A General Strategy for Visible-Light Decaging Based on the Quinone Trimethyl Lock

Article abstract of DOI:10.1021/jacs.7b01548

Visible-light triggered quinone trimethyl locks are reported as a general design for long-wavelength photoremovable protecting groups for alcohols and amines. Intramolecular photoreduction unmasks a reactive phenol that undergoes fast lactonization to release alcohols and amines. Model substrates are released in quantitative yield along with well-defined, colorless hydroquinone byproducts. Substituent modifications of the quinone core allow absorption from 400 to 600 nm.

Full text of DOI:10.1021/jacs.7b01548

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Communication
A General Strategy for Visible-Light Decaging
Based on the Quinone Trimethyl Lock
David P. Walton, and Dennis A. Dougherty
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 21 Mar 2017
Downloaded from http://pubs.acs.org on March 23, 2017
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A General Strategy for Visible-Light Decaging Based on the Quinone
Trimethyl Lock
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David P. Walton and Dennis A. Dougherty*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
Supporting Information Placeholder
Our initial approach to compounds such as 1 is outlined in
Scheme 1. The bromoquinone acid intermediate 4 is quickly obꢀ
tainable in gram quantities as a crystalline solid without the need
for chromatography. Sulfide substituents were then introduced by
conjugate addition to 4 (or esters of 4) in a reaction that is rapid at
room temperature under mild conditions and is highly selective
for displacement of the bromine. Immediate Steglich esterification
affords the methyl, ethyl, and Nꢀhydroxysuccinimide (NHS) esꢀ
ters. Chromatography is only required after esterification, and
simply involves collecting the yellow band that elutes. γꢀ
Aminobutyric acid was caged by reaction of the NHS ester in
acetonitrile / bicarbonate buffer with the free amino acid.
ABSTRACT: Visibleꢀlight triggered quinone trimethyl locks are
reported as a general design for longꢀwavelength photoremovable
protecting groups for alcohols and amines. Intramolecular photoꢀ
reduction unmasks a reactive phenol that undergoes fast lactonizaꢀ
tion to release alcohols and amines. Model substrates are released
in quantitative yield along with wellꢀdefined, colorless hydroquiꢀ
none byproducts. Substituent modifications of the quinone core
allow absorption from 400ꢀ600 nm.
The trimethyl lock (TML) is a highly modular motif that has
found many applications1 as a protecting group for alcohols and
amines (Figure 1). Once a phenolic oxygen is revealed, rapid
cyclization of the propionic ester or amide occurs, releasing HX
in seconds (HX = HOR) to minutes (HX = HNR2) in a process
often referred to as decaging.2 The reaction is quite general, and
scores of examples are found in the literature. The phenol is freꢀ
quently part of a hydroquinone, and chemical, enzymatic, and
electrochemical reductions of the corresponding quinone have
been used to initiate the trimethyl lock (Figure 1). Photochemical
initiation of the trimethyl lock, however, has not been reported,
except for the trivial case of nitrobenzyl decaging of a phenol
using UV light.1ꢀ18
Scheme 1: Synthesis of Quinone TML Derivatives 7-12.
We describe here a photochemicallyꢀinitiated trimethyl lock
system that operates well into the visible range and has the potenꢀ
tial for extension into the nearꢀinfrared. Decaging reactions at
longer wavelengths are rare and would be useful in many conꢀ
texts.19ꢀ29 Importantly, the photochemical step and the trimethyl
lock decaging are completely distinct processes. An intramolecuꢀ
lar photochemical quinone reduction is followed by a thermal
trimethyl lock reaction. As such, this system innately spans the
broad substrate applicability of the trimethyl lock reaction. In
addition, the photochemical step can be optimized separately reꢀ
garding quantum yields, wavelength, etc. without impacting the
decaging step.
There are many examples of intermolecular photochemical reꢀ
ductions of quinones30ꢀ34, and several variants where an appended
amine28, 35ꢀ41 or sulfide42ꢀ44 mediates the photochemical reduction.
As such, we considered compounds such as 1, in which a photoꢀ
chemical reduction would launch the trimethyl lock process. We
describe here the synthesis and photochemistry of variants of 1.
Surprisingly, conjugate addition of amines was unsuccessful for
substrate 4 or 5. Amine derivatives 11-14 were instead syntheꢀ
tized from 10, prepared from 2ꢀchloroꢀ5ꢀmethylphenol (see SI).
Yields of the desired substitution pattern, with the amine para to
Figure 1. Decaging strategies for the quinone trimethyl lock.
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the trimethyl lock chain, were increased by first treating 10 with
thiophenol before addition of amine to give 13 and 14.
carbonyl and oxidation. Interestingly, a ketone masked as the thiol
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substituent can be released simultaneously with an alcohol or
amine in water. In aprotic solvents, the sulfides undergo slow
photolysis to a complex mixture of products.
The UVꢀvis spectra of compounds 6-9 and 11-14 are similar,
with charge transfer absorptions in the visible (Figure 2). In both
systems, the absorption bands are broad enough that efficient and
clean photolysis is possible at wavelengths both below and well
beyond λmax. Irradiation with visible light (300 W Hg arc lamp
with >400 nm glass filter) leads to clean and efficient photolysis
for both compounds. In practice, a simple 455 nm 1 watt portable
LED is suitable for photolysis of both amine and sulfide derivaꢀ
tives. For the amines, a longerꢀwavelength, 565 nm, 1 watt LED
was also practical, allowing for orthogonal decaging (see below).
In contrast to the sulfides 6-9, photolysis of amines 11-14 to the
hydroquinone not only proceeds smoothly in a range of solvents,
but is more efficient in nonꢀpolar solvents. Pyrrolidine derivatives
11 and 13 cyclize to form oxazolidines 19 and 21, whereas isoinꢀ
doline derivatives 12 and 14 form isoindoles 20 and 22. For the
closely related series 11-14, the relative rate of reaction is 10 > 9
> 12 > 11. As with the sulfides, an activated benzylic hydrogen on
the substituent facilitates a more rapid reaction. The aryl sulfide
substituent of 13 and 14 appears to impede the reaction relative to
Cl in 11 and 12. We estimate the quantum yields in methanol for
11 and 12 to be similar to that of 6a and 6b, and that of 13 and 14
to be lower, however, detailed photophysical studies are still in
progress.
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Scheme 2. Photolysis of 6-14.
As described in detail elsewhere45, we have performed extenꢀ
sive mechanistic studies on the photoreaction of sulfides such as
6.41 Based on substituent effects, kinetic isotope effects, stereoꢀ
chemical and radical clock probes, and nanosecond transientꢀ
absorption spectroscopy, we consider the reaction to proceed as
follows. Photochemically induced electron transfer is followed by
a critical and irreversible hydrogen transfer, which leads to a net
twoꢀelectron intramolecular reduction of the quinone and forꢀ
mation of a zwitterionic intermediate such as 15. Capture by solꢀ
vent or the phenolic oxygen, followed by trimethyl lock ring cloꢀ
sure completes the decaging. In some cases, the intermediate hyꢀ
droquinone can be directly observed, and the thermal trimethyl
lock ring closure can be monitored (Supporting Information).
Given the importance of the hydrogen transfer step, it is not surꢀ
prising that the benzylic derivative 6b is more efficient than the
methyl derivative, 6a.
Figure 2. Representative time course UVꢀvis spectra during
photolysis for compounds 6a (top) and 11 (bottom) irradiated with
455 nm and 565 nm LEDs, respectively. Expansions of the visible
bands are shown as insets. Arrows indicate changes in the spectra
over time. Traces are 1 minute apart. For both compounds, 3 mL
of 0.1ꢀ0.2 mM solutions fully converted in under 5 minutes with a
standard 1W mounted LED.
Reduction of the quinone core and subsequent trimethyl lock
occurs in all cases, releasing caged alcohols in quantitative yield.
The reaction is remarkably clean; after removal of the solvent, the
crude reaction is pure by NMR and LCMS with few exceptions.
Monitoring the reaction by NMR in CD3OD shows clean converꢀ
sion and quantitative release of the methanol or ethanol. The hyꢀ
droquinone byproducts vary depending on the solvent and substitꢀ
uent (Scheme 2). Irradiation of 6a in methanol with a 455 nm
LED releases ethanol and generates methanolꢀtrapped 16a (R =
CH3) as the exclusive product (Φ420 = 1.2%). Photolysis of 6b is
more efficient (Φ420 = 6.3%) and produces 16b (R = Ph), along
with a cyclized product 17 (R = Ph).45 In aqueous solution, the
disulfide 18 is cleanly produced, presumably via aqueous trapping
of zwitterionic intermediate 15, subsequent hydrolysis to release a
To illustrate the potential usefulness of these compounds, we
sought to demonstrate release of a fluorophore, release of a bioꢀ
logically relevant molecule, and the orthogonality of the two deꢀ
veloped systems. Figure 3 shows that irradiation of 7 at 455 nm in
aqueous acetonitrile rapidly releases a fluorescence sensor (hyꢀ
mecromone). The uncaged coumarin can be selectively excited at
355 nm in aqueous solvents, allowing release to be monitored by
fluorescence (Figure 3).
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455 nm LED released ethanol from 6. LCMS (see SI) and UVꢀvis
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indicated that no decaging of 6 occurred with the longꢀwavelength
irradiation, allowing complete conversion of 11 orthogonally to 6.
Irradiating the solution with a 455 nm LED led to both comꢀ
pounds bleaching at similar rates. The orthogonality of the two
similar systems allows for rapid release of two different comꢀ
pounds in quick succession.
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Figure 3. Release of 4ꢀMethylumbelliferone (hymecromone)
monitored by fluorescence with 355 nm excitation after samples
were irradiated with a 1 watt 455 nm LED for 0 s, 5 s, 15 s, 30 s,
and 60 s.
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Decaging is also possible in a biological setting, permitting opꢀ
tical control of protein targets. Figure 4 shows decaging of the
neurotransmitter γꢀaminobutyric acid (GABA) using precursor 9.
When expressed in Xenopus oocytes, GABAA receptors are actiꢀ
vated after incubation with 9 and exposure to a 455 nm LED (see
SI for details). For caged amines, such as 9, photolysis is much
faster than trimethyl lock closure, as expected from previously
determined reaction rates of redox triggered quinone trimethyl
lock amine conjugates.2
Figure 5. Orthogonal photolysis of 11 and 6 in methanol. Phoꢀ
tolysis of a solution of 11 and 6 with >530 nm light followed by
>420 nm light allows selective decaging.
We believe compounds of the sort described here will be useful
in many contexts. Photoremovable protecting groups, or photoꢀ
cages, offer the ability to spatioꢀtemporally control the release
(decaging) of molecules with light. The most widely used examꢀ
ples are 2ꢀnitrobenzyl derivatives, which require UV irradiation
and generate toxic byproducts. In fact, there are few transitionꢀ
metal free photoremovable protecting groups that absorb signifiꢀ
cantly above 450 nm.19, 21ꢀ28 Because of the wellꢀestablished inꢀ
crease in tissue penetrance as wavelength increases, longer waveꢀ
length decaging strategies such as that described here have great
potential as chemical biology tools or in a therapeutic setting. The
present systems are already well into the visible range, and a great
deal is known about photoꢀinduced electron transfer reactions of
the sort that initiate this process, providing valuable guidance on
how to move to even longer wavelengths.
In summary, we have created a modular system in which a phoꢀ
toinduced electron transfer of a quinone leads to a thermal trimeꢀ
thyl lock release of an alcohol or amine. The reaction works well
with visible light in aqueous media to produce nonꢀabsorbing
products. The entire range of substrates established to be compatꢀ
ible with the trimethyl lock is now available through visible light
activation. Current work is focused on extending the absorption
wavelength and expanding the scope of photochemical reductions
and thermal cyclizations that are compatible.
Figure 4. Release of the neurotransmitter γꢀamminobutyric acid
(GABA) to activate GABAA receptors in Xenopus oocytes upon
photolysis with a 455 nm LED. Shown is current produced by
activation of the receptor, as probed by wholeꢀcell, twoꢀelectrode,
voltage clamp electrophysiology.
ASSOCIATED CONTENT
Supporting Information
Having demonstrated the release of biologically meaningful
concentrations of model substrates in seconds to minutes, we
sought to demonstrate selectivity possible with the amine and
sulfide systems.25 Solutions of 6 and 11 in methanol with matched
optical densities were mixed 1:1 to give a solution with a broad
absorbance from 360 to 620 nm (Figure 5). Irradiation with first a
565 nm LED through a 530 nm longꢀpass filter cleanly released
only caged methanol from 11 (red). Subsequent irradiation with a
The Supporting Information is available free of charge on the
ACS Publications website.
Synthetic procedures, compound characterization, UVꢀvis during
photolysis, Xenopus oocyte experiments and orthogonal photolyꢀ
sis LCMS traces (PDF).
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(25) Hansen, M. J.; Velema, W. A.; Lerch, M. M.; Szymanski, W.;
AUTHOR INFORMATION
Feringa, B. L., Chem. Soc. Rev. 2015, 44, 3358ꢀ3377.
(26) Kunsberg, D. J.; Kipping, A. H.; Falvey, D. E., Org. Lett. 2015,
17, 3454ꢀ3457.
(27) Šolomek, T.; Wirz, J.; Klán, P., Acc. Chem. Res. 2015, 48,
3064ꢀ3072.
(28) Carling, C.ꢀJ.; Olejniczak, J.; FoucaultꢀCollet, A.; Collet, G.;
Viger, M. L.; Nguyen Huu, V. A.; Duggan, B. M.; Almutairi, A.,
Chem. Sci. 2016, 7, 2392ꢀ2398.
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2
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4
5
6
Corresponding Author
*dadougherty@caltech.edu
Present Addresses
Division
California
of
Chemistry
Institute
and
of
Chemical
Technology,
Engineering
164ꢀ30
7
8
9
(29) Gandioso, A.; Cano, M.; Massaguer, A.; Marchán, V., J. Org.
Chem. 2016, 81, 11556ꢀ11564.
(30) Bruce, J. M., Quart. Rev., Chem. Soc. 1967, 21, 405ꢀ428.
(31) Baxter, I.; Phillips, W. R., J. Chem. Soc., Perkin Trans. 1 1973,
268ꢀ272.
Pasadena, California 91125
Notes
The authors declare no competing financial interests.
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11
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46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(32) Görner, H., Photochem. Photobiol. 2003, 78, 440ꢀ448.
(33) Görner, H., J. Phys. Chem. A. 2003, 107, 11587ꢀ11595.
(34) Görner, H.; von Sonntag, C., J. Phys. Chem. A. 2008, 112,
10257ꢀ10263.
(35) Cameron, D. W.; Giles, R. G. F., Chem. Commun. 1965, 573ꢀ
574.
(36) Cameron, D. W.; Giles, R. G. F., J. Chem. Soc. C 1968, 1461ꢀ
1464.
(37) Giles, R. G. F., Tetrahedron Lett. 1972, 13, 2253ꢀ2254.
(38) Maruyama, K.; Kozuka, T.; Otsuki, T., Bull. Chem. Soc. Jpn.
1977, 50, 2170ꢀ2173.
(39) Jones; Qian, X., J. Phys. Chem. A. 1998, 102, 2555ꢀ2560.
(40) Chen, Y.; Steinmetz, M. G., Org. Lett. 2005, 7, 3729ꢀ3732.
(41) Chen, Y.; Steinmetz, M. G., J. Org. Chem. 2006, 71, 6053ꢀ
6060.
(42) Hidetoshi, I., Bull. Chem. Soc. Jpn. 1989, 62, 3479ꢀ3487.
(43) Iwamoto, H.; Takuwa, A., Bull. Chem. Soc. Jpn. 1991, 64, 724ꢀ
726.
(44) Kallmayer, H.ꢀJ.; Fritzen, W., Pharmazie 1994, 49, 412ꢀ415.
(45) Regan, C. J.; Walton, D. P.; Shafaat, O. S.; Dougherty, D. A., J.
Am. Chem. Soc. DOI: 10.1021/jacs.6b12007.
ACKNOWLEDGMENT
We thank Clint Regan and Oliver Shafaat for helpful discussions
and Chris Marotta for assistance with the electrophysiology exꢀ
periments. We thank the Beckman Institute of Caltech Laser Reꢀ
source Center for use of their fluorimeter. This work was supportꢀ
ed by the W. M. Keck Foundation.
REFERENCES
(1) Levine, M. N.; Raines, R. T., Chem. Sci. 2012, 3, 2412ꢀ2420.
(2) Carpino, L. A.; Triolo, S. A.; Berglund, R. A., J. Org. Chem.
1989, 54, 3303ꢀ3310.
(3) Amsberry, K. L.; Borchardt, R. T., Pharm. Res. 1991, 8, 323ꢀ
330.
(4) Best, Q. A.; Johnson, A. E.; Prasai, B.; Rouillere, A.; McCarley,
R. L., ACS Chem. Biol. 2016, 11, 231ꢀ240.
(5) Borchardt, R. T.; Cohen, L. A., J. Am. Chem. Soc. 1972, 94,
9175ꢀ9182.
(6) Ciampi, S.; James, M.; Le Saux, G.; Gaus, K.; Justin Gooding,
J., J. Am. Chem. Soc. 2012, 134, 844ꢀ847.
(7) Hettiarachchi, S. U.; Prasai, B.; McCarley, R. L., J. Am. Chem.
Soc. 2014, 136, 7575ꢀ7578.
(8) Huang, S.ꢀT.; Lin, Y.ꢀL., Org. Lett. 2006, 8, 265ꢀ268.
(9) Ji, C.; Miller, P. A.; Miller, M. J., ACS Med. Chem. Lett. 2015,
6, 707ꢀ710.
(10) Loew, M.; Forsythe, J. C.; McCarley, R. L., Langmuir 2013, 29,
6615ꢀ6623.
(11) Milstien, S.; Cohen, L. A., J. Am. Chem. Soc. 1972, 94, 9158ꢀ
9165.
(12) Ong, W.; McCarley, R. L., Macromolecules 2006, 39, 7295ꢀ
7301.
(13) Ong, W.; Yang, Y.; Cruciano, A. C.; McCarley, R. L., J. Am.
Chem. Soc. 2008, 130, 14739ꢀ14744.
(14) Prasai, B.; Silvers, W. C.; McCarley, R. L., Anal. Chem. 2015,
87, 6411ꢀ6418.
(15) Shin, W. S.; Han, J.; Verwilst, P.; Kumar, R.; Kim, J.ꢀH.; Kim,
J. S., Bioconjugate Chem. 2016, 27, 1419ꢀ1426.
(16) Silvers, W. C.; Payne, A. S.; McCarley, R. L., Chem. Commun.
2011, 47, 11264ꢀ11266.
(17) Silvers, W. C.; Prasai, B.; Burk, D. H.; Brown, M. L.; McCarley,
R. L., J. Am. Chem. Soc. 2013, 135, 309ꢀ314.
(18) Zheng, A.; Shan, D.; Wang, B., J. Org. Chem. 1999, 64, 156ꢀ
161.
(19) Pelliccioli, A. P.; Wirz, J., Photochem. & Photobiol. Sci. 2002,
1, 441ꢀ458.
(20) Fournier, L.; Aujard, I.; Le Saux, T.; Maurin, S.; Beaupierre, S.;
Baudin, J.ꢀB.; Jullien, L., Chem. Eur. J. 2013, 19, 17494ꢀ17507.
(21) Klán, P.; Šolomek, T.; Bochet, C. G.; Blanc, A.; Givens, R.;
Rubina, M.; Popik, V.; Kostikov, A.; Wirz, J., Chem. Rev. 2013,
113, 119ꢀ191.
(22) Gorka, A. P.; Nani, R. R.; Zhu, J.; Mackem, S.; Schnermann, M.
J., J. Am. Chem. Soc. 2014, 136, 14153ꢀ14159.
(23) Denning, D. M.; Pedowitz, N. J.; Thum, M. D.; Falvey, D. E.,
Org. Lett. 2015, 17, 5986ꢀ5989.
(24) Goswami, P. P.; Syed, A.; Beck, C. L.; Albright, T. R.;
Mahoney, K. M.; Unash, R.; Smith, E. A.; Winter, A. H., J. Am.
Chem. Soc. 2015, 137, 3783ꢀ3786.
ABSTRACT FIGURE
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