Stoichiometric Nitroxyl Photorelease Using the (6-Hydroxy-2-naphthalenyl)methyl Phototrigger

Article abstract of DOI:10.1021/acs.orglett.8b04099

The design and synthesis of a photoactivatable HNO donor incorporating the (6-hydroxynaphthalen-2-yl)methyl (6,2-HNM) photocage coupled to the trifluoromethanesulfonamidoxy analogue of the well-established HNO generator Piloty's acid is described. The photoactive HNO donor stoichiometrically generates HNO (～98%) at neutral pH conditions, and evidence for concerted C-O and N-S bond cleavage was obtained. The methanesulfonamidoxy analogue primarily undergoes undesired N-O bond cleavage.

Full text of DOI:10.1021/acs.orglett.8b04099

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Stoichiometric Nitroxyl Photorelease Using the (6-Hydroxy-2-
naphthalenyl)methyl Phototrigger
Yang Zhou,† Ruth B. Cink,‡,§,∥ Alexander J. Seed,† M. Cather Simpson,§,∥,⊥ Paul Sampson,*,†
and Nicola E. Brasch*,‡,§
†Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44242, United States
‡School of Science, Auckland University of Technology, Auckland 1142, New Zealand
§Dodd-Walls Centre for Quantum and Photonic Technologies, Dunedin 9054, New Zealand
∥The Photon Factory, School of Chemical Sciences, The University of Auckland, Auckland 1142, New Zealand
⊥The Department of Physics, The University of Auckland, The MacDiarmid Institute for Advanced Materials and Nanotechnology,
Auckland 1142, New Zealand
S
* Supporting Information
ABSTRACT: The design and synthesis of a photoactivatable
HNO donor incorporating the (6-hydroxynaphthalen-2-yl)-
methyl (6,2-HNM) photocage coupled to the trifluorome-
thanesulfonamidoxy analogue of the well-established HNO
generator Piloty’s acid is described. The photoactive HNO
donor stoichiometrically generates HNO (∼98%) at neutral
pH conditions, and evidence for concerted C−O and N−S
bond cleavage was obtained. The methanesulfonamidoxy
analogue primarily undergoes undesired N−O bond cleavage.
NO, a redox cousin of nitric oxide (NO), shows
Hydroxysulfonamides incorporating the (3-hydroxy-2-
Hextremely promising pharmacological properties, includ- naphthalenyl)methyl (3,2-HNM) phototrigger release up to
70% HNO, Figure 1.6g,f In addition to the desired HNO
ing treatment of congestive heart failure.1 Furthermore, there is
increasing evidence that HNO is an endogenous signaling
molecule, reacting in particular with thiol residues and
transition metal centers of proteins.2 Since HNO rapidly
dimerizes in aqueous solution,3 HNO studies require
compounds that release HNO upon demand. The release of
HNO from an HNO donor molecule upon photoactivation is
very attractive, given that rapid in situ generation of HNO is
desirable for kinetic studies on fast reactions of HNO with
biomolecules3,4 and that rapid in situ generation of HNO
would be useful for biological studies.5 Several classes of
photoactive HNO donors have been developed.5b−d,6 To the
best of our knowledge, 3-phenyl-5-(2-pyrroyl)-1,2,4-oxadia-
zole-4-oxide has the highest selectivity (99%) for the release of
HNO upon light activation.6a However, 1,2,4-oxadiazole-4-
oxides are reported to release HNO via a triplet excited state;
hence, air-free conditions are required to prevent quenching of
the triplet state by oxygen.
Figure 1. 3,2-HNM-based and 6,2-HNM-based HNO donors.
generation pathway, competing O−N bond cleavage occurs.
The close proximity of the naphthol OH to the nitrogen of the
sulfonamidoxy group in 3,2-HNM-based HNO donors may
promote excited-state intramolecular proton transfer (ESIPT),
resulting in undesired O−N bond cleavage.6f To explore this
possibility further, second-generation 6,2-HNM-based HNO
donors have now been synthesized and preliminary photo-
chemistry experiments undertaken (compounds 1 and 2,
Figure 1). By relocating the naphtholic OH group to a position
distal to the benzylic carbon in the 6,2-HNM chromophore,7
Piloty’s acid (PhSO2NHOH) and related analogues
decompose cleanly to release HNO and the corresponding
sulfinate. Recently, N-hydroxysulfonamides (RSO2NHOH)
have attracted interest as light-activated HNO generators via
photouncaging;5d,6f,g however, significantly less than 1 molar
equiv of HNO has been released from such systems to date. N-
Received: December 23, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b04099
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the formation of an intramolecular hydrogen bond is now
precluded in both 1 and 2, ruling out the possibility of ESIPT.
The synthesis of 6,2-HNM-based HNO donors 1 and 2 was
completed starting from methyl 6-hydroxynaphthalen-2-
carboxylate (3) (see Scheme 1 and section 2, SI).
Scheme 1. Synthesis of HNO Donors 1 and 2
Figure 2. 19F NMR spectra for the anaerobic photolysis of 1 (500
μM, 350 nm, 4W) in 80:20 v/v CD3CN: phosphate buffer solution
−
(5.0 mM, pH 7.0). CF3SO2 −88.7 ppm (∼98%) and CF3SO2NH2
−81.2 ppm (∼2%).
Scheme 2. Two Photolytic Pathways
Triisopropylsilyl (TIPS) protection of naphthol 3 followed
by LiAlH4 reduction of the resulting ester 4 gave alcohol 5.
Alkoxyamine 7 was obtained via Mitsunobu chemistry
followed by hydrazinolysis. N-Sulfonylation of 7 using
RSO2Cl (R = CF3 and Me) gave the corresponding O-TIPS-
protected intermediates 8 and 9. Finally, removal of the TIPS
protecting group using KF in tetraethylene glycol gave the
targets 1 and 2 in 16−29% overall yield over six steps. Target 1
is stable under ambient light not only in solution (at least 7 d)
but also in the presence of the HNO-trapping molecules
aquacobalamin (H2OCbl(III)+) and glutathione (see section 3,
SI). These results indicate HNO release from 1 requires an
intense light source.
carried out under the same solvent conditions, giving an
observed first-order rate constant for photodecomposition of
0.164 0.008 min−1 (half-life 4.2 min, Figure S5b). Slightly
−
less CF3SO2 is obtained in this solvent mixture (92%
−
−
CF3SO2 and 8% CF3SO2NH2; see Figure S5a). CF3SO2 is
−
a much better leaving group compared to MeSO2 , as shown
by the pKa values for the related conjugate acids
[pKa(CF3SO2H) = −0.6, pKa(MeSO2H) = 2.28].6f Therefore,
the alkanesulfonyl group plays a crucial role in determining
both the selectivity for the desired HNO generation pathway
and the observed rate constant for photodecomposition in the
6,2-HNM-based HNO donors.
N-Hydroxysulfonamides typically produce HNO in a 1:1
8
−
ratio with the corresponding sulfinate RSO2 . Hence, initial
studies were carried out using 19F NMR or 1H NMR
spectroscopy to determine if the corresponding chemical
−
−
marker for HNO generation, RSO2 , is formed upon
Observation of CF3SO2 in the photoproduct solution
photolysis of 1 and 2. Figure 2 shows 19F NMR spectra
obtained upon irradiating 1 in an anaerobic aqueous buffered
CD3CN solution as a function of total irradiation time
(Rayonet photoreactor equipped with 350 nm bulbs; 4 W, 8
indicated that HNO generation occurs upon photolysis of 1
(Scheme 2). To unambiguously demonstrate HNO release
upon irradiation, the photodecomposition of 1 was inves-
tigated in the presence of two established HNO trapping
molecules - the vitamin B12 derivative aquacobalamin
(H2OCbl(III)+) and the phosphine trap 3-(diphenylphosphi-
no)-4-(methoxycarbonyl)benzoic acid. H2OCbl(III)+ reacts
stoichiometrically with HNO to form nitrosylcobalamin
(NO−-Cbl(III)).6g,8a As shown in Figure 3, clean isosbestic
points (367 and 493 nm) are observed in the UV−vis spectra
upon irradiation of 1 in the presence of H2OCbl(III)+. These
are consistent with the conversion of H2OCbl(III)+ to NO−-
Cbl(III) and confirm HNO release upon photolysis of 1.6g 1H
and 19F NMR spectra of the product solution showed that
−
lamps). Remarkably, 98% CF3SO2 was generated upon
decomposition of 1, implying nearly stoichiometric HNO
generation (Scheme 2). Importantly, competing photodecom-
position pathways resulting in O−N bond cleavage to generate
trifluoromethanesulfonamide (CF3SO2NH2) are prevented by
the distal arrangement of the hydroxyl and sulfonamidox-
ymethyl groups in 1.6g The photolysis of 2 (4.68 mM) was
1
monitored by H NMR spectroscopy as a function of total
irradiation time in phosphate buffer (pH 7.00, 5.0 mM) and
CD3CN (40:60, v/v; Rayonet photoreactor). After 388 min of
irradiation, 2 was completely converted to MeSO2NH2 (δ 3.08
−
equal amounts of CF3SO2 and HNO (as indicated by NO−-
−
ppm; 95%) and MeSO2 (δ 2.23 ppm; 5%) (Scheme 2; see
Cbl(III) formation) are generated within experimental error
Figure S4a). Thus, the undesired O−N bond cleavage pathway
(leading to MeSO2NH2) is the major pathway observed upon
photolyzing 2. The observed first-order rate constant, kobs, for
(see Table 1 and section 5, SI). Equal concentrations of NO−-
−
Cbl(III) and CF3SO2 were also observed for the CF3SO2
analogue of 3,2-HNM-based HNO donor and
CF3SO2NHOH.6g,8a Finally, the observation of the corre-
sponding phosphine aza-ylide in the presence of the phosphine
steady-state photodecomposition of 2 was 0.018
0.003
min−1 (half-life 38 min, Figure S4b). The photolysis of 1 was
B
DOI: 10.1021/acs.orglett.8b04099
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
mechanism of photodecomposition and the HNO generation
pathway proceeds via singlet excited-state species.
To demonstrate that 1 decomposes more rapidly when
irradiated using a more intense light source, a stopped-flow
instrument operating without a monochromator (150 W xenon
lamp) was also utilized to obtain steady-state kinetic data for
the photodecomposition of 1 in an anaerobic mixture of
phosphate buffer, pH 7.0, and CH3CN. UV−vis spectral
changes (see Figure S16) indicated that 1 underwent much
more rapid decomposition compared to the experiments using
the photoreactor (350 nm light bulbs). On the basis of the
absorbance changes monitored at 305 nm, the best fit of
kinetic data to a first-order reaction gave an observed rate
constant, kobs = 0.15 s−1 (half-life of ∼5 s). As expected, the
observed rate constant is much larger using the more intense
xenon lamp. Given that photouncaging of the 3,2-HNM group
is rapid (krelease= 105 s−110), we expect that upon laser
excitation, HNO donor 1 will generate HNO on the subsecond
time scale. Ultimately time-resolved transient absorption or
vibrational spectroscopy (as opposed to steady-state techni-
ques) are required to gain a better understanding of the exact
rate of HNO generation following excitation of 1.
Photolysis experiments for structurally related systems
suggest that the most likely aromatic product resulting from
C−O heterolytic bond cleavage followed by nucleophilic attack
on the resulting carbocation intermediate in the presence of
water is the diol 6-(hydroxymethyl)naphthalen-2-ol (NuH =
H2O, Scheme 2).7,11 This was confirmed experimentally by 1H
NMR spectroscopy (section 4, SI) and HR-MS (calculated m/
z for [M − H]−, C11H10O2 = 173.0608, observed m/z =
173.0607). Upon irradiation of 1 in the presence of
H2OCbl(III)+ (3.0 molar equiv) in anaerobic 60:40 v/v
CH3OH/5.0 mM phosphate buffer, 6-(methoxymethyl)-
naphthalen-2-ol was instead generated (calcd m/z for
C12H12O2, [M − H]− = 187.0759, obsd m/z = 187.0736;
NuH = CH3OH, Scheme 2). Interestingly, in the absence of a
molecule that rapidly reacts with HNO such as H2OCbl(III)+,
significant amounts of an oxime species were also observed in
the photoproduct mixture (see section 9, SI). This suggests
that under these conditions HNO released from 1 reacts with a
reaction intermediate rather than dimerizing to form N2O.
Experiments are currently underway to further probe this
observation. Photolysis of 2 produced aldehyde 15 as the
major photoproduct arising from O−N bond cleavage, Scheme
2. Small amounts of diol 13 were also observed (C−O/N−S
bond cleavage; see Figure S18). Similar photoproducts were
observed upon the photolysis of the isomeric 3,2-HNM-based
HNO donors.6f
Figure 3. UV−vis spectra for the reaction between H2OCbl(III)+
(30.0 μM) and target 1 (70.0 μM) using xenon light (150 W,
monochromator slit width = 3.0 mm) in an anaerobic mixture of
phosphate buffer (5.0 mM, pH 7.00) and CH3CN (40:60, v/v) at 25
°C.
−
Table 1. Concentrations of CF3SO2 and NO−-Cbl(III)
a
Formed upon Partial Photolysis of 1
b
c
−
expt
[CF3SO2 ] (μM)
[NO−-Cbl(III)] (μM)
A
B
C
111
146
132
3
4
4
103 10
153 15
124 12
a
b
See section 5B, SI, for details. Determined by 19F NMR
c
1
spectroscopy. Determined by H NMR spectroscopy.
trap also provides additional evidence for HNO release upon
photolysis of 1 (see section 6, SI).9
Importantly, evidence was found for HNO release occurring via
concerted C−O and N−S bond cleavage, rather than via
elimination of CF3SO2NHO−, which would subsequently
spontaneously decompose to generate HNO under the solvent
conditions of this study. Specifically, CF3SO2NHO(H) is not
observed as an intermediate in the 19F NMR spectra (t1/2 ∼ 10
min for the spontaneous decomposition of CF3SO2NHO(H)
to give HNO and CF3SO2 8a). The photolysis of 1 was also
−
carried out in an anaerobic mixture of 80:20 v/v CD3CN to
phosphate buffer solution (5.0 mM, pH 3.0; see Figure S6). In
this solvent mixture, CF3SO2NHO− would be rapidly
protonated to give CF3SO2NHOH, which is stable at pH 3.0
(pKa(CF3SO2NHOH) = 5.89
0.058a). Once again,
CF3SO2NHOH was not observed by 19F NMR spectroscopy.
Finally, the absence of MeSO2NHOH in addition to the
−
observation of a small amount of CH3SO2 in the photo-
product solution for 2 are also consistent with a concerted
mechanism for HNO generation. A stepwise mechanism would
afford MeSO2NHO−, which rapidly protonates to the parent
MeSO2NHOH (pKa 9.95).8a A control experiment showed
that MeSO2NHOH is stable in the solvent conditions of this
study in the dark and in irradiated solutions. Hence, multiple
experiments point toward concerted C−O and N−S bond
cleavage.
The mechanism of photodecomposition was probed by
varying the ratio of phosphate buffer, pH 7.0, to CD3CN (% v/
−
v). The selectivity for CF3SO2 (and therefore HNO)
generation upon photolysis of 1 was found to be strongly
solvent dependent (see Figure 4). Approximately 5% v/v
aqueous phosphate buffer is required for HNO generation. A
similar result was found for the 3,2-HNM systems.6g Increasing
the percentage of phosphate buffer above ∼40% v/v decreased
−
Given that CF3SO2 is formed in a 1:1 ratio with HNO
upon photolysis of 1, the photoproduct quantum yield of
HNO generation can be indirectly determined by 19F NMR
spectroscopy using trans-azobenzene as an actinometer. The
−
the selectivity for CF3SO2 (and therefore HNO) generation.
In the absence of water, the HNO generation pathway is totally
shut down, consistent with the results observed for 3,2-HNM-
based HNO donors.6g
In summary, a photoactive Piloty’s acid derivative
comprising an HNO-generating trifluoromethanesulfonami-
doxy moiety tethered to the 6,2-HNM photocage decomposes
−
photoproduct quantum yield for formation of CF3SO2
(=HNO) upon irradiation of 1 was ΦPP = 0.50 0.07; see
section 7, SI. The percentage of CF3SO2− was the same within
experimental error in both anaerobic and aerobic solution in
the same solvent mixture; hence, oxygen does not influence the
C
DOI: 10.1021/acs.orglett.8b04099
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
also to Dr. Sonya Adas (KSU) for assistance with the
CF3SO2NHOH experiments, Mohammad Rahman (KSU)
for synthesizing the oxime, Dr. Mahinda Gangoda (KSU) for
assistance with NMR experiments, and Drs. Jacob Shelley and
You Yi (KSU) for assistance with HRMS analyses.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b04099.
Experimental and synthetic details, characterization data,
and NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: psampson@kent.edu.
*E-mail: nbrasch@aut.ac.nz.
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11892−11899. (b) Du, L.; Zhang, X.; Xue, J.; Tang, W.; Li, M.-D.;
Lan, X.; Zhu, J.; Zhu, R.; Weng, Y.; Li, Y.-L.; Phillips, D. L. J. Phys.
Chem. B 2016, 120, 11132−11141.
ORCID
M. Cather Simpson: 0000-0001-9624-4947
Paul Sampson: 0000-0002-0135-3161
Nicola E. Brasch: 0000-0001-9505-5175
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the U.S. National Science Foundation (CHE-
1306644 and CHE-1545770), Auckland University of
Technology (AUT), and the Dodd-Walls Centre for Photonic
and Quantum Technologies for financial support. Our thanks
D
DOI: 10.1021/acs.orglett.8b04099
Org. Lett. XXXX, XXX, XXX−XXX