Photosensitization of Fluorofuroxans and Its Application to the Development of Visible Light-Triggered Nitric Oxide Donor

Article abstract of DOI:10.1021/acs.joc.7b01709

Nitric oxide (NO) is an endogenous signaling molecule used in multiple biochemical processes. The development of switchable NO donors that deliver an NO payload under spatiotemporal control harbors many medicinal benefits. Previously, 4-fluorofuroxans were found to function as a UV light-induced NO donor under physiological conditions based on the photoinduced isomerization mechanism; however, the isomerization of fluorofuroxans with longer wavelength light is desired for further application into living systems. Herein, we report the use of photosensitizers in the photochemical isomerization of fluorofuroxan, enabling the use of visible light to induce isomerization. Among the tried photosensitizers, anthraquinone derivatives showed a good sensitizing ability to isomerize 4-fluorofuroxan to 3-fluorofuroxan using visible light. This new phenomenon was applied to the synthesis of a water-soluble anthraquinone-fluorofuroxan all-in-one molecule, which demonstrated promising NO-releasing ability using 400-500 nm irradiation. A high level of control is displayed with "on" and "off" NO-release functionality suggesting that photosensitizer-furoxan hybrids would make valuable donors. Furthermore, unlike most furoxans, NO is released in the absence of thiol cofactor.

Full text of DOI:10.1021/acs.joc.7b01709

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Article
Photosensitization of fluorofuroxans and its application to
the development of visible light-triggered nitric oxide donor
Christopher Peter Seymour, Rei Tohda, Motonari Tsubaki, Masahiko Hayashi, and Ryosuke Matsubara
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b01709 • Publication Date (Web): 24 Aug 2017
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Photosensitization of fluorofuroxans and its application to the
development of visible light-triggered nitric oxide donor
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Christopher Peter Seymour, Rei Tohda, Motonari Tsubaki, Masahiko Hayashi and
Ryosuke Matsubara*
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Department of Chemistry, Graduate School of Science, Kobe University, Nada, Kobe 657-8501,
Japan
Abstract: Nitric oxide (NO) is an endogenous signaling molecule used in multiple
biochemical processes. The development of switchable NO donors that deliver an NO payload
under spatiotemporal control harbors many medicinal benefits. Previously, 4-fluorofuroxans
were found to function as a UV light-induced NO donor under physiological conditions based
on the photo-induced isomerization mechanism; however, the isomerization of fluorofuroxans
with longer wavelength light is desired for further application into living systems. Herein, we
report the use of photosensitizers in the photochemical isomerization of fluorofuroxan,
enabling the use of visible light to induce isomerization. Among the tried photosensitizers,
anthraquinone derivatives showed a good sensitizing ability to isomerize 4-fluorofuroxan to 3-
fluorofuroxan using visible light. This new phenomenon was applied to the synthesis of a water
soluble anthraquinone-fluorofuroxan all-in-one molecule which demonstrated promising NO-
releasing ability using 400–500 nm irradiation. A high level of control is displayed with ‘on’
and ‘off’ NO-release functionality suggesting that photosensitizer-furoxan hybrids would make
valuable donors. Furthermore, unlike most furoxans, NO is released in the absence of thiol
cofactor.
1. Introduction
Nitric oxide (NO) is a reactive radical gas with a short lifetime and area of effect.1 NO mediates
a variety of biological effects such as vasodilation, inhibition of platelet aggregation, cell
apoptosis and neurotransmission.2,3,4 Research has been conducted into NO therapeutics given
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the many medicinal benefits. However, gaseous NO cannot be reliably administered, therefore
stable NO donor compounds have been developed to deliver NO exogenously.5 Many NO
donor families exist, the most common being the NONOates6 and S-nitrosothiols.7 The
shortfall of current donors is the lack of quantitative and spatiotemporal control over the NO-
release which limits application.
Photo-induced nitric oxide donors (PINODs) are an attractive solution to the
spatiotemporal control issues of exogenous NO delivery. Light can be used to induce otherwise
impossible reactivity without remaining in the system after completion, furthermore reactivity
can be tuned by altering the parameters of irradiation such as wavelength, amplitude and time.
Various types of PINODs are known such as caged NONOates,8 metal nitrosyls,9 nitroarenes10
and N-nitrosoamines.11
Despite promising advances, there are drawbacks for many PINODs such as toxicity of the
byproducts following uncaging and low stability of the parent donors prior to release.12
Therefore the development of PINODs utilizing new photo-switching mechanisms is necessary.
Recently our laboratory reported a PINOD based on the furoxan structural motif (Scheme
1).13 Furoxan is a known NO donor but prior to our laboratories work has rarely exhibited
PINOD character.14,15 We discovered that fluorofuroxan exhibits PINOD character when the
thermodynamically stable 4-fluoro isomer 1 is isomerized to the 3-isomer 2 under UV
irradiation. In the presence of a thiol cofactor 2 releases NO readily whereas 1 discharges only
marginal quantities of NO. Under continuous irradiation moderate NO-release was achieved.
Following the success of the fluorofuroxan we developed alkoxy furoxan derivatives that
released NO in moderate to high yields, despite lower total isomerization at the photostationary
state (PSS).16
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Scheme 1. Isomerization of furoxans under UV irradiation and following NO-release (our
previous results).
The short UV wavelength required to induce isomerization is a significant drawback in the
previous works because fluorofuroxan does not absorb light above 320 nm which prevents
further application. Additionally, UV irradiation is problematic in a practical sense because
high energy radiation is damaging to organic tissue and penetrates poorly. Furoxan PINODs
that are photoactive under longer wavelengths of irradiation would present an attractive
approach to spatiotemporal in vivo NO-release.
In order to overcome this problem, we focused on photosensitization. Photosensitization
enables reactivity that would not necessarily be available through typical reaction pathways.17
We expected that the use of a photosensitizer that absorbs lower energy light and then transfers
the energy to furoxan could lead to sensitized isomerization and NO-release under biologically
inert wavelengths. Installing both the photosensitizer and fluorofuroxan in one molecule would
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make the self-contained PINOD that is activated by longer wavelengths of irradiation (Figure
1). To the best of our knowledge there have been no reports of furoxan photosensitization so
far. In this report we demonstrate for the first time that 4-fluorofuroxans can be excited under
visible light irradiation ( = 400–500 nm) in the presence of triplet sensitizers and isomerize to
the 3-regioisomer. Based on the finding that anthraquinone is a suitable photosensitizer for 4-
fluorofuroxans, we designed and synthesized a novel PINOD 5 bearing both 4-fluorofuroxan
and anthraquinone units (See Scheme 2 for the structure of 5). PINOD 5 showed visible light-
induced NO-releasing capability in the buffered solution. Unexpectedly, in contrast to most
known furoxans that require a thiol cofactor for NO-release, 5 released NO under irradiation
in the absence of a thiol cofactor.
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Figure 1. Initial working hypothesis for visible light-triggered NO-release via sensitization of
4-fluorofuroxan. 4FF = 4-fluorofuroxan. 3FF = 3-fluorofuroxan.
2. Results and Discussion
2.1 Sensitization of fluorofuroxan
To know the triplet energy of 4-fluorofuroxan phosphorescence of 4-fluorofuroxan 1 was
measured in glassy frozen 2-methyltetrahydrofuroran (2-MeTHF) at 77K (Figure 2). The
phosphorescent spectrum was well-resolved, and maximal phosphorescence wavelength was
above 400 nm. Judging from this spectrum, the photosensitization of 4-fluorofuroxan by light
longer than 400 nm was expected to be possible using appropriate triplet photosensitizers
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Figure 2. Phosphorescence of 1 (1 mM in 2-MeTHF at 77K). Ex = 280 nm.
Potential photosensitizers were screened by irradiating an NMR tube containing a 1:1 molar
solution of sensitizer and furoxan 1 in 5 mM deaerated C6D6 (Table 1). Samples were exposed
to visible light in the range of 400–500 nm and relative isomerization was measured at 15
minute intervals up to 90 minutes by 1H NMR spectroscopy. In most cases the PSS was reached
between 15 and 30 minutes of irradiation.
Table 1. Photosensitized furoxan isomerization under visible irradiationa
Yield of 2 /%b
Entry
Sensitizer
at 15 min
at 30 min
at PSS
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96
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anthraquinone
1,2-benzanthraquinone
benzil
phenothiazine
DDQ
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14
0d
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–
0d
1
d
benzophenone
none
>6e
0
0
0
a Sensitizer (1 equiv), 1 (1 equiv) in deaerated C6D6 (5 mM). b Determined by 1H NMR analysis
using dodecane as an internal standard. c Phenothiazine decomposed and byproduct started to
form after 1 h of irradiation. d DDQ reacted with furoxan. e PSS was not reached after 1.5 h of
irradiation.
Quinone type sensitizers appeared to be quite effective at inducing isomerization in the
furoxan. Anthraquinone produced a good yield of 76% at the PSS which was reached just after
30 minutes of irradiation (entry 1, Table 1). 1,2-Benzanthraquinone was an exceptional
sensitizer reaching the PSS within the first 15 minutes of irradiation, furthermore it displayed
a high yield at the PSS of 96% (entry 2, Table 1). Benzil was also a good photosensitizer
inducing isomerization at a faster rate than anthraquinone and displaying an excellent yield at
the PSS (entry 3, Table 1). Phenothiazine appeared to induce minor isomerization in the furoxan,
however reaction occurred leading to the build-up of by products and rapid decomposition of
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the sensitizer (entry 4, Table 1). 2,3-Dichloro-5,6-dicyano-p-benzoquinone (DDQ) did not
cause isomerization but instead reacted with the furoxan under irradiation. Even if DDQ was a
successful sensitizer it is unlikely that it could be used to synthesize an NO donor hybrid given
its toxic nature (entry 5, Table 1). Benzophenone, a well-known triplet sensitizer, was not
effective inducing only minor isomerization and not reaching the PSS after 90 minutes of
irradiation (entry 6, Table 1). Without a suitable photosensitizer there is no isomerization
induced under visible light irradiation because the furoxan simply does not absorb such low
wavelength light (entry 7, Table 1).13
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We measured the electrochemical properties of anthraquinone and fluorofuroxan 1, and
examined whether or not the electron transfer sensitization is plausible instead of energy
transfer. Cyclic voltammetry experiments on anthraquinone and 1 in CH3CN using 0.1 M
Bu4NClO4 as supporting electrolyte revealed the reduction potentials of
+
anthraquinone/anthraquinone and 1 /1 couples to be 0.95 V and +2.11 V versus saturated
calomel electrode (SCE), respectively.18 The triplet energy of anthraquinone was reported to
be 2.73 eV.19 Following the Rehm–Weller formalism20,21 with the assumption that the work
function  is a small contribution,22 the excited-state reduction potential of anthraquinone was
+
estimated to be +1.78 V versus SCE, which is not comparable to the reduction potential of 1 /1
couple (+2.11 V versus SCE). Thus the G0 for the electron transfer from 1 to the excited
anthraquinone is a large positive value, suggesting that this pathway is unlikely. We currently
believe that the furoxan isomerization using the photosensitizers should proceed via triplet
energy transfer, although further investigation is required to unambiguously determine the
mechanism.
2.2 Synthesis of furoxan-based visible light-induced NO donor
Anthraquinone, 1,2-benzanthraquinone and benzil were considered for a suitable sensitizer-
tethered fluorofuroxan molecule. Previously we found that fast isomerization and subsequent
thiol attack to the furoxan are important factors for successful photo-induced NO-release. With
this in mind, the isomerization ratio at PSS is not so significant because isomerized furoxan is
rapidly replenished upon consumption to maintain said PSS.16 Indeed, when monitored by 1H
NMR analysis, successful alkoxy furoxan PINODs have shown excellent NO-release without
appearing to isomerize to a great extent under irradiation, (e.g. alkoxy furoxan 3, Scheme 1).
Taking this into account we chose not to focus solely on the isomerization properties but also
considered the synthetic utility of potential sensitizers. Although the data suggested 1,2-
benzanthraquinone and benzil were the better sensitizers with respect to isomerization ratio,
after careful consideration they were not selected. We were concerned about the stability of the
benzil during synthesis and biological testing, whereas benzanthraquinone presented a
challenging synthesis which would add unnecessary regioisomeric complications due to the
lack of symmetry. A symmetrical design was preferred to avoid synthetic complexities in
purification and analysis, therefore we decided to use an anthraquinone as the core of the
molecule, the admirable induced furoxan isomerization rate coupled with molecular symmetry
and ease of synthetic modification made the sensitizer attractive.
We anticipated that the synthesis of a molecule featuring an anthraquinone core would
exhibit poor solubility, so to counter this we employed a quaternary ammonium salt side chain.
Another expected issue was the poor stability of the fluorofuroxan, which is prone to reduction
(the furoxan ring is electron deficient) and nucleophilic attack to the C4 position even by weak
nucleophiles.23 This meant that mild reaction conditions were necessary to avoid furoxan ring
decomposition. Based on these considerations, we designed compound 5 as a furoxan-based
visible light-induced NO donor (Scheme 2).
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Scheme 2. Retrosynthesis of designed PINOD 5
For the key step in the synthesis of 5 we envisioned that an organocatalyzed [4 + 2] reaction
based upon work by Lee et al24 would provide the mild conditions necessary for the union
between naphthoquinone 7 and furoxan 9 in one pot. This convergent approach was particularly
attractive because both the anthraquinone core and carbon linker to furoxan would be
simultaneously furnished. With the conjoined intermediate completed, methylation of tertiary
pyrrole amine followed by acid catalyzed Boc deprotection would yield the desired PINOD 5.
Our convergent synthesis of 5 began with the formation of aldehyde fluorofuroxan 9 in four
steps. To gain access to the elongated alkyl frame we first reacted 4-bromostyrene (10) with 5-
bromopentene (11) under iron catalyzed Grignard conditions developed by Nakamura et al.25
The extended styrene 12 was obtained following chromatography in hexanes. Acidic Wieland
conditions constructed the furoxan core and afforded compound 13 as an inseparable mixture
of 13 and 14 in an 87 : 13 ratio respectively.26 It is of note that only the conjugated alkene
reacted and the nitrofuroxan with a pendant alkene was obtained. During the next fluorination
step13 TBAF reacted exclusively with 13 to form 15 which was easily separated from the
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unreacted 14 by chromatography. Finally acrolein and 15 were refluxed in dilute toluene with
Hoveyda-Grubbs second generation catalyst granting 9.27
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Scheme 3. Synthesis of intermediate 9. Reagents and conditions: (i) 4-bromostyrene (10), Mg
turnings, TMEDA, FeCl3, THF, 0 °C, 30 min, 72%; (ii) NaNO2, CH3CO2H, DCM, 25 °C, 2 h
then 2 M HCl, 25 °C, 14 h, 38% (13 : 14 = 87:13); (iii) TBAF, THF, 0 °C, 1 h, 72%; (iv)
acrolein, Hoveyda-Grubbs second generation catalyst, toluene, 100 °C, 18 h, 76%.
Synthesis of the naphthoquinone 7 began with Lewis acid-catalyzed Diels Alder reaction
of 1,4-benzoquinone (8) with 2,3-dimethylbutadiene to afford 16 following recrystallization in
toluene. The oxidation of 16 initially proved difficult, oxidizing agents such as DDQ and IBX
led to poor yields. Utilizing an excess of the relatively mild oxidizing agent MnO2 we obtained
1,4-naphthoquinone 17.28 Benzylic bromination of 17 with NBS led to a mixture of inseparable
byproducts along with the desired dibromo product 18. Recrystallization attempts failed so we
decided to perform a two-step synthesis to 7. Reaction of semi-pure 18 with N-Boc-protected
ethylenediamine under dilute conditions and iodide catalysis granted 7 following overnight
stirring. Initial attempts at accessing 7 were low yielding and tenuous optimization was
required. One of the problems was oxidation of the product, probably leading to the full
unsaturation of nitrogen-containing five-membered ring. The highly activated alkene was also
likely prone to attack from the amine leading to unwanted side reactions. Degassing the solvent
helped offset the low yield, furthermore lower reaction temperature, iodide and high dilution
were necessary to access acceptable yields. Compound 7 is unstable and will readily oxidize
or decompose so should be used immediately in the convergent step.
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Scheme 4. Synthesis of napthoquinone intermediate 7. Reagents and conditions: (i)
benzoquinone, BF3OEt2, 2,3-dimethylbutadiene, toluene, 10 °C, 80 min, 76%; (ii) MnO2,
toluene, 100 °C, 2 h, 70%; (iii) NBS, (PhCO2)2, benzene, 100 °C, 18 h; (iv) N-Boc
ethylenediamine, K2CO3, KI, MeCN, -10 °C, 18 h, 17% (2 steps)
The reaction between 7 and 924 was initially difficult due to decomposition of both
partners during reaction. The use of toluene as the solvent resulted in low yields which were
improved upon by switching to chloroform, this enhanced the solubility of reactants and
increased the yield to 38% (Scheme 5). With the key intermediate 6 in hand, the pyrrole amine
was qauternerized by overnight stirring with excess MeI. Upon completion excess MeI was
evacuated and subsequent Boc deprotection by treatment with trifluoroacetic acid (TFA)
formed a terminal quaternary ammonium ion. The TFA was removed in vacuo and the oily
product sonicated in Et2O whereupon it precipitated out and was collected by filtration, thus
completing the synthesis of 5.
Scheme 5. Organocatalyzed Diels-Alder reaction and subsequent modification to 5. Reagents
and conditions: (i) proline, benzoic acid, CHCl3, 55 °C, 15 h, 38%; (ii) MeI, DCM, 40 °C, 20
h then TFA, DCM, 25 °C, 1 h, 80%.
2.3 NO-releasing ability under visible light
The NO-releasing ability of compound 5 was tested using the Griess method.29 Compound 5
(100 M) was dissolved in a 50 mM phosphate buffer containing 5mM of ʟ-cysteine30 and was
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subjected to seven hours of continuous irradiation (= 400–500 nm). Aliquots were taken at
regular intervals and the NO% determined spectroscopically after treatment with Griess reagent
(Figure 3). To our delight NO was released from 5 under visible light irradiation. The total
release was up to a similar percentage release as 2 under UV irradiation, although the rate was
slightly slower (Figure 3a, blue line).13 Initial release of NO was rapid reaching up to 20% after
one hour of irradiation, under prolonged irradiation the rate of NO-release slowed. This is a
similar trend shared by other NO donors which seem to have intrinsic NO-release limits. Only
background NO-release (up to 2%) was detected without irradiation in the presence of cysteine
after 1 hour (data not shown). To our surprise irradiation in the absence of cysteine led to
significant NO-release with initial rates comparable to that with thiol present (Figure 3a, orange
line). Reports of furoxans that release significant quantities of NO in the absence of thiol are
rare and the exact reason for NO-release is unknown.30 Compound 5 is a particularly unusual
example since the furoxan does not release NO unless irradiated. These results suggest that in
addition to the thiol-mediated irradiation-independent NO-release from the 3-fluorofuroxan,
there might be another NO-release pathway where the photochemically excited fluorofuroxan
or secondary transient intermediate releases NO without thiol cofactor assistance. Besides it
has not been evidently verified that the nitrogen oxide species generated from the furoxans was
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–
only NO; so we do not rule out the possibility that its redox form, NO– or NO2 , was
accompanied. In any case, the exact mechanism of the sensitized release is currently unknown
and is the topic of further research in our laboratory.
Figure 3. (a) NO-release curve of 5 (100 M) in 1% DMSO 50 mM phosphate buffer
containing or omitting 5 mM of ʟ-cysteine under 400–500 nm irradiation at 37 °C. (b)
Pictures of cuvettes containing a solution of 5 (in the presence of ʟ-cysteine) after irradiation
at the indicated period and Griess reagent.
With the NO-release confirmed to be dependent on irradiation we decided to investigate
the level of control over release (Figure 4). Compound 5 was subjected to 1-hour rotations of
irradiation under 400–500 nm and ambient light, aliquots were taken at intervals. Figure 4
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shows that as soon as irradiation is ceased, the NO-release almost completely stops with any
increments attributed to background release. This demonstrates a high level of control over
NO-release with compound 5, therefore it may be possible to release a specific quantity of NO
with appropriate sample concentration and irradiation time. Investigation on spatiotemporal
control of NO release in living systems based on the developed concept is currently underway
in our laboratory.31
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Figure 4. NO-release curve of 5 (100 uM) in 1% DMSO 50 mM phosphate buffer containing
5 mM of ʟ-cysteine under 1-hour rotations of 400–500 nm irradiation and ambient light at
37 °C
3. Conclusion
Triplet photosensitization of furoxans was demonstrated for the first time. The sensitization
led to the visible light-induced isomerization of 4-fluorofuroxan to 3-fluorofuroxan. We have
designed and synthesized a prototype water-soluble furoxan-based visible light-induced NO
donor. The key steps in its synthesis are the selective furoxan ring formation from the diene
and organocatalyzed anthraquinone formation. Water solubility is endowed by
quarternalization of the nitrogen atoms. The synthesized molecule showed visible light-
responsive NO-releasing ability under physiological conditions. The present work provides not
only a concept to utilize biologically inert visible light for furoxan photochemistry, but also a
new mode of NO-release switching of furoxans, which is anticipated to be applicable to the
development of other switching systems for NO-release, e.g. by turning on/off the light
sensitivity of photosensitizer with external stimuli.
Experimental Section
Unless otherwise noted, all reactions were carried out in well cleaned glasswares with magnetic
stirring. Operations were performed under an atmosphere of dry argon using Schlenk and
vacuum techniques, unless otherwise noted. All starting materials were obtained from
commercial sources or were synthesized using standard procedures. Melting points were
1
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measured on a Yanaco MP-500D and are not corrected. H, C NMR, and F spectra (400,
100, and 376 MHz, respectively) were recorded on a Bruker Avance III HD 400 using TMS (0
ppm), CDCl3 (77.0 ppm), and C6F6 (164.9 ppm) as an internal standard, respectively. The
following abbreviations are used in connection with NMR; s = singlet, d = doublet, t = triplet,
q = quartet, quin = quintet, sep = septet, and m = multiplet. Mass spectra were measured using
a JEOL JMS-T100LP (DART method, ambient ionization). Preparative column
chromatography was performed using Kanto Chemical silica gel 60 N (spherical, neutral), Fuji
Silysia BW-4:10MH silica gel or YMC_GEL Silica (6 nm I-40-63 µm). Thin layer
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chromatography (TLC) was carried out on Merck 25 TLC silica gel 60 F254 aluminium sheets.
Phosphorescence spectrum was recorded on a spectrofluorometer (Jasco FP-6500) using
phosphorescence measurement mode. Cyclic voltammetric measurements were performed
using an ALS CHI606S electrochemical analyzer. 4-Fluorofuroxan 1 was synthesized
according to a previously reported method.13
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Phosphorescence measurement of 4-fluorofuroxan 1. Phosphorescence of 4-fluorofuroxan
1 was measured for a 1 mM 2-MeTHF solution of 1 kept in a quartz ESR tube ( 5 mm) at 77
K. The delay time, gate time and excitation wavelength were set to be 9 ms, 9 ms and 280 nm,
respectively.
Photosensitized isomerization of 4-fluorofuroxan 1 to 3-fluorofuroxan 2 (Table 1). A
solution of 4-fluorofuroxan (0.5 mg, 2.4 µM) and sensitizer (2.4 µM) in deaerated C6D6 (0.5
mL) was prepared in a Pyrex NMR tube. A small quantity of dodecane (approximately 1 µL)
was added as an internal standard. Before irradiation of the sample, the integration ratio of the
1H NMR peaks corresponding to the 4-fluorofuroxan and dodecane were measured. The
solution was irradiated with 400–500 nm light (a 300W Xenon lamp, Asahi Spectra MAX-303
equipped with a 300- to 600-nm ultraviolet-visible module, and a combination of a 400-nm
1
long-pass and 500-nm short-pass filters). The reaction progress was monitored by H NMR
analysis (Figure S1). The yield was determined from the peak integration relative to dodecane
internal standard under the assumption that the internal standard was not affected by the
experiment.
Redox property. Cyclic voltammetric measurements were performed at 298 K with solvents
being deaerated by Ar bubbling for 30 min before each measurement. The supporting
electrolyte was 0.10 M TBAClO4. Aconventional three-electrode cell was used with a platinum
working electrode and a platinum wire as a counter electrode. The cyclic voltammograms were
recorded with respect to the Ag/AgNO3 (10 mM) reference electrode at a sweep rate of 50
mV/s. The oxidation potential (determined as the peak potential) was corrected to the SCE
scale by adding 0.31 V, on the basis of the measurement of the redox potential (+0.09 V in the
cell system used) of the Fc/Fc+ couple as the internal standard.
Synthesis of furoxan 9 (Scheme 3). 1-Ethenyl-4-(pent-4-en-1-yl)benzene (12). Compound
12 was synthesized according to the reported method.25 To a mixture of 5-bromopent-1-ene
(11.3 mL, 95.6 mmol), FeCl3 (47.8 mL of a 0.1 M THF solution, 4.78 mmol) was added at 0 °C
a mixture of 4-ethenylphenylmagnesium bromide (115 mLof a 1 M THF solution, 114.8 mmol)
and TMEDA (17.2 mL, 114.8 mmol) dropwise over 2.5 hours at a rate that keeps the reaction
mixture yellow (4-ethenylphenylmagnesium bromide was prepared by dropwise addition of a
1 M solution of 4-ethenylphenyl bromide in THF to Mg turnings at a rate which maintained
gentle reflux, following addition the mixture was stirred for a further 30 minutes). Once
addition was completed the reaction was stirred for a further 20 minutes. Saturated aqueous
NH4Cl was added followed by 1 M HCl and water, the reaction mixture was extracted thrice
with EtOAc and the organics washed with brine, dried over MgSO4 and filtered. Solvent was
removed in vacuo and the residue purified by column chromatography to yield a colorless oil
1
(11.82 g, 68.6 mmol, 72%). Rf = 0.47 (hexane); H NMR (400 MHz, CDCl3) δH(ppm): 7.33
(2H, d, J = 8.0 Hz), 7.15 (2H, d, J = 8.0 Hz), 6.70 (1H, dd, J = 17.6, 11.0 Hz), 5.84 (1H, ddt, J
= 17.2, 10.2, 6.8 Hz ), 5.71 (1H, dd, J = 17.6, 1.0 Hz), 5.20 (1H, dd, J = 11.0, 1.0 Hz), 5.03
(1H, dq, J = 17.2, 2.0 Hz), 4.98 (ddt, J = 10.2 Hz, 3.2 Hz, 1.2 Hz), 2.62 (2H, t, J = 7.6 Hz),
2.01 (2H, q, J = 6.8 Hz), 1.72 (2H, quin, 7.6 Hz); δC (100 MHz, CDCl3) 142.3, 138.6, 136.7,
135.2, 128.7, 126.2, 114.8, 112.9, 35.1, 33.3, 30.6; IR (neat) 3081, 3000, 2973, 2927, 2853,
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1641, 1620, 1511, 1438, 1406, 988, 902, 836, 817 cm-1. HRMS (DART) m/z: [M + H]+ Calcd
for C13H17 173.1325; Found 173.1330.
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4-Nitro-3-[4-(pent-4-en-1-yl)phenyl]furoxan (13). Nitrofuroxans 13 and 14 were synthesized
according to a reported method.26 To a stirred suspension of NaNO2 (961 mg, 13.9 mmol) in a
0.2 M CH2Cl2 solution of 12 (300 mg, 1.74 mmol) was added AcOH (796 µL, 13.9 mmol) over
30 minutes at room temperature. Two hours after starting the AcOH addition, a 2 M solution
of HCl (5.2 mL, 10.5 mmol) was added. The reaction mixture was stirred for 14 h at rt, and
then extracted three times with CH2Cl2. The combined organic layer was washed with brine,
dried over MgSO4, and filtered. The filtrate was concentrated in vacuo. The residue was
purified by silica gel to yield 13 and 14 (181 mg, 0.66 mmol, 38%) as an 87:13 (13:14) mixture.
The reaction can be conducted on a larger scale at the cost of diminished yield as byproduct
formation increases. Data reported is for the major isomer. Rf = 0.25 (30% DCM in hexane);
1H NMR (400 MHz, CDCl3) δH(ppm) 7.52 (2H, apparent dt, J = 8.6, 2.0 Hz), 7.37 (2H, apparent
d, J = 8.6 Hz), 5.83 (1H, ddt, J = 17.0, 10.4, 6.4 Hz), 5.04 (1H, dq, J = 17.0, 2.0 Hz), 5.01 (1H,
ddt, J = 10.4, 2.0, 1.2 Hz), 2.71 (2H, t, J = 8 Hz), 2.11 (2H, q, J = 6.4 Hz), 1.76 (2H, quin, J =
7.2 Hz); δC (100 MHz, CDCl3) Major isomer 13; 158.2, 147.1, 138.1, 129.4, 128.6, 116.5,
115.2, 109.2, 35.3, 33.2, 30.2; distinguishable minor isomer 14; 147.4, 138.1, 129.2, 128.6; IR
(neat) 3075, 2924, 2856, 1612, 1560, 1519, 1492, 1357, 1270, 1115, 1069, 983, 909, 839, 812,
785, 755 cm-1. HRMS (DART) m/z: [M + H]+ Calcd for C13H14N3O4 276.0979; Found
276.0984.
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4-Fluoro-3-[4-(pent-4-en-1-yl)phenyl]furoxan (15). Fluorofuroxan 15 was synthesized
according to the reported method.13 To a solution containing the mixed isomers of 13 and 14
(1 g, 3.63 mmol) in THF (7.3 mL) was added dropwise a 1 M solution of tetrabutylammonium
fluoride (TBAF) in THF (3.63 mL, 3.63 mmol) at 0 °C. After 1 hour the reaction was diluted
with water and extracted thrice with DCM. The organics were dried over MgSO4, filtered and
solvent removed in vacuo. The residue was purified by column chromatography to yield 15 as
a colourless oil (652 mg, 2.63 mmol, 72%). Rf = 0.36 (30% DCM in hexane); 1H NMR (400
MHz, CDCl3) δH(ppm): 7.89 (2H, d, J = 8.6 Hz), 7.37 (2H, d, J = 8.6 Hz), 5.83 (1H, ddt, J =
17.2, 10.4, 6.4 Hz), 5.06–4.98 (2H, m), 2.70 (2H, t, J = 8.0 Hz), 2.10 (2H, q, J = 7.6 Hz), 1.75
(2H, quin, J = 8.0 Hz); δC (100 MHz, CDCl3) 161.0 (d, JC–F = 258.4 Hz), 145.5, 137.1, 128.5,
124.8 (d, JC–F = 3.8 Hz), 116.9, 114.1, 104.9 (d, JC–F = 26.4 Hz), 34.2, 32.1, 29.1; IR (neat)
3072, 2975, 2924, 2856, 1612, 1568, 1522, 1476, 1411, 1338, 1321, 1154, 1124, 1093, 977,
909, 825, 800, 733, 634, 608, 590 cm-1. HRMS (DART) m/z: [M + H]+ Calcd for C13H14FN2O2
249.1034; Found 249.1039.
(2E)-6-[4-(4-Fluoro-furoxan-3-yl)phenyl]hex-2-enal
(9). Furoxan 9 was synthezised
according to the reported method.27 To a stirred solution of 15 (300 mg, 1.21 mmol) and
Hoveyda-Grubbs second generation catalyst (22.7 mg, 36.3 µmol) was added acrolein
monomer and the mixture heated to 100 °C for 28 hours. Upon reaction completion the solvent
was removed in vacuo and crude material purified by column chromatography on silica gel to
give 9 (252.2 mg, 0.913 mmol, 76%) as a pale yellow crystalline solid. Rf = 0.44 (35% EtOAc
1
in hexane); H NMR (400 MHz, CDCl3) δH(ppm): 9.52 (1H, d, J = 7.8 Hz), 7.91 (2H, d, J =
8.4 Hz), 7.37 (2H, d, J = 8.4 Hz), 6.85 (1H, dt, J = 15.6, 6.8 Hz), 6.14 (1H, ddt, J = 15.6, 7.8,
1.2 Hz), 2.75 (2H, t, J = 8.0 Hz), 2.39 (2H, qd, J = 8.0, 1.6 Hz), 1.89 (2H, quin, J = 7.6 Hz); δC
(100 MHz, CDCl3) 193.8, 162.0 (d, JC–F = 258.4 Hz), 157.4, 145.4, 133.4, 129.4, 126.0 (d, JC–
= 3.7 Hz), 118.3, 105.8 (d, JC–F = 26.5 Hz), 35.2, 32.0, 29.0; IR (neat): 2951, 2935, 2870,
F
2829, 2739, 1679, 1612, 1568, 1522, 1473, 1408, 1340, 1326, 1305, 1292, 1150, 1120, 1091,
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974, 831, 814, 725, 598 cm-1 HRMS (DART) m/z: [M + H]+ Calcd for C14H14FN2O3 277.0983;
Found 277.0989; m.p. 53–56 °C.
.
6
Synthesis of naphthalene-1,4-dione
7
(Scheme 4). 6,7-Dimethyl-4a,5,8,8a-
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tetrahydronaphthalene-1,4-dione (16). To a stirred solution of 1,4-benzoquinone (10.0 g, 92.5
mmol) and BF3OEt2 (1.14 mL, 9.25 mmol) in toluene was added 2,3-dimethyl-1,3-butadiene
(12.6 mL, 111 mmol) dropwise at 16 °C over 10 minutes. After addition completion the
reaction was allowed to warm to rt over 80 minutes. Upon completion the mixture was washed
twice with brine and the organics dried over Na2SO4, filtered and solvent condensed under
vacuum. The product was recrystallized from toluene to yield yellow crystals (13.3 g, 70.0
mmol, 76%). 1H NMR (400 MHz, CDCl3) δH(ppm): 6.65 (2H, s), 3.22–3.17 (2H, m), 2.40
(2H, apparent d, J = 16.9 Hz), 2.08 (2H, apparent d, J = 16.9 Hz), 1.63 (6H, s); δC (100 MHz,
CDCl3) 200.3, 139.3, 123.3, 47.1, 30.3, 18.9; IR (neat) 2919, 2876, 2828, 1679, 1602, 1441,
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1427, 1373, 1354, 1262, 1196, 1095, 887, 864, 848, 805, 749, 682 cm-1 HRMS (DART) m/z:
[M + H]+ Calcd for C12H15O2 191.1067; Found 191.1072; m.p. 119–121 °C.
;
6,7-Dimethylnaphthalene-1,4-dione (17). Naphthoquinone 17 was synthesized according to
the reported method.28 To a stirred solution of 16 (3.54 g, 18.6 mmol) in toluene (140 mL)
was added portion wise activated MnO2 (17.4 g, 200 mmol) and refluxed for 2 hours. The
reaction mixture was cooled and filtered through celite by washing with EtOH. The solvent
was condensed and crude material purified by column chromatography to yield a yellow solid
1
(2.42 g, 12.7 mmol, 69%). Rf = 0.30 (5% EtOAc in hexane); H NMR (400 MHz, CDCl3)
δH(ppm): 7.83 (2H, s), 6.90 (2H, s), 2.41 (6H, s); δC (100 MHz, CDCl3) 185.3, 143.8, 138.5,
129.9, 127.4, 20.2; IR (neat) 2948, 2920, 1664, 1595, 1447, 1392, 1341, 1326, 1307, 1221,
1144, 1057, 1017, 994, 972, 835, 769, 712, 626 cm-1 HRMS (DART) m/z: [M + H]+ Calcd for
;
C12H11O2 187.0754; Found 187.0759; m.p. 121–123 °C.
tert-Butyl [2-(5,8-dioxo-1,3,5,8-tetrahydro-2H-benzo[f]isoindol-2-yl)ethyl]carbamate (7). A
stirred solution of 17 (340 mg, 1.79 mmol), NBS (732.6 mg, 4.12 mmol) and benzoyl peroxide
(21.7 mg, 0.09 mmol) in benzene (18 mL) was heated to reflux for 24 h. Upon reaction
completion the mixture was washed twice with distilled water, the organic layer was dried over
MgSO4, filtered and solvent condensed in vacuo. The residue was filtered through a plug of
silica with EtOAc/hexane to yield 6,7-bis(bromomethyl)naphthalene-1,4-dione (18) as a crude
green solid. The impure material 18 was stirred in a mixture containing K2CO3 (795 mg, 5.76
mmol) and KI (120 mg, 0.72 mmol) in degassed MeCN (720 mL). To the mixture was added
tert-butyl (2-aminoethyl)carbamate (248.2 mg, 1.44 mmol) at 10 °C and stirred for 15 hours.
Upon reaction completion the reaction mixture was concentrated and water was added, the
mixture was extracted thrice with EtOAc. The combined organics were dried over MgSO4,
filtered and solvent removed in vacuo. The crude material was purified by column
chromatography to yield 7 (103 mg, 0.301 mmol, 17% over two steps) as an orange solid.
Compound 7 should be used immediately in the next step as it is unstable and prone to
1
spontaneous oxidation. Rf = 0.31 (40% EtOAc in hexane); H NMR (400 MHz, CDCl3)
δH(ppm): 7.91 (2H, s), 6.96 (2H, s), 5.01 (1H, broad s), 4.06 (4H, s), 3.37-3.32 (2H, m), 2.90
(2H, t, J = 6.0 Hz), 1.45 (9H, s); δC (100 MHz, CDCl3): 185.0, 156.0, 149.7, 146.6, 138.5, 131.5,
120.4, 58.6, 54.7, 39.0, 28.4; IR (neat) 3368, 3048, 3013, 2980, 2980, 2930, 281, 2761, 1688,
1663, 1608, 1524, 1435, 1385, 1362, 1327, 1305, 1273, 1240, 1153, 1129, 1040, 964, 838 cm-
1; HRMS (DART) m/z: [M + H]+ Calcd for C19H23N2O4 343.1652; Found 343.1658; m.p.
110 °C (decomposed).
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Synthesis of fluorofuroxan 5 (Scheme 5). tert-Butyl [2-(6-{2-[4-(4-fluorofuroxan-3-
yl)phenyl]ethyl}-5,10-dioxo-1,3,5,10-tetrahydro-2H-naphtho[2,3-f]isoindol-2-
yl)ethyl]carbamate (6). Compound 6 was synthesized according to the reported method.24 A
mixture of 7 (100 mg, 0.292 mmol), 9 (129 mg, 0.468 mmol), ʟ-proline (6.7 mg, 0.058 mmol)
and benzoic acid (3.4 mg, 0.03 mmol) in CHCl3 (1.5 mL) was stirred at 50 °C for 15 hours.
After reaction completion the solvent was condensed in vacuo and the residue purified by silica
chromatography to afford 6 (67.1 mg, 0.112 mmol, 38%) as an orange solid. Rf = 0.35 (20%
EtOAc in hexane); 1H NMR (400 MHz, CDCl3) δH(ppm): 8.31 (1H, dd, J = 7.8, 1.2 Hz), 8.12
(1H, s), 8.10 (1H, s), 7.92 (2H, d, J = 8.0 Hz), 7.65 (1H, t, J = 7.8 Hz), 7.52 (2H, d, J = 8.0 Hz),
7.48 (1H, dd, J = 8.0, 1.2 Hz), 5.05 (1H, broad s), 4.10 (4H, s), 3.56 (2H, t, J = 7.8 Hz), 3.40-
3.33 (2H, m), 3.05 (2H, t, J = 7.8 Hz), 2.93 (2H, apparent t, J = 6.4 Hz), 1.46 (9H, s); δC (100
MHz, CDCl3): 184.9, 183.4, 162.1 (d, JC–F = 258.3 Hz), 156.0, 147.0, 146.4, 145.9, 144.6,
137.8, 135.5, 134.5, 133.3, 132.3, 131.0, 129.8, 126.8, 125.9 (d, JC–F = 3.7 Hz), 121.2, 120.5,
118.1 (d, JC–F = 4.2 Hz), 105.9 (d, JC–F = 26.6 Hz), 79.4, 58.7, 58.6, 54.8, 39.1, 37.6, 37.2 28.4;
IR (neat) 2958, 2927, 2846, 1712, 1666, 1606, 1584, 1522, 1471, 1321, 1298 1252, 1168, 1153,
1115, 981, 726 cm-1; HRMS (DART) m/z: [M + H]+ Calcd for C33H32FN4O6 599.2300; Found
599.2306; m.p. 209 °C (decomposed).
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2-(2-Aminoethyl)-6-[4[(4-fluorofuroxan-3-yl)phenyl]ethyl]-2-methyl-5,10-dioxo-2,3,5,10-
tetrahydro-1H-naphtho[2,3-f]isoindol-2-ium bistrifluoroacetate (5). Compound 6 (60 mg,
0.100 mmol) was stirred in DCM (1 mL) and MeI (32 µL, 0.500 mmol) was added and the
reaction warmed to 40 °C for 20 hours. The solvent and excess MeI were removed under
vacuum. To the crude material was added DCM (0.5 mL) and cooled to 10 °C, TFA (0.5 mL)
was added the mixture stirred for 1 hour. The solvent and excess TFA were removed in vacuo
and the brown oil sonicated in Et2O for 1 hour whereupon it solidified and was filtered to afford
5 (58.9 mg, 0.08 mmol, 80%) as a brown solid. 1H NMR (400 MHz, D2O) δH(ppm): 8.14 (1H,
s), 8.09 (1H, dd, J = 7.6, 1.2 Hz), 8.03 (1H, s), 7.68–7.64 (3H, m), 7.56 (1H, t, J = 7.6 Hz),
7.23 (2H, d, J = 8.0 Hz), 5.17–5.06 (4H, m), 4.05–4.01 (2H, m), 3.62–3.58 (2H, m), 3.51–3.46
(2H, m), 3.33 (3H, s), 2.94 (2H, t, J = 7.2 Hz); δC (100 MHz, d6-DMSO): 183.9, 182.4, 162.5
(d, JC–F = 256.3 Hz), 158.2 (q, JC–F = 32.5 Hz), 145.5, 144.6, 139.8, 139.2, 138.2, 135.2, 134.6,
134.0, 133.0, 130.4, 129.4, 126.3 (d, JC–F = 3.1 Hz), 122.6, 121.8, 118.2, 106.9 (d, JC–F = 27.4
Hz), 68.9, 59.6, 49.9, 36.6, 36.3, 33.3; IR (neat) 3006, 2929, 2860, 1667, 1614, 1523, 1473,
1415, 1327, 1298, 1197, 1173, 1125, 976, 836, 796, 719 cm-1 HRMS (DART) m/z:
.
[M2CF3CO2–C2H4NH2]+ Calcd for C27H21FN3O4 470.1511; Found 470.1516. [M + H]+ was
not observed due to facile fragmentation.
Measurement of the NO-releasing ability of compound 5 with visible light-irradiation
(Figure 3). A 100 M stock solution of 5 was prepared by adding a 50 mM phosphate buffer
(pH 7.4) either containing or omitting 5 mM ʟ-cysteine to a 10 mM solution of 5 in DMSO
(100 L, 1 µmol) to bring the total volume up to 10 mL. 8 mL of the stock solution was added
to a Pyrex vial, which was placed in a 37 °C oil bath and then irradiated using 400–500 nm
light. At intervals, aliquots (0.5 mL) were taken out, to which the Griess reagent (40 L) was
added. After the samples were stood for 30 min, the absorbance of the samples at 520 nm was
–
measured, and the percent nitrite (NO2 ) (mol/mol) was determined from a calibration curve
prepared in advance by using NaNO2 standard solutions (20–100 M) treated with the Griess
reagent.
The Griess reagent was prepared by diluting a mixture of 1 g of sulfanilamide, 50 mg of N-
naphthylethylenediamine dihydrochloride, and 2.5 mL of 85% phosphoric acid with distilled
water to a final volume of 25 mL.
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Measurement of NO-releasing ability of compound 5 without light-irradiation (Figure 3).
A 100 M stock solution of 5 was prepared by adding a 50 mM phosphate buffer (pH 7.4)
containing 5 mM ʟ-cysteine to a 10 mM solution of 5 in DMSO (100 L, 1 µmol) to bring the
total volume up to 10 mL. 8 mL of the stock solution was added to a Pyrex vial, which was
placed in a 37 °C oil bath. The solution was maintained at 37 °C for 1 h. Then, 3 mL of the
solution was added to an empty quartz cuvette, followed by the addition of 250 Lof the Griess
reagent. After 10 min of stirring, the absorbance at 520 nm was measured, and the percent
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–
nitrite (NO2 )(mol/mol) was determined from a calibration curve prepared in advance by using
NaNO2 standard solutions (20–100 M) treated with the Griess reagent.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publication website at DOI:
The detailed photosensitized isomerization data, CV data, in vivo NO imaging and copies of
1H, 13C, and 19F NMR spectra of all new compounds (PDF).
AUTHOR INFORMATOION
Corresponding Author
*E-mail: matsubara.ryosuke@people.kobe-u.ac.jp
ORCID
Ryosuke Matsubara: 0000-0001-8941-2391
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Prof. Atsunori Mori and Prof. Kentaro Okano for their help on mass analysis. We
also thank Prof. Yasuhiro Kobori, Prof. Takashi Tachikawa and Mr. Fumitoshi Ema for their
help on phosphorescence analysis. This work was financially supported by JSPS KAKENHI
Grant Numbers JP16K18844 and JP17J00025, Futaba Electronics Memorial Foundation,
Suzuken Memorial Foundation, Inamori Foundation, and Daiichi Sankyo Foundation of Life
Science. CPS wishes to thank the MEXT for the Japanese Government Scholarship Program.
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