3-Nitro-2-naphthalenemethanol: A photocleavable protecting group for carboxylic acids

Article abstract of DOI:10.1016/j.tet.2005.08.019

Photocleavable protecting groups are important in synthesis and caging. Among many such groups, 2-nitrobenzyl and related groups have been found useful in many applications. However, most of the known 2-nitrobenzyl-based caging chromophores show either low quantum yield or the photolysis wavelength is not suitable for various applications. In this paper, we report 2-nitro-3- naphthalenemethanol (NNM) as an efficient photocleavable protecting group for molecules containing a carboxylic function. NNM possesses photochemical properties better than the 2-nitrobenzyl chromophores as it is photoactivatable at 380 nm in aqueous medium (CH₃CN/H₂O, 3:2 v/v) showing the desired photochemistry. The carboxylic acids are efficiently photoreleased from NNM-based esters in almost quantitative yield.

Full text of DOI:10.1016/j.tet.2005.08.019

Tetrahedron 61 (2005) 10007–10012
3-Nitro-2-naphthalenemethanol: a photocleavable protecting
group for carboxylic acids
*
Anil K. Singh and Prashant K. Khade
Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India
Received 31 May 2005; revised 18 July 2005; accepted 4 August 2005
Available online 26 August 2005
Abstract—Photocleavable protecting groups are important in synthesis and caging. Among many such groups, 2-nitrobenzyl and related
groups have been found useful in many applications. However, most of the known 2-nitrobenzyl-based caging chromophores show either low
quantum yield or the photolysis wavelength is not suitable for various applications. In this paper, we report 2-nitro-3-naphthalenemethanol
(NNM) as an efficient photocleavable protecting group for molecules containing a carboxylic function. NNM possesses photochemical
properties better than the 2-nitrobenzyl chromophores as it is photoactivatable at 380 nm in aqueous medium (CH3CN/H2O, 3:2 v/v) showing
the desired photochemistry. The carboxylic acids are efficiently photoreleased from NNM-based esters in almost quantitative yield.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
chromophores.8 NNM is photoactivatable at 380 nm in
aqueous medium (0.9% aqueous NaCl) giving the desired
photochemistry with quantum yield of w0.60. We also
demonstrated the application of NNM in protection and
deprotection of –NH2 group.8 The improved photochemical
properties of NNM prompted us to examine its usefulness in
caging of molecules bearing the carboxyl group.
Light-induced deprotection of functional groups is
important in synthesis and biomolecular caging.1–5 For
biomolecular caging applications, the photoactivation of the
cage is required to be done with high rate and quantum
efficiency under physiological conditions at wavelengths
that are non-detrimental to biological systems. Several
photoactivatable groups such as benzoin, cinnamoyl,
coumaryl, 2-nitrobenzyl, phenacyl, polyaromatics etc.
have been developed and employed in diverse synthetic
and caging applications. Among these, the 2-nitrobenzyl
derivatives have attracted much attention because of their
compatibility with many functional groups (e.g., phosphate,
carboxylates, hydroxyl and amines). Besides its application
as an orthogonal protecting group in organic synthesis,6 it
has been extensively used in biology for photoactivation of
antibodies,7,8 synthesis of difficult cyclic peptide,9 photo-
release of Ca2C ions,10 synthesis of DNA arrays on glass
substrates11 and construction of a light-activated protein by
unnatural amino acid mutagenesis.12 However, most of the
known 2-nitrobenzyl-based caging chromophores show
either low quantum yield or photolysis wavelength is not
suitable for various applications.3 Recently, we have
reported a new caging chromophore namely 2-nitro-3-
naphthalenemethanol (NNM, 1), which possesses
photochemical properties better than the 2-nitrobenzyl
This paper describes photocleavable protecting group
properties of NNM for carboxylic acids. For comparison
purpose, we have also synthesized the widely used 4,5-
dimethoxy-2-nitrophenyl methanol3,13 (DMNM, 2) and
examined its efficacy for protection and deprotection of
carboxylic acids.
2. Results and discussion
NNM8 (1) and DMNM14 (2) were prepared according to the
given procedures. In order to investigate the photocleavable
protecting group properties of NNM for carboxylic
functional groups, it was attached to four different
carboxylic acids to obtain esters 1a–1d (Scheme 1) and
photochemistry of these esters was examined. Similar
studies were done on DMNM-based ester 2a also.
The UV–vis absorption of NNM-based esters (1a–1d) is
similar to the UV–vis absorption of NNM, with bands
located at 214, 258, 302 and 352 nm. Hence, the presence of
carboxylic acid moiety in NNM does not change its
absorption wavelength. Similarly, the absorption spectrum
of ester 2a is similar to the absorption spectrum of DMNM,
Keywords: 3-Nitro-2-naphthalenemethanol; Photocleavable protecting
group; Caging.
Corresponding author. Tel.: C91 22 25767167; fax: C91 22 25767152;
e-mail: retinal@chem.iitb.ac.in
*
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.08.019

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A. K. Singh, P. K. Khade / Tetrahedron 61 (2005) 10007–10012
Scheme 1.
with bands located at 226 and 346 nm. However, a 1.0!
10K3 M solution of these esters (1a–1d, 2a) in CH3CN–
H2O (3/2 v/v) shows absorption up to w420 nm. Irradiation
of a solution of 1a in CH3CN–H2O (3/2 v/v) at R370 nm
showed no considerable changes at 352 and 304 nm and it
decreased at 258 (Fig. 1). When 2a was photolysed under
similar condition, absorbance at 346 nm decreased with
concomitant increase at 220 nm (Fig. 2). HPLC analysis of
the photomixture of ester 1a revealed formation of parent
carboxylic acids (o-chlorobenzoic acid), which can be
generated if the ester undergoes the expected known
photochemical reaction of NNM (Fig. 3).8 Similarly,
DMNM-based ester 2a showed photorelease of parent
carboxylic acid as evident by HPLC analysis (Fig. 4). This
photorelease is again due to the expected photochemical
reaction of DMNM.3 The photochemical mechanism
involved in these photoreleases is depicted in Scheme 2.
The formation of nitrosoaldehyde is confirmed by proton
NMR studies of the photomixture obtained from 1c. The
photomixture shows a peak at 11.20 ppm due –CHO group.
It is similar to the mechanism observed in the case of
2-nitrobenzyl-based protecting groups.2–4,8,15
HPLC analysis showed formation of a few photoproducts in
addition to the expected nirosoaldehyde. These could have
generated from the secondary photoreaction of the primary
photoproduct nitrosoaldehyde. The nitrosoaldehyde is
known to undergo secondary photoreactions like dimerisa-
tion and oxidation.13,16,17 Similar side reactions are also
Figure 1. UV–vis absorption spectral changes during photolysis of 1a
(1.0!10K4 M) in acetonitrile–water (3/2, v/v).
Figure 2. UV–vis absorption spectral changes during photolysis of 2a
(1.0!10K4 M) in acetonitrile–water (3/2, v/v).

A. K. Singh, P. K. Khade / Tetrahedron 61 (2005) 10007–10012
10009
Figure 3. HPLC trace: (a) standard o-chlorobenzoic acid, RtZ4.3 min;
(b) ester 1a, RtZ37.6 min; (c) photomixture after 7 h photolysis of ester 1a
(O370 nm) with the released o-chlorobenzoic acid.
envisaged in NNM as well. However, these secondary
reactions do not affect the percent release of the carboxylic
acid, as the release is due to the primary photochemical
process.
Scheme 2.
An action plot was constructed to obtain photodeprotection
profiles for the esters. As shown in Figure 5 as a typical
example, excellent deprotection yields of 96–100% were
obtained in case of esters 1a–1d (1.0!10K3 M) in about
6–7 h of photolysis at O370 nm. However, when 2a was
irradiated under identical conditions the yield of deprotec-
tion was 88% but in about 12 h of photolysis. Quantum
yields of disappearance (F) of the esters 1a–1d were in
range of 0.14–0.16 at 380 nm. However, ester 2a
disappeared under similar photochemical condition with a
F of only 0.08 (Table 1). The acid moieties on NNM do not
significantly affect the photochemical properties of NNM as
no considerable differences in the quantum yield of
disappearance of esters 1a–1d have been observed.
Photolysis of esters was done in solvents like 1,4-dioxane,
ethanol, and acetonitrile. The maximum photorelease
Figure 5. Time dependent decrease of esters (1b and 2a) and release of
hippuric acid. Ester 1b (B); Ester 2a (6); % Deprotection of hippuric acid
from ester 1b (C); % Deprotection of hippuric acid from ester 2a (:).
Figure 4. HPLC trace: (a) standard hippuric acid, RtZ2.9 min; (b) ester 2a,
RtZ6 min; (c) photomixture, after 12 h photolysis (O370 nm) of ester 2a
with the released hippuric acid.

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A. K. Singh, P. K. Khade / Tetrahedron 61 (2005) 10007–10012
Table 1. Photochemical properties of esters 1a–1d and 2a (1.0!10K3 M in 3:2 acetonitrile/water)
Ester
Carboxylic acids
3 at 380 nm
(molK1 cmK1 L)
Photolysis
time (h)a
Unreacted
(%)b
Deprotection
yield (%)b
F
1a
1626
7
3
97
0.16
1b
1c
1280
1886
6
6
4
0
96
0.15
0.15
100
1d
2a
1755
3180
6
4
5
96
88
0.13
0.08
12
a Time of disappearance of ester.
b Determined by HPLC.
(w75%) of the acid was observed in acetonitrile. However,
the photorelease of acids in 1,4-dioxane and ethanol was
only 25 and 35%, respectively. The photorelease of the
acids can be further improved by irradiation of the esters in
aqueous acetonitrile. Thus, the photoreleases could be
maximized to O95% by irradiating the esters in acetonitrile
containing water (up to 40%). Thus, polar solvents favour
the photorelease. Control experiments showed that no
thermal decomposition of the esters occurred as evidenced
by HPLC analysis of the samples of esters solutions in
CH3CN–H2O (3/2 v/v) kept in dark for about 8 days. Thus,
NNM can be used to cage molecules bearing carboxyl
functions. Its usage for other functional groups like
phosphate and alcohol is also envisaged. It may, however,
be noted that currently low solubility of NNM in water and
additional synthetic work involved in its preparation limit
its usage.
suppliers and distilled before use. Other solvents (AR,
UV–vis spectroscopic and HPLC grade) were purchased
from SRL India Ltd, Mumbai. Thin-layer chromatography
(TLC) was performed on silica gel (GF254 from SRL India
Ltd, Mumbai) plates prepared by coating thin film of silica
gel slurry prepared in ethyl acetate on glass plates. Column
chromatography was performed on silica gel of 60–120 or
100–200 mesh (SRL India Ltd, Mumbai/Thomas Baker).
All the organic extracts were dried over anhydrous sodium
sulphate.
Melting points (Mp) were determined with a Veego melting
point apparatus and are uncorrected. Electronic absorption
spectra were recorded on Shimadzu UV-160 or Shimadzu
UV-260 spectrophotometer. Nicolet Impact 400 series
Fourier transform spectrometer (FTIR) was used to record
the IR spectra of the compounds. NMR spectra were
measured in a Fourier transform mode on Varian 300/
400 MHz magnetic resonance spectrometer. The high-
resolution mass spectra were recorded on Bruker Daltonics
APEX 3 T Fourier Transform mass spectrophotometer and
Q-Tof micro (YA-105) mass spectrometer. HPLC analyses
were done using Hitachi L-6250 intelligent pump attached
with U-2000 spectrophotometer under the following
conditions: ALTEX ODS 5 m, 4.6 mm!25 cm, 55%
acetonitrile in water containing 0.1% phosphoric acid,
1.0 mL/min, 226 nm.
3. Conclusions
We have found that NNM is an efficient photocleavable
protecting group for the carboxylic acids. It is photo-
activatable in aqueous medium at 380–400 nm with the
desired photochemical reaction. The carboxylic acids are
released by irradiating the corresponding ester solution.
Polar solvents like aqueous acetonitrile favour the photo-
release of the acid. As compared to DMNM, the
photorelease of acids in case of NNM-based esters is almost
quantitative and with higher quantum efficiency.
Irradiations were done using a 400 W medium pressure
mercury lamp (Applied Photophysics Ltd, London UK,
Model R-607), fitted either with a f/3.4 monochromator or
with a glass filter (R370 nm having 47 and 96%
transmittance at 370 and 420 nm, respectively, with distance
between sample and lamp being 2 cm) to isolate the desired
wavelength for irradiation. UV–vis spectroscopic grade
solvents were used for all photochemical works. The
photochemical yield was defined as the ratio of ester
molecules disappeared (calculated using HPLC as described
above) to the amount of photons absorbed using the
potassium ferrioxalate actinometer.18
4. Experimental
4.1. General
Starting materials for synthesis were from M/s. Lancaster
(UK) and SRL India Ltd. All organic solvents were dried
using standard procedures. Petroleum ether (pet-ether,
60–80 8C fraction) was procured in bulk from local

A. K. Singh, P. K. Khade / Tetrahedron 61 (2005) 10007–10012
10011
NNM was prepared as described earlier.8 It is a yellowish
solid and remains as such under ambient laboratory light.
However, the light yellowish solution of NNM in common
organic solvents darkens under ambient laboratory light
over a period of time. However, no significant absorption
changes are observed in NNM solutions (10K3–10K5 M)
under ambient laboratory light for about a week.
(m, 4H, Ar-H), 7.40–7.26 (m, 5H, Ar-H), 5.64 (s, 2H,
CH2O) and 3.78 (s, 2H, CH2).
4.1.4. 4-Methyl-benzoic acid 3-nitro-naphthalen-2-yl-
methyl ester (1d). Yield: 94%; light yellow solid. Mp:
120–122 8C; FTIR (KBr) nmax (cmK1): 1716 (OCO), 1539
and 1341 (NO2); ES-MS: m/z 344.0901 (MCCNa) (found),
344.0899 (MCCNa) (calcd for C19H15NO4); 1H NMR
(400 MHz, CDCl3): d 8.69 (s, 1H, Ar-H), 8.06–7.90 (m, 5H,
Ar-H), 7.69–7.64 (m, 2H, Ar-H), 7.27 (d, JZ8.8 Hz, 2H,
Ar-H), 5.87 (s, 2H, CH2O) and 2.43 (s, 3H, CH3).
Esters were prepared according to literature procedure.19 In
a typical case, solution of carboxylic acid (0.24 mmol),
alcohol (0.24 mmol) and 4-(dimethylamino)pyridine
(DMAP, 0.024 mmol) in anhydrous dichloromethane
(5 mL) was stirred for 10 min. To the reaction mixture
N,N-dicyclohexylcarbodiimide (DCC, 0.24 mmol) was
added and stirred for 12 h at ambient temperature.
Subsequently water (20 mL) was added into reaction
mixture and compound was extracted with dichloro-
methane. The organic layer was washed with sodium
bicarbonate solution and then with water and dried with
anhydrous sodium sulphate. Solvent evaporation in vacuo
gave solid, which was further purified by column
chromatography.
4.1.5. Benzoylamino-acetic acid 4, 5-dimethoxy-2-nitro-
benzyl ester (2a). Yield: 63%; light brown solid. Mp: 158–
160 8C; FTIR (KBr) nmax (cmK1): 3429 (NH), 1741 (OCO),
1659 (NHCO), 1528 and 1327 (NO2); ES-MS: m/z 397.1024
(MCCNa) (found), 397.1012 (MCCNa) (cacld for
1
C18H18N2O7); H NMR (400 MHz, CDCl3): d 7.81 (d, JZ
8.4 Hz, 1H, Ar-H), 7.73 (s, 1H, Ar-H), 7.54 (t, JZ7.2 Hz,
2H, Ar-H), 7.46 (t, JZ7.2 Hz, 2H, Ar-H), 7.05 (s, 1H,
Ar-H), 6.70 (br s, 2H, NH), 5.65 (s, 2H, CH2O), 4.36 (d, JZ
4.8 Hz, 1H, CH2NH), 4.00 (s, 3H, OCH3) and 3.96 (s, 3H,
OCH3).
4.1.1. 2-Chlorobenzoic acid-3-nitro-naphthalen-2-yl-
methyl ester (1a). 2-Chlorobenzoic acid (0.038 g,
0.24 mmol), 3-nitro-naphthalenemethanol (0.049 g,
0.24 mmol) and DMAP (0.003 g, 0.024 mmol) in anhydrous
dichloromethane (5 mL) was stirred for 10 min. To the
reaction mixture DCC (0.049 g, 0.24 mmol) was added and
stirred for 12 h at ambient temperature. Subsequently water
(20 mL) was added into reaction mixture and compound
was extracted with dichloromethane. The organic layer was
washed with sodium bicarbonate solution and then with
water and dried with anhydrous sodium sulphate. Solvent
evaporation in vacuo gave light brown solid, which was
further purified by column chromatography.
4.2. Photodeprotection of esters and calculation of
deprotection yield by HPLC
In a typical experiment, 2 mL of 1.0!10K3 M solution of
ester in solvents (e.g., acetonitrile–water, 3:2 v/v) was taken
in a quartz cuvette. It was photolysed by a 400 W medium
pressure mercury lamp and the progress of the photoreaction
(deprotection) was monitored by HPLC. For the calculation
of percent disappearance of ester and percent appearance of
the corresponding acid (e.g., 2-chlorobenzoic acid in case of
1a), aliquots of 20 mL of the photomixture was removed
periodically and analysed by HPLC. The photochemical
deprotection yields were calculated by comparing the HPLC
trace (peak area) due to released acid (e.g., 2-chlorobenzoic
acid in case of 1a) with the corresponding peak area due to
respective standard acid. The HPLC analysis data are
average of three to four independent runs.
Yield: 89%. Mp: 94–96 8C; FTIR (KBr) nmax (cmK1): 1729
(OCO), 1525 and 1361 (NO2); ES-MS: m/z 364.0367
(MCCNa) (found), 364.0353 (MCCNa) (calcd for
1
C18H12ClNO4); H NMR (400 MHz, CDCl3): d 8.73 (s,
1H, Ar-H), 8.14 (s, 1H, Ar-H), 8.02 (d, JZ8.4 Hz, 1H,
Ar-H), 7.95–7.89 (m, 2H, Ar-H), 7.73–7.64 (m, 2H, Ar-H),
7.50–7.43 (m, 2H, Ar-H), 7.37–7.33 (m, 1H, Ar-H) and 5.91
(s, 2H, CH2O).
Acknowledgements
Research grant [01(1509)/98/EMR-II] from the Council of
Scientific and Industrial Research, New Delhi, Government
of India is gratefully acknowledged.
4.1.2. Benzoylamino acetic acid 3-nitro-naphthalen-2-yl-
methyl ester (1b). Yield: 97%; light yellow solid. Mp:
152–154 8C; FTIR (KBr) nmax (cmK1): 3293 (NH), 1752
(OCO), 1639 (NHCO), 1530 and 1334 (NO2); ES-MS: m/z
365.1130 (MCC1) (found), 365.1137 (MCC1) (calcd for
1
C20H16N2O5); H NMR (300 MHz, CDCl3): d 8.72 (s, 1H,
Supplementary data
Ar-H), 8.04–7.93 (m, 3H, Ar-H), 7.85–7.82 (m, 2H, Ar-H),
7.75–7.66 (m, 2H, Ar-H), 7.56–7.43 (m, 3H, Ar-H), 6.72 (br
s, 1H, NH), 5.87 (s, 2H, CH2O) and 4.40 (d, JZ5.1 Hz, 2H,
CH2NH).
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tet.2005.08.019.
4.1.3. Phenyl-acetic acid 3-nitro-naphthalen-2-yl-methyl
ester (1c). Yield: 63%; light yellow solid. Mp: 90–92 8C;
FTIR (KBr) nmax (cmK1): 1729 (OCO), 1525 and 1334
(NO2); ES-MS: m/z 344.0911 (MCCNa) (found), 344.0899
References and notes
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(MCCNa) (calcd for C19H15NO4); H NMR (400 MHz,
CDCl3): d 8.66 (s, 1H, Ar-H), 7.97 (d, 1H, Ar-H), 7.72–7.60
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