Anthracene-9-methanol - A novel fluorescent phototrigger for biomolecular caging

Article abstract of DOI:10.1016/j.tetlet.2005.06.026

Photoexcitation of a solution of anthracene-9-methanol derived esters at ～386 nm in CH₃CN/H₂O (3:2 v/v) results in fluorescence emission in the 380-480 nm range, with quantum yields of fluorescence (Φ_f) in the 0.01-0.09 range and releases of the carboxylic acids in good chemical yields (43-100%), with quantum yields of photoreaction (Φ_PR, i.e., the photodisappearance of the esters) in the 0.067-0.426 range.

Full text of DOI:10.1016/j.tetlet.2005.06.026

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 5563–5566
Anthracene-9-methanol—a novel fluorescent phototrigger
for biomolecular caging
Anil K. Singh* and Prashant K. Khade
Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India
Received 8 March 2005; revised 2 June 2005; accepted 8 June 2005
Available online 1July 2005
Abstract—Photoexcitation of a solution of anthracene-9-methanol derived esters at ꢀ386 nm in CH3CN/H2O (3:2 v/v) results in
fluorescence emission in the 380–480 nm range, with quantum yields of fluorescence (Uf) in the 0.01–0.09 range and releases of
the carboxylic acids in good chemical yields (43–100%), with quantum yields of photoreaction (UPR, i.e., the photodisappearance
of the esters) in the 0.067–0.426 range.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Photochemical release of bioactive components from a
Ôcaged moleculeÕ is an important strategy in the study
of numerous processes in biology, medicine and materi-
als science.1–3 The prerequisites for a chromophore to
act as a phototrigger in caging applications include
photoactivatability of the chromophore under physio-
logical conditions and at biologically acceptable wave-
lengths, with a high rate and quantum yield of the
photoreaction, etc. Many phototriggers such as 2-
nitrobenzyl, benzoin, phenacyl, etc. have been synthes-
ised and utilised in the caging of molecules.1c Recently,
polycyclic aromatics have been found to act as good
phototriggering groups for alcohols, amines, peptides,
phosphates and carboxylic acids. Some of the known
polycyclic aromatics are anthraquinon-2-ylmethoxycar-
bonyl4 (Aqmoc, kmax 327 nm), pyren-1-ylmethoxycarbonyl5
(Pmoc, kmax 323 nm), 7-methoxycoumarin-4-ylmethoxy-
carbonyl6a,b (Mmoc, kmax 343 nm) and phenanthren-9-
ylmethoxycarbonyl4 (Phmoc, kmax 297 nm). Attempts
to synthesise either fluorescent phototriggers or to con-
vert a photolabile protecting group into fluorescence
phototriggers are limited.5,6 Fluorescent phototriggers
have a few advantages over photocleavable protecting
groups. They may be useful in monitoring changes in
concentration and thus allow tracing of the location of
Ôcaged moleculesÕ inside living cells by the fluorescent
microscope technique.6,7 Fluorescent phototriggers also
allow visualisation of support grafting during in situ
synthesis of oligonucleotides and peptides.8 To the best
of our knowledge, fluorescent phototriggers other than
coumarin and pyrene are not known so far. However,
the caging applications of these phototriggers are re-
stricted as they have low quantum yields, require specific
solvents for the photoreaction and generally the photo-
lysis wavelength is not more than 350 nm. Herein, we
report a new fluorescent phototrigger, namely anthracene-
9-methanol (1), which shows improved photochemical
properties useful in caging carboxylic acids.
In order to study the phototrigger properties of 1 for
carboxylic acids, ester derivatives from five different car-
boxylic acids, viz o-chlorobenzoic acid, hippuric acid,
m-nitrobenzoic acid, phenylacetic acid and acetic acid
were prepared. Ester derivatives 1a–d were prepared
by DCC/DMAP mediated condensation of 1 (1mmol)
with the carboxylic acids in dichloromethane at ambient
temperature (Scheme 1).9 Ester 1e was prepared by the
reaction of 1 (1mmol) with acetic anhydride (2 mmol)
in the presence of DMAP (0.1mmol) and pyridine
(1mmol) in dichloromethane. All ester derivatives 1a–
e were purified by column chromatography and charac-
1
terised by melting point, FT-IR, H NMR, 13C NMR
and mass spectral analysis.10
Keywords: Caging; Anthracene-9-methanol; Esters; Photochemistry;
Phototrigger.
Solutions (1.0 · 10À3 M) of esters 1a–e in CH3CN–H2O
(3:2) showed UV–vis absorptions in the 300–400 nm
range with kmax values at ca. 383–386 nm. The
*
Corresponding author. Tel.: +9122 2576 7167; fax: +9122 2576
7152; e-mail: retinal@chem.iitb.ac.in
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.06.026

5564
A. K. Singh, P. K. Khade / Tetrahedron Letters 46 (2005) 5563–5566
OH
OX
DCC / DMAP
CH2Cl2, XOH
1
O
Cl
X =
CH2Cl2,
Pyridine,
DMAP
N
H
(CH3CO)2O
O
O
1b
1a
OCOCH3
NO2
O
O
1e
1c
1d
Scheme 1.
Table 1. Photochemical properties of esters 1a–e
Ester
Carboxylic acid
e (molÀ1 cmÀ1 L) at 386 nm
in 3:2 v/v CH3CN–H2O
Uf (kex 386 nm)
UPR (kht 386 nm)
ht timea (min)
% Photoreleaseb
Cl
1a
7590
0.018
0.087
40
100
HO
O
O
HO
N
1b
1c
6705
8572
0.089
0.010
0.335
0.067
10
51
68
H
O
HO
O
360
NO2
HO
1d
1e
8220
7292
0.093
0.077
0.148
0.426
20
20
43
O
HO CH3
ND
O
ND: Release of acetic acid could not be detected by HPLC.
a Time for complete disappearance of starting esters (1.0 · 10À3 M).
b Yield calculated using HPLC.
fluorescence emission spectra of the esters (1.0 · 10À6 M
in CH3CN–H2O, 3:2) are typical of anthracene fluores-
cence emission with four bands located at 389, 411,
434 and 460 nm. The fluorescence quantum yield (Uf)
of the esters changes with the nature of the carboxylic
acid group.11 In comparison to 1(Uf, 0.036), the Uf is
higher for 1b, 1d and 1e and lower for 1a and 1c (Table
1). The low Uf for 1a and 1c could be due to intersystem
crossing to the triplet state because of the presence of
nitro- and chloro groups in the corresponding esters.
ance of 1a being 75 and 40 min in ethanol and
CH3CN–H2O, respectively. In THF–H2O and 1,4-diox-
ane–H2O systems, the yields, however, were relatively
low (95% and 89%, respectively) and the photolysis
times were 90 and 60 min, respectively. Thus, the photo-
release in CH3CN–H2O (3:2 v/v) was found to be better
than that found in other solvent systems. Therefore, the
CH3CN–H2O (3:2 v/v) solvent system was used to study
the photochemical properties of esters 1b–1e. The
quantum yields of photoreaction (UPR, that is, the
photodisappearance of the esters 1a–e) were in the range
0.067–0.426 (Table 1). Good photorelease of carboxylic
acids was obtained in the cases of 1a and 1c. However,
with 1b and 1d, the photorelease yields were less, which
could be due to secondary photoreactions of the esters
or of esters with the parent alcohol 1 generated during
the photolysis. Minor photo-by-products were detected
To examine the effect of solvent, a 1.0 · 10À3 M solution
of ester 1a was photolysed in various solvent systems
including ethanol, 3:2 (v/v) THF–H2O, 1,4-dioxane–
H2O and CH3CN–H2O.11 Its photolysis in ethanol and
CH3CN–H2O resulted in 100% release of o-chloroben-
zoic acid, with the time for complete photodisappear-

A. K. Singh, P. K. Khade / Tetrahedron Letters 46 (2005) 5563–5566
5565
OCOR
CH2
Homolytic cleavage
OCOR
e transfer
hv
1
CH2
Heterolytic cleavage
+
OCOR
H2O
EtOH
OCH2CH3
OH
+
RCOOH
+
RCOOH
2
1
Scheme 2.
by HPLC analysis of the photo-mixtures but these by-
products could not be characterised. Attempts were
made to determine the origin of these photo-by-prod-
ucts and it was found that the majority of the by-prod-
ucts originated from secondary photoreactions of the
released parent alcohol 1. To confirm the secondary
photoreaction, photolysis of a 1.0 · 10À3 M solution of
1 in CH3CN–H2O (3:2) was performed and it was found
that the photoproducts obtained were similar to those of
the by-products which were formed during the photoly-
sis of 1a–e.
References and notes
1. (a) Methods Enzymol; Marriott, G., Ed.; Caged Com-
pounds; Academic Press: New York, 1998; Vol. 291; (b)
Pelliccioli, A. P.; Wirz, J. Photochem. Photobiol. Sci. 2002,
1, 441; (c) Bochet, C. G. J. Chem. Soc., Perkin Trans. 1
2002, 125.
2. Nerbonne, J. M. Curr. Opin. Neurobiol. 1996, 6, 379.
3. Shigeri, Y.; Tatsu, Y.; Yumoto, N. Pharmacol. Ther. 2001,
91, 85.
4. Furata, T.; Hirayama, Y.; Iwamura, M. Org. Lett. 2001, 3,
1809.
5. Iwamura, M.; Hodota, C.; Ishibashi, M. Synlett 1991, 35.
6. (a) Schade, B.; Hagen, V.; Schmidt, R.; Herbrich, R.;
Krause, E.; Eckardt, T.; Bendig, J. J. Org. Chem. 1999, 64,
9109; (b) Hagen, V.; Frings, S.; Bendig, J.; Lorenz, D.;
Wiesner, B.; Kaupp, U. B. Angew. Chem., Int. Ed. 2002,
41, 3625; (c) Welch, M. B.; Martinez, C. I.; Zhang, A. J.;
Jin, S.; Gibbs, R.; Burgess, K. Chem. Eur. J. 1999, 5, 951;
(d) Eckardt, T.; Hagen, V.; Schade, B.; Schmidt, R.;
Schweitzer, C.; Bending, J. J. Org. Chem. 2002, 67, 703.
7. Politz, J. C. Trends Cell Biol. 1999, 9, 284.
8. (a) Muller, C.; Even, P.; Viriot, M.-L.; Carre, M.-C. Helv.
Chim. Acta 2001, 84, 3753; (b) McGall, G. H.; Barone, A.
D.; Diggelmann, M.; Fodor, S. P. A.; Gentalen, E.; Ngo,
N. J. Am. Chem. Soc. 1997, 119, 5081.
Chromophore 1 and the released o-chlorobenzoic acid
were the major photoproducts in the photolysis of 1a
in the CH3CN–H2O (3:2) system. However, photolysis
of 1a in ethanol gave 10-(ethoxymethyl) anthracene (2)
and o-chlorobenzoic acid as the major photoproducts.
A plausible photochemical mechanism is shown in
Scheme 2. Homolytic cleavage of the C–O bond fol-
lowed by electron transfer can yield an anthryl-9-meth-
ylene carbocation, which on nucleophilic attack by the
solvent molecule leads to the respective products. An-
other possible mechanism could involve heterolytic
cleavage of the C–O bond, which can directly give the
9-anthryl-methylene carbocation.
9. Hassner, A.; Alexanian, V. Tetrahedron Lett. 1978, 19,
4475.
10. 1a: Yield 98%; Mp: 122–124 ꢁC; FTIR (KBr) mmax (cmÀ1):
Thus, anthracene-9-methanol (1) can be an efficient fluo-
rescent phototrigger for molecules containing a carboxyl
function. Furthermore, 1 can also be used for caging via
other functional groups such as –NH2, –PO3H, –OH, etc.
1736 (OCO); ES-MS: m/z found: 369.091(M ++Na) calcd
1
for C22H15O2Cl 369.810 (M++Na); H NMR (400 MHz,
CDCl3): d 8.54 (1H, s, Ar-H), 8.44 (d, J = 8.8 Hz, 2H, Ar-
H), 8.04 (d, J = 8.4 Hz, 2H, Ar-H), 7.71(d, J = 7.6 Hz,
1H, Ar-H), 7.60 (t, J = 6.8 Hz, 1H, Ar-H), 7.50 (t,
J = 7.6 Hz, 1H, Ar-H), 7.40 (d, J = 8 Hz, 1H, Ar-H),
7.34 (t, J = 8.8 Hz, 2H, Ar-H), 7.18 (t, J = 7.2 Hz, 2H, Ar-
H) and 6.41(s, 2H, CH 2O); 13C NMR (75 MHz): d 165.9,
133.9, 132.7, 131.7, 131.4, 131.2, 131.1, 129.8, 129.5, 129.2,
126.8, 126.6, 125.8, 125.2, 124.0 and 59.9. 1b: Yield 90%;
Mp: 182–184 ꢁC; FTIR (KBr) mmax (cmÀ1): 3329 (NH),
1738 (OCO) and 1632 (NHCO); ES-MS: m/z found:
Acknowledgements
We thank the Council of Scientific and Industrial
Research, New Delhi for financial assistance [01(1509)/
98/EMR-II] to this project.

5566
A. K. Singh, P. K. Khade / Tetrahedron Letters 46 (2005) 5563–5566
392.1268 (M++Na) calcd for C24H19NO3 392.1263
and 41.3. 1e: Yield 50%; Mp: 108–110 ꢁC; FTIR (KBr)
mmax (cmÀ1): 1723 (OCOCH3); GC–MS (% rel int): 250
(M+, 33), 191 (100), 165 (7) and 94 (9); 1H NMR
(300 MHz, CDCl3): 8.51(s, H1, Ar-H), 8.33 (d,
J = 8.8 Hz, 2H, Ar-H), 8.03 (d, J = 8.4 Hz, 2H, Ar-H),
7.58 (t, J = 7.6 Hz, 2H, Ar-H), 7.49 (t, J = 7.5 Hz, 2H, Ar-
H), 6.15 (s, 2H, OCH2) and 2.09 (s, 3H, CH3); 13C NMR
(75 MHz): d 171.2, 131.3, 131.0, 129.2, 129.1, 126.6, 126.1,
125.1, 123.9, 58.9 and 21.2.
(M++Na); 1H NMR (400 MHz, CDCl3): d 8.54 (s, 1H,
Ar-H), 8.36 (d, J = 10 Hz, 2H, Ar-H), 8.04 (d, J = 8.4 Hz,
2H, Ar-H), 7.77 (d, J = 7.2 Hz, 2H, Ar-H), 7.59 (t,
J = 4.8 Hz, 2H, Ar-H), 7.50 (t, J = 7.6 Hz, 2H, Ar-H),
7.43 (t, J = 7.2 Hz, 2H, Ar-H), 6.62 (br s, 2H, NH), 6.30 (s,
2H, CH2O) and 4.25 (d, J = 4.8 Hz, 2H, CH2); 13C NMR
(100 MHz): d 170.3, 167.6, 133.8, 131.9, 131.4, 131.1,
129.7, 129.3, 128.7, 127.1, 126.9, 125.3, 123.8, 60.1 and
41.9. 1c: Yield 81%; Mp: 152–154 ꢁC; FTIR (KBr) mmax
(cmÀ1): 1723 (OCO), 1538 and 1354 (NO2); GC–MS (% rel
int): 380 (M++Na, 11), 362 (12), 300 (100), 281 (42), 204
(23), 165 (17), 140 (33), 115 (41) and 91 (38); 1H NMR
(300 MHz, CDCl3): d 8.81(t, J = 2.4 Hz, 1H, Ar-H), 8.56
(s, 1H, Ar-H), 8.42 (d, J = 9 Hz, 2H, Ar-H), 8.36–8.27 (m,
2H, Ar-H), 8.06 (d, J = 8.4 Hz, 2H, Ar-H), 7.64–7.49 (m,
5H, Ar-H) and 6.46 (s, 2H, CH2O); 13C NMR (75 MHz): d
164.8, 135.5, 131.9, 131.5, 131.3, 129.7, 129.6, 129.3, 127.5,
127.0, 125.5, 125.3, 124.8, 123.8 and 60.4. 1d: Yield 64%;
Mp: 112–114 ꢁC; FTIR (KBr) mmax (cmÀ1): 1744 (OCO);
GC–MS (% rel int): 326 (M+, 17), 191 (100), 165 (6) and 91
11. Rabek, J. F. Radiometry and Actinometry. In Experi-
mental Methods in Photochemistry and Photophysics;
Wiley: New York, 1982; Vol. 2, pp 944–946, The Uf
against anthracene (Uf, 0.2 in ethanol) as a standard was
determined at ambient temperature. The UPR was calcu-
lated for ester disappearance at 386 nm, and the number
of quanta absorbed were calculated by ferrioxalate acti-
nometry. Photolyses were performed using a 400 W
medium pressure Hg lamp and the desired wavelengths
were obtained by using a monochromator/>370 nm glass
filter having 47% and 96% transmittance at 370 and
420 nm, respectively. The distance between the sample and
the lamp was 2 cm. HPLC analyses were performed on a
Hitachi instrument consisting of a L-6250 intelligent pump
and a U-2000 spectrophotometer under the following
conditions: column, ALTEX ODS (5l, 4.6 mm · 25 cm),
solvent, CH3CN–H2O (3:2 v/v), flow rate, 0.8 mL/min,
detection k 226 nm.
1
(12); H NMR (400 MHz, CDCl3): d 8.48 (s, 1H, Ar-H),
8.27 (d, J = 8 Hz, 2H, Ar-H), 8.01(d, J = 8.4 Hz, 2H, Ar-
H), 7.78–7.45 (m, 4H, Ar-H), 7.32–7.23 (m, 5H, Ar-H),
6.15 (s, 2H, CH2O) and 3.62 (s, 2H, CH2); 13C NMR
(75 MHz): d 171.9, 134.2, 133.9, 131.4, 131.1, 129.4, 129.3,
129.1, 128.6, 127.3, 127.2, 126.7, 126.1, 125.2, 124.0, 59.4