Perylen-3-ylmethyl: Fluorescent photoremovable protecting group (FPRPG) for carboxylic acids and alcohols

Article abstract of DOI:10.1016/j.tet.2011.11.074

Perylen-3-ylmethyl demonstrated as a new fluorescent photoremovable protecting group (FPRPG) for carboxylic acids and alcohols. Carboxylic acids including amino acids were protected as their corresponding esters by coupling with FPRPG, perylen-3-ylmethyl. Photophysical studies of caged esters showed that they all exhibited strong fluorescence properties and their fluorescence quantum yields were in the range of 0.85-0.95. Irradiation of the caged esters using visible light (≥410 nm) in aqueous acetonitrile released the corresponding carboxylic acids in high chemical (94-97%) and quantum (0.072-0.093) yields. The results obtained from the photolysis of the caged ester in different solvents indicated that solvent has influence on the rate of photorelease. Further, we also explored the ability of FPRPG, perylen-3-ylmethyl for the protection of alcohols and phenols.

Full text of DOI:10.1016/j.tet.2011.11.074

Tetrahedron 68 (2012) 1128e1136
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Perylen-3-ylmethyl: fluorescent photoremovable protecting group (FPRPG)
for carboxylic acids and alcohols
*
Avijit Jana, Mohammed Ikbal, N.D. Pradeep Singh
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 June 2011
Received in revised form 23 November 2011
Accepted 24 November 2011
Available online 1 December 2011
Perylen-3-ylmethyl demonstrated as a new fluorescent photoremovable protecting group (FPRPG) for
carboxylic acids and alcohols. Carboxylic acids including amino acids were protected as their corre-
sponding esters by coupling with FPRPG, perylen-3-ylmethyl. Photophysical studies of caged esters
showed that they all exhibited strong fluorescence properties and their fluorescence quantum yields
were in the range of 0.85e0.95. Irradiation of the caged esters using visible light (ꢀ410 nm) in aqueous
acetonitrile released the corresponding carboxylic acids in high chemical (94e97%) and quantum
(0.072e0.093) yields. The results obtained from the photolysis of the caged ester in different solvents
indicated that solvent has influence on the rate of photorelease. Further, we also explored the ability of
FPRPG, perylen-3-ylmethyl for the protection of alcohols and phenols.
Keywords:
Fluorescent photoremovable protecting
group
Perylen-3-ylmethyl
Caged esters
Ó 2011 Elsevier Ltd. All rights reserved.
Caged carbonates
Photorelease
1. Introduction
ꢀ410 nm. Based on the above facts and together with our recent
research interest on FPRPGs,16 herein, we report a new FPRPG
Photoremovable protecting groups (PRPGs) with fluorescent
properties have thrown light in the study of numerous processes in
biology1e3 and medicine.4,5 Fluorescent PRPGs (FPRPGs) have certain
advantages over PRPGs, sincethey notonly releaseactive molecules of
interest at desired location for a specific period of time, but also allow
us to visualize, quantify, and follow the spatial distribution, localiza-
tion, and depletion of the active molecules by monitoring its fluo-
rescent caged precursors using fluorescent microscope techniques.6
Although FPRPGs are of great utility, attempts to synthesize ei-
ther FPRPGs or to modify a PRPG into fluorescence photolabile
group are limited. Initially, Burgess et al.7 reported fluorophore
based on dansyl derivative, which on irradiation using UV light
(365 nm) resulted in deprotection of carboxylic acids, rather with
low photolysis efficiency and chemical yield. Based on the above
strategy, polycyclic aromatic compounds, which are fluorophores
namely anthraquinone,8 pyrene,8,9 phenanthrene,8 anthracene,10
and coumarins11 moieties have been targeted as FPRPGs.
namely perylen-3-ylmethyl (1), with improved photophysical and
photochemical properties for carboxylic acids and alcohols.
2. Results and discussion
2.1. Synthesis of caged esters (4aeh)
A series of carboxylic acids including amino acids were pro-
tected by 3-(bromomethyl)perylene (2) in the form of caged esters
(4aeh) as outlined in Schemes 1 and 2 were readily prepared from
3-(hydroxymethyl)perylene (1) by reaction with phosphorus tri-
bromide (PBr3) in carbon tetrachloride (CCl4). Treatment of car-
boxylic acids (3aec) with 1 equiv of 2 in the presence of K2CO3/KI in
dry N,N-dimethylformamide (DMF) at room temperature for a pe-
riod of 8e12 h afforded the caged esters in excellent yields (Table 1,
4aec). While, for the protection of amino acids we used corre-
sponding Boc-protected amino acids (3deh), and they were pro-
tected similar to carboxylic acids in good yields (Table 1, 4deh),
except KF was used in place of KI. All the caged esters (4aeh) were
characterized by IR, 1H, 13C NMR, and mass spectral analysis.
So far, 7-methoxycoumarin-4-yl,11,12 anthracene-9-methanol,10
and pyren-1-ylmethyl8e10,13e15 were demonstrated as FPRPGs for
various functional groups. However, the above-mentioned FPRPGs
have certain limitations like moderate quantum yields, needs spe-
cific solvent for the photoreaction, and no stronger absorption band
2.2. Photophysical properties of caged esters (4aeh)
UV/vis absorption and emission spectra of degassed 2ꢁ10ꢂ6 M
solution of caged esters (4aeh) and 3-(hydroxymethyl)perylene (1)
* 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 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.11.074

A. Jana et al. / Tetrahedron 68 (2012) 1128e1136
1129
O
Br
R1
OH
R1CO2H (3a-c)
O
R1 = (4a) CH2Ph,
(4b) p-CH3Ph,
(4c) p-OCH3Ph
ii
i
4a-c
O
N-Boc amino
R2
acids (3d-h)
1
2
O
iii
R2 = (4d) CH2NHBoc,
(4e) (CH2)3NHBoc,
(4f) CH(CH3)NHBoc,
(4g) CH(CH2Ph)NHBoc,
(4h) CH(1H-indol-3-yl)NHBoc
4d-h
Reagents: i, PBr3/CCl4, 80 °C, 4 h; ii, K2CO3/KI/DMF, rt, 8-12 h; iii, K2CO3/KF/DMF, rt, 8-12 h;
Scheme 1. Synthesis of caged esters (4aeh) of perylen-3-ylmethyl.
Table 1
Synthetic yields of the caged esters (4aeh), UV/vis absorption and fluorescence data
of caged esters (4aeh) and its precursor 3-(hydroxymethyl)perylene (1) in absolute
ethanol
Absorption
Emission
Caged ester
Synthetic
yielda (%)
UV/vis absorption
Fluorescence
b
c
F e
lmax (nm)
log
3
lmaxd (nm)
f
4a
4b
4c
4d
4e
4f
4g
4h
1
91
88
86
80
76
75
79
82
–
438
438
439
439
439
437
440
435
439
4.38
4.39
4.55
4.40
4.39
4.39
4.40
4.40
4.55
445
445
447
446
446
443
447
447
446
0.92
0.93
0.93
0.87
0.87
0.88
0.86
0.86
0.95
a
Based on isolated yield.
Maximum absorption wavelength.
Molar absorption coefficient at the maximum absorption wavelength.
Maximum emission wavelength.
b
c
d
e
Fluorescence quantum yield (excitation wavelength 390 nm, error limit within
350
400
450
500
550
ꢃ5%).
Wavelength (nm)
in absolute ethanol (EtOH) were recorded. The absorption and
emission maxima, molar absorptivities, and fluorescence quantum
yield of the above esters along with 1 are summarized in Table 1.
Fluorescence quantum yield was calculated using 9,10-
diphenylanthracene as standard (Vf¼0.95 in ethanol).17
Fig. 1. Normalized UV/vis absorption spectrum (black line) and emission spectrum
(red line) of caged ester 4c in EtOH.
caged esters their corresponding carboxylic acids (3aeh) were re-
leased in high chemical (94e97%) and quantum (0.072e0.093)
yields (Vp) (Table 2).
For all caged esters mentioned in Table 2, the photoproducts
were isolated and analyzed by spectroscopy and in every case we
found the released carboxylic acid was the only significant
photoproduct in addition to the FPRPG (1). The quantum
yield (Vp) was calculated using potassium ferrioxalate as an
actinometer.19,20
As a representative example, we have shown in Fig. 3 the HPLC
profile of caged ester 4c at regular intervals of irradiation. The HPLC
chart shows gradual depletion of the peak at tR 6.77 min with an
increase in irradiation time, indicating the photodecomposition of
the caged ester 4c. On the other hand, we also noted a gradual
increase of two new major peaks at tR 4.25 min and tR 2.20 min,
corresponding to the photoproducts 3-(hydroxymethyl)perylene
(1) and 4-methoxybenzoic acid (3c), respectively.
Fig.1 shows the normalized absorption and the emission spectra
of caged ester 4c in ethanol. The absorption spectrum of 4c showed
three strong absorption bands centered at 390, 410, and 439 nm
(pep* transition), and its emission spectrum found to be a mirror
image of its absorption spectrum with three emission bands at 445,
480, and 510 nm (Fig. 1), typical characteristics of the perylene
moiety.18
We observed similar absorption and emission behavior for all
other caged esters (Fig. 2 a and b).
Further, the fluorescence quantum yields, Vf, for caged esters
(4aeh) in absolute ethanol (EtOH) at room temperature were in the
range of 0.86e0.93 (Table 1), indicates that caged esters are highly
fluorescent. Moreover, the photophysical properties of caged esters
were found to be similar to FPRPG (1), which suggested that perylene
moiety only dictates the absorption and emission maxima of caged
esters ruling out the influence of its counterpart carboxylic acids.
Further, we also monitored the course of photorelease of caged
ester 4c using UV/vis and fluorescence spectroscopy (Supplemen-
tary data Figs. S.1.a and S.1.b).
2.3. Photolysis of caged esters (4aeh)
Considering our main interest to study the application of per-
ylene-3-ylmethyl as an FPRPG, we irradiated caged esters (4aeh)
(1.0ꢁ10ꢂ4 M) individually in acetonitrile/H2O (75/25 v/v) using
125 W medium pressure Hg lamp as visible light source (ꢀ410 nm)
and 1 M NaNO2 solution as UV cut-off filter. We found for all the
2.4. The effect of solvent on photorelease
To understand the solvent effect on the rate of photorelease of
carboxylic acids, we irradiated caged ester 4c in different solvents

1130
A. Jana et al. / Tetrahedron 68 (2012) 1128e1136
Table 2
1.25
1.00
0.75
0.50
0.25
4a
Photolytic data of caged esters (4aeh) on irradiation by visible light (ꢀ410 nm) in
(a)
4b
4c
4d
4e
4f
acetonitrile/H2O (75/25) solution
Ester Photolytic data of caged esters (4aeh)
Acid
% of ester % of acid
Quantum
Fp
depleteda releasedb yieldc
(
)
4g
4h
CO2H
CO2H
4a
4b
98
97
96
96
0.090
0.091
H3C
CO2H
360
375
390
405
420
435
450
465
4c
98
97
0.093
Wavelength (nm)
H3CO
Boc
3500
3000
2500
2000
1500
1000
500
4a
(b)
4b
4c
4d
4e
4f
4d
4e
95
95
95
94
0.089
0.072
N
CO2H
H
Boc
N
CO2H
CO2H
NH
H
4g
4h
H3C
Boc
4f
98
95
96
97
94
95
0.087
0.087
0.077
CO2H
4g
HN
Boc
CO2H
0
440
460
480
500
520
540
Wavelength (nm)
4h
Fig. 2. (a) UV/vis absorption spectra of caged esters (4aeh). (b) Fluorescence spectra of
caged esters (4aeh).
HN
HN
Boc
a
% of caged ester depleted as determined by HPLC.
% of carboxylic acid released as determined by HPLC.
Photochemical quantum yield for the release of carboxylic acids (error limit
and the results are summarised in Table 3. We noticed caged ester
4c released 4-methoxybenzoic acid more efficiently in methanol/
H2O (95/5) compared to acetonitrile/H2O (95/5) and dioxane/H2O
(75/25). The above fact can be attributed due to the formation of
ionic intermediates during photolysis (Scheme 2) and on the nature
of the solvent (since hydrogen bonding and the polarity of the
solvent have an influence on the excited states). Though the pho-
tocleavage of caged esters in methanol is efficient their solubility in
methanol is not as good as compared to acetonitrile.
Further, we also noticed the efficiency of the photorelease of
caged ester 4c increases continuously with increasing the amount
of water (quantum yield for the release of 4-methoxybenzoic acid
by caged ester 4c increased by w30 times as we move from 5% to
25% of water in acetonitrile). The interpretation for the increase of
quantum yield is due to the better stabilization of the newly gen-
erated ion pair (Scheme 2) by solvation, the similar phenomenon
was also noted in the case of caged coumarins.11
b
c
within ꢃ5%).
carboxylate anion. After ion-pair separation in a polar solvent, the
methylenic carbocation and carboxylate anion reacts with solvent
molecules to yield 3-(hydroxymethyl) perylene (1) and carboxylic
acid, respectively (Scheme 2).
To support the formation of perylene-3-ylmethyl carbocation
intermediate, we carried out photolysis of caged ester 4c in MeOH/
H2O (95/5 v/v) and analysed the course of photolysis by HPLC at
P-CH2OH
1
aquous
CH3CN
*
[P-CH2OCOR]
P-CH2 OCOR
+
RCOOH
2.5. Mechanism of photorelease
abs
emi
Literature precedence10,11,16 and the above solvent study suggest
that the photolysis of caged esters proceeds through an ionic
mechanism. The initial photochemical step involves excitation of
the perylene-3-ylmethyl chromophore to its singlet excited state,
which then undergoes heterolysis of the CeO ester bond (photo-
SN1) to produce an ion pair of perylene-3-ylmethyl carbocation and
P-CH2OH
(minor)
P-CH2OCH3
+
[P-CH2OCOR]
P = Perylene-3-yl
abs = absorbance, emi = emission
MeOH
RCOOH
Scheme 2. Possible mechanism for the photorelease.

A. Jana et al. / Tetrahedron 68 (2012) 1128e1136
1131
Fig. 3. HPLC profile of caged ester 4c (1ꢁ10ꢂ4 M) in ACN/H2O (75/25 v/v) at regular intervals of irradiation (0e50 min, time interval¼5 min).
Table 3
Further to find out among two closely spacing strong absorp-
tion bands centered at 410 nm and 440 nm is responsible for the
photocleavage of CeO bond, we irradiated ester conjugate 4c
(3 ml, ACN/H2O, v/v 75/25) in a quartz cuvette at 410 nm and
440 nm UV light individually using fluorescence spectrophotom-
eter with a 2.5 nm slit and the fluorescence spectra were recorded
at regular interval of photolysis. The fluorescence spectra in-
dicated photocleavage of 4c was achieved on irradiation at
410 nm, while we noticed no change in emission spectra of 4c on
excitation at 440 nm (see Supplementary data, page no. S15),
which suggest absorption band centered at 410 nm is responsible
for photocleavage.
Photolytic data of caged ester 4c on irradiation by visible light (ꢀ410 nm) in different
solvents
Solvent
Photolytic data of caged ester 4c
% of 4c
% of 3c
Quantum
depleteda
releasedb
yieldc
(Fp)
CH3CN
5
11
69
98
82
96
98
5
10
65
97
75
86
89
0.002
0.004
0.042
0.093
0.062
0.089
0.093
CH3CN/H2O (95/5)
CH3CN/H2O (90/10)
CH3CN/H2O (75/25)
Dioxane/H2O (75/25)
CH3OH
CH3OH/H2O (95/5)
a
% of 4c depleted as determined by HPLC.
% of acid released as determined by HPLC.
Photochemical quantum yield for the release of 4c (error limit within ꢃ5%).
b
c
2.6. Synthesis of caged carbonates (7aed)
To explore the versatility of our FPRPG (1), we carried out the
protection of a series of alcohols including phenol as outlined in
Scheme 3. Initially, we converted alcohols (6aed) into their cor-
responding chloroformates by treating with triphosgene ((COCl2)3)
at 0 ꢄC. Treatment of alcohol chloroformates with FPRPG (1) in the
presence of 4-dimethylaminopyridine (DMAP) in dry DCM resul-
ted the corresponding caged carbonates (7aed) in good yields
(Table 4).
regular intervals of irradiation. The HPLC chart (Fig. 4) showed the
appearance of new peak at tR 5.33 min in addition to peak at tR
2.11 min, which correspond to 4-methoxybenzoic acid. The new
peak at tR 5.33 min was identified as 3-(methoxymethyl) perylene
by isolating and analysing by spectroscopy. The formation of
3-(methoxymethyl) perylene suggests the trapping of perylene-3-
ylmethyl carbocation by methanol.
Fig. 4. HPLC profile of caged ester 4c (1ꢁ10ꢂ4 M) in MeOH/H2O (95/5 v/v) at regular intervals of irradiation (0e50 min, time interval¼10 min).

1132
A. Jana et al. / Tetrahedron 68 (2012) 1128e1136
OH
O
O
R3
O
R3 = (7a) cyclohexyl,
(7b) menthyl,
O
i
ii
R3-OH (6a-d) + (COCl2)3
R3
+
(7c) cholesterol,
(7d) phenyl
Cl
O
1
7a-d
Reagents: i, K2CO3, toluene, 0 °C, 4-5 h; ii, DMAP, DCM, rt, 8-12 h.
Scheme 3. Synthesis of caged carbonates (7aed) of perylen-3-ylmethyl.
spectra of carbonates (7aed), see Supplementary data Figs. S.4.a
and S.4.b).
Table 4
Synthetic yields, UV/vis absorption, and fluorescence data of caged carbonates
(7aed) in absolute ethanol
2.8. Photolysis of caged carbonates (7aed)
Caged
carbonate
Synthetic
yielda (%)
UV/vis
Fluorescence
c
d
lmaxb (nm)
log
3
lmax (nm)
Ffe
Irradiation of carbonates (7aed) using the procedure described for
caged esters, resulted in the release of alcohols in quantitative yield
(95e96%, Table 5). In each case, the photolysis was stopped when
conversion reached at least 95% (as indicated by HPLC). For all caged
compounds mentioned in Table 5, the photolysis products (corre-
sponding alcohol and the protecting group 1) were confirmed by iso-
lating and matching their 1H NMR spectra to those of authentic samples.
In similar to previously discussed caged esters, the mechanism
of the photocleavage of carbonates involves heterolysis of the CeO
ester bond to produce an ion pair of perylene-3-ylmethyl carbo-
cation and carbonate anion. The newly generated carbonate anion
undergoes decarboxylation21,22 followed by proton abstraction
from the solvent to produce alcohol.
The stability of caged compounds was performed by keeping the
solution containing caged compound (4aec, e, h and 7bed) in-
dividually in dark at three different initial pH values (4.5, 6, and 7.5)
for a period of 30 days. We observed 7e15% decomposition for
caged compounds by HPLC at three pH values (Supplementary data
Table 1s).
7a
7b
7c
7d
76
75
79
82
440
438
440
439
4.39
4.39
4.40
4.40
447
445
447
446
0.92
0.91
0.91
0.91
a
Based on isolated yield.
Maximum absorption wavelength.
Molar absorption coefficient at the maximum absorption wavelength.
Maximum emission wavelength.
Fluorescence quantum yield (error limit within ꢃ5%).
b
c
d
e
2.7. Photophysical properties of caged carbonates (7aed)
The photophysical properties of caged carbonates (7aed)
were recorded as explained for caged esters and the results are
summarized in Table 4. The absorption and emission maxima,
molar absorptivities, and fluorescence quantum yield of carbon-
ates were found to be analogous to caged esters, thus confirming
the photophysical properties of caged compounds is dictated
only by the perylene moiety (for absorption and emission
Table 5
Photolytic data of carbonates (7aed) on irradiation by visible light (ꢀ410 nm) in acetonitrile/H2O (75/25) solution
Carbonate
Photolytic data carbonates (7aed)
Alcohol/Phenol
% of carbonate
depleteda
% of alcohol
releasedb
Quantum
yieldc
(Fp)
OH
OH
7a
95
90
0.087
7b
7c
96
96
91
89
0.086
0.085
H
H
H
HO
OH
7d
95
90
0.088
a
% of carbonate ester depleted as determined by HPLC.
b
c
% of alcohol released as determined by 1H NMR spectroscopy.
Photochemical quantum yield for the release of alcohols (error limit within ꢃ5%).

A. Jana et al. / Tetrahedron 68 (2012) 1128e1136
1133
3. Conclusion
acid (0.039 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 4b
(0.102 g, 88%) as a yellow solid, mp: 139e143 ꢄC; TLC Rf 0.68 (15%
In summary, the newly developed FPRPG, perylene-3-ylmethyl
exhibited unique properties like, good molar absorption co-
efficient in the visible wavelength region (ꢀ410 nm) compared to
anthracene-9-methanol and pyrene-1-ylmethyl, strong fluores-
cence and more important, releases carboxylic acids and alcohols
with high chemical and quantum yield in aqueous condition by
visible light. Though the present perylene chromophore can act as
an efficient FPRPG still their water solubility needs to be improved.
In future we will publish highly fluorescent and good water-soluble
perylene chromophore having polar side chain on its bay area as an
efficient FPRPG.
EtOAc in pet. ether); 1H NMR (CDCl3, 200 MHz):
d
¼8.22e8.11 (m,
4H), 7.97 (d, J¼8.2 Hz, 2H), 7.90 (d, J¼8.4 Hz, 1H), 7.68 (d, J¼8.2 Hz,
2H), 7.53e7.43 (m, 4H), 7.22 (d, J¼8.0 Hz, 2H), 5.71 (s, 2H), 2.39 (s,
3H); 13C NMR (CDCl3, 50 MHz):
d
¼166.6, 143.8, 134.6, 133.1, 132.1,
131.8, 131.1, 130.9, 129.8, 129.1, 128.4, 128.1, 127.4, 127.1, 126.6, 123.4,
120.5, 120.3, 119.6, 65.0, 21.6; FTIR (KBr, cmꢂ1): 1717; HRMS calcd
for C28H18O2: 386.1307, found: 386.1306.
4.2.3. (Perylen-3-yl)methyl 4-methoxybenzoate (4c). 3-(Bromo-
methyl)perylene (0.100 g, 0.29 mmol), potassium iodide (0.057 g,
0.34 mmol), potassium carbonate (0.048 g, 0.34 mmol), and 4-
methoxybenzoic 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
4c (0.104 g, 86%) as a yellow solid, mp: 151e155 ꢄC; TLC Rf 0.53 (15%
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 reported
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 resonance
the internal standard (deuterochloroform: 77.0 ppm). UV/vis ab-
sorption spectra were recorded on a Shimadzu UV-2450 UV/vis
spectrophotometer, fluorescence emission spectra were recorded
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 spec-
trometer. Photolysis of all the ester conjugates was 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.
EtOAc in pet. ether); 1H NMR (CDCl3, 200 MHz):
d
¼8.18 (q, J¼7.8 Hz,
4H), 8.07 (d, J¼9.0 Hz, 2H), 7.92 (d, J¼8.4 Hz, 1H), 7.71 (d, J¼8.0 Hz,
2H), 7.59e7.45 (m, 4H), 6.93 (d, J¼9.0 Hz, 2H), 5.72 (s, 2H), 3.86 (s,
3H); 13C NMR (CDCl3, 50 MHz):
131.8, 131.2, 131.1, 130.9, 129.2, 128.9, 128.4, 128.0, 127.0, 126.6, 123.4,
122.6, 120.5, 120.3, 119.6, 117.3, 113.7, 112.5, 64.9, 55.4; FTIR (KBr,
cmꢂ1): 1700; HRMS calcd for C29H20O3: 416.1412, found: 416.1422.
d
¼166.3, 163.5, 134.6, 133.0, 132.1,
4.3. General procedure for the synthesis of caged esters (4deh)
3-(Bromomethyl)perylene (1 equiv) was dissolved in dry DMF
(2 ml) and the corresponding Boc-protected amino acid (3deh)
(1 equiv) was added followed by 1.2 equiv of K2CO3 and KF. 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 eluant.
4.3.1. tert-Butyl (((perylen-3-yl) methoxy) carbonyl) methyl-
carbamate (4d). 3-(Bromomethyl)perylene (0.100 g, 0.29 mmol),
potassium fluoride (0.020 g, 0.34 mmol), potassium carbonate
(0.048 g, 0.34 mmol), and N-(tert-butoxycarbonyl)glycine (0.051 g,
0.29 mmol). The reaction mixture was stirred for 10 h. Crude re-
action mixture was purified by column chromatography using 35%
EtOAc in pet. ether to give compound 4d (0.102 mg, 80%) as a light
yellow solid, mp: 175e178 ꢄC; TLC Rf 0.75 (30% EtOAc in pet. ether);
4.2. General procedure for the synthesis of caged esters (4aec)
3-(Bromomethyl)perylene (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 (1 equiv) were added. The reaction mixture was stirred at room
temperature for 8e12 h. The solvent was removed by rotary evap-
oration under reduced pressure and the crude residue was purified
by column chromatography using ethylacetate (EtOAC) in pet. ether.
1H NMR (CDCl3, 200 MHz):
d
¼8.11e7.99 (m, 4H), 7.71 (d, J¼8.0 Hz,
1H), 7.64 (d, J¼8.0 Hz, 2H), 7.50e7.38 (m, 4H), 5.49 (s, 2H), 5.04 (s,
1H, NH), 3.95 (d, J¼4.6 Hz, 2H), 1.42 (s, 9H); 13C NMR (CDCl3,
100 MHz):
d
¼170.5, 155.8, 134.6, 132.9, 132.4, 131.8, 131.0, 130.8,
4.2.1. (Perylen-3-yl)methyl 2-phenylacetate (4a). 3-(Bromomethyl)
perylene (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 (0.039 g, 0.29 mmol) were used. The reaction mixture was
stirred for 10 h. The crude residue was purified by column chroma-
tography with 20% EtOAc in pet. ether to give the title compound 4a
(0.105 g, 91%) as a yellow solid, mp: 109e112 ꢄC; TLC Rf 0.63 (15%
130.2, 128.9, 128.5, 128.3, 128.1, 127.2, 126.6, 123.2, 120.6, 120.4,
119.5, 80.2, 65.5, 42.6, 28.4; FTIR (KBr, cmꢂ1): 2925, 1761; HRMS
calcd for C28H25NO4: 439.1784, found: 439.1791.
4.3.2. tert-Butyl 3-(((perylen-3-yl) methoxy)carbonyl) propylcarba-
mate (4e). 3-(Bromomethyl)perylene (0.100 g, 0.29 mmol), potas-
sium fluoride (0.020 g, 0.34 mmol), potassium carbonate (0.048 g,
0.34 mmol), and 4-(tert-butoxycarbonylamino)butyric acid (0.059 g,
0.29 mmol) were used. The reaction mixture was stirred for 10 h. A
brown solid obtained which on purification by column chromatog-
raphy using 30% EtOAc in pet. ether gave the cage ester 4e (0.103 g,
76%) as a yellow solid, mp: 165e168 ꢄC; TLC Rf 0.65 (30% EtOAc in pet.
EtOAc in pet. ether); 1H NMR (CDCl3, 200 MHz):
6H), 7.68 (d, J¼7.4 Hz, 4H), 7.47 (t, J¼7.2 Hz, 5H), 7.31 (s, 1H), 5.50 (s,
2H), 3.70 (s, 2H); 13C NMR (CDCl3, 50 MHz):
133.0, 132.3, 131.9, 131.2, 130.8, 129.5, 128.7, 128.3, 128.1, 127.3, 127.1,
126.7, 123.4, 120.6, 120.4, 119.7, 65.3, 41.6; FTIR (KBr, cmꢂ1): 1718;
HRMS calcd for C29H20O2: 400.1463, found: 400.1461.
d
¼8.21e8.07 (m,
d
¼171.7, 134.7, 134.0,
ether); 1H NMR (CDCl3, 200 MHz):
d
¼8.31e8.18 (m, 4H), 7.95 (d,
J¼8.6 Hz, 1H), 7.65 (d, J¼8.2 Hz, 2H), 7.60e7.36 (m, 4H), 5.51 (s, 2H),
4.58 (s, NH), 3.16 (q, J¼6.6 Hz, 2H), 2.43 (t, J¼7.4 Hz, 2H),1.84 (m, 2H),
4.2.2. (Perylen-3-yl)methyl 4-methylbenzoate (4b). 3-(Bromomethyl)
perylene (0.100 g, 0.29 mmol), potassium iodide (0.057 g, 0.34 mmol),
potassium carbonate (0.048 g, 0.34 mmol), and 4-methyl benzoic
1.42 (s, 9H); 13C NMR (CDCl3, 100 MHz):
d¼173.2, 155.9, 134.5, 132.4,
131.8, 131.1, 130.8, 129.3, 128.9, 128.4, 128.1, 127.9, 126.9, 126.6, 126.0,

1134
A. Jana et al. / Tetrahedron 68 (2012) 1128e1136
123.4, 120.5, 120.3, 120.1, 119.5, 79.3, 64.7, 39.9, 31.6, 28.4; FTIR (KBr,
cmꢂ1): 2927, 1722; HRMS calcd for C30H29NO4: 467.2097, found:
467.2106.
evaporation under reduced pressure and the crude residue was
purified by column chromatography with EtOAc in pet. ether as an
eluant.
4.3.3. tert-Butyl (S)-1-(((perylen-3-yl)methoxy)carbonyl) ethyl-
carbamate (4f). 3-(Bromomethyl)perylene (0.100 g, 0.29 mmol),
potassium fluoride (0.020 g, 0.34 mmol), potassium carbonate
4.4.1. Cyclohexyl (perylen-3-yl)methyl carbonate (7a). 3-(Hydrox-
ymethyl)perylene (0.100 g, 0.36 mmol), cyclohexyl chloroformate
(0.059 g, 0.36 mmol), and DMAP (0.053 g, 0.43 mmol) were used
and the reaction mixture was stirred for 6 h at room temperature.
The crude reaction mixture was purified by column chromatogra-
phy using 10% EtOAc in pet. ether to give the carbonate ester 7b
(0.112 mg, 76%) as a yellow solid, mp: 158e160 ꢄC; TLC Rf 0.60 (15%
(0.048 g, 0.34 mmol), and N-(tert-butoxycarbonyl)-L-alanine
(0.055 g, 0.29 mmol) were used. The reaction mixture was stirred
for 12 h. A deep brown solid residue obtained which on purification
by column chromatography using 30% EtOAc in pet. ether gave
compound 4f (0.099 mg, 75%) as a yellow solid, mp: 144e147 ꢄC;
TLC Rf 0.30 (15% EtOAc in pet. ether); 1H NMR (CDCl3, 200 MHz):
EtOAc in pet. ether); 1H NMR (CDCl3, 200 MHz):
d
¼8.23e8.10 (m,
4H), 7.86 (d, J¼8.2 Hz, 1H), 7.69 (d, J¼8.0 Hz, 2H), 7.58e7.43 (m, 4H),
d
¼8.24e8.11 (m, 4H), 7.79 (d, J¼8.2 Hz, 1H), 7.69 (d, J¼8.2 Hz, 2H),
5.54 (s, 2H), 4.71e4.62 (m,1H),1.98e1.26 (m, 10H); 13C NMR (CDCl3,
7.58e7.44 (m, 4H), 5.55 (dd, J¼12.4, 7.6 Hz, 2H), 5.08 (s, NH), 4.38 (t,
50 MHz):
d
¼154.9, 134.7, 133.1,132.5,131.9,131.2,130.9,130.6,129.1,
J¼6.8 Hz, 1H), 1.43 (s, 9H), 1.38 (d, J¼7.2 Hz, 3H); 13C NMR (CDCl3,
128.5, 128.3, 128.1, 127.3, 126.7, 123.4, 120.7, 120.5, 119.7, 72.8, 67.74,
31.7, 25.4, 23.9; FTIR (KBr, cmꢂ1): 1734; HRMS calcd for C28H24O3:
408.1725, found: 408.1727.
100 MHz):
d¼173.3, 155.0, 134.5, 132.8, 132.3, 131.8, 130.9, 130.7,
130.3, 128.9, 128.4, 128.3, 128.2, 128.0, 127.9, 127.1, 126.6, 126.5,
123.1, 120.6, 120.5, 120.3, 119.5, 79.8, 65.5, 49.3, 28.2, 18.6; FTIR (KBr,
cmꢂ1): 2924, 1711; HRMS calcd for C29H27NO4: 453.1940, found:
453.1948.
4.4.2. 2-Isopropyl-5-methylcyclohexyl (perylen-3-yl)methyl carbon-
ate (7b). 3-(Hydroxymethyl)perylene (0.100 g, 0.36 mmol), men-
thol chloroformate (0.079 g, 0.36 mmol), and DMAP (0.053 g,
0.43 mmol) were used and the reaction mixture was stirred for 8 h
at room temperature. The crude reaction mixture was purified by
column chromatography using 10% EtOAc in pet. ether to give the
4.3.4. tert-Butyl (S)-1-(((perylen-3-yl)methoxy)carbonyl)-2-
phenylethylcarbamate (4g). 3-(Bromomethyl)perylene (0.100 g,
0.29 mmol), potassium fluoride (0.020 g, 0.34 mmol), potassium
carbonate (0.048 g, 0.34 mmol), and N-(tert-butoxycarbonyl)-
L
-
carbonate ester 7c (0.125 mg, 75%) as
132e135 ꢄC; TLC Rf 0.72 (15% EtOAc in pet. ether); 1H NMR (CDCl3,
200 MHz):
a yellow solid, mp:
phenylalanine (0.077 g, 0.29 mmol) were used. The reaction mix-
ture was stirred for 12 h. The crude reaction mixture was purified
by column chromatography using 25% EtOAc in pet. ether to give
d
¼8.17e8.05 (m, 4H), 7.79 (d, J¼8.4 Hz, 1H), 7.64 (d,
J¼8.2 Hz, 2H), 7.52e7.39 (m, 4H), 5.51 (s, 2H), 4.62e4.49 (m, 1H),
2.10e1.88 (m, 3H), 1.67e1.60 (2H), 1.40 (m, 1H), 1.11e0.994 (m, 3H),
0.89e0.83 (m, 6H), 0.76 (d, J¼7.0 Hz, 3H); 13C NMR (CDCl3,
ester conjugate 4g (0.121 mg, 79%) as
147e150 ꢄC; TLC Rf 0.30 (15% EtOAc in pet. ether); 1H NMR (CDCl3,
200 MHz):
a yellow solid, mp:
d
¼8.28e8.18 (m, 3H), 8.12 (d, J¼7.8 Hz, 1H), 7.76e7.68
50 MHz):
d
¼155.0, 134.5, 132.8, 132.2, 131.7, 131.0, 130.8, 130.4,
(m, 3H), 7.56e7.45 (m, 4H), 7.14 (s, 3H), 6.98 (s, 2H), 5.52 (dd,
J¼12.4, 4.0 Hz, 2H), 4.98 (d, J¼8.8 Hz, NH), 4.659e4.629 (m, 1H),
3.05 (d, J¼5.4 Hz, 2H), 1.39 (s, 9H); 13C NMR (CDCl3, 50 MHz):
128.9, 128.4, 128.1, 127.9, 127.0, 126.5, 123.2, 120.5, 120.3, 119.5, 78.8,
67.7, 47.1, 40.8, 34.1, 31.5, 26.1, 23.4, 22.0, 20.7, 16.3; FTIR (KBr,
cmꢂ1): 1736; HRMS calcd for C32H32O3: 464.2351, found: 464.2348.
d
¼171.9, 154.8, 135.8, 134.6, 133.0, 132.5, 131.9, 131.0, 130.8, 130.2,
129.3, 129.0, 128.7, 128.5, 128.3, 128.1, 127.2, 126.9, 126.7, 123.3,
120.6, 120.4, 119.5, 79.9, 65.5, 54.5, 38.4, 28.3; FTIR (KBr, cmꢂ1):
2925, 1733; HRMS calcd for C35H31NO4: 529.2253, found: 529.2368.
4.4.3. (8S,9S,10R,13R,14S,17R)-2,3,4,7,8,9,10,11,12,13,14,15,16,17-
Tetradecahydro-10,13-dimethyl-17-((R)-6-methylheptan-2-yl)-1H-
cyclopenta[a]phenanthren-3-yl
(perylen-3-yl)methyl
carbonate
(7c). 3-(Hydroxymethyl)perylene (0.100 g, 0.36 mmol), cholesterol
chloroformate (0.162 g, 0.36 mmol), and DMAP (0.053 g,
0.43 mmol) were used and the reaction mixture was stirred for 6 h
at room temperature. The crude reaction mixture was purified by
column chromatography using 10% EtOAc in pet. ether to give the
4.3.5. tert-Butyl(S)-1-(((perylen-3-yl)methoxy)carbonyl)-2-(1H-in-
dol-3-yl)ethylcarbamate (4h). 3-(Bromomethyl)perylene (0.100 g,
0.29 mmol), potassium fluoride (0.020 g, 0.34 mmol), potassium
carbonate (0.048 g, 0.34 mmol), and N-(tert-butoxycarbonyl)-L-
tryptophan (0.088 g, 0.29 mmol) were used. The reaction mixture
was stirred for 12 h. The crude reaction mixture was purified by
column chromatography using 30% EtOAc in pet. ether to give
caged ester 4h (0.135 mg, 82%) as a yellow solid, mp: 162e164 ꢄC; Rf
0.50 (30% EtOAc in pet. ether); 1H NMR (CDCl3, 200 MHz):
carbonate ester 7d (0.198 mg, 79%) as
168e170 ꢄC; TLC Rf 0.72 (15% EtOAc in pet. ether); 1H NMR (CDCl3,
200 MHz):
J¼8.0 Hz, 2H), 7.55e7.40 (m, 4H), 5.51 (s, 2H), 5.35 (d, J¼4.0 Hz, 1H),
4.59e4.43 (m, 1H), 2.39e2.34 (m, 2H), 1.98e1.09 (m, 23H), 0.95 (s,
6H), 0.89e0.82 (m, 9H), 0.63 (s, 3H); 13C NMR (CDCl3, 50 MHz):
a yellow solid, mp:
d
¼8.20e8.08 (m, 4H), 7.83 (d, J¼8.4 Hz, 1H), 7.66 (d,
d
¼8.27e8.16 (m, 3H), 8.11 (d, J¼7.6 Hz, 1H), 7.75e7.60 (m, 3H),
7.52e7.34 (m, 4H), 7.20e7.06 (m, 4H), 6.69 (s, 1H), 5.48 (dd, J¼12.2,
6.2 Hz, 2H), 5.11 (s, 1H), 4.74 (m, 1H), 3.27 (d, J¼5.0 Hz, 2H), 1.42 (s,
d
¼154.6, 139.3, 134.5, 132.8, 132.2, 131.6, 130.9, 130.7, 130.2, 128.8,
128.4,128.2,127.9,127.0,126.5,123.2,122.9,120.5,120.2,119.4, 78.2,
71.8, 67.6, 56.6, 56.1, 49.9, 42.3, 39.7, 39.5, 38.1, 36.8, 36.5, 36.2, 35.8,
31.81, 28.2, 28.0, 27.7, 24.3, 23.9, 22.9, 22.6, 21.0, 19.2, 18.7; FTIR
(KBr, cmꢂ1): 1741; HRMS calcd for C49H58O3: 694.4386, found:
694.4388.
9H), 13C NMR (CDCl3, 50 MHz):
d¼172.3, 155.2, 136.0, 134.6, 132.9,
132.2, 131.7, 131.0, 130.8, 130.3, 129.6, 128.98, 128.5, 128.2, 128.0,
127.6, 127.1, 126.6, 123.2, 122.8, 122.1, 120.6, 120.3, 119.6, 119.5, 118.7,
111.1, 109.9, 79.9, 65.4, 54.4, 29.9, 28.3; FTIR (KBr, cmꢂ1): 2926,
1706; HRMS calcd for C37H32N2O4: 568.2362, found: 568.2479.
4.4.4. (Perylen-4-yl)methyl phenyl carbonate (7d). 3-(Hydrox-
ymethyl)perylene (0.100 g, 0.36 mmol), phenyl chloroformate
(0.056 g, 0.36 mmol), and DMAP (0.053 g, 0.43 mmol) were used
and the reaction mixture was stirred for 10 h at room temperature.
The crude reaction mixture was purified by column chromatogra-
phy using 15% EtOAc in pet. ether to give the carbonate ester 7e
(0.119 mg, 82%) as a yellow solid, mp: 130e132 ꢄC; TLC Rf 0.48 (15%
4.4. General procedure for the synthesis of caged carbonates
(7aed)
3-(Hydroxymethyl)perylene (1 equiv) was dissolved in dry DCM
(5 ml), and to the solution corresponding alcohol chloroformate
(1 equiv) was added followed by 1.2 equiv of N,N-dimethylpyridin-
4-amine (DMAP). The reaction mixture was stirred at room tem-
perature for 8e10 h. The solvent was removed by rotary
EtOAc in pet. ether); 1H NMR (CDCl3, 200 MHz):
4H), 7.88 (d, J¼8.2 Hz, 1H), 7.67 (d, J¼8.0 Hz, 2H), 7.63e7.15 (m, 9H),
d
¼8.22e8.09 (m,

A. Jana et al. / Tetrahedron 68 (2012) 1128e1136
1135
5.54 (s, 2H); 13C NMR (CDCl3, 50 MHz):
d
¼153.9, 151.4, 134.7, 132.9,
Solution (0.006 M) of potassium ferrioxalate was irradiated using
125 W medium pressure Hg lamp as visible light source (ꢀ410 nm)
and 1 M NaNO2 solution as UV cut-off filter. At regular interval of time
(3 min), 1 ml of the aliquots was taken out and to it 3 ml of 1,10-
phenanthroline solution and 2 ml of the buffer solution were added
and the whole solution was kept in dark for 30 min. The absorbance
of red phenanthrolineeferrous complex formed was then measured
spectrophotometrically at 510 nm. The amount of Fe2þ ion was de-
termined from the calibration graph. The calibration graph was
plotted by measuring the absorbance of phenanthrolineeferrous
complex at several known concentration of Fe2þ ion in dark. From the
slope of the graph the molar absorptivity of the phenan-
throlineeferrous complex was calculated to be 1.10ꢁ104 Mꢂ1 cmꢂ1 at
510 nm, which is found to be similar to reported value.19a Using the
known quantum yield for potassium ferrioxalate actinometer at
406.7 nm,24 the number of Fe2þ ion formed during photolysis and the
fraction of light absorbed by the actinometer, the incident intensity
132.7,131.9, 131.1,130.8, 129.8, 129.7, 128.8, 128.4, 128.2, 127.3, 126.7,
126.4, 126.5, 126.3, 123.2, 121.3, 121.1, 120.8, 120.7, 120.5, 119.5, 68.9;
FTIR (KBr, cmꢂ1): 1744; HRMS calcd for C28H18O3: 402.1256, found:
402.1260.
4.5. Photophysical properties of caged esters (4aeh) and
carbonates (7aed)
The UV/vis absorption spectra of degassed 2ꢁ10ꢂ6 M solution of
the caged esters (4aeh) and carbonates (7aed) in absolute ethanol
were recorded on a Shimadzu UV-2450 UV/vis spectrophotometer,
and the fluorescence emission spectra were recorded on a Hitachi
F-7000 fluorescence spectrophotometer. Fluorescence quantum
yield of the caged compounds was calculated using the Eq. 1.23
À
Á
ꢀ
ꢁ
ꢀ
ꢁ
h2CG
ðGradCG
Þ
Ff
¼
Ff
À
Á
(1)
(I0) of the 125 W Hg lamp was determined as 5.767ꢁ1017 quanta Sꢂ1
.
h2ST
CG
ST ðGradST
Þ
where, the subscript CG and ST denotes caged compound and
standard, respectively. 9,10-Diphenylanthracene in ethanol was
taken as standard. Ff is fluorescence quantum yield; Grad is the
gradient from the plot of integrated fluorescence intensity versus
4.8. Preparative photolysis
A solution of caged compound (4aeh) and (7bed) (0.05 mmol)
in acetonitrile/H2O (75/25) individually was irradiated using the
procedure described under deprotection photolysis. The irradiation
was monitored by TLC at regular intervals. After completion of
photolysis, solvent was removed under vacuum and the photo-
products (3-(hydroxymethyl)perylene, and the corresponding car-
boxylic acid or alcohol) were isolated by column chromatography
using increasing percentage of EtOAc in hexane as an eluant.
3-(Hydroxymethyl)perylene (1): 1H NMR (CDCl3, 200 MHz):
absorbance, and
h the refractive index of the solvent.
Since, fluorescence for both caged compounds and standard
were recorded in the same solvent Eq. 1a was used for the calcu-
lation of fluorescence quantum yield.
ꢀ
ꢁ
ꢀ
ꢁ
ðGradCG
ST ðGradST
Þ
Ff
¼
Ff
(1a)
CG
Þ
d
¼8.23 (m, 4H), 7.93 (d, J¼8.6 Hz, 1H), 7.72 (d, J¼7.8 Hz, 2H),
4.6. Deprotection photolysis of caged esters (4aeh) and
carbonates (7aed)
7.56e7.33 (m, 4H), 5.07 (s, 2H).
A solution of caged compound (4c) (0.05 mmol) in MeOH/H2O
(95/5 v/v) was photolysed and the irradiation was monitored by
HPLC at regular intervals. After completion of photolysis, solvent
was removed under vacuum and the photoproducts (3-(methox-
ymethyl)perylene and 4-methoxybenzoic acid) were isolated by
column chromatography using increasing percentage of EtOAc in
hexane as an eluant.
A solution of 10ꢂ4 M of the caged esters (4aeh) and carbonates
(7aed) was prepared in acetonitrile/H2O (75/25) individually. Half
of the solution was kept in dark and to the remaining half nitrogen
was passed and irradiated using 125 W medium pressure Hg lamp
as visible light source (ꢀ410 nm) and 1 M NaNO2 solution as UV
cut-off filter. At regular interval of time, 20 ml of the aliquots was
3-(Methoxymethyl)perylene: 1H NMR (CDCl3, 200 MHz):
taken and analyzed by RP-HPLC using mobile phase methanol, at
a flow rate of 1 ml/min (detection: UV 254 nm). Peak areas were
determined by RP-HPLC, which indicated gradual decrease of the
caged compound with time, and the average of three runs. The
reaction was followed until the consumption of the caged com-
pound is less than 5% of the initial area. Based on HPLC data for each
caged compounds, we plotted normalized [A] (HPLC peak area)
versus irradiation time. We observed an exponential correlation for
the disappearance of the caged compounds, which suggested a first
order reaction. Further, the quantum yield for the photolysis of
caged compounds was calculated using Eq. 2
d
¼8.23e8.14 (m, 4H), 7.94 (d, J¼8.2 Hz, 1H), 7.69 (d, J¼8.2 Hz, 2H),
7.58e7.44 (m, 4H), 4.85 (s, 2H), 3.48 (s, 3H).
Acknowledgements
We thank DST (SERC Fast Track Scheme) for financial support,
DST-FIST for 400 MHz NMR and CDRI Lucknow for HRMS analysis,
we also thank Prof. M. Halder for his valuable suggestions. A.J. and
M.I. are thankful to CSIR for their fellowship.
À
Á
Supplementary data
À
Á
kp
I0ðFCG
CG
Þ
Fp
¼
(2)
CG
Supplementary data related to this article can be found online at
doi:10.1016/j.tet.2011.11.074.
where, the subscript ‘CG’ denotes caged compound. Fp is the
photolysis quantum yield, kp is the photolysis rate constant, and I0
is the incident photon flux and F is the fraction of light absorbed.
Potassium ferrioxalate was used as an actinometer.
References and notes
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4.7. Determination of incident photon flux (I0) of the UV lamp
by potassium ferrioxalate actinometry
Potassium ferrioxalate actinometry19,20 was used for the de-
termination of incident photon flux (I0) of the UV lamp used for irra-
diation. Solution of potassium ferrioxalate, 1,10-phenanthroline and
thebuffersolutionwerepreparedfollowingtheliteratureprocedure.19

1136
A. Jana et al. / Tetrahedron 68 (2012) 1128e1136
8. Furuta, T.; Hirayama, Y.; Iwamura, M. Org. Lett. 2001, 3, 1809.
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