Influences of alpha-substituent in 4,5-dimethoxy-2-nitrobenzyl-protected esters on both photocleavage rate and subsequent photoreaction of the generated 2-nitrosophenyl ketones: A novel photorearrangement of 2-nitrosophenyl ketones

Article abstract of DOI:10.1016/j.jphotochem.2016.01.012

Ultraviolet(UV)-induced photodeprotection of 2-nitrobenzyl esters to release the acid with formation of the corresponding 2-nitrosoketone proceeds rapidly when the α-position in the 2-nitrobenzyl group is substituted by a branched alkyl group. The generat

Full text of DOI:10.1016/j.jphotochem.2016.01.012

Journal of Photochemistry and Photobiology A: Chemistry 321 (2016) 41–47
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Influences of alpha-substituent in 4,5-dimethoxy-2-nitrobenzyl-
protected esters on both photocleavage rate and subsequent
photoreaction of the generated 2-nitrosophenyl ketones: A novel
photorearrangement of 2-nitrosophenyl ketones
Noriko Chikaraishi Kasugaa,b, Yusuke Saitoa, Naomichi Okamuraa, Tatsuya Miyazakia,
Hikaru Satoua, Kazuhiro Watanabea, Takaaki Ohtaa, Shu-hei Morimotoa,
Kazuo Yamaguchia,b,
*
a
Department of Chemistry, Faculty of Science, Kanagawa University, Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan
Research Institute for Photofunctionalized Materials, Kanagawa University, Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Ultraviolet(UV)-induced photodeprotection of 2-nitrobenzyl esters to release the acid with formation of
Received 1 December 2015
Received in revised form 12 January 2016
Accepted 14 January 2016
Available online 15 January 2016
the corresponding 2-nitrosoketone proceeds rapidly when the
substituted by a branched alkyl group. The generated 2-nitrosophenyl ketones undergo multiple
photoreactions, including unique photorearrangement, depending upon the nature of the
-substituent. 2-Nitrosoketone bearing an isopropyl substituent mainly undergoes this rearrangement
a-position in the 2-nitrobenzyl group is
a
a
to afford a bicyclic oxazole via a shift of the isopropyl group to the bridgehead position, resulting in loss of
aromaticity of the six-membered ring. 2-Nitrosopheyl ketone bearing a tertiary-butyl substituent gives
mainly azoxy and azo compounds via intermolecular reaction of nitrosoketones with loss of isobutene.
The photorearrangement does not proceed in the case of the phenyl-substituted compound. These
findings will be helpful for the selection and/or design of photolabile protecting groups.
ã 2016 Elsevier B.V. All rights reserved.
Keywords:
Alpha-substitutent
2-Nitrobenzyl
Esters
2-Nitrosoarene
Branched-acyl-group
Photorearrangment
1. Introduction
recent reviews [21–24]. Therefore, to avoid side reactions of 2-
nitrosophenyl ketones formed as by-products of photodeprotec-
The 2-nitrobenzyl group (NB) is widely used as a photo-
removable protecting group (PPG) for caged compounds, which are
inert, but can be deprotected under light irradiation to release
(uncage) an active species, such as ATP or biomodulators, in situ
[1–7]. Caged compounds are also used in kinetics studies and to
examine the concentration dependence of cellular reactions [8–
10]. In addition to such biological applications, NBs have been
widely employed to protect various functional groups in the fields
of organic synthesis and materials science [1,2,11–20].
It is well-known that deprotection of 2-nitrobenzyl derivatives
through light-irradiation produces reactive 2-nitrosoarenes, which
can undergo nitroso Diels–Alder reactions, N-nitroso aldol
reactions, and nitroso-ene reactions, and are also intermediates
for preparation of functional azo compounds, as described in
tion reactions, several chemical modifications of the 2-nitrobenzyl
group have been investigated [1,7,25].
Our recent research interests have focused on NB-protected
silane coupling agents, which are surface-modification agents used
to control the hydrophilicity and hydrophobicity of inorganic
surfaces, and also to form masking patterns for the production of
organic thin film transistors and intelligent materials for biotech-
nology [26–28]. We have frequently employed PPGs based on 6-
nitroveratryl analogs, 4,5-dimethoxy-2-nitrobenzyl derivatives of
NB, because they can be deprotected with lower-energy ultraviolet
light than unsubstituted NBs [3,7,12].
Structural modifications of NB that have so far been investigat-
ed can be divided primarily into two categories: i) modification of
the aromatic ring and ii) substitution at the benzylic carbon atom
[6]. During the course of our studies of these photolabile materials,
we noted that PPG removal proceeded smoothly when the
a
-position was substituted with a branched alkyl group such as
isopropyl (iPr) or tertiary-butyl (tBu). In this paper, we report the
effect of various -substituents on the photochemical reactions of
* Corresponding author at: Department of Chemistry, Faculty of Science,
Kanagawa University, Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan. Fax: +81
463 58 9684.
a
E-mail address: kazu@kanagawa-u.ac.jp (K. Yamaguchi).
http://dx.doi.org/10.1016/j.jphotochem.2016.01.012
1010-6030/ã 2016 Elsevier B.V. All rights reserved.

42
N.C. Kasuga et al. / Journal of Photochemistry and Photobiology A: Chemistry 321 (2016) 41–47
four simple 4-bromobenzoates (1–4), in which the carboxylic
group is protected by an -substituted 4,5-dimethoxy-2-nitro-
Crystallographic Data Centre as supplementary publications no.
CCDC 988420, 1417307 and 1417308, respectively.
a
benzyl group, as well as the four 4,5-dimethoxy-2-nitrosoarenes
(5–8) that are released upon photodeprotection of the corre-
sponding esters, as shown in Fig. 1.
2.3. Synthesis of 1-(4,5-dimethoxy-2-nitrophenyl)-2-methylpropyl 4-
bromobenzoate, 2 and the related acetate 20
A mixture of 1.33 g of 4-bromobenzoic acid (6.63 mmol), 1.27 g
of EDC (6.63 mmol), 1.13 g of 1-(4,5-dimethoxy-2-nitrophenyl)-2-
methylpropan-1-ol (4.42 mmol) and 0.65 g of DMAP dissolved in
55 mL of THF was stirred at room temperature for a few hours until
the alcohol was completely consumed, as confirmed by TLC.
Concentration on a rotary evaporator afforded a residual yellow
solid, which was taken up in 80 mL of CHCl3. The organic layer was
washed with diluted hydrochloric acid (40 mL of H2O and 10 mL of
2 M HCl) and saturated NaHCO3 (30 mL ꢀ 3), then dried over
anhydrous MgSO4, and concentrated. A solution of the residue in
10 mL of ethyl acetate solution was slowly evaporated to give pale-
yellow block crystals (1.55 g, 80% yield), which were soluble in
CHCl3, CH2Cl2, EtOAc and acetone; sparingly soluble in MeOH. Anal.
2. Experimental
2.1. Materials
The following reagent-grade chemicals were used as received:
iodine, HNO3, veratrol, KBr, dimethyl sulfoxide (DMSO), CHCl3,
CH2Cl2, MeOH, EtOAc, CH3CN, tetrahydrofuran, acetone, 1,4-
dioxane and acetic anhydride (Wako); 1-ethyl-3-(3-dimethyl-
aminopropyl) carbodiimide hydrochloride (EDC), 4-dimethyla-
minopyridine (DMAP) and propionic anhydride, pivalic anhydride
(TCI); EtOH (Kanto); CDCl3, DMSO-d6 and CD3CN (99.9 D atom %,
Isotec).
Found: C, 52.07; H, 4.36; N, 3.18. Calcd for C19H20BrNO6 C, 52.07; H,
1
4.60; N, 3.20%. H NMR (400 MHz, 21.5 ꢁC, CDCl3):
d 1.07 (3H, d,
2.2. Instrumentation/analytical procedures
J = 6.8 Hz),1.12 (3H, d, J = 6.8 Hz), 2.35 (1H, m), 3.87 (3H, s), 3.94 (3H,
s), 6.54 (1H, d, J = 5.8 Hz), 6.93 (1H, s), 7.61 (2H, d, J = 8.6 Hz), 7.62
(1H, s), 7.93 (2H, d, J = 8.6 Hz). 13C NMR (126 MHz, 22.0 ꢁC, CDCl3):
d
Photoirradiation was performed with a high-pressure mercury
lamp, using an Ushio HB-50106-AA-A/UHC-500-D system.
Illumination intensity was measured with a Ushio UIT-201 in-
strument. HPLC measurements were performed on a Shimadzu
LC-20AD system fitted with an Ultron ODS column, with CH3CN:
H2O = 6:1 as the eluent. CHN elemental analyses were performed
with a PerkinElmer PE2400 series II CHNS/O analyzer. ESI MS
spectra in positive-ion mode were recorded using a JEOL JMS-
T100LC system in the range of m/z 100–1000. Reserpine was used
as an internal reference. 1H and 13C NMR spectra were recorded on
a JEOL ECP300 NMR or a JEOL ECP500 NMR spectrometer with TMS
as the internal reference.
17.51, 19.45, 33.47, 56.29, 56.35, 76.33, 107.73, 108.73, 128.45,
128.92, 130.83, 131.00, 131.97, 140.96, 148.05, 153.13, 164.92.
Acetate ester 20 was similarly prepared, using acetic anhydride
instead of 4-bromobenzoic acid and EDC. 20. 1H NMR (400 MHz,
24.2 ꢁC, CDCl3):
d
0.98 (3H, d, J = 6.4 Hz), 1.00 (3H, d, J = 6.4 Hz), 2.09
(3H, s), 2.18 (1H, m), 3.94 (3H, s), 3.96 (3H, s), 6.33 (1H, d, J = 5.6 Hz),
6.90 (1H, s), 7.59 (1H, s). 13C NMR (126 MHz, 22.1 ꢁC, CDCl3):
17.42,
d
19.23, 20.96, 33.29, 56.34, 56.36, 75.22, 107.80, 108.92, 131.09,
141.01, 147.93, 153.06, 170.04. ESI MS 320.1079, calcd for
C
14H19NO4Na [M + Na+] 320.1110.
The other related esters 1, 3, 30 and 4 were also similarly
A single crystal of 10, 13 or 14 was mounted on a loop, and
intensity data were collected at 120 K on a Rigaku Saturn CCD
prepared. Characterization data are presented in supporting
information.
diffractometer with Mo-Ka radiation for calculation of cell
constants. Structures were solved by direct methods and refined
by full-matrix least-squares calculation on F2 using the Yadokari
program package [30]. All non-hydrogen atoms were refined
anisotropically. The hydrogen atoms were placed geometrically
and treated using the riding model. Crystal data and the results of
structural refinement of 10, 13 and 14 are summarized in
Table S1 and have been deposited with the Cambridge
2.4. Isolation of two photolysis products, 2-nitrosoketone (6) and its
cyclic isomer (10), from irradiated solution of isopropyl-substituted 2-
nitrobenzyl acetate (20)
Compounds 6 and 10 were isolated using acetate 20 instead of 4-
bromobenzoate 2, because the generated acetic acid could be
easily removed under reduced pressure. A THF solution (100 mL) of
Fig. 1. Photo-cleavage reaction of four 4,5-dimethoxy-2-nitrobenzyl esters.

N.C. Kasuga et al. / Journal of Photochemistry and Photobiology A: Chemistry 321 (2016) 41–47
43
20 (3 mM) was prepared in a quartz beaker in air at ambient
temperature. Photolysis of the solution was performed using an
2.5.3. 2-[2-[2-(2,2-Dimethylpropyl-1-oxo)-4,5-dimethoxyphenyl]-1-
oxidodiazenyl]benzoic acid, 13
Ushio UHC-500-D/HB-50106-AA-A system
(
l
> 300 nm with
Yellow crystals obtained by liquid-liquid diffusion of ethyl
50 mW/cm2 intensity) for 6 h. The reaction progress was moni-
tored by HPLC. Two products, 6 and 10, were isolated by column
chromatography (silica gel, Wako C-300, EtOAc: hexane = 3:1) in
10% and 50% yield, respectively; their structures were established
by means of 1H NMR, 13C NMR and ESI MS analyses.
acetate solution of 13 and hexane. H NMR (400 MHz, 25.0 ꢁC,
1
CDCl3)
s), 6.69 (1H, s), 7.52 (1H, s), 7.89 (1H, s), 8.46 (1H, s). 13C NMR
(125 MHz, 25.8 ꢁC, CDCl3)
: 27.47, 45.47, 56.31, 56.38, 56.48, 56.56,
d: 1.25 (9H, s), 3.97 (3H, s), 3.98 (3H, s), 4.00 (3H, s), 4.02 (3H,
d
104.76,105.05,108.83,114.40,118.70,130.29,137.00,138.00,149.33,
149.96, 152.00, 152.31, 166.22, 210.59. ESI MS 274.1070, calcd for
2.4.1. 1-(4,5-Dimethoxy-2-nitrosophenyl)-2-methylpropan-1-one, 6
C
13H17NO4Na [M + Na+] 274.1055.
Green viscous liquid. 1H NMR (400 MHz, 24.5 ꢁC, CDCl3)
d
: 1.26
(6H, d, J = 6.9 Hz), 3.57 (1H, septet, J = 6.9 Hz), 3.89 (3H, s), 4.06 (3H,
s), 6.23 (1H, s), 7.11 (1H, s). 13C NMR (125 MHz, 25.5 ꢁC, CDCl3)
2.5.4. 2,20-Azobis[4,5-dimethoxybenzoic acid], 14
d
:
Brown crystals suitable for single-crystal X-ray analysis were
grown by vapor-diffusion of an inner solution of the powder
dissolved in hot DMSO with hexane as the external solvent. The
crystals were soluble in CHCl3 and DMSO, but insoluble in CH3CN.
Anal. Found: C, 54.14; H, 4.74; N, 6.41. Calcd for 14.0.5H2O or
18.56, 43.69, 56.25, 56.84, 91.71, 109.62, 138.57, 150.84, 155.84,
159.40, 209.68. ESI MS 260.0855, calcd for C12H15NO4Na [M + Na+]
260.0899.
2.4.2. 5,6-Dimethoxy-3a-(1-methylethyl)-2,1-benzisoxazol-3(3aH)-
one, 10
C
22H27N2O8.5: C, 54.14; H, 4.75; N, 6.41. 1H NMR (400 MHz, 23.3 ꢁC,
DMSO-d6) d C
: 3.84 (3H, s), 3.91 (3H, s), 7.23 (1H, s), 7.38 (1H, s). 13
The isolated powder was dissolved in methanol at 70 ꢁC and
brown crystals suitable for single-crystal X-ray analysis were
NMR (125 MHz, 25.3 ꢁC, DMSO-d6)
d: 55.65, 56.08, 99.40, 111.71,
124.61, 145.08, 150.74, 151.35.
grown by slow evaporation. 1H NMR (400 MHz, 24.1 ꢁC, CDCl3)
d:
0.94 (3H, d, J = 6.9 Hz), 1.12 (3H, d, J = 6 .9 Hz), 1.99 (1H, septet,
J = 6.9 Hz), 3.71 (3H, s), 3.84 (3H, s), 5.36 (1H, s), 5.65 (1H, s). 13
NMR (125 MHz, 27.7 ꢁC, CDCl3)
: 16.21, 17.36, 40.26, 55.16, 55.80,
56.57, 88.26, 97.31, 149.92, 158.41, 168.44, 177.11. Anal. Found: C,
60.51; H, 6.57; N, 5.58. Calcd for C12H15NO4: C, 60.75; H, 6.37; N,
5.90.
3. Results and discussion
C
d
The 4,5-dimethoxy-2-nitrobenzyl esters 1–4 were prepared
quantitatively by EDC-mediated condensation reaction of the
corresponding 4,5-dimethoxy-2-nitrobenzyl alcohol and 4-bro-
mobenzoic acid, and purified by crystallization from a mixed
solution of ethyl acetate and hexane, or by column chromatogra-
phy.
Deprotection rates of the 4,5-dimethoxy-2-nitrobenzyl ben-
zoates were determined from HPLC measurements of remaining
amounts of each benzoate (1–4) in 0.1 mM CH3CN solution
irradiated with a high-pressure mercury lamp. The plots of ln
(Ct/C0) (concentration of esters at time t over t = 0 s) versus the
irradiation time were linear, as shown in Fig. 2, indicating that the
photoreactions of these four model compounds were pseudo first-
order within the measurement time.
2.5. Isolation of photolysis products 7, 12, 13 and 14 from a solution of
a
-tBu-substituted 2-nitrobenzyl acetate (30)
Isolation of products 7, 12, 13 and 14 from an irradiated solution
of tBu-substituted derivative 30 was difficult because of low yields
and instability of the products. Nitroso compound 7 was isolated in
4% yield from an irradiated CH3CN solution of 30 (61 mM,
l
= 365 nm, intensity 20 mW/cm2, ambient temperature, 15.5 h).
The azoxy and azo compounds 12, 13 and 14 were obtained from a
solution of 30 in CH3CN (23 mM, = 300 nm, 50 mW/cm2 intensity,
l
Photoirradiation of the four benzoates generated 4-bromoben-
zoic acid and the corresponding 2-nitrosoarenes formed via the
aci-nitro intermediate [3,31,32]. The area ratio of each ester
ambient temperature, 5 h) in 10%, 12% and 1% yield, respectively.
These products were isolated by column chromatography (silica
gel, Wako C-300, EtOAc: hexane = 3:1) followed by recycling
preparative HPLC. The structures of 7,12,13 and 14 were confirmed
by 1H, 13C NMR and ESI MS. The molecular structures of 13 and 14
were also established by single-crystal X-ray analysis.
showed that photocleavage of a-branched alkyl-substituted esters,
i.e. iPr-substituted (2) and tBu-substituted esters (3), occurred
faster than that of the methyl-substituted analog (1) (Fig. 2), in
2.5.1. 1-(4,5-Dimethoxy-2-nitrosophenyl)-2,2-dimethylpropan-1-
one, 7
Green visous liquid. 1H NMR (400 MHz, 22.7 ꢁC, CDCl3)
d
: 1.28
(9H, s), 3.91 (3H, s), 4.04 (3H, s), 6.63 (1H, s), 6.80 (1H, s). 13C NMR
(125 MHz, 24.3 ꢁC, CDCl3)
: 27.33, 45.48, 56.21, 56.79, 107.62,
149.52, 155.70, 159.42, 213.46. ESI MS 274.1070, calcd for
13H17NO4Na [M + Na+] 274.1055.
d
C
2.5.2. 2-[2-[2-(2,2-Dimethylpropyl-1-oxo)-4,5-dimethoxyphenyl]-1-
oxidodiazenyl]-4,5-dimethoxy-2,2-dimethylpropan-1-one, 12
Orange crystals obtained by liquid-liquid diffusion of ethyl
1
acetate solution of 12 and hexane. H NMR (400 MHz, 23.7 ꢁC,
CDCl3)
(3H, s), 6.60 (1H, s), 6.72 (1H, s), 7.56 (1H, s), 8.64 (1H, s). 13C NMR
(125 MHz, 22.9 ꢁC, CDCl3)
: 27.32, 27.50, 45.20, 45.46, 56.07, 56.17,
d: 1.23 (18H, s), 3.93 (3H,s), 3.94 (3H, s), 3.95 (3H, s), 3.95
d
56.31, 56.39, 105.17, 106.41, 107.83, 108.38, 129.59, 133.93, 134.49,
138.14, 148.10, 148.71, 149.54, 151.58, 211.23, 212.99. ESI MS
509.2229, calcd for C26H34N2O7Na [M + Na+] 509.2264.
Fig. 2. Plots of ln(Ct/C0) versus irradiation time of 0.1 mM CH3CN solutions of 1–4.

44
N.C. Kasuga et al. / Journal of Photochemistry and Photobiology A: Chemistry 321 (2016) 41–47
Fig. 3. (a) ORTEP drawing of 10 showing thermal ellipsoids at the 50% probability level. (b) Molecular structure of 5,6-dimethoxy-1-(4-tolylsulfonyl)-benzo[c]isoxazol-3(1H)-
one [33]. (c) Molecular structure of 13. (d) Molecular structure of 14 in the crystal. (For interpretation of the references to color in the text, the reader is referred to the web
version of this article.)
agreement with Metzker’s findings on nucleotide protection by
ether bonding [29].
can be seen in Fig. 5(b). Although we could not identify all of the
products, four compounds were identified. We could not isolate 11,
a cyclic isomer of 7, but its existence was indicated by the 1H NMR
signals between 5 and 6 ppm (Fig. 5(b), marked with green
asterisks), at similar chemical shift values to those of 7. Two azoxy
compounds (12 and 13) and an azo compound (14) were isolated
from the mixture by column chromatography and recycling
preparative HPLC. Their molecular structures were determined
by ESI MS (12) or single-crystal X-ray analysis (13 and 14), as shown
in Fig. 3(c) and (d). Their 1H NMR spectra are shown in Fig. 5(c) and
(d). Formation of the intermolecular reaction products, azoxy and
azo compounds, proceeded with release of isobutene, as confirmed
by GC and 1H NMR (red asterisks). One of the reasons for
intermolecular condensation of tBu derivative would be explained
To our surprise, however, continued UV irradiation of a solution
of 2 produced a large amount of another compound. Using acetate
derivative 20 instead of the benzoate 2 for convenience, we isolated
2-nitrosophenylketone (6) and a brown solid (10), which was
crystallized from methanolic solution. Single-crystal X-ray analysis
of 10 revealed the structure depicted in Fig. 3(a).
Interestingly, the isopropyl group in 10 is 1,2-shifted onto the 6-
membered ring and the nitroso group is incorporated in a 5-
membered ring. A similar compound, 5,6-dimethoxy-1-(4-tolyl-
sulfonyl)benzo[c]isoxazol-3(1H)-one (Fig. 3(b)) [33], has a planar
aromatic ring, but in 10, the ring was distorted. Additionally, the
C7–C7A and C3A–C4 bond lengths (1.438(2) and 1.4864(19) Å,
respectively) were longer than that of a typical aromatic CꢂꢂC bond
(1.39 Å) as shown in Table 1. Loss of aromaticity in the six-
membered ring was supported by the 1H NMR chemical shifts of
two signals of 10 in the region between 5 and 6 ppm (marked with
orange asterisks in Fig. 4). The signals of the 6-membered ring
protons of 10 appeared at 5.39 and 5.74 ppm in CDCl3, i.e., at higher
field than the aromatic ring protons of 2-nitrosoarene 6, which
were observed at 6.42 and 7.16 ppm. To our knowledge, this type of
rearrangement has not been described before.
t
by elimination of the bulky Bu group occurs easily compared to
i
that for the Me and Pr derivatives. Unexpected and unclarified
t
i
behavior of Bu compound via Pr one has recently reported for
Newman–Kwart rearrangement [34]. The yield of 12 was less than
that of 13, judging from the signal areas.
The relative amounts of the photoreaction products are
summarized in Table 2.
When methyl-substituted ester 1 was photoirradiated, depro-
tection proceeded relatively slowly compared to the other three
esters. The main product was the 2-nitroso compound, 1-(4,5-
dimethoxy-2-nitrosophenyl) ethanone, 5 [35]. The cyclic isomer 9
In contrast to the photoreaction of iPr-substituted esters 2 and
20, UV irradiation of tBu ester 3 rapidly gave a complex mixture, as

N.C. Kasuga et al. / Journal of Photochemistry and Photobiology A: Chemistry 321 (2016) 41–47
45
Table 1
Selected bond distances (Å), angles (ꢁ) and torsion angles (ꢁ) of compounds 10,13 and 14.
10
5,6-Dimethoxy-1-(4-tolylsulfonyl)benzo[c]isoxazol-3(1H)-onea
C7–C6
C6–C5
C5–C4
C4–C3A
C3A–C7A
C7A–C7
C7A–N1
N1–O2
O2–C3
1.3554(19)
1.4871(18)
1.3578(16)
1.5081(18)
1.4862(19)
1.438(2)
1.2944(18)
1.4688(17)
1.3708(18)
1.5115(19)
1.5935(19)
115.83(12)
121.63(12)
121.35(12)
118.07(12)
120.01(12)
ꢂ179.81(12)
4.93(19)
C9–C8
C8–C6
C6–C4
C4–C3
C3–C2
C2–C9
C9–N1
N1–O6
O6–C1
C1–C2
1.387(3)
1.381(3)
1.424(3)
1.371(3)
1.396(4)
1.365(3)
1.426(3)
1.436(3)
1.392(3)
1.451(4)
C3–C3A
C3A–C8
C7A–C7–C6
C5–C6–C7
C4–C5–C6
C3A–C4–C5
C7–C7A–C3A
C5–C6–C7–C7A
C6–C5–C4–C3A
C6–C7–C7A–N1
C5–C4–C3A–C3
C9–C8–C6
C8–C6–C4
C6–C4–C3
116.9(2)
121.4(2)
120.1(2)
152.61(14)
ꢂ144.14(13)
13
14
1.265(3)
i
N1–N2
1.271(2)
N1 –N1
N2–C10
1.462(2)
N1–C2
1.425(2)
N1–C2
1.410(3)
N2–C10
1.462(2)
O1–C7
O2–C7
N2–N1–C2
O5–N2–N1
N1–N2–C10
C2–N1–N2–O5
1.228(2)
1.322(2)
115.24(15)
128.04(16)
114.61(15)
3.4(3)
O1–C7
1.201(3)
1.324(2)
115.5(2)
O2–C7
N1i–N1–C2
i
N1 –N1–C2–C3
ꢂ5.7(3)
Symmetry operation; i = ꢂx, ꢂy, 2 ꢂ z.
a
Data from Ref. [33].
was also formed from 5, but the yield was very much lower than in
the case of isopropyl derivative 2, and this may be the main reason
why this rearrangement has not been reported before. When a
solution of iPr-substituted ester 2 was irradiated in CD3CN, signals
of 6 and its isomer 10 appeared within 30 min, while signals of the
compound formed by intermolecular reaction corresponding to 13
for tBu one were observed only after irradiation for 15 h. Irradiation
t
of a solution of Bu-substituted derivative 3 afforded a complex
mixture, though azo compound 14 was accumulated as a brown
solid due to its low solubility in CD3CN. Other photolysis products
of 3 could not be isolated because of their low yield and instability.
This would account for the fact that the sum of the yields of 7,11,12,
13 and 14 is smaller than the amount of the 4-bromobenzoic acid.
Compounds 7 and 11 readily undergo intermolecular condensa-
tion, in contrast to the corresponding isopropyl-substituted
compounds 6 and 10.
Fig. 4. (a) 1H NMR spectra of 6 in CD3CN, and (b) after irradiation for 10 min. The
orange asterisks show signals of 10. Two signals near 2 ppm are those of water and
acetonitrile and the signal at 3.6 ppm is due to the internal reference, 1,4-dioxane.
(For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
Photoreaction of the isolated 2-nitrosophenyl ketones, 5–8, was
next performed to examine whether their successive reactions
were influenced by protons from the deprotected acids (Table 2).
The rearrangement product
9 was observed when 1-(4,5-
dimethoxy-2-nitrosophenyl) ethan-1-one 5 was irradiated. Intra-
molecular isomerization of iPr-substituted compound 6 also
proceeded efficiently. The sum of 6 and 10 was more than 90%
and the combined yield of other intermolecular products was less
than one percent. Photodegradation of tBu-substituted derivative 7
proceeded more rapidly than that of ester 3. These results
indicated that successive photoreaction of the three 2-nitro-
sophenyl ketones proceeded readily in the absence of protons,
even in the case of tBu-substituted compound 7. The phenyl-
substituted 2-nitroso compound 8 [36], which can be stored for
weeks in a refrigerator at around ꢂ18 ꢁC was little changed after
30-min UV irradiation. The photoreaction products of the
Fig. 5. (a) 1H NMR spectra of 7 in CD3CN, (b) after irradiation for 10 min, (c) 12 in
CD3CN and (d) 13 in CD3CN. The signals marked with green asterisks are assigned to
7. Two signals near 2 ppm are those of water and acetonitrile and the one at 3.6 ppm
is due to the internal reference, 1,4-dioxane. The signals at at 1.72 and 4.66 ppm
marked with red asterisks are due to isobutene. (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of this article.)

46
N.C. Kasuga et al. / Journal of Photochemistry and Photobiology A: Chemistry 321 (2016) 41–47
Table 2
Ratios (%) of products formed by UV irradiation of 2-nitrobenzyl esters and the isolated 2-nitroarenes.
Starting compound
Ester remaining
4-Bromo-benzoic acid
5
6
7
8
9
10
11
12
13
14
2-nitrobenzyl ester
1a
2a
3a
4a
48
22
5
37
66
85
69
31
–
–
–
–
7
–
–
1
–
–
–
–
–
–
–
2
–
–
–
17
–
–
28
–
–
–
–
–
–
13
–
<1
–
16
–
22
–
–
67
–
2-nitrosophenyl ketone
5a
6b
7a
8a
98
–
–
–
–
<1
–
–
–
–
8
–
–
–
–
–
7
–
45
–
–
–
47
–
–
–
–
<1
–
–
–
<1
–
11
–
–
–
99
–
–
Quantification was done based on the peak area ratio (measured in CD3CN in quartz NMR tubes) to the internal reference of 1,4-dioxane at 3.6 ppm, except in the case of 7.
Photoirradiation of 3 and 7 gave an insoluble brown precipitate of 14, which was isolated and weighed.
a
Irradiation conditions;
Irradiation conditions;
l
l
> 300 nm, 50 mW/cm2.
> 300 nm, 25 mW/cm2.
b
Fig. 6. Photoreaction products 9–14 obtained from 2-nitrosophenyl ketones 5–7.
2-nitrosophenyl ketones (5–8) identified in this work are
summarized in Fig. 6. These results show that the stability of 2-
nitrosophenyl ketones and their isomers is significantly influenced
group, especially tBu, decreases the stability of the 2-nitrosoarene.
From the point of view of usage as a protecting group, an isopropyl-
substituted 2-nitrobenzyl group may be very effective, because
deprotection proceeds faster and the 2-nitrosophenylketone
by the nature of the a-acyl substituent. A bulky branched aliphatic

N.C. Kasuga et al. / Journal of Photochemistry and Photobiology A: Chemistry 321 (2016) 41–47
47
formed mainly undergoes intramolecular rearrangement or
remains intact, so that unwanted intermolecular reactions are
minimized.
[10] X. Tang, J. Zhang, J. Sun, Y. Wang, J. Wu, L. Zhang, Caged nucleotides and their
photochemical biology, Org. Biomol. Chem. 11 (2013) 7814–7824.
[11] M.C. Pirrung, V.S. Rana, Photoremovable Protecting Groups in Dna Synthesis
and Microarray Fabrication Dynamic Studies in Biology: Phototriggers,
Photoswitches and Caged Biomolecules, in: M. Goeldner, R. Givens (Eds.),
Wiley-VCH, Weinheim, 2016, pp. 341–368 Chapter 6.
[12] H. Rhee, J.-S. Lee, C. Joo, H. Han, M. Cho, Photolytic control and infrared probing
of amide I mode in the dipeptide backbone-caged with the 4,5-dimethoxy-2-
nitrobenzyl group, J. Phys. Chem. B 112 (2008) 2128–2135.
4. Conclusions
We found that the substituent group at the
a-position of 2-
[13] S.A.A. Ahmad, L.S. Wong, E. ul-Haq, J.K. Hobbs, G.J. Leggett, J. Micklefield,
nitrobenzyl esters has a significant influence not only on the
uncaging rate, but also on the stability of the 2-nitrosophenylke-
tone formed. The nature of the acyl group also has a substantial
influence on the photoreaction products of 2-nitrosophenyl
ketones. Isopropyl-substituted 2-nitrosoarene 6 rearranges to a
unique bicyclic oxazole, accompanied by 1,2-shift of the alkyl
group onto the bridgehead, which breaks delocalization of the
aromatic ring. The tBu-substituted derivative 7 affords a more
complex mixture of products, including intermolecularly formed
azoxy and azo compounds, with release of isobutene gas.
Photorearrangement of methyl-substituted nitrosoketone 5 to
the cyclic isomer 9 also occurs, but is very much slower than that of
6. The phenyl-substituted derivative 8 was stable under UV
irradiation. These findings will be helpful for the selection and/or
design of photolabile protecting groups for various purposes.
Further mechanistic studies on the photoreaction of 2-nitro-
sophenyl ketones are in progress.
Micrometer- and nanometer-scale photopatterning using 2-
nitrophenylpropyloxycarbonyl-protected aminosiloxane monolayers, J. Am.
Chem. Soc. 131 (2009) 1513–1522.
[14] S.K. Choi, M. Verma, J. Silpe, R.E. Moody, K. Tang, J.J. Hanson, J.R. Baker Jr., A
photochemicap approach for controlled drug release in targeted drug delivery,
Bioorg. Med. Chem. 20 (2012) 1281–1290.
[15] H. Zhao, E.S. Sterner, E.B. Coughlin, P. Theato, o-Nitrobenzyl alcohol
derivatives: opportunities in polymer and materials science o-Nitrobenzyl
alcohol derivatives: opportunities in polymer and materials science,
Macromolecules 45 (2012) 1723–2736.
[16] S. Impellizzeri, B. McCaughan, F. Callan, F.M. Raymo, Photoinduced
enhancement in the luminescence of hydrophilic quantum dots coated with
photocleavable ligands, J. Am. Chem. Soc. 134 (2012) 2276–2283.
[17] J.-F. Gohy, Y. Zhao, Photo-responsive block copolymer micelles: design and
behavior, Chem. Soc. Rev. 42 (2013) 7117–7129.
[18] J. Cui, V.S. Miguel, A. del Campo, Light-triggered multifunctionality at surfaces
mediated by photolabile protecting groups, Macromol. Rapid Commun. 34
(2013) 310–329.
[19] J. Hemphill, E.K. Borchardt, K. Brown, A. Asokan, A. Deiters, Optical control of
CRISPR/Cas9 gene editing, J. Am. Chem. Soc. 137 (2015) 5642–5645.
[20] X. Tan, B.B. Li, X. Lu, F. Jia, C. Santori, P. Menon, H. Li, B. Zhang, J.J. Zhao, K. Zhang,
Light-triggered: self-immolative nucleic acid-drug nanostructures, J. Am.
Chem. Soc. 137 (2015) 6112–6115.
Acknowledgements
[21] B.G. Gowenlock, G.B. Richter-Addo, Preparations of C-nitroso compounds,
Chem. Rev. 104 (2004) 3315–3340.
[22] H. Yamamoto, M. Kawasaki, Nitroso and azo compounds in modern organic
synthesis: late blooming but very rich, Bull. Chem. Soc. Jpn. 80 (2007) 595–607.
[23] S. Carosso, M.J. Miller, Nitroso Diels–Alder (NDA) reaction as an efficient tool
for the functionalization of diene-containing natural products, Org. Biomol.
Chem. 12 (2014) 7445–7468.
[24] D.J. Dixton, O.P. Morejon, in: P. Knochel, G.A. Molander (Eds.), Photochemistry
of Nitro and Nitroso Compounds Comprehensive Organic Synthesis II, vol. 8,
Elsevier Ltd., Amsterdam, 2014, pp. 479–492.
We acknowledge financial support (grant for specified joint
research) from Kanagawa University, Japan. We thank Professor
Tadashi Sugawara of Kanagawa University for use of the recycling
preparative HPLC instrument. We also thank Professor Nobuhiro
Kihara for helpful discussions.
[25] M. Wilcox, R.W. Viola, W. Randall, K.W. Johnson, A.P. Billington, B.K. Carpenter,
J.A. McCray, A.P. Guzikowski, G.P. Hess, Synthesis of photolabile precursors of
amino acid neurotransmitters, J. Org. Chem. 55 (1990) 1585–1589.
[26] Y. Edagawa, J. Nakanishi, K. Yamaguchi, N. Takeda, Spatiotemporally controlled
navigation of neurite outgrowth in sequential steps on the dynamically photo-
patternable surface, Colloids Surf. B 99 (2012) 20–26.
[27] S. Kaneko, K. Yamaguchi, J. Nakanishi, Dynamic substrate based on
photocleavable poly(ethylene glycol): zeta potential determines the capability
of geometrical cell confinement, Langmuir 29 (2013) 7300–7308.
[28] T. Arai, N. Sato, K. Yamaguchi, M. Kawasaki, M. Fujimori, T. Shiba, M. Ando, K.
Torii, Self-aligned fabrication process of electrode for organic thin-film
transistors on flexible substrate using photosensitive self-assembled
monolayers, Jpn. J. Appl. Phys. 46 (2007) 2700–2703.
[29] B.P. Stupi, H. Li, J. Wang, W. Wu, S.E. Morris, V.A. Litosh, J. Muniz, M.N. Hersh, M.
L. Merzker, Stereochemistry of benzylic carbon substitution coupled with ring
modification of 2-nitrobenzyl groups as key determinations for fast-cleaving
reversible terminators, Angew. Chem. Int. Ed. 51 (2012) 1724–1727.
[30] Yadokari-XG, Software for Crystal Structure Analyses, K. Wakita (2001);
Release of Software (Yadokari-XG 2009) for Crystal Structure Analyses, C.,
Kabuto, S., Akine, T. Nemoto, and E. Kwon, J. Cryst. Soc. Jpn. 51 (2009) 218–224.
[31] Y.V. Il’ichev, M.A. Schwoerer, J. Wirz, Photochemical reaction mechanisms of 2-
nitrobenzyl compounds: methyl ethers and caged ATP, J. Am. Chem. Soc. 126
(2004) 4581–4595.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.
jphotochem.2016.01.012.
References
[1] C.G. Bochet, Photolabile protecting groups and linkers, J. Chem. Soc. Perkin
Trans. 1 (2002) 125–142.
[2] J.E.T. Corrie, in: M. Goeldner, R. Givens (Eds.), Photoremoval Protecting Groups
Use for the Caging of Biomolecules in Dynamic Studies in Biology,
Phototriggers, Photoswitches and Caged Biomolecules, Wiley-VCH, Weinheim,
2005, pp. 1–94.
[3] F. Bley, K. Schaper, H. Görner, Photoprocesses of molecules with 2-nitrobenzyl
protecting groups and caged organic acids, Photochem. Photobiol. 84 (2008)
162–171.
[4] H. Yu, J. Li, D. Wu, Z. Qiu, Y. Zhang, Chemistry and biological applications of
photo-labile organic molecules, Chem. Soc. Rev. 39 (2010) 464–473.
[5] P. Klán, T. Šolomek, C.G. Bochet, A. Blanc, R. Givens, M. Rubina, V. Popik, A.
Kostikov, J. Wirz, Photoremovable protecting groups in chemistry and biology:
reaction mechanisms and efficacy, Chem. Rev. 113 (2013) 119–191.
[6] P. Wang, Photolabile protecting groups: structure and reactivity, Asian J. Org.
Chem. 2 (2013) 452–464.
[7] P.G.M. Wuts, Photochemically Cleaved Phosphate Protective Groups Greene’s
Protective Groups in Organic Synthesis, 5th ed., John Wiley & Sons, Ltd.
Hoboken, N.J, 2014, pp. 1254–1258 Chapter 9.
[8] V. Hogen, K. Benndorf, U.B. Kaupp, in: M. Goeldner, R. Givens (Eds.), Control of
Cellular Activity Dynamic Studies in Biology: Phototriggers, Photoswitches
and Caged Biomolecules, Wiley-VCH, Weinheim, 2005, pp. 55–251 Chapter 4.
[9] S. Loudwig, H. Bayley, in: M. Goeldner, R. Givens (Eds.), Photoregulation of
Proteins Dynamic Studies in Biology: Phototriggers, Photoswitches and Caged
Biomolecules, Wiley-VCH, Weinheim, 2006, pp. 253–340 Chapter 5.
[32] A.P. Pelliccioli, J. Wirz, Photoremoval protecting groups: reaction mechanisms
and applications, Photochem. Photobiol. Sci. 1 (2002) 441–458.
[33] Z. Shang, K. Zhao, 5,6-Dimethoxy-1-(4-tolylsulfonyl)benzo[c]-isoxazol-3(1H)-
one, Acta Crystllogr. 61 (Sect. E) (2005) o132–o133.
[34] V. Novakoca, M. Miletin, T. Filandrova, J. Lenco, A. Ruzicka, P. Zimcik, Role of
steric hindrance in the Newman–Kwart rearrangement and in the synthesis
and photophysical properties of arylsulfanyltetrapyrazinoporphyrazines, J.
Org. Chem. 79 (2014) 2082–2093.
[35] R.C. Kevwitch, D.V. McGrath, Synthesis of photolabile dendrimer cores,
Synthesis 117 (2002) 1171–1176.
[36] P.J. Serafinowski, P.B. Garland, Novel photo-acid generators for photo-directed
oligonucleotide synthesis, J. Am. Chem. Soc. 125 (2003) 962–965.