Phenacyl ester derivatives bearing heterocycles as models for photocleavable linkers: Synthesis and photolysis studies

Article abstract of DOI:10.1016/j.tet.2012.06.100

The synthesis of several phenacyl ester derivatives linked to oxygen and nitrogen heterocycles, benzoquinolone and (benzo)coumarins, was carried out in an effort to obtain systems that could be applied as photocleavable (bi)functional linkers for solid phase peptide synthesis. The heterocycles were attached to a spacer, with different lengths, followed by coupling to 2-bromo-1-phenylpropan-1-one, acting as a model for the solid support. Photolysis studies of the resulting phenacyl ester derivatives were carried out by irradiation in a photochemical reactor at different wavelengths (300, 350 and 419 nm), in methanol/HEPES buffer solution (80:20).

Full text of DOI:10.1016/j.tet.2012.06.100

Tetrahedron 68 (2012) 8024e8032
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Phenacyl ester derivatives bearing heterocycles as models for photocleavable
linkers: synthesis and photolysis studies
*
Andrea S.C. Fonseca, M. Sameiro T. Gonc¸ alves, Susana P.G. Costa
Centre of Chemistry, University of Minho, Campus of Gualtar, 4710-057 Braga, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of several phenacyl ester derivatives linked to oxygen and nitrogen heterocycles, benzo-
quinolone and (benzo)coumarins, was carried out in an effort to obtain systems that could be applied as
photocleavable (bi)functional linkers for solid phase peptide synthesis. The heterocycles were attached to
a spacer, with different lengths, followed by coupling to 2-bromo-1-phenylpropan-1-one, acting as
a model for the solid support. Photolysis studies of the resulting phenacyl ester derivatives were carried
out by irradiation in a photochemical reactor at different wavelengths (300, 350 and 419 nm), in
methanol/HEPES buffer solution (80:20).
Received 1 February 2012
Received in revised form 21 June 2012
Accepted 26 June 2012
Available online 4 July 2012
Keywords:
Coumarin
Quinolone
Ó 2012 Elsevier Ltd. All rights reserved.
Phenacyl derivatives
Photocleavable linkers
Photolysis
1. Introduction
It has been proven the possibility of using different wavelengths
of irradiation to achieve orthogonality within a set of photolabile
The possibility of breaking bonds efficiently by irradiation
with light of appropriate wavelength without the need of any
other reagents is very appealing for solid phase peptide synthesis
(SPPS) where the substrate is bound to a polymeric matrix,
usually through a linker that can be cleaved by light in the very
last step of the sequence, in order to liberate the desired prod-
uct.1 Additionally, photocleavable protecting groups can be used
in the intermediate coupling cycles as temporary protection for
the amino acids terminal function, since during the synthesis it is
necessary that side chain protecting groups remain in place while
those at the main chain terminal should be cleaved before
starting a new cycle.2 The use of photolabile linkers became
widespread in the generation of combinatorial libraries of or-
ganic molecules because it allows a release of the library com-
pounds under very mild conditions. Light-induced detachment is
orthogonal to acid and basic reaction conditions and therefore
affords additional flexibility in the synthesis on solid support.
The prerequisites for a photochemically removable linker are the
same as those described for photochemically removable pro-
tecting groups.3
protecting groups. Based on these studies, the concept of chro-
matic orthogonality has been coined as ‘the possibility of
transforming photochemically a specific chromophore at a spe-
cific wavelength, without affecting other photosensitive
moieties’.5
In recent years, we have been involved in the design of het-
erocyclic photocleavable protecting groups, derived from oxygen
and nitrogen such as (benzo)coumarins, (benzo)quinolones and
their thionated analogues, and benzozaxoles.6 These moieties have
been used in the protection of amino acids, as models for bi-
functional biomolecules, by coupling through ester and urethane
bonds. Attempts were made to optimise the photolytic process at
longer wavelengths, by tailoring of the heterocycle structure, and
interesting results were obtained for some systems, resulting in fast
cleavage of the bond between the amino acid and the heterocycle at
350 and 419 nm. Considering our previous experience in this field,
we now report the synthesis of phenacyl ester derivatives linked to
heterocycles, (benzo)coumarins and (benzo)quinolones, in order to
obtain systems that could be applied as photocleavable (bi)func-
tional linkers for solid phase synthesis, allowing photolysis at
longer wavelengths with short irradiation times. In principle, a bi-
functional linker would allow the selective cleavage of each bond
(resin-linker, with l1, and linker-substrate, with l2) leaving the
other bond intact. If the linker possessed a property like fluores-
cence, one could choose to cleave the resin-linker bond as a means
to obtain a fluorescently labelled substrate (Scheme 1).
One of the critical issues in protecting group chemistry is
orthogonality, i.e., the possibility of selectively removing one
group in the presence of others in any chronological sequence.4
* Corresponding author. E-mail address: spc@quimica.uminho.pt (S.P.G. Costa).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.06.100

A.S.C. Fonseca et al. / Tetrahedron 68 (2012) 8024e8032
8025
Benzocoumarins bearing a different ring fusion (angular and
linear) were also obtained by a Pechmann reaction between 2,
7-dihydroxynaphthalene with ethyl 3-oxobutanoate, resulting
a mixture of the two isomers that were very difficult to separate by
chromatographic methods, and as a result, the mixture of the iso-
meric benzocoumarins was used in the subsequent synthetic steps,
being only separated in the final product (Scheme 4).
The heterocycles bearing a hydroxyl group 1 and 6aec were
coupled to ethyl bromoacetate (in the case of 1) or ethyl
4-bromobutanoate (in the case of 1 and 6aec), in the presence of
potassium carbonate in N,N-dimethylformamide,9 in order to in-
troduce a spacer of different length, resulting in the corresponding
coumarins 2a,b, benzoquinolone 7a and benzocoumarins 7b,c.
Coumarin 2c was obtained from 2b by a classical nitration pro-
cedure in the presence of nitric acid and sulfuric acid. The ethyl
esters were then hydrolysed to the corresponding carboxylic acids
3aec, 8aec (Schemes 2e4).9
phenacyl
ester bond
peptide
Heterocycle
ester bond
λ2
λ1
cleavage
cleavage
for
for analysis
peptide release
λ2
ester bond
peptide
peptide
Heterocycle
labelled peptide
Scheme 1. Schematic representation of a photocleavable bifunctional linker for solid
phase synthesis.
2. Results and discussion
2.1. Synthesis
Finally, the phenacyl ester derivatives bearing different hetero-
cycles were synthesised by coupling of 2-bromo-1-phenylpropan-
1-one, acting as a model for the solid support (such as the bromi-
nated Wang resin), to the carboxylic acid terminal of heterocycle
7-Hydroxy-4-methylcoumarin
1 was synthesised through
a Pechmann reaction between 1,3-dihydroxybenzene and ethyl 3-
oxobutanoate, catalysed by aqueous sulfuric acid at room temper-
ature, through a method reported earlier (Scheme 2).6c
O
O
O
Br
O
n
O
R
O
DMF, K2CO3
aq 70% H2SO4
O
HO
OH
O O
HO
O
O
n
O
1
R
H
H
n
2a
2b
1
3
3
HNO3
H2SO4
2c NO2
O
Br
Ph
R
R
H+
O
O
HO
KF, DMF
Ph
O
O
X
O
O
O
n
n
O
O
R
X
n
1
3
3
3
R
H
H
n
1
3
3
4a
4b
4c NO2
4d
H
O
3a
3b
H
O
O
S
LR
3c NO2
toluene
H
Scheme 2. Synthesis of coumarin phenacyl derivatives 4aed.
7-(N-Methylamino)naphthalen-2-ol 5 was obtained from 2,
7-dihydroxynaphthalene by reductive treatment with methyl-
amine.7 Through a modified Knorr synthesis8 between 5 and ethyl
3-oxobutanoate, in acidic media, 9-hydroxy-1,4-dimethylbenzo[f]
quinolin-3(4H)-one 6a was obtained (Scheme 3).
derivatives 3aec, and 8aec, in N,N-dimethylformamide, at room
temperature, in the presence of potassium fluoride.10 The corre-
sponding coumarin 4aec, benzoquinolone 9a and benzocoumarin
9b,c phenacyl esters were obtained as solids in moderate to good
yields (Table 1). The coumarin phenacyl ester 4b was further
O
O
O
Na2S2O5
HO
N
O
aq 70% H2SO4
HO
OH
MeNH2
HO
N
H
O
O
6a
5
K2CO3
DMF
Br
3
O
Br
O
Ph
R
O
RO
3O
N
Ph
3O
N
7a Et
H+
KF, DMF
O
O
8a
H
O
O
9a
Scheme 3. Synthesis of benzoquinolone phenacyl derivatives 9a.

8026
A.S.C. Fonseca et al. / Tetrahedron 68 (2012) 8024e8032
O
O
O
+
HO
6b
O
aq 70% H2SO4
HO
OH
HO
O O
O
6c
O
K2CO3
DMF
Br
O
3
R
R
+
RO
RO
O
7b Et
8b
7c Et
3O
O
O
O O
H+
H+
3
O
O
O
H
8c
H
O
KF
Br
Ph
DMF
O
O
O
O
9b
9c
+
Ph
Ph
3O
3O
O
O
O
O
O
Scheme 4. Synthesis of benzocoumarin phenacyl derivatives 9b,c.
Table 1
the presence of the nitro group (a known fluorescence quencher)
and the presence of a thiocarbonyl group, due to the heavy atom
induced spineorbit coupling by the sulfur atom that gives rise to an
effective intersystem crossing mechanism that lowers the fluores-
cence emission.14 As for the maximum wavelengths of absorption
and emission, the most striking difference, a 77 and 52 nm bath-
ochromic shift, occurred after conversion of the carbonyl (4b) to
a thiocarbonyl group (4d), thus confirming the benefits of such
change for shifting absorption and emission to longer wavelengths
(see Fig. 1). Absorption at longer wavelengths could indicate that
photolysis at higher wavelengths might be possible with shorter
irradiation times.
UVevis absorption and emission data for phenacyl derivatives 4aed and 9aec in
absolute ethanol
Compound
Absorption
Emission
labs (nm)
log
3
lem (nm)
fF
Stokes’ shift (nm)
4a
4b
4c
4d
9a
9b
9c
316
320
321
397
335
347
343
4.22
4.28
4.00
4.28
4.00
4.05
4.21
384
381
394
433
407
441
466
0.07
0.13
0.04
0.01
0.21
0.82
0.26
68
61
73
36
72
94
123
reacted with Lawesson’s reagent (LR), by heating at reflux in dry
toluene,11 affording the expected thionated derivative 4d. All final
products were fully characterised by the usual techniques (1H and
13C NMR spectroscopy, mass spectrometry, IR, UVevis absorption
and fluorescence spectroscopy) and the obtained data were in
agreement with the structure of the compounds.
The photophysical properties (absorption and emission max-
ima, molar absorptivities and fluorescence quantum yields, fF) of
the newly synthesised compounds 4aed and 9aec were evaluated
by tracing the UVevis absorption and emission spectra of degassed
10ꢀ5 M solutions in absolute ethanol (Table 1). Relative fluores-
1.2
1
4a
4b
0.8
4c
0.6
0.4
0.2
0
4d
9a
9b
9c
cence
quantum
yields
were
calculated
using
9,10-
diphenylanthracene as standard (fF¼0.95 in ethanol),12 for all
compounds, except for thionated coumarin 4d, which required
a 0.05 M solution of quinine in sulfuric acid as standard (fF¼0.55).13
For the fF determination, the fluorescence standard was excited at
the wavelengths of maximum absorption found for each of the
compounds to be tested and in all fluorimetric measurements, the
absorbance of the solution did not exceed 0.1.
300
350
400
450
500
Wavelength (nm)
Fig. 1. Normalised UVevis absorption spectra of coumarins 4aed, benzoquinolone 9a
and benzocoumarin 9b,c conjugates.
By analysis of the data in Table 1 for phenacyl esters 4 and 9, it
was found that the fluorescence quantum yield was influenced by
the nature of the heterocycle: benzoquinolone 9a and benzocou-
marins 9b,c had the highest quantum yields and, in the latter case,
the fusion of the ring was also significant as the angular derivative
9b showed higher quantum yield when compared to the linear
derivative 9c. Among coumarins 4, nitration (in the case of 4c) and
thionation (in the case of 4d) result in an expected decrease of the
fluorescence quantum yield when compared to coumarin 4b, due to
2.2. Photolysis studies of phenacyl derivatives 4aed and 9aec
The evaluation of the various phenacyl ester derivatives 4aed
and 9aec bearing different heterocycles as photocleavable linkers
was carried out by photolysis under irradiation at different wave-
lengths (300, 350 and 419 nm). Solutions of the mentioned com-
pounds (1ꢁ10ꢀ4 m) in an 80:20 mixture of methanol and HEPES

A.S.C. Fonseca et al. / Tetrahedron 68 (2012) 8024e8032
8027
buffer, were irradiated in a Rayonet RPR-100 reactor in order to
determine the most favourable cleavage conditions. The course of
the photocleavage reaction was followed by reverse phase HPLC
with UV detection. The determined irradiation time represents the
time necessary for the consumption of the starting materials until
less than 5% of the initial peak area (A) was detected. Based on HPLC
data, the plot of ln A versus irradiation time showed a linear cor-
relation for the disappearance of the starting material, which sug-
gested a first order reaction, obtained by the linear least squares
methodology for a straight line, with high correlation coefficients,
and rate constants were calculated (Table 2).
trend was reversed and the thionated coumarin derivative 4d
showed the shortest irradiation time of all compounds (82 min),
with the irradiation times for 4a and 4b being impractical (Fig. 3).
Due to this promising result, irradiation at 419 nm was carried out
for 4d and the result improved with an irradiation time of about
59 min.
15
14
4a
Table 2
4d
13
Irradiation times (tirr, in min), photochemical quantum yields (FP,ꢁ10ꢀ3) and rate
constants (in minꢀ1) for derivatives 4aed, 9aec and phenylalanine ester conjugates
4e, 9d,e at different wavelengths in methanol/HEPES buffer (80:20) solution
9a
9b
12
Compound 300 nm
tirr FP
350 nm
tirr FP
419 nm
9c
k
k
tirr
FP
k
11
4a
4b
4c
30.8 0.187 0.097 3269 0.027 0.001
28.4 0.189 0.105 1576 0.017 0.002
d
d
d
d
d
d
d
d
d
0
20
40
60
80
100
Irradiation time (min)
260 0.018 0.012 784
0.015 0.004
4d
110 0.207 0.028 82.4 0.066 0.038 58.8 0.056 0.053
33.7 0.235 0.090 253 0.054 0.013
101 0.272 0.032 45.7 0.106 0.072 36.6 0.071 0.084
39.2 0.079 0.077 40.4 0.133 0.075 6003 0.027 0.001
4e6c
4f15
9a
d
d
d
Fig. 3. Plot of ln A versus irradiation time for the photolysis of conjugates 4a (-), 4d
(:), 9a (C), 9b (3) and 9c (A) at 350 nm in methanol/HEPES (80:20) solution.
9b
9c
d
d
d
85.8 0.048 0.035 4989 0.050 0.001
837 0.004 0.004
d
d
d
d
d
d
As for compounds 9a and 9b, it was found that both cleaved in
similar rates at 350 and 419 nm, which were faster for irradiation at
350 nm (between 40 and 86 min). Benzoquinolone 9a cleaved two
times faster than benzocoumarin 9b at 350, while at 419 nm the
difference in the irradiation times was minimal.
9d
44.2 0.489 0.067 43.0 0.619 0.070 2996 0.114 0.001
33.6 0.851 0.089 38.1 0.441 0.083 301 0.659 0.010
9e6e
Since the purpose of this work was to evaluate the possibility of
using the above mentioned heterocycles as photocleavable linkers
for solid phase synthesis, their behaviour towards irradiation was
compared to that of the related coumarins 4e,f, benzoquinolone 9d
and benzocoumarin 9e phenylalanine conjugates, previously
reported by us (Fig. 2).6e,15
The photochemical quantum yields (FP) were calculated based
on half-lives (t1/2), molar absorptivities at the irradiation wave-
length ( ) and the incident photon flux (I0), which was determined
3
by potassium ferrioxalate actinometry.16 The photocleavage pro-
cess was not as efficient as desirable (see FP in Table 2), possibly by
deactivation via fluorescence pathways that compete with the
photochemical reaction, as well as the type of reactor used (open
chamber reactor).
As for the benzocoumarin derivatives 9b and 9c, their structural
difference being the ring fusion, the angular 9b cleaved faster than
the linear 9c at 350 nm (at about 10 times faster). For this reason,
photolysis at 419 nm was only done for derivative 9b, but it resulted
in a very long and unfeasible irradiation time.
O
Ph
O
O
X
Ph
O
O
O
O
X
Ph
O
N
H
Ph
O
N
H
O
O
O
As a result of the above comments, the most promising het-
erocyclic linker would be the thionated coumarin 4d at 419 nm if its
application was considered in connection with a short-wavelength
and short tirr photocleavable protecting group: in this scenario, the
protecting group would be cleaved without meaningful cleavage of
the bond to the linker and in the end the linker would be cleaved by
irradiation at 419 nm. On the other hand, if a substrate was to be
cleaved from the solid support bearing its temporary protecting
groups, compound 4d would allow fast cleavage at 419 nm, with
the remaining structure of the substrate being unaffected.
We envisaged the application of coumarin, benzoquinolone and
benzocoumarins as bifunctional linkers (with a phenacyl ester
bond to the solid support by the fused benzene ring and an ester
bond to the substrate by the ring containing the heteroatom). For
this to be feasible, the photolytic behaviour of the ester and phe-
nacyl ester bonds should be very distinct. Thus, a comparison be-
tween the obtained photolysis results with previously reported
structurally related coumarins 4e,f, benzoquinolone 9d and ben-
zocoumarin 9e phenylalanine conjugates (Fig. 2) was carried out.
Based on the data presented in Table 2, the behaviour of cou-
marins 4a and 4e was similar at 300 nm but at 350 nm a distinction
was possible with the ester bond in 4e cleaving more than 10 times
faster than the phenacyl ester bond in 4a (ca. 4 h vs 54 h). For the
4e X = O
4f X = S
9d X = NCH3
9e X = O
Fig. 2. Structure of phenylalanine coumarin 4e,f, benzoquinolone 9d and benzocou-
marin 9e ester conjugates.
In these conjugates, the model amino acid is linked by an ester
bond to the heterocycle at the ring containing the heteroatom. In
the new reported phenacyl derivatives, the link is at the fused
benzene ring. With this in mind, and the different type of link to the
amino acid (ester) and to the model of the solid support (phenacyl
ester), it was expected to have some differentiation on the photo-
cleavage behaviour. The photolysis data of conjugates 4e,f and 9d,e,
previously reported by us, is presented in Table 2 for easier
comparison.
Comparison of the data for phenacyl coumarin derivatives 4aed
revealed the influence of the structure of the compound on the
photocleavage rates: the unsubstituted coumarins 4a and 4b
cleaved with similar rates at 300 nm and much faster than de-
rivatives 4c,d. The length of the spacer was not critical for the
photolytic behaviour in the case of compounds 4a,b. At 350 nm, the

8028
A.S.C. Fonseca et al. / Tetrahedron 68 (2012) 8024e8032
thionated coumarins 4d and 4f, cleavage rates of the ester and
phenacyl ester bonds were very similar and the difference seen at
350 nm was not high enough for a clearly distinct performance. For
the benzoquinolones 9a and 9d, also no clear difference was seen
between the cleavage rates of the ester and phenacyl ester bonds.
Comparison of the results for benzocoumarins 9b and 9e revealed
that at 419 nm the ester bond cleaved in much shorter irradiation
times (ca. 5 h) when compared to the phenacyl ester bond (ca.
83 h).
Phenacyl esters have been widely used as caging groups for
biological applications and the cleavage mechanism is thought to
involve the release of a carboxylic acid and a ketone as the by-
product.17 It is reasonable to assume a related pathway yielding the
same photoproducts, carboxylic acids 3aed and 8aec and
1-phenylpropan-1-one. Esters, on the other hand, are cleaved by
a homolytical/heterolytical fission of the OeCH2 bond, resulting in
the release of a carboxylic acid and an alcohol as the by-product.6c
performed using a Licrospher 100 RP18 (5 mm) column in an HPLC
system composed by a Jasco PU-980 pump, a Shimadzu SPD-GAV
UV/Vis detector and a Shimadzu C-RGA Chromatopac register. All
commercial reagents were used as received.
4.2. Synthesis of compounds 1e9
4.2.1. General procedure for the synthesis of compounds 1 and
6b,c. Compounds 1 and 6b,c were prepared by reaction of 1,
3-dihydroxybenzene or 2,7-dihydroxynaphthalene (1 equiv), re-
spectively, with ethyl 3-oxobutanoate (1.5 equiv), and aqueous 70%
sulfuric acid (5 mL) was added. The reaction mixture was stirred at
room temperature until reaction completion. The reaction mixture
was poured into ice, the formed precipitate was filtered, washed
with water and dried at 50 ꢂC.
4.2.1.1. 7-Hydroxy-4-methylcoumarin, 1. After stirring at room
temperature for 17 h, compound 1 was obtained as a white solid
(2.135 g, 12.1 mmol, 89%). Mp¼185.3e186.8 ꢂC. 1H NMR (300 MHz,
3. Conclusions
DMSO-d6):
d
¼2.34 (3H, s, CH3), 6.10 (1H, s, H3), 6.68 (1H, d, J 2.7 Hz,
The synthesis of several phenacyl ester derivatives linked to
oxygen and nitrogen heterocycles, benzoquinolone and (benzo)
coumarins, was described by reaction of 2-bromo-1-phenylpropan-
1-one, acting as a model for a solid support, with a carboxylic acid
group. The resulting compounds were tested as photocleavable
linkers with a spacer, with different lengths, by irradiation in
a photochemical reactor at different wavelengths (300, 350 and
419 nm), in methanol/HEPES buffer solution (80:20). It was found
that the most promising heterocyclic linker would be the thionated
coumarin 4d at 419 nm if it was considered in connection with
a fast-cleaving short-wavelength photocleavable protecting group.
As for the use of the synthesised phenacyl esters as bifunctional
linkers, involving the simultaneous presence of an ester and
a phenacyl ester bond in the same molecule, it was possible to
confirm a different photolytic behaviour for these two types of
bond, when comparing the results for the phenacyl ester bond in
coumarin 4a and benzocoumarin 9b with the corresponding ester
bond conjugates 4e and 9e.
H8), 6.78 (1H, dd, J 2.7 and 8.7 Hz, H6), 7.56 (1H, d, J 8.7 Hz, H5),
10.51 (1H, s, OH). 13C NMR (75.4 MHz, DMSO-d6):
d¼18.12 (CH3),
102.18 (C8), 110.25 (C3), 112.01 (C4a), 112.87 (C6), 126.62 (C5),
153.56 (C4), 154.85 (C8a), 160.31 (C2), 161.18 (C7). FTIR (KBr 1%,
cmꢀ1):
n
¼3190, 1679, 1652, 1600, 1515, 1451, 1390, 1369, 1336, 1274,
1239, 1213, 1160, 1134, 1068, 1018, 983, 898, 864, 846, 806, 761, 747,
693, 641, 583, 510. UVevis (ethanol, nm): lmax (log
3
)¼323 (4.12).
MS m/z (ESI, %): 176 ([MþH]þ, 68). HRMS: m/z (ESI) calcd for
C10H8O3 176.0473, found 176.0479.
4.2.1.2. Benzocoumarins 6b and 6c. After stirring for 5 days,
a mixture of compounds 6b,c was obtained as a light pink solid
(0.367 g, 74%). The solid mixture was used in the next reaction
without further purification or separation.
4.2.1.2.1. 1-Methyl-9-hydroxybenzo[f]coumarin, 6b. 1H NMR
(400 MHz, DMSO-d6):
d
¼2.86 (3H, d, J 0.8 Hz, CH3), 6.41 (1H, d, J
1.2 Hz, H2), 7.13 (1H, dd, J 8.8 and 2.0 Hz, H8), 7.28 (1H, d, J 8.8 Hz,
H5), 7.90 (1H, d, J 8.8 Hz, H7), 8.00 (1H, d, J 2.0 Hz, H10), 8.04 (1H, d,
J 8.8 Hz, H6). 13C NMR (100.6 MHz, DMSO-d6):
d¼18.17 (CH3),
4. Experimental section
4.1. General
108.66 (C10), 110.67 (C10b), 113.90 (C5), 115.77 (C2), 117.25 (C8),
125.22 (C6a), 130.31 (C10a), 131.33 (C7), 134.35 (C6), 152.27 (C1),
155.35 (C4a), 157.80 (C9), 159.38 (C3).
4.2.1.2.2. 4-Methyl-8-hydroxybenzo[g]coumarin, 6c. 1H NMR
All melting points were measured on a Stuart SMP3 melting
point apparatus. TLC analyses were carried out on 0.25 mm thick
precoated silica plates (Merck Fertigplatten Kieselgel 60F254) and
spots were visualised under UV light. Chromatography on silica gel
was carried out on Merck Kieselgel (230e240 mesh). IR spectra
were determined on a BOMEM MB 104 spectrophotometer. UVevis
absorption spectra (200e700 nm) were obtained using a Shimadzu
UV/2501PC spectrophotometer. NMR spectra were obtained on
a Varian Unity Plus Spectrometer at an operating frequency of
300 MHz for 1H and 75.4 MHz for 13C or a Bruker Avance III 400 at
an operating frequency of 400 MHz for 1H and 100.6 MHz for 13C
using the solvent peak as internal reference at 25 ꢂC. All chemical
shifts are given in parts per million using dH Me4Si¼0 ppm as ref-
erence and J values are given in hertz. Assignments were made by
comparison of chemical shifts, peak multiplicities and J values and
were supported by spin decoupling-double resonance and bidi-
mensional heteronuclear correlation techniques. Low and high
resolution mass spectrometry analyses were performed at the
‘C.A.C.T.I.dUnidad de Espectrometria de Masas’, at University of
Vigo, Spain. Fluorescence spectra were collected using a Fluo-
roMax-4 spectrofluorometer. Photolyses were carried out using
a Rayonet RPR-100 chamber reactor equipped with 10 lamps of 300
(21 W), 350 (24 W) and 419 (14 W) nm. HPLC analyses were
(400 MHz, DMSO-d6): d 2.50 (3H, s, CH3), 6.33 (1H, s, H3), 7.11 (1H,
dd, J 2.4 and 8.8 Hz, H7), 7.15 (1H, d, J 2.0 Hz, H9), 7.61 (1H, s, H10),
7.94 (1H, d, J 8.8 Hz, H6), 8.27 (1H, s, H5), 10.19 (1H, s, OH). 13C NMR
(100.6 MHz, DMSO-d6):
d
¼18.71 (CH3), 105.22 (C9), 110.46 (C10),
114.50 (C3), 117.90 (C4a), 119.63 (C7), 125.02 (C5), 125.28 (C5a),
130.43 (C6), 136.79 (C9a), 151.00 (C4), 152.08 (C10a), 158.72 (C8),
161.15 (C2).
4.2.2. Synthesis of 7-(N-methylamino)naphthalen-2-ol, 5. 2,7-
Dihydroxynaphthalene (1 equiv, 0.500 g, 3.12 mmol) was mixed
to sodium metabisulfite (2.44 equiv, 1.448 g, 7.62 mmol), sodium
hydroxide (5.2 equiv, 0.650 g, 16.20 mmol), water (12 mL) and
methylamine hydrochloride (5.2 equiv, 1.094 g, 16.20 mmol). The
reaction mixture was stirred, in an oil bath, at 90 ꢂC for 3 days. The
reaction mixture was extracted with ethyl acetate (3ꢁ10 mL), and
the organic layer was dried with anhydrous magnesium sulfate,
filtered and evaporated under reduced pressure. The resulting
crude was purified by silica gel column chromatography using
dichloromethane/methanol (98:2) as eluent, yielding compound 5
as dark yellow solid (0.300 g, 1.73 mmol, 56%). Mp¼101.4e103.3 ꢂC.
1H NMR (300 MHz, DMSO-d6):
5.76e5.81 (1H, m, NH), 6.42 (1H, d, J 2.1 Hz, H8), 6.62e6.68 (2H, m,
H3 and H6), 6.80 (1H, d, J 2.7 Hz, H1), 7.40e7.45 (2H, m, H4 and H5),
d
¼2.72 (3H, d, J 4.8 Hz, CH3),

A.S.C. Fonseca et al. / Tetrahedron 68 (2012) 8024e8032
8029
9.32 (1H, s, OH). 13C NMR (75.4 MHz, DMSO-d6):
d
¼29.70 (CH3),
CH2CH3), 6.13 (1H, t, J 1.3 Hz, H3), 6.80 (1H, d, J 2.4 Hz, H8), 6.85 (1H,
dd, J 8.8 and 2.4 Hz, H6), 7.49 (1H, d, J 8.8 Hz, H5). 13C NMR
100.48 (C8), 106.96 (C1), 113.16 (C3), 114.92 (C6), 121.09 (C8a),
128.04 (C5), 128.84 (C4), 136.79 (C4a), 147.94 (C7), 155.52 (C2). FTIR
(100.6 MHz, CDCl3):
30.62 ( CH2), 60.52 (CH2CH3), 67.30 (
112.44 (C6), 113.60 (C4a), 125.50 (C5), 152.48 (C4), 155.23 (C8a), 161.16
(C2), 161.84 (C7), 172.92 (C]O ester). FTIR (KBr 1%, cmꢀ1):
d
¼14.20 (CH2CH3), 18.63 (CH3), 24.35 (
bCH2),
(liquid film, cmꢀ1):
n
¼3335, 1634, 1616, 1525, 1418, 1351, 1241, 1205,
a
g
CH2), 101.46 (C8), 111.96 (C3),
1120, 1045, 965, 951, 877, 830, 613. MS m/z (EI, %): 173 (Mþ, 100).
HRMS: m/z (EI) calcd for C11H11ON 173.0841, found 173.0850.
n¼3017,
1744, 1615, 1561, 1513, 1478, 1457, 1419, 1393, 1368, 1346, 1281, 1264,
1204, 1182, 1157, 1135, 1070, 1019, 1118, 1100, 995, 951, 811, 781, 665,
650. MS m/z (EI, %): 290 (Mþ, 79). HRMS: m/z (EI) calcd for C16H18O5
290.1154, found 290.1160.
4.2.3. Synthesis of 9-hydroxy-1,4-dimethylbenzo[f]quinolin-3(4H)-
one, 6a. Ethyl 3-oxobutanoate (1.5 equiv, 0.33 mL, 2.60 mmol) was
heated at 180 ꢂC and compound 5 (0.300 g, 1.73 mmol) was added.
The reaction mixture was heated at reflux for 45 min. After
evaporation of the excess ethyl 3-oxobutanoate under vacuum,
the residue was stirred with aqueous 70% sulfuric acid (5 mL) at
95 ꢂC for 45 min. After cooling to room temperature, water
(10 mL) was added to the mixture, followed by extraction with
ethyl acetate (3ꢁ15 mL). The organic layer was dried with anhy-
drous magnesium sulfate, filtered and evaporated under reduced
pressure. The resulting solid was purified by silica gel column
chromatography using chloroform/methanol (100:1) of increasing
4.2.4.3. Ethyl 4-(1,4-dimethylbenzo[f]quinolin-3(4H)-one-9-
yloxy)butanoate, 7a. Compound 7a was obtained as beige solid
(0.040 g, 1.13 mmol, 55%). 1H NMR (400 MHz, CDCl3):
d
¼1.28 (3H, t,
CH2), 2.59 (2H, t, J 7.2 Hz,
CH2), 2.94 (3H, s, CH3), 3.86 (3H, s, CH3), 4.15e4.20 (4H, m, CH2
J 7.2 Hz, CH2CH3), 2.20e2.23 (2H, m,
b
a
g
and CH2CH3), 6.71 (1H, s, H2), 7.18 (1H, dd, J 8.8 and 2.4 Hz, H8), 7.46
(1H, d, J 9.2 Hz, H5), 7.82 (1H, d, J 8.8 Hz, H7), 7.91 (1H, d, J 9.2 Hz,
H6), 8.00 (1H, d, J 2.0 Hz, H10). 13C NMR (100.6 MHz, CDCl3):
polarity as eluent, yielding compound 6a as
a
dark solid
d
¼14.23 (CH2CH3), 24.65 (
b
CH2), 26.77 (CH3), 30.48 (NCH3), 30.81
(0.269 g, mmol, 65%). Mp¼269.1e271.3 ꢂC. 1H NMR (400 MHz,
(a
CH2), 60.50 (CH2CH3), 66.96 (g
CH2), 107.65 (C10), 112.84 (C5),
DMSO-d6):
d
¼2.84 (3H, s, CH3), 3.72 (3H, s, NCH3), 6.57 (1H, d, J
115.63 (C10b), 115.87 (C8), 122.46 (C2), 125.17 (C6a), 130.53 (C7),
131.98 (C6), 132.43 (C10a), 140.80 (C4a), 147.51 (C1), 157.98 (C9),
0.4 Hz, H2), 7.07 (1H, dd, J 8.8 and 2.0 Hz, H8), 7.54 (1H, d, J 9.2 Hz,
H5), 7.86 (1H, d, J 8.8 Hz, H7), 8.01 (1H, d, J 9.6 Hz, H6), 8.02 (1H, d,
J 2.0 Hz, H10), 9.96 (1H, s, OH). 13C NMR (100.6 MHz, DMSO-d6):
161.51 (C3), 173.13 (C]O ester). FTIR (KBr 1%, cmꢀ1):
n
¼3021, 2995,
1718,1620, 1535,1517, 1458, 1367, 1233,1200, 1151, 1102, 1048, 1003,
993, 940, 777. MS m/z (EI, %): 353 (Mþ, 88). HRMS: m/z (EI) calcd for
C21H23O4N 353.1628, found 353.1661.
d
¼26.31 (CH3), 29.96 (NCH3), 108.63 (C10), 112.44 (C5), 113.90
(C10b), 116.12 (C8), 121.22 (C2), 123.89 (C6a), 130.73 (C7), 132.14
(C10a), 132.24 (C6), 140.64 (C4a), 147.59 (C1), 156.78 (C9), 160.16
(C3). FTIR (KBr 1%, cmꢀ1):
n
¼3165, 1625, 1614, 1590, 1522, 1495,
4.2.4.4. Benzocoumarins 7b and 7c. The compounds were
obtained as a beige solid mixture (0.260 g, 89%). The mixture was
used in the next reaction without further purification.
1470, 1428, 1416, 1291, 1234, 1100, 1069, 1009, 921, 887, 847, 666.
MS m/z (EI, %): 239 (Mþ, 81). HRMS: m/z (EI) calcd for C15H13O2N
239.0947, found 239.0893.
4.2.4.4.1. Ethyl 4-(1-methyl-benzo[f]coumarin-9-yloxy)butanoate,
7b. 1H NMR (400 MHz, CDCl3):
CH2CH3), 2.17e2.20 (2H, m,
(3H, s, CH3), 4.13e4.19 (4H,
d
¼1.24e1.29 (3H, t, J 7.2 Hz,
CH2), 2.57e2.59 (2H, m, CH2), 2.90
CH2 and CH2CH3), 6.31 (1H, s, H2), 7.17
4.2.4. General procedure for the synthesis of compounds 2a,b and
7aec. Compounds 1 and 6aec (1 equiv) were added to a suspen-
sion of potassium carbonate (1.5 equiv) in dry N,N-dime-
thylformamide. Ethyl bromoacetate (in the case of 1) or ethyl
b
a
g
(1H, dd, J 7.2 and 2.4 Hz, H8), 7.26 (1H, d, J 8.8 Hz, H5), 7.73e7.77
(1H, m, H7), 7.87 (1H, d, J 2.4 Hz, H10), 8.02 (1H, d, J 8.6 Hz, H-6). 13C
4-bromobutanoate (in the case of compounds
1
and 6aec)
NMR (100.6 MHz, CDCl3):
d
¼14.14 (CH2CH3), 18.17 (CH3), 24.61
(1.1 equiv) was added dropwise to the mixture. The reaction mix-
ture was stirred at room temperature for 1 day. Water (10 mL) was
added to the mixture, followed by extraction with ethyl acetate
(3ꢁ15 mL). The organic layer was washed with aqueous 5% HCl,
dried with anhydrous magnesium sulfate, filtered and evaporated
under reduced pressure. The resulting oil was purified by silica gel
column chromatography using chloroform/methanol (100:1) as
eluent, yielding compounds 2a,b and 7aec.
(b
CH2), 30.41 ( CH2), 61.05 (CH2CH3), 66.70 (g
a
CH2), 108.66 (C10),
110.67 (C10b), 113.90 (C5), 115.77 (C2), 117.25 (C8), 125.22 (C6a),
130.31 (C10a), 131.33 (C7), 134.35 (C6), 152.27 (C1), 155.35 (C4a),
157.80 (C9), 159.38 (C3).
4.2.4.4.2. Ethyl
noate, 7c. 1H NMR (300 MHz, CDCl3):
CH2CH3), 2.20e2.24 (2H, m, CH2), 2.54e2.60 (5H, m,
CH3), 4.15e4.21 (4H, m, CH2 and CH2CH3), 6.30 (1H, d, J 1.2 Hz, H3),
7.12e7.16 (2H, m, H7 and H9), 7.59 (1H, s, H10), 7.82 (1H, d, J 9.0 Hz,
H6), 8.01 (1H, s, H5). 13C NMR (100.6 MHz, CDCl3):
¼14.15
(CH2CH3), 18.45 (CH3), 24.54 ( CH2), 30.42 ( CH2), 61.03 (CH2CH3),
66.74 ( CH2), 105.25 (C9), 110.45 (C10), 114.51 (C3), 117.89 (C4a),
4-(4-methyl-benzo[g]coumarin-8-yloxy)buta-
¼1.26e1.28 (3H, t, J 7.2 Hz,
CH2 and
d
b
a
g
4.2.4.1. Ethyl 2-(4-methylcoumarin-7-yloxy)acetate, 2a. Compo-
und 2a was obtained as colourless oil (0.100 g, 0.38 mmol, 67%). 1H
d
b
a
NMR (300 MHz, CDCl3):
d
¼1.32 (3H, t, J 6.9 Hz, CH2CH3), 2.41 (3H, d, J
g
0.9 Hz, CH3), 4.26e4.33 (2H, m, CH2CH3), 4.69 (2H, s, CH2), 6.16 (1H, d,
J 0.9 Hz, H3), 6.78 (1H, d, J 2.7 Hz, H8), 6.92 (1H, dd, J 8.7 and 2.4 Hz,
119.90 (C7), 125.11 (C5), 125.31 (C5a), 130.44 (C6), 136.83 (C9a),
151.04 (C4), 152.09 (C10a), 158.70 (C8), 161.19 (C2).
H6), 7.53 (1H, d, J 9 Hz, H5). 13C NMR (75.4 MHz, CDCl3):
d¼14.13
(CH2CH3), 18.65 (CH3), 61.70 (CH2CH3), 65.33 (CH2), 101.67 (C8),
112.49 (C3), 112.55 (C6), 114.39 (C4a), 125.76 (C5), 152.36 (C4), 155.05
(C8a), 160.61 (C7), 161.03 (C2), 167.94 (C]O ester). FTIR (liquid film,
4.2.4.5. Ethyl 4-(4-methyl-6-nitrocoumarin-7-yloxy)butanoate,
2c. Compound 2b (0.072 g, 0.25 mmol) was dissolved in 96% sul-
furic acid (0.5 mL), in an ice bath, and 65% nitric acid (0.1 mL) was
added. The reaction mixture was stirred at room temperature for
30 min and then poured into ice, the formed precipitate was fil-
tered and dried. The solid was purified by silica gel column chro-
matography using dichloromethane/methanol (98:2) as eluent,
yielding compound 2c as yellow solid (0.033 g, 0.09 mmol, 36%). 1H
cmꢀ1):
n
¼3064, 2982, 2938, 1756, 1723, 1615, 1561, 1506, 1444, 1424,
1389, 1370, 1301, 1268, 1212, 1154, 1083, 1029, 981, 853, 818, 736, 703,
666, 634. MS m/z (EI, %): 265 (Mþ, 100). HRMS: m/z (EI) calcd for
C14H14O5 265.0841, found 265.0827.
4.2.4.2. Ethyl 4-(4-methyl-coumarin-7-yloxy)butanoate, 2b. Com-
pound 2b was obtained as white solid (0.130 g, 0.45 mmol, 82%),
NMR (300 MHz, CDCl3):
(2H, m, CH2), 2.42 (3H, d, J 1.2 Hz, CH3), 2.57e2.62 (2H, m,
3.01e3.40 (2H, m, CH2), 4.22e4.26 (2H, m, CH2CH3), 6.26 (1H, d, J
1.2 Hz, H3), 6.96 (1H, s, H8), 8.19 (1H, s, H5). FTIR (KBr 1%, cmꢀ1):
¼3054, 2987, 2868, 1745, 1714, 1623, 1546, 1422, 1385, 1363, 1301,
d
¼1.25e1.30 (3H, m, CH2CH3), 2.17e2.22
b
a
CH2),
Mp¼81.3e82.7 ꢂC. 1H NMR (400 MHz, CDCl3):
d
¼1.27 (3H, t, J 7.2 Hz,
g
CH2CH3), 2.12e2.19 (2H, m, bCH2), 2.40 (3H, d, J 1.2 Hz, CH3), 2.53 (2H,
t, J 7.2 Hz,
aCH2), 4.08 (2H, t, J 6.4 Hz,
gCH2), 4.16 (2H, q, J 7.2 Hz,
n

8030
A.S.C. Fonseca et al. / Tetrahedron 68 (2012) 8024e8032
1265, 1163, 1094, 1064, 896, 842, 738, 705, 666. MS m/z (EI, %): 335
(Mþ, 100). HRMS: m/z (EI) calcd for C16H17O7N 335.1005, found
335.1019.
(EI, %): 325 (Mþ, 100). HRMS: m/z (EI) calcd for C19H19O4N
325.1315, found 325.1306.
4.2.5.5. Benzocoumarins 8b and 8c. The compounds were
obtained as a light pink solid mixture (0.075 g, 75%). The mixture
was used in the next reaction without further purification.
4.2.5.5.1. 4-(1-Methyl-benzo[f]coumarin-9-yloxy)butanoic acid,
4.2.5. General procedure for the synthesis of compounds 3aec and
8aec. Compounds 2aec and 7aec (1 equiv) were added to a solu-
tion of water/acetic acid/hydrochloric acid (3:1:0.1, 5 mL/equiv) and
the mixture was heated at reflux for 30 min. The solution was
allowed to cool slowly and the precipitate was filtered off. Com-
pounds 3aec and 8aec were obtained as solids.
8b. 1H NMR (400 MHz, DMSO-d6):
2.41e2.50 (2H, m,
CH2), 6.46 (1H,s, H2), 7.28 (1H, dd, J 9.0 and 2.1 Hz, H-8), 7.37 (1H,
d, J 8.7 Hz, H5), 7.97e8.00 (2H, m, H7 and H10), 8.11 (1H, d, J 9.0 Hz,
H6), 12.1 (1H, s, OH). 13C NMR (100.6 MHz, DMSO-d6):
¼18.25
(CH3), 24.66 ( CH2), 30.32 ( CH2), 66.75 ( CH2), 108.69 (C10),
d
¼2.02 (2H, t, J 6.3 Hz,
bCH2),
a
CH2), 2.93 (3H, s, CH3), 4.19 (2H, t, J 6.3 Hz,
g
4.2.5.1. 2-(4-Methylcoumarin-7-yloxy)acetic acid, 3a. The com-
d
pound was obtained as white solid (0.065 g, 0.28 mmol, 73%).
b
a
g
Mp¼201.2e202.8 ꢂC. 1H NMR (300 MHz, DMSO-d6):
d
¼2.38 (3H, d,
110.70 (C10b), 114.01 (C5), 115.73 (C2), 117.34 (C8), 125.24 (C6a),
130.32 (C10a), 131.39 (C7), 134.40 (C6), 152.22 (C1), 155.36 (C4a),
157.82 (C9), 159.36 (C3).
J 0.9 Hz, CH3), 4.82 (2H, s, CH2), 6.21 (1H, d, J 1.2 Hz, H3), 6.94e6.98
(2H, m, H6 and H8), 6.68 (1H, d, J 9.0 Hz, H5). 13C NMR (75.4 MHz,
DMSO-d6):
d
¼18.14 (CH3), 64.82 (CH2), 101.51 (C8), 111.39 (C3),
4.2.5.5.2. Ethyl 4-(4-methyl-benzo[g]coumarin-8-yloxy)butanoic
112.30 (C6), 113.55 (C4a), 126.52 (C5), 153.40 (C4), 154.55 (C8a),
acid, 8c. 1H NMR (400 MHz, DMSO-d6):
d
¼1.98e2.07 (2H, m,
CH2), 2.52 (3H, d, J 1.2 Hz, CH3), 4.13 (2H,
gCH2), 6.38 (1H, d, J 1.2 Hz, H3), 7.19 (1H, dd, J 9.2 and
bCH2),
160.11 (C2), 160.79 (C7), 169.67 (C]O acid). FTIR (KBr 1%, cmꢀ1):
2.41e2.45 (2H, t, J 7.2 Hz,
t, J 6.4 Hz,
a
n
¼2923, 1765, 1674, 1660, 1621, 1606, 1557, 1511, 1428, 1392, 1374,
1326,1299,1206,1169,1153,1085,1020, 977, 921, 857, 848, 813, 752,
738, 655. MS m/z (EI, %): 234 (Mþ, 100). HRMS: m/z (EI) calcd for
C12H10O5 234.0528, found 234.0559.
2.4 Hz, H7), 7.38 (1H, d, J 2.4 Hz, H9), 7.13 (1H, s, H10), 8.00 (1H, d, J
9.2 Hz, H6), 8.33 (1H, s, H5), 12.12 (1H, s, OH). 13C NMR (100.6 MHz,
DMSO-d6):
d
¼18.44 (CH3), 24.55 (
bCH2), 30.42 (aCH2), 66.73
(g
CH2), 105.27 (C9), 110.44 (C10), 114.58 (C3), 117.94 (C4a), 119.92
4.2.5.2. 4-(4-Methyl-coumarin-7-yloxy)butanoic acid, 3b. The
(C7), 125.08 (C5), 125.36 (C5a), 130.45 (C6), 136.92 (C9a), 151.08
(C4), 152.11 (C10a), 158.61 (C8), 161.23 (C2).
compound was obtained as white solid (0.100 g, 0.38 mmol, 96%).
Mp¼170.3e171.4 ꢂC. 1H NMR (300 MHz, DMSO-d6):
¼1.90e2.00
d
(2H, m,
6.6 Hz,
H8), 6.67 (1H, d, J 9.3 Hz, H5). 13C NMR (75.4 MHz, DMSO-d6):
CH2), 29.99 ( CH2), 67.41 ( CH2), 101.20
b
CH2), 2.36e2.41 (5H, m,
a
CH2 and CH3), 4.09 (2H, t, J
4.2.6. General procedure for the synthesis of compounds 4aec and
9aec. Compounds 3aec and 8aec (1 equiv) were dissolved in dry
N,N-dimethylformamide. 2-Bromo-1-phenylpropan-1-one (1 equiv)
and potassium fluoride (3 equiv) were added to the mixture. The
mixture was stirred, in an oil bath, at 50 ꢂC for 1 day. The reaction
mixture was filtered and the solvent was removed in a rotary
evaporator. The crude residue was purified by silica gel column
chromatography using chloroform/methanol (100:1) as eluent.
gCH2), 6.19 (1H, d, J 1.2 Hz, H3), 6.93e6.96 (2H, m, H6 and
d
¼18.14 (CH3), 23.99 (
b
a
g
(C8), 111.13 (C3), 112.42 (C6), 113.12 (C4a), 126.50 (C5), 153.46 (C4),
154.76 (C8a), 160.19 (C2), 161.60 (C7), 174.04 (C]O acid). FTIR (KBr
1%, cmꢀ1):
n
¼3095, 1700, 1687, 1607, 1558, 1512, 1480, 1427, 1392,
1366, 1720, 1211, 1162, 1073, 1020, 986, 891, 872, 848, 828, 810, 770,
753, 713, 666. MS m/z (EI, %): 262 (Mþ, 100). HRMS: m/z (EI) calcd
for C14H14O5 262.0841, found 262.0888.
4.2.6.1. 1-Oxo-1-phenylpropan-2-yl 2-(4-methylcoumarin-7-
yloxy)acetate, 4a. The compound was obtained as white solid
(0.065 g, 0.18 mmol, 83%). Mp¼178.1e179.0 ꢂC. 1H NMR (400 MHz,
4.2.5.3. 4-(4-Methyl-6-nitrocoumarin-7-yloxy)butanoic
acid,
3c. The compound was obtained as yellow solid (0.030 g,
CDCl3):
d
¼1.59 (3H, d, J 7.2 Hz,
b
CH3), 2.39 (3H, d, J 1.2 Hz, CH3),
H), 6.15 (1H, d, J 1.2 Hz,
9.77 mmol, 98%). Mp¼190.1e193.0 ꢂC. 1H NMR (400 MHz, DMSO-
4.79e4.88 (2H, m, CH2), 6.08e6.13 (1H, m,
a
d6):
4.27 (2H, t, J 6.4 Hz,
H8), 8.31 (1H, s, H5). 13C NMR (100.6 MHz, DMSO-d6):
(CH3), 23.68 ( CH2), 29.66 ( CH2), 69.23 ( CH2), 102.66 (C8),
d
¼1.90e1.98 (2H, m,
b
CH2), 2.38e2.42 (5H, m,
a
CH2 and CH3),
H3), 6.82 (1H, d, J 2.8 Hz, H8), 6.92 (1H, dd, J 8.4 and 2.8 Hz, H6),
7.47e7.53 (3H, m, H5, H30 and H50), 7.59e7.63 (1H, m, H40), 7.92 (2H,
dd, J 8.4 and 1.2 Hz, H20 and H60). 13C NMR (100.6 MHz, CDCl3):
g
CH2), 6.38 (1H, d, J 1.4 Hz, H3), 7.39 (1H, s,
¼18.00
d
b
a
g
d
¼17.10 (
bCH3), 18.61 (CH3), 65.04 (CH2), 72.58 (aC), 102.06 (C8),
112.53 (C4a), 113.10 (C3), 122.84 (C5), 136.09 (C6), 152.76 (C4),
112.19 (C6), 112.50 (C3), 114.46 (C4a), 125.76 (C5), 128.41 (C20 and
C60), 128.85 (C30 and C50), 133.83 (C40), 133.97 (C10), 152.31 (C4),
154.97 (C8a), 160.49 (C7), 161.01 (C2), 167.55 (C]O ester), 195.77
153.97 (C7), 156.61 (C8a), 159.11 (C2), 173.86 (C]O acid). FTIR
(KBr 1%, cmꢀ1):
n
¼3046, 1732, 1624, 1537, 1382, 1356, 1309, 1207,
þ
1162, 1062, 991, 904, 845, 666. MS m/z (ESI, %): 308 ([MþH]
,
(C]O ketone). FTIR (KBr 1%, cmꢀ1):
n
¼1769, 1697, 1609, 1601, 1560,
100). HRMS: m/z (ESI) calcd for C14H14O7N 308.07648; found
1513, 1499, 1449, 1425, 1387, 1365, 1347, 1286, 1271, 1228, 1218,
1200, 1166, 1140, 1091, 1062, 1027, 1017, 990, 974, 948, 898, 864, 847,
308.07650.
703. UVevis (ethanol, nm): lmax (log
3
)¼316 (4.22). MS m/z (ESI, %):
4.2.5.4. 4-(1,4-Dimethylbenzo[f]quinolin-3(4H)-one-9-yloxy)bu-
tanoic acid, 8a. The compound was obtained as beige solid
(0.035 g, 1.08 mmol, 95%). Mp¼114.8e116.1 ꢂC. 1H NMR (400 MHz,
367 ([MþH]þ, 100). HRMS: m/z (ESI) calcd for C21H19O6 367.11761;
found 367.11748.
DMSO-d6):
d¼2.00e2.07 (2H, m,
b
CH2), 2.42e2.55 (2H, m,
a
CH2),
CH2),
4.2.6.2. 1-Oxo-1-phenylpropan-2-yl 4-(4-methylcoumarin-7-
yloxy)butanoate, 4b. The compound was obtained as a colourless
2.91 (3H, s, CH3), 3.73 (3H, s, NCH3), 4.18 (2H, t, J 6.0 Hz,
g
6.61 (1H, s, H2), 7.21 (1H, dd, J 8.8 and 2.4 Hz, H8), 7.62 (1H, d, J
9.2 Hz, H5), 7.94 (1H, d, J 9.2 Hz, H7), 7.80 (1H, d, J 2.0 Hz, H10),
8.01 (1H, d, J 9.2 Hz, H6). 13C NMR (100.6 MHz, DMSO-d6):
oil (0.025 g, 0.08 mmol, 65%). 1H NMR (400 MHz, CDCl3):
d
¼1.54 (2H,
CH2), 2.39 (3H, s, CH3),
CH2), 4.08 (2H, t, J 6.4 Hz, CH2), 5.97e6.02 (1H,
d, J 6.8 Hz,
2.63e2.67 (2H, m,
m, aH), 6.12 (1H, d, J 1.2 Hz, H3), 6.80 (1H, d, J 2.4 Hz, H8), 6.85 (1H,
bCH3), 2.16e2.19 (2H, m, b
a
g
d
¼24.32 (
a
CH2), 26.03 (CH3), 30.07 (
b
CH2), 30.21 (NCH3), 66.67
(g
CH2), 107.11 (C10), 113.43 (C5), 114.39 (C10b), 115.85 (C8), 121.61
dd, J 8.4 and 2.8 Hz, H6), 7.46e7.49 (3H, m, H5, H30 and H50),
7.56e7.61 (1H, m, H40), 7.94 (2H, dd, J 8.0 and 3.2 Hz, H20 and H60).
(C2), 124.67 (C6a), 130.61 (C7), 131.76 (C6), 132.05 (C10a), 140.69
(C4a), 147.52 (C1), 157.50 (C9), 160.16 (C3), 174.10 (C]O acid). FTIR
13C NMR (100.6 MHz, CDCl3):
d
¼17.10 (
CH2), 71.56 (
(C3), 112.45 (C6), 113.57 (C4a), 125.48 (C5), 128.38 (C20 and C60),
b
CH3), 18.61 (CH3), 24.28
(KBr 1%, cmꢀ1):
n
¼3020, 2994, 1728, 1622, 1542, 1519, 1461, 1366,
(b
CH2), 30.28 (
a
CH2), 67.07 (
g
aC), 101.49 (C8), 111.90
1233, 1200, 1150, 1100, 1050, 1000, 993, 938, 776, 717, 677. MS m/z

A.S.C. Fonseca et al. / Tetrahedron 68 (2012) 8024e8032
8031
128.78 (C30 and C50), 133.60 (C40), 134.27 (C10), 152.48 (C4), 155.19
(KBr 1%, cmꢀ1):
n
¼3063, 2984, 2938, 1730, 1698, 1624, 1596, 1552,
(C8a), 161.26 (C2), 161.82 (C7), 172.36 (C]O ester), 196.76 (C]O
1518, 1449, 1434, 1401, 1375, 1357, 1312, 1281, 1251, 1227, 1175, 1137,
1090, 1050, 973, 931, 838, 735, 702, 666, 580. UVevis (ethanol, nm):
ketone). FTIR (liquid film, cmꢀ1):
1472,1449,1426,1388,1369,1294,1265,1229,1202,1091,1071,1036,
n
¼1732, 1699, 1615, 1558, 1510,
lmax (log
3
)¼347 (4.05). MS m/z (ESI, %): 445 ([MþH]þ, 100). HRMS:
1016, 973, 879, 849, 792, 702, 666. UVevis (ethanol, nm): lmax
m/z (ESI) calcd for C27H25O6 445.16456, found 445.16459.
4.2.6.5.2. 1-Oxo-1-phenylpropan-2-yl 4-(4-methyl-benzo[g]cou-
marin-8-yloxy)butanoate, 9c. Yield 0.045 g, 0.10 mmol, 66%.
(log
3
)¼320 (4.28). MS m/z (ESI, %): 395 ([MþH]þ, 100). HRMS: m/z
(ESI) calcd for C23H23O6 395.14891, found 395.14885.
Mp¼136.2e137.5 ꢂC. 1H NMR (400 MHz, CDCl3):
d
¼1.55 (2H, d,
4.2.6.3. 1-Oxo-1-phenylpropan-2-yl 4-(4-methyl-6-nitrocoumarin-
J 6.8 Hz, bCH3), 2.18e2.26 (2H, m, bCH2), 2.53 (3H, d, J 1.2 Hz, CH3),
7-yloxy)butanoate, 4c. Thecompoundwasobtainedasacolourlessoil
2.68e2.72 (2H, m,
aCH2), 4.17 (2H, t, J 6.4 Hz, gCH2), 5.99e6.04 (1H,
(0.030 g, 0.07 mmol, 72%).1H NMR (400 MHz, CDCl3):
d
¼1.54 (2H, d, J
CH2), 2.44 (3H, d, J 1.2 Hz, CH3),
CH2), 4.24e4.27 (2H, m, CH2), 5.96e6.01 (1H, m,
H), 6.26 (1H, d, J 1.2 Hz, H3), 6.97 (1H, s, H8), 7.46e7.50 (2H, m, H30
and H50), 7.56e7.60 (1H, m, H40), 7.93 (2H, dd, J 8.4 and 1.6 Hz, H20 and
H60), 8.18 (1H, s, H5). 13C NMR (100.6 MHz, CDCl3):
¼17.10 ( CH3),
CH2), 71.77 ( C),
m, aH), 6.28 (1H, d, J 1.2 Hz, H3), 7.11 (1H, d, J 2.4 Hz, H9), 7.14 (1H,
7.2 Hz,
bCH3), 2.17e2.27 (2H, m,
b
dd, J 8.8 and 2.4 Hz, H7), 7.48 (3H, dt, J 6.0, 1.6 Hz, H30 and H50),
7.56e7.60 (2H, m, H40 and H10), 7.81 (2H, dd, J 9.2 Hz, H6), 7.95 (2H,
dd, J 8.4 and 1.2 Hz, H20 and H60), 7.99 (1H, s, H5). 13C NMR
2.69e2.73 (2H, m,
a
g
a
(100.6 MHz, CDCl3):
30.46 ( CH2), 66.74 (
d
¼17.13 (
b
CH3), 18.65 (CH3), 24.43 (
bCH2),
d
b
a
gCH2), 71.55 (a
C), 105.40 (C9), 111.64 (C10),
18.55 (CH3), 24.08 (
b
CH2), 29.95 (
a
CH2), 68.89 (
g
a
114.56 (C3), 117.92 (C4a), 119.57 (C7), 124.64 (C5), 125.58 (C5a),
128.41 (C20 and C60), 128.79 (C30 and C50), 130.14 (C6), 133.62 (C40),
134.32 (C10), 136.42 (C9a), 150.84 (C4), 152.12 (C10a), 158.72 (C8),
160.98 (C2),172.53 (C]O ester),196.85 (C]O ketone). FTIR (KBr 1%,
102.39 (C8), 112.73 (C4a), 113.99 (C3), 122.94 (C5), 128.40 (C20 and
C60),128.78 (C30 and C50),133.60 (C40),134.25 (C10),151.49 (C4),155.17
(C7), 157.25 (C8a), 159.40 (C2), 172.24 (C]O ester), 196.82 (C]O ke-
tone). FTIR (liquid film, cmꢀ1
1533, 1501, 1449, 1384, 1363, 1316, 1291, 1256, 1229, 1207, 1161, 1135,
)
n¼3037, 2937, 1737, 1698, 1622, 1598,
cmꢀ1):
n
¼3427, 3058, 2934, 1279, 1729, 1632, 1597, 1484, 1464,
1449, 1389, 1358, 1323, 1255, 1228, 1188, 1148, 1052, 1034, 977, 942,
1095,1064,1035,1001, 973, 909, 846, 736, 702, 666. UVevis (ethanol,
891, 841, 810, 694. UVevis (ethanol, nm): lmax (log
3
)¼343 (4.21).
nm): lmax (log
3
)¼321 (4.00). ES m/z (ESI, %): 440 ([MþH]þ, 100).
MS m/z (ESI, %): 445 ([MþH]þ, 100). HRMS: m/z (ESI) calcd for
HRMS: m/z (ESI) calcd for C23H22O8N 440.13399, found 440.13407.
C27H25O6 445.16456, found 445.16467.
4.2.6.4. 1-Oxo-1-phenylpropan-2-yl 4-(1,4-dimethylbenzo[f]qui-
nolin-3(4H)-one-9-yloxy)butanoate, 9a. The compound was
obtained as beige oil (0.020 g, 4.37 mmol, 89%). 1H NMR (400 MHz,
4.2.7. Synthesis of 1-oxo-1-phenylpropan-2-yl 2-(4-methylcoumarin-
2-thione-7-yloxy)butanoate, 4d. Compound 4b (0.093 g, 0.23 mmol)
was dissolved in dry toluene (5 mL) and Lawesson’s reagent was
added (0.8 equiv, 0.076 g, 0.18 mmol). The reaction mixture was
heated at reflux for 1 day. The mixture was filtered hot and the
solvent removed by rotary evaporation. The crude residue was pu-
rified by silica gel column chromatography using chloroform as el-
uent, yielding compound 4d as a yellow solid (0.060 g, 0.15 mmol,
CDCl3):
2.69e2.73 (2H, m,
(2H, t, J 6.0 Hz, CH2), 5.98e6.03 (1H, m,
d
¼1.54 (2H, d, J 7.2 Hz,
CH2), 2.92 (3H, s, CH3), 3.85 (3H, s, NCH3), 4.19
H), 6.71 (1H, s, H2), 7.18
bCH3), 2.05e2.27 (2H, m, bCH2),
a
g
a
(1H, d, J 8.8 and 2.4 Hz, H8), 7.43e7.49 (3H, m, H30, H50 and H5),
7.55e7.59 (1H, m, H40), 7.80 (1H, d, J 9.2 Hz, H7), 7.90 (1H, d, J 9.2 Hz,
H6), 7.92e7.95 (2H, m, H20 and H60), 7.99 (1H, d, J 2.4 Hz, H10). 13C
65%). Mp¼115.3e118.0 ꢂC. 1H NMR (400 MHz, CDCl3):
¼1.55 (3H, d,
d
NMR (100.6 MHz, CDCl3):
d
¼17.11 (
b
CH3), 24.49 (
b
CH2), 26.76
C), 107.64
J 6.8 Hz, bCH3), 2.12e2.23 (2H, m, bCH2), 2.35 (3H, d, J 1.2 Hz, CH3),
(CH3), 30.46 ( CH2), 30.50 (NCH3), 66.73 (
a
gCH2), 71.53 (
a
2.60e2.73 (2H, m,
aCH2), 4.11 (2H, t, J 6.4 Hz, gCH2), 5.98e6.03 (1H,
(C10), 112.76 (C5), 115.67 (C10b), 115.96 (C8), 122.28 (C2), 125.14
(C6a), 128.37 (C20 and C60), 128.77 (C30 and C50), 130.47 (C7), 131.99
(C6), 132.38 (C10a), 133.60 (C40), 134.29 (C10), 140.70 (C4a), 147.71
(C1), 157.96 (C9), 161.50 (C3), 172.55 (C]O ester), 196.79 (C]O
m, aH), 6.93 (1H, dd, J 8.8 and 2.4 Hz, H6), 6.96 (1H, d, J 2.4 Hz, H8),
7.07 (1H, d, J 1.2 Hz, H3), 7.47e7.51 (2H, m, H30 and H50), 7.54 (1H, d, J
8.8 Hz, H5), 7.58e7.62 (1H, m, H40), 7.95 (2H, dd, J 8.4 and 1.2 Hz, H20
and H60). 13C NMR (100.6 MHz, CDCl3):
d
¼17.13 (
b
CH3), 18.00 (CH3),
C), 101.04 (C8),
ketone). FTIR (liquid film, cmꢀ1):
n
¼1737, 1698, 1652, 1652, 1603,
24.26 (
b
CH2), 30.30 (
a
CH2), 67.29 (
g
CH2), 71.60 (a
1598,1576,1548,1519,1449,1441,1416,1366,1323,1226,1172,1133,
114.32 (C6), 115.29 (C4a), 125.46 (C5), 126.58 (C3), 128.41 (C20 and
C60), 128.80 (C30 and C50), 133.64 (C40), 134.29 (C10), 144.92 (C4),
157.92 (C8a), 162.26 (C7), 172.30 (C]O ester), 196.75 (C]O ketone),
1096, 1041, 1001, 973, 859, 832, 793, 735, 701, 666. UVevis (ethanol,
nm): lmax (log
3
)¼335 (4.00). MS m/z (ESI, %): 458 ([MþH]þ, 100).
HRMS: m/z (ESI) calcd for C23H23O5S 458.19620, found 458.19623.
195.35 (C2). FTIR (KBr 1%, cmꢀ1):
n¼3043, 2985, 2944, 1731, 1699,
1625, 1603, 1546, 1467, 1449, 1390, 1348, 1302, 1286, 1250, 1228,
4.2.6.5. Benzocoumarins 9b and 9c. The resulting crude oil was
purified by silica gel column chromatography using chloroform/
methanol (100:1) as eluent. Fractions containing the separated
products were combined and evaporated, yielding compound 9b as
colourless oil and compound 9c as white solid.
1212, 1175, 1156, 1135, 1072, 1017, 971, 935, 842, 697, 666. UVevis
(ethanol, nm): lmax (log
3
)¼397 (4.28). MS m/z (ESI, %): 411
([MþH]þ, 100). HRMS: m/z (ESI) calcd for C23H23O5S 411.12607,
found 411.12585.
4.2.6.5.1. 1-Oxo-1-phenylpropan-2-yl 4-(1-methyl-benzo[f]cou-
4.3. General photolysis procedure
marin-9-yloxy)butanoate, 9b. Yield 0.030 g, 0.07 mmol, 78%. 1H
NMR (400 MHz, CDCl3):
(2H, m, CH2), 2.67e2.77 (2H, m,
J 6.0 Hz, CH2), 5.98e6.03 (1H, m,
d
¼1.54 (2H, d, J 6.8 Hz,
CH2), 2.91 (3H, s, CH3), 4.19 (2H, t,
H), 6.32 (1H, d, J 0.8 Hz, H2), 7.21
b
CH3), 2.21e2.27
A 1ꢁ10ꢀ4 m methanol/HEPES buffer (80:20) solution of conju-
gates 4 and 9 (5 mL) was placed in a quartz tube and irradiated in
a Rayonet RPR-100 reactor at the desired wavelength. The lamps
used for irradiation were of 300, 350 and 419ꢃ10 nm. HEPES buffer
solution was prepared in distilled water with HEPES (4-(2-
hydroxyethyl)-1-piperazine ethanesulfonic acid) (10 mM), sodium
chloride (120 mM), potassium chloride (3 mM), calcium chloride
(1 mM) and magnesium chloride (1 mM) and the pH adjusted to 7.2
with aqueous 1 M sodium hydroxide.
b
g
a
a
(1H, d, J 8.8 and 2.4 Hz, H8), 7.31 (1H, dd, J 8.8 Hz, H5), 7.47 (3H, dt, J
6.4 and 1.6 Hz, H30 and H50), 7.58 (1H, dt, J 7.6 and 1.2 Hz, H40), 7.81
(1H, d, J 8.8 Hz, H7), 7.88 (1H, d, J 8.8 Hz, H6), 7.92e7.95 (3H, m, H10,
H20 and H60). 13C NMR (100.6 MHz, CDCl3):
d
¼17.13 ( CH3), 24.55
b
(b
CH2), 26.22 (CH3), 30.39 (
a
CH2), 66.75 (
g
CH2), 71.56 (aCH), 107.08
(C10), 113.74 (C10b), 115.31 (C5), 115.89 (C2), 116.45 (C8), 126.45
(C10a), 128.37 (C20 and C60), 128.79 (C30 and C50), 131.01 (C7), 131.69
(C6a), 133.30 (C6), 134.27 (C10), 154.17 (C1), 155.29 (C4a), 158.36
(C9), 160.45 (C3), 172.53 (C]O ester), 196.79 (C]O ketone). FTIR
Aliquots of 100
by RP-HPLC. The eluent was acetonitrile/water (3:1) previously
filtered through a Millipore, type HN 0.45 m filter and degassed by
mL were taken at regular intervals and analysed
m

8032
A.S.C. Fonseca et al. / Tetrahedron 68 (2012) 8024e8032
4. Kocienski, P. Protecting Goups, 3rd ed.; Thieme: Stuttgart, 2005.
5. del Campo, A.; Miguel, V. S.; Bochet, C. G. J. Am. Chem. Soc. 2011, 133, 5380e5388.
6. (a) Fonseca, A. S. C.; Gonc¸ alves, M. S. T.; Costa, S. P. G. Tetrahedron 2007, 63,
1353e1359; (b) Fernandes, M. J. G.; Gonc¸ alves, M. S. T.; Costa, S. P. G. Tetrahe-
dron 2008, 64, 3032e3038; (c) Fonseca, A. S. C.; Gonc¸ alves, M. S. T.; Costa, S. P.
G. Amino Acids 2010, 39, 699e712; (d) Soares, A. M. S.; Costa, S. P. G.; Gonc¸ alves,
M. S. T. Amino Acids 2010, 39, 121e133; (e) Piloto, A. M.; Soares, A. M. S.; Costa,
S. P. G.; Gonc¸ alves, M. S. T. Amino Acids 2012, 42, 2275e2282; (f) Soares, A. M. S.;
Piloto, A. M.; Hungerford, G.; Costa, S. P. G.; Gonc¸ alves, M. S. T. Eur. J. Org. Chem.
2012, 922e930; (g) Soares, A. M. S.; Costa, S. P. G.; Gonc¸ alves, M. S. T. Tetra-
hedron 2010, 66, 8189e8195.
ultra-sound for 30 min, at a flow rate of 0.8 mL/min for all com-
pounds. The chromatograms were traced by detecting UV absorp-
tion at the wavelength of maximum absorption for each conjugate
(retention time: 4a, 5.6; 4b, 4.5; 4c, 5.3; 4d, 7.8; 9a, 8.3; 9b, 7.7; 9c,
8.4 min).
Acknowledgements
ˇ
~
Thanks are due to the Fundac¸ ao para a Ciencia e Tecnologia (FCT,
7. Kim, H. M.; Choo, H. J.; Jung, S. Y.; Ko, Y. G.; Park, W. H.; Jeon, S. J.; Kim, C. H.; Joo,
T.; Cho, B. R. ChemBioChem 2007, 8, 553e559.
8. Uray, G.; Niedereeiter, K. S.; Belaj, F.; Fabian, W. M. F. Helv. Chim. Acta 1999, 82,
1408e1417.
9. Akerblom, E. B.; Nygren, A. S.; Agback, K. H. Mol. Divers. 1998, 3, 137e148.
10. Tjoeng, F. S.; Heavner, G. A. Synthesis 1981, 897e899.
Portugal) for financial support to the NMR Portuguese network
(PTNMR, Bruker Avance III 400-Univ. Minho), FCT and FEDER (Eu-
ropean Fund for Regional Development)-COMPETE-QREN-EU for
financial support to the Research Centre, CQ/UM [PEst-C/QUI/
UI0686/2011 (FCOMP-01-0124-FEDER-022716)] and project PTDC/
QUI/69607/2006 (FCOMP-01-0124-FEDER-007449). A Ph.D. grant
to A.S.C.F. (SFRH/BD/32664/2006) is also acknowledged.
11. Jesberger, M.; Davis, T. P.; Barner, L. Synthesis 2003, 13, 1929e1958.
12. Morris, J. V.; Mahaney, M. A.; Huber, J. R. J. Phys. Chem. 1976, 80, 969e974.
13. Montalti, L.; Credi, A.; Prodi, T.; Gandolfi, M. T. Handbook of Photochemistry, 3rd
ꢀ
ed.; Taylor and Francis: Boca Raton, FL, 2006.
14. Seixas de Melo, J. S.; Burrows, H. D.; Svensson, M.; Andreson, M. R.; Monkman,
A. P. J. Chem. Phys. 2003, 118, 1550e1556.
References and notes
15. Fonseca, A. S. C.; Soares, A. M. S.; Gonc¸ alves, M. S. T.; Costa, S. P. G. Tetrahedron
2012, 68, 7892e7900.
ꢀ
16. Muller, C.; Even, P.; Viriot, M.-L.; Carre, M. C. Helv. Chim. Acta 2001, 84,
1. Glatthar, R.; Giese, B. Org. Lett. 2000, 2, 2315e2317.
3735e3741.
2. Jones, J. Amino Acids and Peptide Synthesis; Oxford Science: Oxford, 1997.
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3. Bochet, C. G. Angew. Chem., Int. Ed. 2001, 40, 2071e2073.