, Stepwise Chemical Redox Cycling James L. Bachman, P. Rogelio Escamilla, Alexander J. Boley, Cyprian I. Pavlich, and Eric V. Anslyn* Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States
Org. Lett. 2019.21:206-209. Downloaded from pubs.acs.org by UNITED ARAB EMIRATES UNIV on 01/09/19. For personal use only.
S Supporting Information *
ABSTRACT: A base-catalyzed direct oxidation of rhodamine, carborhodamine, and siliconrhodamine pyronines to the corresponding xanthones is studied. This methodology utilizes addition of water to split pyronines into xanthone and reduced xanthene, the latter of which is returned to pyronine by oxidation with iodine. The transformation is general, working on the three most recalcitrant versions of N,N,N′,N′tetramethylpyronines in good to excellent yields. extended π system competes with ketone formation, especially for the rhodamine cores with O, C, and Si. However, rhodamine derivatives with heavier atoms such as P, Ge, and Sn proceed more readily to the ketone with oxygen-transfer oxidants.5,6 Despite the utility of these intermediates, few good-yielding methodologies exist for their syntheses. In their efforts to make CalFluors, Bertozzi et al. demonstrated that a3 can be prepared by reducing a1 with borohydride followed by permanganate oxidation.9 Akin to that, classic carbopyronine synthesis proceeds through two aromatic substitutions followed by permanganate oxidation.3 In one of the few examples of direct pyronine oxidation, Nagano et al. reported a method for preparing Si-xanthones using potassium cyanide and iron trichloride.4 The unreliable oxidations of these described methods can be attributed to the instability of both the pyronines and xanthenes under redox conditions, and thus yields in the 20−40% range are observed for these three methods. In this work, we present direct oxidation of the three most used rhodaminepyronines (O, CMe2, SiMe2) to their corresponding xanthones that does not rely on harsh oxidants. We utilize the electrophilicity of the pyronine 9-position for a base-catalyzed addition of water, giving the desired xanthone and a xanthene byproduct, which is returned to the starting pyronine via iodine oxidation (Scheme 1). The addition of cesium carbonate to a solution of Pyronin Y (a1) in 5:1 N-methylpyrrolidone (NMP):DMSO-d6 with heating resulted in the formation of xanthone (a3) in a 53:47 ratio with xanthene (a2). By 1H NMR, the initial formation of an aqua-addition complex (4) was observed, as the vinylic proton (8.8 ppm) shifted upfield as a methyne (5.5 ppm) within 30 min at room temperature (SI16). Addition of aqueous media reversed the complex back to starting pyronine with the reappearance of orange fluorescence. However, under anhydrous conditions, further heating took the reaction to the
he use of fluorescent dyes in chemical and biological applications is extremely widespread. The utility of these molecules in elucidating structure and mechanism cannot be overstated.1 Among the many classes of fluorophores, the xanthene dye rhodamine has seen considerable adoption due to its favorable fluorescence properties, including high absorptivity, photostability in harsh pH and temperature conditions, as well as scaffold tunability.2 In efforts toward making fluorophores with bathochromic shifts for far-red imaging, many analogues have been prepared by replacement of the core oxygen with C, Si, Ge, Sn, and P.3−6 A common intermediate in these rhodamine syntheses is the xanthone of the given derivative (Scheme 1). Addition to the ketone by
T
Scheme 1. Oxidation Reaction Cycle
lithiating aryl halides is used in rhodamine synthesis with this intermediate, but triflation followed by Suzuki−Miyaura cross coupling has also been demonstrated as a suitable synthetic route.7 Accessing the xanthone intermediate in the multistep syntheses is among the synthetically challenging steps, in part due to low yielding oxidations of xanthene by KMnO4 or quinones. Additionally, as noted by Hell and others, carbopyroninexanthenes (e.g., b2 and other alkyl-substituted anilines) undergo degradation in ambient conditions to the corresponding carbopyronine (b1).8 The stability of the © 2018 American Chemical Society
Received: November 15, 2018 Published: December 24, 2018 206
DOI: 10.1021/acs.orglett.8b03661 Org. Lett. 2019, 21, 206−209
Letter
Organic Letters observed formation of a2 and a3. To probe the mechanism, the scope of bases suitable for reactivity was evaluated (Table 1). Table 1. Base and Solvent Effects
Figure 1. Proposed oxidation reduction mechanism. entry
solvent
base (2 equiv)
1 2 3b 4c 5 6 7 8d 9 10 11 12c 13 14
DMF-d7 DMF-d7 NMP/DMSO-d6 DMSO-d6 CH3CN-d3 DMF-d7 DMF-d7 DMF-d7 DMF-d7 DMF-d7 DMF-d7 DMF-d7 DMF-d7 DMF-d7
Cs2CO3a Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 K2CO3 NaHCO3 Na3PO4 CsOPiv DIPEA NaOH NaOtBu NaOAc CsF
equiv of H2O
%1
%2
%3
5 6 2 3 4 4 30 4 5 5 4 3 4
23 0 0 0 45 37 43 0 0 0 32 0 25 0
39 53 53 37 29 35 31 51 52 39 38 41 39 54
35 47 47 36 26 28 26 49 48 61 30 42 36 46
of a sacrificial hydride acceptor, and the second was the addition of a co-oxidant to reoxidize the intermediate xanthene back to pyronine. Several quinones were evaluated as sacrificial oxidants: chloranil, anthraquinone, 1,2-napthoquinone, and benzoquinone. Under the reaction conditions, regardless of the electron-donating or -withdrawing quinone, heating the pyronines in their presence resulted in degradation. The reaction solutions quickly turned brown with little to no observable product formation. The second approach taken was to oxidize the byproduct xanthene back to starting material, creating a cyclic mechanism. While monitoring pyronine Y under the reaction conditions described in Table 1, two interesting observations were made. First, subjection of a pyronine-spotted silica gel TLC plate to extended times of ultraviolet light turned the xanthene spot to the orange red fluorescence seen in the pyronine starting material. Second, using an iodine chamber resulted in the immediate oxidation of a short-wave active xanthene spot back to the fluorescence of the pyronine. Under reaction conditions described in Table 1, no product was formed when silica gel was added into the reaction. However, when I2 was added after formation of a3 and a2, the reaction went from brown back to the dark red color of the pyronine. Xanthene oxidation by iodine back to pyronine proceeded within seconds. Further heating following iodine addition resulted in the oxidation cycle proceeding to complete xanthone formation. Shown in Figure 2, monitoring the reaction by 1H NMR displays clean oxidations between starting material and the split products, followed by iodine oxidation back to pyronine and complete xanthone formation. The bases that most readily produced the desired oxidation from Table 1 were tested for compatibility with iodine as a cooxidant. As shown in Table 2, Cs2CO3, Na3PO4, and CsF all performed similarly in this oxidation, but Na3PO4 was the best, giving a 96% yield of the desired product a3 in DMF and up to 97% in NMP. With the optimized conditions in hand, generalization of the method to the other common rhodamine analogues was performed. We chose to concentrate on the species that are the most recalcitrant to oxidation to the ketone, rather than the Ge, Sn, and P analogues that readily give ketone with the previous methods. Thus, replacement of the core oxygen in pyronin Y a1 with carbon or silicon gives the rhodamine analogues carbopyronine and Si-pyronine, respectively. The red-shifted fluorescence of these compounds relative to rhodamine is especially useful for biological imaging. To evaluate the oxidation of these analogues, N,N,N′,N′tetramethylcarbopyronine b1 and N,N,N′,N′-tetramethylsiliconpyronine c1 were prepared following literature precedence. The tetramethylaniline pyronines were selected because they represent a general tetralkyl substitution pattern and should be representative of the reactivity of other common pyronines, such as the julolidine-containing Atto 647N fluorophores10 and the azetidine-substituted Janeliafluors.11
a
Entry 1 was run with 0.5 equiv of Cs2CO3. bNMP-H9 to DMSO-d6 (5:1). Cannot determine water equivalents by 1H NMR. cReaction components do not add up to 1.0 because of the presence of a nonisolated intermediate 4 that decayed back to 1. dWater from dodecahydrate base.
The most effective at catalyzing the oxidation/reduction were Cs2CO3, CsF, CsOPiv, and Na3PO4, with the amine N,Ndiisopropylethylamine yielding a greater ratio of xanthene byproduct. The improved reactivity for similar bases such as Cs2CO3 compared to K2CO3 can be explained by increased solubility.
The effect of solvent was evaluated with the high-performing base Cs2CO3, with results indicating that polar aprotic solvents are necessary, and amide solvents DMF and NMP producing the desired reaction in the shortest time. For CH3CN, the reaction did not go to completion, while DMSO stabilized the aqua-addition intermediate (4) and required several hours to completely generate 2 and 3. To further elucidate the mechanism of water addition to pyronine, isotopically labeled water H2O18 was added to the reaction with an equimolar amount of water to the hydrated base, Na3PO4. Mass spectral data revealed a slight preference for the H2O16 a3 compared to H2O18 in a 2.0:1.16 ratio. These data convince us that the source of oxygen is from water added to the reaction, not ambient O2 or solvent. Based on these results, and that 0.5 equiv of base (Table 1, entry 1) does allow the reaction to reach completion, we speculate that the mechanism (Figure 1) is dependent upon a base-catalyzed addition of water to the pyronine, followed by another deprotonation step in which the hydrogen at the 9 position undergoes a hydride transfer to another molecule of pyronine, reducing it to xanthene. To push the xanthone yield beyond 50%, it was envisioned that two approaches could be taken. The first was the addition 207
DOI: 10.1021/acs.orglett.8b03661 Org. Lett. 2019, 21, 206−209
Letter
Organic Letters
Figure 2. 1H NMR reaction of pyronine Y (a1).
product degradation, as a slightly lower yield of 70% was observed. The other most widely used rhodamine fluorophore is the silicon variant. In preparation, the silicon-pyronine degraded during silica gel purification, and the reported yield is for a 0.1 mmol scale of HPLC purified product. To avoid flash chromatography, at the 1 mmol scale siliconpyronine was taken crude following DDQ oxidation and subjected directly to the methodology conditions, giving a 54% yield for the four steps, comparable to the ∼50% yield observed for the threestep sequence involving direct oxidation of xanthene with permanganate8 (SI4). For the HPLC purified c1, under the optimized reaction conditions oxidation to c3 proceeded in 71% yield. Compared to the efficiency of oxidation for the oxygen rhodamine derivative, the group 15 pyronines performed slightly worse under the same conditions. The difference in reactivity appears to depend on the increased electronegativity of oxygen, making the 9-position of the pyronine more electrophilic, promoting the addition of water more readily. In conclusion, we have developed an efficient method for the direct oxidation of the most common rhodamine pyronine analogues in good to excellent yields. For oxygen-substituted rhodamines, yields of 97% represent a 5-fold increase compared to the two-step reduction followed by oxidation. For silicon pyronine, again, this method nearly doubles the reported direct oxidation.5 For carbopyronine species, this method offers an alternative to KMnO4 oxidation and would be especially useful in circumstances where accessing the carbopyronine is facile. This is the first such pyronine reaction scheme to utilize water as an oxidant and iodine as a terminal
Table 2. Generalization of Pyronine Oxidation
entry
compound
solvent
base
max temp. (°C)
yield (%)
1 2 3 4 5 6 7 8 9
a3 a3 a3 a3 b3 b3 b3 c3 c3
DMF DMF DMF NMP DMF NMP NMP NMP NMP
Cs2CO3 CsF Na3PO4 Na3PO4 Na3PO4 Na3PO4 Na3PO4 Na3PO4 Na3PO4
110 110 110 110 110 110 125 110 110
84 83 96 97 47a 86 73a 71b 54c
a
Refers to yield determined by internal standard 1,3,5-trimethoxybenzene, otherwise isolated. bHPLC purified material (0.1 mmol scale). cYield determined for a four-step reaction sequence without purification of the pyronine.
When heating b1 to 110 °C in DMF, significant carbopyronine xanthene and pyronine remained following reaction times similar to use for a1. Changing the solvent to NMP but maintaining the same temperature lead to a 2-fold increase in yield (Table 2). At 110 °C, the reaction reached a maximum yield with 86%. Increasing the temperature to 125 °C under the ideal reaction conditions lead to a faster reaction time and appeared to fully convert b1 to b3, but the higher temperature gave some 208
DOI: 10.1021/acs.orglett.8b03661 Org. Lett. 2019, 21, 206−209
Letter
Organic Letters oxidant to cycle the reaction to completion. The general nature of the tetramethyl-aniline moieties indicates that this reaction should proceed readily for the commonly used alicyclic rhodamine compounds.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03661. General experimental procedures, synthesis, and characterization data for all key compounds (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Eric V. Anslyn: 0000-0002-5137-8797 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by fellowships from the NIH (DP1 GM106408) and the Welch Regents Chair (F-0046). Additionally, we thank NSF (1 S10 OD021508-01) for the Bruker AVANCE III 500 NMR.
■
REFERENCES
(1) Miyawaki, A.; Sawano, A.; Kogure, T. Nat. Cell Biol. 2003. (2) Lang, K.; Chin, J. Nat. Chem. 2013, 5, 81. (3) Kolmakov, K.; Belov, V. N.; Wurm, C. A.; Harke, B.; Leutenegger, M.; Eggeling, C.; Hell, S. W. Eur. J. Org. Chem. 2010, 2010 (19), 3593. (4) Koide, Y.; Urano, Y.; Hanaoka, K.; Piao, W.; Kusakabe, M.; Saito, N.; Terai, T.; Okabe, T.; Nagano, T. J. Am. Chem. Soc. 2012, 134 (11), 5029−5031. (5) Koide, Y.; Urano, Y.; Hanaoka, K.; Terai, T.; Nagano, T. ACS Chem. Biol. 2011, 6 (6), 600. (6) Chai, X.; Cui, X.; Wang, B.; Yang, F.; Cai, Y.; Wu, Q.; Wang, T. Chem. - Eur. J. 2015, 21 (47), 16754. (7) Calitree, B.; Detty, M. Synlett 2010, 2010 (01), 89. (8) Pastierik, T.; Š ebej, P.; Medalová, J.; Š tacko, P.; Klán, P. J. Org. Chem. 2014, 79, 3374−3382. (9) Shieh, P.; Dien, V. T.; Beahm, B. J.; Castellano, J. M.; WyssCoray, T.; Bertozzi, C. R. J. Am. Chem. Soc. 2015, 137 (22), 7145. (10) Eggeling, C.; Ringemann, C.; Medda, R.; Schwarzmann, G.; Sandhoff, K.; Polyavoka, S.; Belov, V. N.; Hein, B.; Middendorff, C. V.; Schonle, A.; Hell, S. Nature 2009, 457, 1159−1162. (11) Grimm, J. B.; Muthusamy, A. K.; Liang, Y.; Brown, T. A.; Lemon, W. C.; Ronak, P.; Lu, R.; Macklin, J. J.; Keller, P. J.; Ji, N.; Lavis, L. D. Nat. Methods 2017, 14, 987−994.
209
DOI: 10.1021/acs.orglett.8b03661 Org. Lett. 2019, 21, 206−209
