1-Acetylpyrene with dual functions as an environment-sensitive fluorophore and fluorescent photoremovable protecting group

Article abstract of DOI:10.1016/j.tet.2010.10.090

A series of new fluorescent ester conjugates of carboxylic acids including amino acids was synthesized by coupling with an environment-sensitive fluorophore 1-acetylpyrene. Interestingly, the fluorescence properties of the ester conjugates and 1-acetylpyrene were found to be highly sensitive to its surrounding environment. The results obtained from the photolysis of the ester conjugates indicated that various factors like solvent, irradiation wavelength, and the structure of the conjugates govern the rate of the photocleavage.

Full text of DOI:10.1016/j.tet.2010.10.090

Tetrahedron 66 (2010) 9798e9807
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
1-Acetylpyrene with dual functions as an environment-sensitive fluorophore
and fluorescent photoremovable protecting group
*
Avijit Jana, Sanghamitra Atta, Sujan K. Sarkar, N.D. Pradeep Singh
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2010
Received in revised form 21 October 2010
Accepted 31 October 2010
Available online 5 November 2010
A series of new fluorescent ester conjugates of carboxylic acids including amino acids was synthesized by
coupling with an environment-sensitive fluorophore 1-acetylpyrene. Interestingly, the fluorescence
properties of the ester conjugates and 1-acetylpyrene were found to be highly sensitive to its sur-
rounding environment. The results obtained from the photolysis of the ester conjugates indicated that
various factors like solvent, irradiation wavelength, and the structure of the conjugates govern the rate of
the photocleavage.
Keywords:
1-Acetylpyrene
Ó 2010 Elsevier Ltd. All rights reserved.
Fluorescence
Photocleavage
Photoremovable protecting group
1. Introduction
with neither fluorescent nor strong absorption in the UV/vis region
by a far more sensitive technique than common UV absorption.21
Photoremovable protecting groups (PRPGs) for various func-
tional groups are of great interest since they have demonstrated
potential applications in synthetic organic chemistry,1 bio-
chemistry,2 and materials science.2a,3 To date several PRPGs have
been developed to mask different functional groups including
carboxylic acids,4 alcohols,5 amines,6 phosphates,7 aldehydes,8 and
ketones.8 The carboxyl group containing molecules can be effec-
tively protected by a variety of PRPGs, such as 2-(dimethylamino)-
Secondly, it helps us to visualize the amino acids involved in bi-
ological process.22 Finally, since the amino acids are tagged by
fluorescent PRPG, they can be released for a specific period of time
at a desired location. For example, release of neuroactive amino
acids (e.g.,
g-aminobutyric acid, glycine, glutamic acid, etc.) by
using fluorescent PRPG in the treatment of neuropsychiatric dis-
eases has been reported.23
Although fluorescent PRPGs are of great utility, only limited
number of PRPG of above class have been reported. In recent years
PRPGs of polycyclic aromatic compounds namely anthraquinone,24
pyrene,24,25 phenanthrene,24 anthracene,15 and coumarins18a,b
moieties have been targeted as fluorescent photolabile protecting
groups. Among them use of pyren-1-yl methyl as fluorescent
photolabile protecting group for alcohols,24 carboxylic acids,26
phosphates,27 and amines28 has been demonstrated. Although,
pyrene is a well known fluorophore, to the best of our knowledge,
pyren-1-yl methyl is the only fluorescent PRPG derived from the
pyrene skeleton. Hence a search of another set of pyrene derivatives
with improved photophysical and photochemical properties led us
to investigate, 1-acylpyrenes.
5-nitrophenol,9
3-nitro-2-naphthalenemethanol,10
a-carboxy
nitrobenzyl,11 p-hydroxyphenacyl,12 -keto amides,13 1-acyl-nitro-
a
indolines,14 anthracene-9-methanol,15 and derivatives of quino-
line16 as well as coumarin.17
Among the aforementioned PRPGs some groups are fluorescent
and have greater advantage over non-fluorescent protecting groups
since they not only release molecules of interest at desired location
for a specific period of time, but also allow us to visualize, quantify
and follow the spatial distribution, localization, and depletion of
the released molecules.18 The above strategy of using fluorescent
PRPGs has been successfully employed for temporal and spatially
controlled delivery of bioactive molecules in the study of numerous
processes in biological19 and medical research field.20 In particular,
labeling of amino acids by fluorescent PRPGs have offered great
benefits. Firstly, it allows detection of small aliphatic amino acids
1-Acylpyrenes are used as photoprobes29,30 since their fluo-
rescence properties are highly sensitive to medium polarity and
the hydrogen bonding of the microenvironment. For example, 1-
heptanoylpyrene29 exhibits very low fluorescence in non-polar
solvents like hexane or diethyl ether. But as we move to polar
solvents like methanol and acetonitrile the fluorescence efficiency
of 1-heptanoylpyrene increases and becomes almost identical to
* Corresponding author. Tel.: þ91 3222 282324; fax: þ91 3222 282252; e-mail
address: ndpradeep@chem.iitkgp.ernet.in (N.D.P. Singh).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.10.090

A. Jana et al. / Tetrahedron 66 (2010) 9798e9807
9799
1-heptylpyrene and also the fluorescence emission band shifts
from violet to blue. More interestingly, 1-heptanoylpyrene showed
further increase in the fluorescence efficiency and red shift of the
fluorescence emission band in binary aqueous mixtures of aceto-
nitrile and methanol. The above fluorescence behavior of 1-hep-
a band at around 1730 cmꢁ1 due to the stretching vibration of the
newly formed ester carbonyl group. In addition the spectra also
showed the carbonyl band of the 1-acetylpyrene protecting group
in the range of 1650e1700 cmꢁ1. The confirmation of the presence
of the newly formed ester group was further supported by 13C NMR
tanoylpyrene is due to their close lying (
p
e
p
*
) and (ne
p
*
) singlet
spectra, which showed the ester carbonyl at
d
171 ppm in addition
excited states. Similar fluorescence trends were also observed in
1-octanoylpyrene30 and pyrene-1-carbaldehyde.31 Considering
1-acylpyrenes’ high fluorescence efficiency, environment-sensi-
tive emission properties and absorption maximum greater than
350 nm prompted us to develop one of its derivatives to act both as
an environment-sensitive fluorophore and as a fluorescent PRPG.
In this paper we report a novel environment-sensitive fluo-
rophore namely 1-acetylpyrene (1) as a photocage for carboxylic
acids and amino acids. The synthesis and the characterization of 1-
acetylpyrenyl ester conjugates were discussed. The absorption and
emission properties of 1-acetylpyrene and its ester conjugates were
also measured. The photorelease ability of 1-acetylpyrene was
studied by irradiating the ester conjugates at different UV wave-
lengths (254, 350, and 410 nm). Further the effect of solvent on the
rate of photorelease was also investigated.
to carbonyl signal of the 1-acetylpyrene at
d 196 ppm.
2.2. Photophyiscal properties of ester conjugates (4aen)
The photophysical properties of all the ester conjugates and its
fluorophore 1-acetylpyrene were investigated. The UV/vis absorption
and emission spectra of degassed 2ꢂ10ꢁ6 M solution of esters (4aen)
and 1-acetylpyrene (1) in absolute ethanol (EtOH) were recorded. The
absorption and emission maxima, molar absorptivities, and fluores-
cencequantumyieldoftheaboveestersalongwith1-acetylpyrene(1)
are summarized in Table 1. Fluorescence quantum yields were cal-
culated using anthracene as standard (
F
¼0.27 in ethanol).32
Fig. 1a shows the normalized absorption and the emission
spectra of GABA conjugate 4h in ethanol. The absorption spectrum
of 4h shows an intense band centered at 354 nm with
3
20.7ꢂ103 molꢁ1 L cmꢁ1 while in the emission spectrum the emis-
sion maxima was red shifted to about 440 nm. We observed similar
absorption and emission maxima for all the other ester conjugates
(Table 1), which clearly suggest that 1-acetylpyrene moiety only
dictates the position of the absorption and emission maxima ruling
out the influence of its counterpart carboxylic acids. The Stokes’
shift has been calculated from the difference in the absorption and
the emission maxima and the magnitude of the Stokes’ shift of all
the conjugates varies between 86 and 98 nm. Further the conju-
gated esters also showed moderate fluorescence quantum yield
2. Results and discussion
2.1. Synthesis of ester conjugates (4aen)
A series of carboxylic acids including amino acids were pro-
tected by 1-(Bromoacetyl) pyrene (2) in the form of esters (4aen)
as outlined in Scheme 1. 1-(Bromoacetyl) pyrene (2) was readily
prepared from the commercially available 1-acetylpyrene (1) by
reaction with cupric bromide. Protection of carboxylic acids (3aeg)
was straightforward. Treatment of carboxylic acids with 1 equiv of 2
in the presence of K2CO3/KI in dry N,N-Dimethylformamide (DMF)
at 50 ꢀC for a period of 8e12 h afforded the corresponding esters in
good to excellent yield (Table 1, entries 4aeg). However, for pro-
tecting amino acids (3hen) we followed slightly different protocol.
Boc-protected amino acids were treated with 2 using 1,8-Dia-
zabicyclo[5.4.0]undec-7-ene (DBU) as catalyst at 0 ꢀC in 1,4-dioxan,
which provided high yield of protection (Table 1, entries 4hen).
All ester conjugates were characterized by IR, 1H, 13C NMR, and
mass spectral analysis. The IR spectra of conjugates 4aen showed
(0.028<F<0.056).
2.2.1. Fluorescence spectra of ester conjugate (4h) in neat sol-
vents. The GABA ester conjugate 4h (2ꢂ10ꢁ6 M) was excited at
356 nm and the emission spectra were recorded in various solvents.
Fig. 1b indicates the fluorescence of conjugate 4h is due to the
monomeric excited singlet state and displays strong solvent de-
pendence like other 1-acyl derivatives of pyrene.30,31 The fluores-
cence solvatochromism of the conjugate 4h is due to their closely
lying 1p
ep
*
and 1ne
*
p single states. In non-polar solvent (e.g.,
O
CH3
O
1
O
Boc
N
H
CuBr2
Boc
O
HO2C
N
EtOAc, CHCl3
r. t
3h
H
O
O
DBU, 1, 4-dioxan,
0 °C
O
R
Br
O
4h
(3a-g)
RCOOH
K2CO3, KI
DMF, 50 °C
Boc
R
O
HN
R
O
HO
Boc
N
(3i-n)
O
H
4a-g
2
O
R = (3a) CH2C6H5,
(3b) CH=CHC6H5,
(3c) C6H5,
DBU, 1, 4-dioxan,
0 °C
(3d) p-CH3C6H4,
(3e) p-CH3OC6H4,
(3f) o-BrC6H4,
R = (3i) H,
(3j) CH3,
4i-n
(3k) CH2Ph,
(3l) CH2OH,
(3g) p-ClC6H4.
(3m) CH(OH)CH3,
(3n) C4H6NO.
Scheme 1. Synthesis of 1-acetylpyrene-carboxylic acid ester conjugates.

9800
A. Jana et al. / Tetrahedron 66 (2010) 9798e9807
Table 1
Synthetic yields, UV/vis and fluorescence data for the ester conjugates (4aen) and its precursor 1-acetylpyrene (1) in absolute ethanol
Compound
Carboxylic acid
Synthetic yield (%)a
UV/vis
Fluorescence
c
f
lmax (nm)b
3
ꢂ103 (molꢁ1 L cmꢁ1
)
lmax (nm)d
Stokes’ shift (nm)e
VF
4a
4b
80
76
356
355
20.5
20.2
450
451
94
96
0.032
0.035
HO2C
HO2C
HO2C
Ph
Ph
4c
88
91
355
356
22.4
20.9
453
453
98
97
0.028
0.030
HO2C
4d
CH3
HO2C
4e
92
356
21.0
453
97
0.035
OCH3
HO2C
4f
70
83
356
356
20.4
20.9
451
449
95
93
0.044
0.033
Br
HO2C
4g
Cl
Boc
4h
4i
HO2C
N
H
80
76
82
354
355
356
20.7
20.4
20.4
440
450
451
86
95
95
0.030
0.037
0.035
Boc
HO2C
N
H
Boc
HN
HO2C
4j
CH3
Boc
HN
HO2C
4k
4l
75
79
82
356
357
356
20.5
21.2
21.1
452
450
451
96
93
95
0.034
0.041
0.056
Ph
Boc
HN
HO2C
OH
Boc
HN
4m
CH3
HO2C
HO2C
OH
H
N
4n
86
357
359
20.9
15.0
450
439
93
80
0.040
0.023
O
1
–
–
a
Based on isolated yield.
b
c
d
e
f
Maximum absorption wavelength.
Molar absorption coefficient (in molꢁ1 L cmꢁ1) maximum absorption wavelength.
Maximum emission wavelength.
Difference between maximum absorption wavelength and maximum emission wavelength.
Fluorescence quantum yield (error limit within ꢃ5%).
n-hexane) the fluorescence spectrum of conjugate 4h is structured
with peaks around 385e430 nm with weak emission. This partic-
solvent the emissive 1p
creased intensity of fluorescence.
Further, we also observed strong red shift of the fluorescence
emission band in hydroxylic polar solvents similar to pyrene-1-
carbaldehyde, which can be attributed due to geometric re-
arrangement of the molecule or of the solvation shell in the
ep becomes the LUMO and results in in-
*
ular fluorescence is from the non-fluorescent lowest 1ne
p
*
state.
However in the weakly or moderately interacting polar solvents the
energy of the 1p is brought below the 1ne
by solvent re-
laxation during the lifetime of the excited state.31c Thus in the polar
ep
*
p
*

A. Jana et al. / Tetrahedron 66 (2010) 9798e9807
9801
350
(a)
(a)
H
R
Absorption
Emission
H-bonding
O
H
O
300
O
250
200
150
100
50
O
ACN
10% H O
20% H2O
30% H2O
40% H2O
45% H2O
50% H2O
60% H22O
0
250
300
350
400
450
500
550
600
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
600
500
400
300
200
100
0
350
300
250
200
150
100
50
(b)
ACN
(b)
10% H O
20% H2O
30% H2O
40% H2O
45% H2O
50% H2O
60% H22O
Hexane
ACN
DCE
EtOH
MeOH
20
15
10
5
ACN
0
400 450 500 550 600
400
450
500
550
600
0
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 2. a) Corrected fluorescence spectra of the ester 4h in ACN and in increasing
percentage of water in ACN (2.0ꢂ10ꢁ6 M). (b) Corrected fluorescence spectra of 1-
acetylpyrene (1) in ACN and in increasing percentage of water in ACN (2.0ꢂ10ꢁ6 M).
Fig. 1. a) Normalized UV/vis absorption spectrum (black line) and emission spectrum
(red line) of ester 4h in EtOH (2.0ꢂ10ꢁ6 M). (b) Corrected fluorescence spectra of the
ester 4h in Hexane, ACN, Dichloroethane (DCE), EtOH, and MeOH (2.0ꢂ10ꢁ6 M).
fluorescent 1p excited state.29 Moreover the red shift is also
ep
*
4h and its hydrogen bonded complex. Similar phenomenon was
also manifested in the emission spectra (Fig. 2b) of 1-acetylpyrene
(1) in acetonitrile/water mixtures.
The photophysical studies showed that 1-acetylpyrene (1) and
its ester conjugates exhibited strong fluorescence, large Stokes’
shift, moderate fluorescence quantum yield, and moreover their
fluorescence properties were sensitive to its surrounding environ-
ment, thus makes 1-acetylpyrene to be an environment-sensitive
fluorophore.
accompanied by a loss of fine structure and an increase of band
width, which indicates the occurrence of an intramolecular
charge transfer (amine group is an electron donor and the car-
bonyl moiety of 1-acetylpyrene is electron deficient) allowing the
involvement of an excited state with intramolecular charge
transfer (ICT) character.33
2.2.2. Fluorescence spectra of ester conjugate (4h) in aqueous binary
mixtures. In order to understand the effect of hydrogen bonding
interaction on the fluorescence spectra, we measured the emission
spectra of 4h in acetonitrile/water mixtures. In similar to 1-hep-
tanoylpyrene, we also noted that upon addition of increasing
amount of water the fluorescence spectra of GABA conjugate (4h) is
red shifted with a concomitant increase in fluorescence intensity
(Fig. 2a). This can be attributed to the fact that hydrogen bonding
2.3. Photolysis of ester conjugates (4aen)
Considering our main interest, we investigated the application
of 1-acetylpyrene as a fluorescent PRPG for various carboxylic
acids including amino acids. Photolysis of conjugates (4aen) in
ACN/4-(2-hydroxyethyl)piperazine-1-ethanesulfonicacid (HEPES)
buffer solution (50:50) using 125 W medium pressure Hg lamp at
different irradiation wavelengths (254, 350, and 410 nm) were
studied in order to determine the best photocleavage condition.
The course of the photocleavage reaction was monitored by 1H
NMR spectroscopy. A known amount of photolysate was taken at
regular intervals of time, then solvent was evaporated under
raises the energy of the triplet 3ne
cinity of lowest singlet 1p
excited states) above the lowest sin-
p
*
(which was found in the vi-
ep
*
glet 1p
ep excited states thereby reducing the efficiency of the
*
intersystem crossing.29
Further we also observed isoemmisive point at
which indicates existence of an equilibrium between the conjugate
l
¼427 nm,

9802
A. Jana et al. / Tetrahedron 66 (2010) 9798e9807
vacuum and redissolved in CDCl3 with 1,2-dichloroethane as the
internal standard and the 1H NMR was recorded. The photo-
cleavage of the conjugate esters were then quantified by com-
life was calculated for all the conjugates and the results are tabu-
lated in Table 2.
The course of photolysis of conjugated esters was also moni-
tored by fluorescence spectroscopy (As a representative example
fluorescence spectra of 4h at regular interval of time of irradiation
is presented in Supplementary data Fig. S3).
paring the integration of the a-CH2 signal to that of the internal
standard. The reaction was followed until the consumption of the
starting material is less than 5% of the initial area. In all cases 1H
NMR spectra showed clean photocleavage of the ester conjugates
to produce their corresponding carboxylic acids (Scheme 2). In
several examples as indicated in Table 2 the photoproducts (car-
boxylic acids and 1-(hydroxyl acetyl) pyrene) were confirmed by
isolating and comparing their spectra with corresponding au-
thentic samples. The quantum yield was calculated using potas-
sium ferrioxalate as an actinometer.34
2.4. Effect of the conjugate structure on photorelease
Like pyrene-1-ylmethyl,20 we also observed similar influence of
the structure of amino acids on the rate of photocleavage. Conju-
gates of amino acids with polar side chain (4l and 4m) and amino
acids with longer chain length (4h) had lower quantum yield for
photocleavage (Table 2). Further, in the case of aromatic carboxylic
acids we found that electron donating substituent like p-methyl
(4d) and p-methoxy (4e) were efficiently released in higher quan-
tum yield compared to halogen substituted carboxylic acids (4feg).
O
OCOR
CH2
Homolytic
O
R
cleavage
O
2.5. Effect of irradiation wavelength on photorelease
O
The influence of the irradiation wavelength on the extent of
photocleavage of conjugates (4aen) in acetonitrile/HEPES buffer
(50:50) is shown in Table 3. The time required for photocleavage of
conjugates increases as the irradiation wavelength increases. Since
CeO bond disassociation energy is 86 kcal/mol and as we increase
the irradiation wavelength from 254 nm to 410 nm, the excitation
energy decreases from 112 kcal/mol to 70 kcal/mol. Hence, GABA
conjugate 4h was fully depleted at 254 nm after 26 min of irradi-
ation, while it required 65 min at 350 nm and 304 min at 410 nm
(Table 3, entry 4h). The incident photon flux (I0) at 254, 350, and
h
ν
e- transfer
O
CH2
OCOR
Heterolytic
cleavage
R = alkyl, aryl or
amino acid residue
MeOH/H2O
ACN/H2O
410 nm is 1.8ꢂ1017, 1.55ꢂ1017, and 2.13ꢂ1016 photons sꢁ1cmꢁ2
,
O
O
OMe
OH
R
respectively.
O
O
+
+
6
+
H
H
2.6. Solvent effect on photorelease
R
O
O
To understand the role of the solvent on the rate of photo-
cleavage, we carried out the photolysis of GABA conjugate 4h in
aqueous mixtures of methanol, acetonitrile, and THF and the results
are included in Table 4. The photocleavage of 4h in methanol/H2O
5
6
Scheme 2. Possible photocleavage pathway for the photolysis of 1-acetylpyrene-car-
boxylic acid ester conjugates.
Table 2
Photolytic data of ester conjugates (4aen) at different irradiation wavelengths in acetonitrile/HEPES (50:50) solution
Ester
254 nm
350 nm
410 nm
.103 (molꢁ1 L cmꢁ1
a
a
a
3
ꢂ103 (molꢁ1 L cmꢁ1
)
t1/2 (min)b
Vc
3
ꢂ103 (molꢁ1 L cmꢁ1
)
t1/2 (min)b
Vc
3
)
t1/2 (min)b
Vc
4ad
4b
19.2
18.6
17.6
17.8
18.4
17.5
17.2
18.0
17.8
17.4
17.8
18.4
17.6
19.1
2.7
3.5
4.1
3.8
3.0
4.3
4.6
6.1
3.8
5.3
5.6
6.4
6.4
5.5
0.62
0.51
0.41
0.45
0.56
0.40
0.37
0.28
0.45
0.32
0.30
0.27
0.27
0.31
22.8
22.4
21.5
22.3
22.8
21.4
18.7
20.7
22.5
21.1
18.0
18.4
17.2
17.7
6.9
9.7
11.6
7.4
5.8
9.3
10.4
15.0
9.3
0.42
0.30
0.25
0.40
0.51
0.32
0.28
0.19
0.32
0.29
0.23
0.23
0.24
0.26
3.9
3.7
3.3
3.6
3.8
3.0
2.8
3.2
3.6
2.8
2.9
3.9
3.3
3.1
55.5
58.8
64.1
57.6
55.4
75.7
78.7
70.4
60.8
61.3
61.1
74.0
72.9
72.4
0.39
0.38
0.35
0.39
0.41
0.29
0.28
0.32
0.36
0.36
0.36
0.30
0.30
0.31
4cd
4dd
4ed
4f
4gd
4hd
4i
4jd
4kd
4l
9.9
12.9
12.7
12.0
11.1
4m
4nd
a
Molar absorption coefficient (in molꢁ1 L cmꢁ1) at the irradiation wavelength.
Half-life under photolytic conditions (in minutes).
b
c
Photochemical quantum yield (error limit within ꢃ5%).
d
Esters conjugates for which the photoproducts were isolated and compared with authentic samples.
Based on 1H NMR data for each compound, the natural loga-
(50:50) found to be more efficient compared to similar system of
acetonitrile (though the cleavage in methanol is efficient the sol-
ubility of the conjugates is not as good as compared to acetonitrile).
This can be attributed not only due to the formation of ionic in-
termediates (Scheme 2) but also depends on the hydrogen bonding
rithm of the concentration of ester conjugate (lnC) vs irradiation
time was plotted (see Fig. S2 in Supplementary data) and we ob-
served a linear correlation for the disappearance of the starting
material, which suggested a first order reaction kinetics. The half-

A. Jana et al. / Tetrahedron 66 (2010) 9798e9807
9803
CeO bond followed by an electron transfer15 to produce an ion-pair
of 1-pyrene-1-yl-ethanoyl carbocation and carboxylate anion or
concurrently, the heterolytic cleavage20 of the CeO bond could
directly afford the already mentioned ion-pair. After ion-pair sep-
aration in polar solvent, the newly formed carbocation undergoes
nucleophilic attack by the solvent molecule to yield 1-(hydrox-
yacetyl)pyrene (6) (while in aqueous methanolic solvent we ob-
served both 1-hydroxy and 1-(methoxyacetyl)pyrene (5)).
Table 3
Irradiation time required for photolysis of ester conjugates (4aen) at different
wavelengths in acetonitrile/HEPES (50:50) solution
Compound
Time required for photocleavage of (4aen)
254 nm (min)
350 nm (min)
410 nm (min)
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
12
15
18
16
13
19
20
26
17
24
24
28
28
24
30
42
50
32
25
40
45
65
40
43
56
55
52
48
240
254
277
250
240
327
340
304
263
265
264
320
315
313
3. Conclusion
Carboxylic acids including amino acids were protected by 1-
acetylpyrene to give their corresponding ester conjugates in good
to excellent yields using simple procedure. Photophysical studies
revealed that 1-acetylpyrene and its ester conjugates showed good
fluorescence properties and more interestingly their fluorescence
properties were found to be highly sensitive to its surrounding
environment. Further we also demonstrated the ability of the 1-
acetylpyrene to release carboxylic acids and amino acids in aqueous
organic solvents using visible light. In conclusion, we have showed
that 1-acetylpyrene can act as an environment-sensitive fluo-
rophore as well as a fluorescent photoremovable protecting group.
and polarity of the solvent since they have influence on the excited
states.
Further to mimic the biological condition, we also carried out
the photolysis of 4h in acetonitrile/HEPES mixtures. We noticed the
efficiency of the photoreaction of conjugate 4h increases continu-
ously with increasing amount of water (For example, efficiency of
photocleavage of conjugate 4h increased by a factor of two times
(350 nm) as we move from 10% to 50% of HEPES in acetonitrile),
which is in contrast to its fluorescence behavior, which increases at
the beginning and slows down after reaching a maximum (Fig. 2a).
The interpretation of the increase of quantum yield of photo-
chemistry is due to the better stabilization of the newly generated
ion-pair (Scheme 2) by solvation, which leads to the acceleration of
the rate of photolysis thereby reducing the fluorescence efficiency,
the similar phenomenon is also observed in caged coumarins.18a
The stability of the ester conjugates was also tested by keeping
them in the dark in an aqueous solvent for a period of 15 days. The
rate constant for the hydrolysis of the ester conjugates was calcu-
lated using 1H NMR, and were found to be in the range of 2.28ꢂ10ꢁ3
dayꢁ1 to 9.28ꢂ10ꢁ3 dayꢁ1 (see Table S1 in the Supplementary data
for the hydrolysis constant of all the ester conjugates).
4. Experimental section
4.1. General
1H NMR (200 MHz) spectra were recorded on a BRUKER-AC
200 MHz spectrometer. Chemical shifts are reported in parts per
million from tetramethylsilane with the solvent resonance as the
internal standard (deuterochloroform: 7.26 ppm). Data are repor-
ted as follows: chemical shifts, multiplicity (s¼singlet, d¼doublet,
t¼triplet, m¼multiplet), coupling constant (Hz). 13C NMR (50 MHz)
spectra were recorded on a BRUKER-AC 200 MHz Spectrometer
with complete proton decoupling. Chemical shifts are reported in
parts per million from tetramethylsilane with the solvent reso-
nance as the internal standard (deuterochloroform: 77.0 ppm). UV/
vis absorption spectra were recorded on a Shimadzu UV-2450 UV/
vis spectrophotometer, fluorescence emission spectra were recor-
ded on a Hitachi F-7000 fluorescence spectrophotometer, FTIR
spectra were recorded on a PerkineElmer RXI spectrometer and
HRMS spectra were recorded on a JEOL-AccuTOF JMS-T100L mass
spectrometer. Photolysis of all the ester conjugates were carried out
using 125 W medium pressure mercury lamp supplied by SAIC
(India). Chromatographic purification was done with 60e120 mesh
silica gel (Merck). For reaction monitoring, precoated silica gel 60
F254 TLC sheets (Merck) were used.
2.7. Mechanism of the photorelease
The solvent study indicates that the photocleavage of the ester
conjugates proceeds through an ionic mechanism. The initial
photochemical step involves excitation of the 1-acetylpyrene
chromophore to its singlet excited state, which then undergoes
rapid singletetriplet intersystem crossing. Subsequently, from the
triplet excited state it then undergoes homolytic cleavage of the
Table 4
Photolytic data of ester conjugate 4h at different irradiation wavelengths in different solvent systems
Solvent system
350 nm
410 nm
a
c
a
c
3
ꢂ103 (molꢁ1 L cmꢁ1
)
t1/2 (min)b
Vphot
3
ꢂ103 (molꢁ1 L cmꢁ1
)
t1/2 (min)b
Vphot
MeOH
MeOH/H2O (50:50)
ACN
ACN/H2O (50:50)
THF
THF/H2O (50:50)
ACN/HEPES (90:10)
ACN/HEPES (70:30)
ACN/HEPES (50:50)
20.7
20.7
20.7
20.7
20.6
20.7
20.6
20.7
20.7
32.2
11.4
40.1
15.0
55.0
21.1
30.7
20.4
15.0
0.09
0.26
0.07
0.19
0.05
0.14
0.10
0.14
0.19
3.9
3.9
3.9
3.9
3.9
3.7
3.6
3.4
3.2
263.4
40.9
341.0
70.4
542.2
80.1
86.0
79.4
70.4
0.08
0.54
0.07
0.32
0.04
0.28
0.26
0.28
0.30
a
Molar absorption coefficient (in molꢁ1 L cmꢁ1) at the irradiation wavelength.
Half-life under photolytic conditions (in minutes).
Photochemical quantum yield (error limit within ꢃ5%).
b
c

9804
A. Jana et al. / Tetrahedron 66 (2010) 9798e9807
4.2. General procedure for the synthesis of the ester
conjugates 4aeg
(0.039 g, 0.29 mmol), potassium carbonate (0.048 g, 0.34 mmol),
and potassium iodide (0.057 g, 0.34 mmol) were used. The re-
action mixture was stirred for 8 h. A brown residue was obtained,
which on purification using 20% EtOAc in pet. ether affords com-
pound 4d (0.100 g, 91%) as a yellow solid, mp: 165 ꢀC; TLC Rf 0.48
1-(Bromoacetyl)pyrene (1 equiv) was dissolved in dry N,N-
Dimethylformamide (DMF) (2 mL), potassium iodide (1.2 equiv),
potassium carbonate (1.2 equiv), and the corresponding carboxylic
acid 3aeg (1 equiv) were added. The reaction mixture was stirred at
50 ꢀC for 8e12 h. The solvent was removed by rotary evaporation
under reduced pressure and the crude residue was purified by
column chromatography using ethylacetate (EtOAC) in pet. ether.
(20% EtOAc in pet. ether); 1H NMR (CDCl3, 200 MHz):
d
¼9.02 (d,
J¼9.4 Hz, 1H), 8.36 (d, J¼8.2 Hz, 1H), 8.26e8.14 (m, 5H), 8.08e8.01
(m, 4H), 7.27 (d, J¼8.0 Hz, 2H), 5.69 (s, 2H), 2.42 (s, 3H); 13C NMR
(CDCl3, 50 MHz):
d
¼196.3, 166.3, 144.1, 134.3, 131.0, 130.5, 130.0
(2C), 129.9, 129.2 (2C), 128.6, 127.0, 126.7, 126.5 (3C), 126.3, 125.8,
125.0, 124.6, 124.1 (2C), 123.9, 68.1, 21.7; FTIR (KBr) nmax (cmꢁ1):
4.2.1. 2-Oxo-2-(pyren-3-yl)ethyl 2-phenylacetate (4a). 1-(Bromoa-
cetyl)pyrene (0.100 g, 0.29 mmol), potassium iodide (0.057 g,
0.34 mmol), potassium carbonate (0.048 g, 0.34 mmol), and phenyl
acetic acid (0.056 g, 0.29 mmol) were used. The reaction mixture
was stirred for 10 h. The crude residue was purified by column
chromatography with 20% EtOAc in pet. ether to give the title
compound (4a) (0.088 g, 80%) as a yellow solid, mp: 135 ꢀC; TLC Rf
1719 (OCO), 1637 (CO); UV/vis (EtOH): lmax
(
3
) 356 (20.9ꢂ103), 400
(6.2ꢂ103); LCMS: m/z (%): 379.4 (100) [MþH]þ, 177.4 (15), 119.2
(17); HRMS (ESþ) m/z calcd for C26H19O3 [MþH]þ: 379.1334; found
379.1334.
4.2.5. 2-Oxo-2-(pyren-3-yl)ethyl 4-methoxybenzoate (4e). 1-(Bro-
moacetyl)pyrene (0.100 g, 0.29 mmol), potassium iodide
(0.057 g, 0.34 mmol), potassium carbonate (0.048 g, 0.34 mmol),
and 4-methoxy benzoic acid (0.044 g, 0.29 mmol) were used. The
reaction mixture was stirred for 8 h. A reddish-brown residue
was obtained, which on purification using 25% EtOAc in pet. ether
gave compound 4e (0.105 g, 92%) as a yellow solid, mp: 137 ꢀC;
TLC Rf 0.45 (20% EtOAc in pet. ether); 1H NMR (CDCl3, 200 MHz):
0.43 (20% EtOAc in pet. ether); 1H NMR (CDCl3, 200 MHz):
(d, J¼9.6 Hz, 1H), 8.28 (m, 8H), 7.25 (s, 5H), 5.49 (s, 2H), 3.85 (s, 2H);
13C NMR (CDCl3, 50 MHz):
d
¼9.04
d
¼196.5, 171.5, 134.5, 133.8, 131.2, 130.7,
130.3, 130.2, 129.6 (2C), 128.8 (2C), 127.4, 127.2, 126.7 (2C), 126.5
(2C), 125.9, 125.1, 124.7, 124.3, 124.1 (2C), 68.4, 41.1; FTIR (KBr) nmax
(cmꢁ1): 1737 (OCO), 1636 (CO); UV/vis (EtOH): lmax
(3) 356
(20.5ꢂ103), 400 (6.3ꢂ103); LCMS: m/z (%): 401.1(100) [MþNa]þ,
379.1(5), 301.1(10); HRMS (ESþ) m/z calcd for C26H19O3 [MþH]þ:
379.1334; found: 379.1328.
d
¼9.02 (d, J¼9.4 Hz, 1H), 8.37 (d, J¼8.2 Hz, 1H), 8.27e7.90 (m, 9H),
6.95 (d, J¼8.8 Hz, 2H), 5.68 (s, 2H), 3.87 (s, 3H); 13C NMR (CDCl3,
50 MHz):
d
¼196.5, 165.9, 163.7, 134.3, 132.1 (2C), 131.0, 130.5,
130.0, 129.9, 128.6, 127.0, 126.5 (2C), 126.3, 125.8, 125.0, 124.6,
4.2.2. 2-Oxo-2-(pyren-3-yl)ethyl cinnamate (4b). 1-(Bromoacetyl)
pyrene (0.100 g, 0.29 mmol), potassium iodide (0.057 g,
0.34 mmol), potassium carbonate (0.048 g, 0.34 mmol), and cin-
namic acid (0.043 g, 0.29 mmol) were used. The reaction mixture
was stirred for 8 h. Purification of the crude residue by column
chromatography, using EtOAc/pet. ether mixtures of increasing
polarity as the eluent gave compound 4b as a yellow solid (0.086 g,
76%), mp: 161 ꢀC; TLC Rf 0.45 (20% EtOAc in pet. ether); 1H NMR
124.1, 123.9 (2C), 121.8, 113.8 (2C), 68.0, 55.4; FTIR (KBr) nmax
(cmꢁ1): 1717 (OCO), 1638 (CO); UV/vis (EtOH): lmax
(3) 356
(21.0ꢂ103), 400 (6.3ꢂ103); LCMS: m/z (%): 395.1 (100) [MþH]þ,
193.1 (12), 135.0(18); HRMS (ESþ) m/z calcd for C26H19O4
[MþH]þ: 395.1283; found 395.1274.
4.2.6. 2-Oxo-2-(pyren-3-yl)ethyl 2-bromobenzoate (4f). 1-(Bro-
moacetyl)pyrene (0.100 g, 0.29 mmol), potassium iodide (0.057 g,
0.34 mmol), potassium carbonate (0.048 g, 0.34 mmol), and 2-
bromo benzoic acid (0.058 g, 0.29 mmol) were used. The reaction
mixture was stirred for 12 h. Purification of the crude residue by
column chromatography, using EtOAc/pet. ether mixtures of in-
creasing polarity as the eluent gave compound 4f (0.089 g, 70%) as
a yellow solid, mp: 128 ꢀC; TLC Rf 0.50 (20% EtOAc in pet. ether); 1H
(CDCl3, 200 MHz):
d
¼9.02 (d, J¼9.4 Hz, 1H), 8.31 (d, J¼8.2 Hz, 1H),
8.26e8.00 (m, 7H), 7.86 (d, J¼16.0 Hz, 1H), 7.56 (s, 2H), 7.34 (s, 3H),
6.65 (d, J¼16.0 Hz, 1H), 5.62 (s, 2H); 13C NMR (CDCl3, 50 MHz):
d
¼196.3, 166.5, 146.3, 134.4, 134.3, 130.9, 130.6, 130.2, 130.0, 128.9
(2C), 128.5 (2C), 128.3 (2C), 127.0, 126.6 (2C), 126.4, 125.8, 125.0,
124.6, 123.9 (3C), 117.0, 76.9; FTIR (KBr) nmax (cmꢁ1): 1720 (OCO),
1684 (CO); UV/vis (EtOH): lmax
(3
) 355 (20.2ꢂ103), 400 (6.2ꢂ103);
NMR (CDCl3, 200 MHz):
1H), 8.29e8.04 (m, 8H), 7.71 (t, J¼7.2 Hz, 1H), 7.47e7.29 (m, 2H),
5.76 (s, 2H); 13C NMR (CDCl3, 50 MHz):
d¼9.03 (d, J¼9.4 Hz, 1H), 8.37 (d, J¼8.2 Hz,
LCMS: m/z (%): 391.2 (100) [MþH]þ, 377.2 (75), 229.1(20); HRMS
(ESþ) m/z calcd for C27H19O3 [MþH]þ: 391.1334; found 391.1350.
d
¼195.6, 165.6, 134.4 (2C),
132.95, 132.0, 131.4, 131.0, 130.5, 130.2, 130.1, 129.6, 128.3, 127.2,
127.0, 126.5, 126.4, 125.8, 125.0, 124.5, 124.1, 123.9 (2C), 122.1, 68.4;
FTIR (KBr) nmax (cmꢁ1): 1734 (OCO), 1677 (CO); UV/vis (EtOH): lmax
4.2.3. 2-Oxo-2-(pyren-3-yl)ethyl benzoate (4c). 1-(Bromoacetyl)
pyrene(0.100 g, 0.29 mmol), benzoic acid (0.035 g, 0.29 mmol),
potassium carbonate (0.048 g, 0.34 mmol), and potassium iodide
(0.057 g, 0.34 mmol) were used and the reaction mixture was
stirred for 10 h. A reddish precipitate was obtained, which on pu-
rification by column chromatography, using EtOAc/pet. ether mix-
tures of increasing polarity as the eluent gave compound 4c
(0.093 g, 88%) as a brown solid, mp: 130 ꢀC; TLC Rf 0.48 (20% EtOAc
(
3
) 360 (20.4ꢂ103), 400 (6.7ꢂ103); LCMS: m/z (%): 445.2 (98), 443.1
(100) [MþH]þ, 229.1 (36), 242.8 (8), 240.8 (5); HRMS (ESþ) m/z
calcd for C25H16BrO3 [MþH]þ: 443.0283; found 443.0291.
4.2.7. 2-Oxo-2-(pyren-3-yl)ethyl 4-chlorobenzoate (4g). 1-(Bro-
moacetyl)pyrene(0.100 g, 0.29 mmol), 4-chloro benzoic acid
(0.039 g, 0.29 mmol), potassium carbonate (0.048 g, 0.34 mmol),
and potassium iodide (0.057 g, 0.34 mmol) were used. The reaction
mixture was stirred for 10 h. A brown residue was obtained, which
on purification using 20% EtOAc in pet. ether affords compound 4g
(0.096 g, 83%) as a yellow solid, mp: 125 ꢀC; TLC Rf 0.55 (20% EtOAc
in pet. ether); 1H NMR (CDCl3, 200 MHz):
8.33 (d, J¼8.2 Hz, 1H), 8.22e7.99 (m, 9H), 7.56 (t, J¼7.2 Hz, 1H),
7.56e7.43 (m, 2H), 5.71 (s, 2H); 13C NMR (CDCl3, 50 MHz):
d¼9.02 (d, J¼9.4 Hz, 1H),
d¼196.0,
166.2, 134.4, 133.3, 130.9, 130.5, 130.1, 130.0 (3C), 129.5, 128.5 (3C),
127.0, 126.5 (2C), 126.3, 125.8, 124.9, 124.5, 124.0, 123.9 (2C), 68.2;
FTIR (KBr) nmax (cmꢁ1): 1721 (OCO), 1640 (CO); UV/vis (EtOH): lmax
in pet. ether); 1H NMR (CDCl3, 200 MHz):
8.36 (d, J¼8.2 Hz,1H), 8.29e7.96 (m, 9H), 7.38 (d, J¼6.2 Hz, 2H), 5.74
(s, 2H); 13C NMR (CDCl3, 50 MHz):
d
¼9.05 (d, J¼9.4 Hz, 1H),
(
3
) 355 (22.4ꢂ103), 400 (7.2ꢂ103); LCMS: m/z (%): 365.4 (100)
[MþH]þ, 229.2 (8), 163.2 (14); HRMS (ESþ) m/z calcd for C25H17O3:
365.1178 [MþH]þ; found: 365.1190.
d
¼195.7,165.4,139.8,134.4,131.4
(2C), 130.9, 130.4, 130.2, 130.0, 129.2, 128.8 (2C), 128.2, 127.9, 126.9,
126.6, 126.5, 126.4, 125.8, 125.0, 124.5, 124.0, 123.9, 68.3; FTIR (KBr)
4.2.4. 2-Oxo-2-(pyren-3-yl)ethyl 4-methylbenzoate (4d). 1-(Bro-
moacetyl)pyrene(0.100 g, 0.29 mmol), 4-methyl benzoic acid
nmax (cmꢁ1): 1723 (OCO), 1701 (CO); UV/vis (EtOH): lmax
(3) 356
(20.9ꢂ103), 400 (6.7ꢂ103); LCMS: m/z (%): 401.6 (44), 399.4 (100)

A. Jana et al. / Tetrahedron 66 (2010) 9798e9807
9805
[MþH]þ, 229.4 (43), 197.2 (14), 141.0 (9); HRMS (ESþ) m/z calcd for
C25H16ClO3 [MþH]þ: 399.0788; found 399.0787.
(6.3ꢂ103); LCMS: m/z (%): 432.2 (55) [MþH]þ, 376.4 (91), 332.2
(100), 261.2 (35); HRMS (ESþ) m/z calcd for C26H25NO5 [M]þ:
431.1733; found: 431.1741.
4.3. General procedure for the synthesis of compound 4hen
4.3.4. tert-Butyl(S)-1-((2-oxo-2-(pyren-3-yl)ethoxy)carbonyl)-
1-(Bromoacetyl)pyrene (1 equiv) was dissolved in dry dioxan
and the corresponding Boc-protected amino acid 3hen (1 equiv)
was added followed by 1.2 equiv of 1,8-Diazabicyclo[5.4.0]undec-
7-ene (DBU) at 0 ꢀC. The reaction mixture was stirred at room
temperature for 8e12 h. The solvent was removed by rotary
evaporation under reduced pressure and the crude residue was
purified by column chromatography with EtOAc in pet. ether as an
eluent.
2-phenylethyl carbamate (4k). 1-(Bromoacetyl)pyrene (0.100 g.
0.29 mmol), N-(tert-butoxycarbonyl)-L-phenylalanine (0.147 g,
0.29 mmol), and DBU (0.053 g, 0.35 mmol) were used. The re-
action mixture was stirred for 12 h. The crude reaction mixture
was purified by column chromatography using 25% EtOAc in pet.
ether to give ester conjugate 4k (0.110 mg, 75%) as a deep yellow
solid, mp: 139 ꢀC; TLC Rf 0. 54 (25% EtOAc in pet. ether); 1H NMR
(CDCl3, 400 MHz):
d
¼8.97 (d, J¼9.4 Hz, 1H), 8.21e8.14 (m, 4H),
8.09e7.95 (m, 4H), 7.306 (s, 5H), 5.63 (d, J¼16.4 Hz, 1H), 5.43 (d,
J¼16.4 Hz, 1H), 5.09 (d, J¼8.2 Hz, 1H), 4.84 (m, 1H), 3.42 (dd,
J¼14.0 Hz, 5.2 Hz, 1H), 3.19 (dd, J¼14.0 Hz, 7.2 Hz, 1H), 1.43 (s,
4.3.1. tert-Butyl 3-((2-oxo-2-(pyren-3-yl)ethoxy)carbonyl)propylcar-
bamate (4h). 1-(Bromoacetyl) pyrene (0.100 g, 0.29 mmol), 4-(tert-
butoxycarbonylamino)butyric acid (0.059 g, 0.29 mmol), and DBU
(0.053 g, 0.35 mmol) were used. The reaction mixture was stirred
for 10 h. A deep brown semi-solid residue obtained, which on pu-
rification by column chromatography using 30% EtOAc in pet. ether
gave the ester conjugate 4h (0.103 g, 80%) as a pale yellow solid,
mp: 148 ꢀC; TLC Rf 0.31 (25% EtOAc in pet. ether); 1H NMR (CDCl3,
9H); 13C NMR (CDCl3, 50 MHz):
d
¼195.5, 171.9, 155.4, 136.3, 134.6,
131.1, 130.6, 130.4, 130.2, 129.7 (2C), 128.7 (2C), 128.1, 127.1, 127.0,
126.8, 126.7, 126.6, 125.9, 125.0, 124.5, 124.2, 124.0 (2C), 80.2,
68.3, 54.6, 38.4, 28.5 (3C); FTIR (KBr) nmax (cmꢁ1): 3371 (NH),
1756 (OCO), 1684 (CO), 1595 (CONH); UV/vis (EtOH): lmax (3) 356
(20.5ꢂ103), 400 (6.5ꢂ103); LCMS: m/z (%): 530.1 (100) [MþNa]þ,
474.1 (12), 302.0 (16), 292.0 (8), 260.9 (6), 130.1 (12), 102.0 (11);
HRMS (ESþ) m/z calcd for C32H29NO5 [M]þ: 507.2046; found:
507.2050.
200 MHz):
4.81 (s, NH), 3.24 (q, J¼6.4 Hz, 2H), 2.59 (t, J¼7.4 Hz, 2H), 1.93 (m,
2H), 1.45 (s, 9H); 13C NMR (CDCl3, 50 MHz):
d
¼8.96 (d, J¼9.4 Hz, 1H), 8.29e8.02 (m, 8H), 5.49 (s, 2H),
d
¼196.3, 173.0, 155.9,
134.5, 131.1, 130.6, 130.3, 130.2 (2C), 128.5, 127.1, 126.7, 126.5, 125.9,
125.0, 124.6, 124.2 (2C), 124.1, 79.9, 67.9, 40.0, 31.3, 28.6 (3C), 25.4;
FTIR (KBr) nmax (cmꢁ1): 3365 (NH), 1737 (OCO), 1684 (CO), 1653
4.3.5. tert-Butyl(S)-1-((2-oxo-2-(pyren-3-yl)ethoxy)carbonyl)-2-hy-
droxyethyl carbamate (4l). 1-(Bromoacetyl)pyrene (0.100 g. 0.29
mmol), N-(tert-butoxycarbonyl)-L-serine (0.059 g, 0.29 mmol), and
(CONH); UV/vis (EtOH): lmax
(
3
) 354 (20.7ꢂ103), 400 (7.0ꢂ103);
LCMS: m/z (%): 446.2 (56) [MþH]þ, 390.4 (18), 346.4 (100), 261.2
(53); HRMS (ESþ) m/z calcd for C27H27NO5 [M]þ: 445.1889; found:
445.1908.
DBU (0.053 g, 0.35 mmol) were used. The reaction mixture was
stirred for 10 h. A dark brown liquid obtained, which on purification
by column chromatography using 50% EtOAc in pet. ether gave ester
conjugate 4l (0.102 mg, 79%) as a brown viscous liquid, which on
keeping in refrigerator for 24 h gives a brown solid, mp: 99 ꢀC; TLC
4.3.2. tert-Butyl ((2-oxo-2-(pyren-3-yl)ethoxy)carbonyl)methylcar-
bamate (4i). 1-(Bromoacetyl) pyrene (0.100 g. 0.29 mmol), N-(tert-
butoxycarbonyl)glycine (0.051 g, 0.29 mmol), and DBU (0.053 g,
0.35 mmol) were used. The reaction mixture was stirred for 10 h.
Crude reaction mixture was purified by column chromatography
using 35% EtOAc in pet. ether to give compound 4i (0.092 mg, 76%)
as a light yellow solid, mp: 141 ꢀC; TLC Rf 0.32 (25% EtOAc in pet.
Rf 0.57 (50% EtOAc in pet. ether); 1H NMR (CDCl3, 200 MHz):
d¼8.97
(d, J¼9.4 Hz, 1H), 8.22e7.85 (m, 8H), 5.89 (d, H¼16.8 Hz, 1H), 5.82
(d, J¼9.4 Hz, 1H), 5.36(d, J¼12.0 Hz, 1H), 4.48 (d, J¼10.0 Hz, NH),
4.16 (d, J¼7.2 Hz, 1H), 4.04 (dd, J¼11.8 Hz, 3.2 Hz, 1H), 3.65 (s, OH),
1.55 (s, 9H); 13C NMR (CDCl3, 50 MHz): 196.0, 171.1, 155.9, 135.0,
130.9, 130.7, 130.6, 130.4, 130.3, 127.0, 126.9, 126.8, 126.4, 126.2,
124.8, 124.4, 123.9 (2C), 123.8, 80.4, 68.3, 64.5, 56.5, 28.6 (3C); FTIR
(KBr) nmax (cmꢁ1): 3443 (OH), 2923 (NH), 1752 (OCO), 1690 (CO),
ether); 1H NMR (CDCl3, 200 MHz):
8.19e7.89 (m, 8H), 5.51 (s, 2H), 5.214 (s, NH), 4.21 (d, J¼5.4 Hz, 2H),
1.51 (s, 9H); 13C NMR (CDCl3, 50 MHz):
d
¼8.92 (d, J¼9.4 Hz, 1H),
d
¼195.2, 170.3, 155.8, 134.5,
1622 (CONH); UV/vis (EtOH): lmax
(
3
)
357 (21.2ꢂ103), 400
130.9, 130.4, 130.2, 130.1 (2C), 127.7, 126.9, 126.7, 126.6, 126.4, 125.7,
(8.4ꢂ103); LCMS: m/z (%): 470.1 (100) [MþNa]þ, 414.1 (28), 403.1
(21), 370.1 (14), 302.2 (24), 288.2 (31), 284.2 (26), 215.2 (25); HRMS
(ESþ) m/z calcd for C25H23NO5 [M]þ: 417.1576; found: 417.1576;
HRMS (ESþ) m/z calcd for C26H25NO6 [M]þ: 447.1682; found:
447.1676.
124.9, 124.4, 123.9 (2C), 80.2, 68.2, 42.4, 18.4 (3C); FTIR (KBr) nmax
(cmꢁ1): 3364 (NH), 1733 (OCO), 1695 (CO); UV/vis (EtOH): lmax
(3)
355 (20.4ꢂ103), 400 (6.7ꢂ103); LCMS: m/z (%): 440.1 (100)
[MþNa]þ, 385.0 (6), 384.1 (25), 362.1 (12), 318.0 (11), 261.1 (11),
130.0 (5); HRMS (ESþ) m/z calcd for C25H23NO5 [M]þ: 417.1576;
found: 417.1583.
4.3.6. tert-Butyl (1S,2S)-1-((2-oxo-2-(pyren-3-yl)ethoxy)carbonyl)-
2-hydroxypropyl carbamate (4m). 1-(Bromoacetyl)pyrene (0.100 g.
4.3.3. tert-Butyl (S)-1-((2-oxo-2-(pyren-3-yl)ethoxy)carbonyl)ethyl-
0.29 mmol), N-(tert-butoxycarbonyl)-L-threonine (0.064 g, 0.29
carbamate (4j). 1-(Bromoacetyl) pyrene (0.100 g, 0.29 mmol), N-
(tert-butoxycarbonyl)-L-alanine, and DBU (0.053 g, 0.35 mmol)
mmol), and DBU (0.053 g, 0.35 mmol) were used. The reaction
mixture was stirred for 8 h. A dark yellowish solid obtained, which
on purification by column chromatography using 40% EtOAc in pet.
ether gave ester conjugate 4m (0.110 mg, 82%) as a yellow solid, mp:
76 ꢀC; TLC Rf 0.67 (50% EtOAc in pet. ether); 1H NMR (CDCl3,
were used. The reaction mixture was stirred for 12 h. A semi-
solid residue obtained, which on purification by column chro-
matography using 30% EtOAc in pet. ether gave compound 4j
(0.103 mg, 82%) as a brown solid, mp: 129 ꢀC; TLC Rf 0.46 (25%
200 MHz):
d
¼8.87 (d, J¼9.4 Hz, 1H), 8.12e7.87 (m, 6H), 7.71e7.64
EtOAc in pet. ether); 1H NMR (CDCl3, 200 MHz):
d
8.98 (d,
(m, 2H), 5.75 (d, J¼16.0 Hz, 1H), 5.35 (d, J¼16.4 Hz, 1H), 5.30 (s, NH),
4.74 (q, J¼6.0 Hz, 1H), 4.58 (d, J¼9.6 Hz, 1H), 3.79 (s, OH), 1.52 (s,
J¼9.4 Hz, 1H), 8.32e8.05 (m, 8H), 5.68 (d, J¼16.4 Hz, 1H), 5.42 (d,
J¼16.2 Hz, 1H), 5.09 (s, 1NH), 4.54 (m, 1H), 1.50 (d, J¼9.0 Hz, 3H),
9H), 1.42 (d, J¼6.2 Hz, 3H); 13C NMR (CDCl3, 50 MHz):
¼195.8,
d
1.46 (s, 9H); 13C NMR (CDCl3, 50 MHz):
d
¼195.4, 173.2, 155.2,
171.4, 156.4, 134.7, 130.7, 130.4, 130.3, 130.1, 126.8, 126.7, 126.5 (3C),
125.9,124.5,124.2,123.6 (2C),123. 5, 79.9, 68.6, 68.0, 59.5, 28.4 (3C),
18.9; FTIR (KBr) nmax (cmꢁ1): 3449 (OH), 3353 (NH), 1755 (OCO),
134.4, 130.9, 130.4, 130.2, 130.1, 128.0, 126.9, 126.6, 126.5, 126.4,
125.7, 124.9, 124.4, 124.0, 123.9 (2C), 79.9, 68.1, 49.3, 28.4 (3C),
18.8; FTIR (KBr) nmax (cmꢁ1): 3387 (NH), 1761 (OCO), 1717 (CO),
1689 (CO), 1624 (CONH); UV/vis (EtOH): lmax
(
3
) 356 (21.0ꢂ103),
1683 (CONH); UV/vis (EtOH): lmax
(
3
) 356 (20.4ꢂ103), 400
400 (8.4ꢂ103); LCMS: m/z (%): 462.2 (53) [MþH]þ, 406.2 (53), 362.4

9806
A. Jana et al. / Tetrahedron 66 (2010) 9798e9807
(100), 261.2 (9); HRMS (ESþ) m/z calcd for C27H27NO6 [M]þ:
Acknowledgements
461.1838; found: 461.1842.
We thank DST (SERC Fast Track Scheme) for financial support,
DSTFIST for 400 MHz NMR, Central Drug Research Institute,
Lucknow for HRMS analysis, Prof. Soumen Hazra for helping with
LCMS analysis. We are also grateful to Prof. J. Dey and Prof. N.
Sarkar for their valuable suggestions regarding the photophysical
behavior of the ester conjugates. Avijit is thankful to CSIR (India)
and Sanghamitra is grateful to UGC (India) for their research
fellowship.
4.3.7. 2-Oxo-2-(pyren-3-yl)ethyl
5-oxopyrrolidine-2-carboxylate
(4n). 1-(Bromoacetyl)pyrene (0.100 g. 0.29 mmol), pyroglutamic
acid (0.037 g, 0.29 mmol), and DBU (0.053 g, 0.35 mmol) were used.
The reaction mixture was stirred for 12 h. Crude reaction mixture
was purified by column chromatography using 80% EtOAc in pet.
ether to give the ester conjugate 4n (0.092 mg, 86%) as a dark
yellow solid, mp: 139 ꢀC; TLC Rf 0.55 (100% EtOAc in pet. ether); 1H
NMR (CDCl3, 200 MHz):
d
¼8.88 (d, J¼9.4 Hz, 1H), 8.23e7.81 (m,
8H), 6.90 (s, NH), 5.56 (d, J¼16.2 Hz, 1H), 5.44 (d, J¼16.2 Hz, 1H),
Supplementary data
4.50 (t, J¼5.2 Hz, 1H), 2.61e2.36 (m, 4H); 13C NMR (CDCl3, 50 MHz):
d
¼194.8, 178.8, 172.1, 134.5, 130.8, 130.3, 130.1 (2C), 130.0, 127.4,
Supplementary data related to this article can be found online,
at doi:10.1016/j.tet.2010.10.090.
126.9, 126.7, 126.6, 126.5, 125.7, 124.8, 124.3, 123.9 (2C), 68.2, 55.6,
29.3, 25.2; FTIR (KBr) nmax (cmꢁ1): 3222 (NH), 1746 (OCO), 1695
(CO), 1595 (CONH); UV/vis (EtOH): lmax
(
3
) 357 (20.9ꢂ103), 400
(8.1ꢂ103); LCMS: m/z (%): 372.4 (100) [MþH]þ, 354.2 (20), 261.0
(31), 243.2 (9), 170.2 (16); HRMS (ESþ) m/z calcd for C23H18NO4
[MþH]þ: 372.1236; found: 372.1233.
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