Expeditious photochemical reaction toward the preparation of substituted chroman-4-ones

Article abstract of DOI:10.1016/j.tetlet.2014.06.081

A facile photochemical preparation of 5-, 6-, and 7-substituted chroman-4-ones from aryl 3-methyl-2-butenoate esters is described. The two-phase base-catalyzed method relies upon two consecutive processes in one-pot reaction through a photo-Fries rearrangement and a based-catalyzed intramolecular oxa-Michael addition to afford the desired products.

Full text of DOI:10.1016/j.tetlet.2014.06.081

Tetrahedron Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Expeditious photochemical reaction toward the preparation
of substituted chroman-4-ones
⇑
Daniela Iguchi, Rosa Erra-Balsells, Sergio M. Bonesi
CIHIDECAR—CONICET, Departamento de Química Orgánica, 3[er] Piso, Pabellón 2, Ciudad Universitaria, FCEyN, University of Buenos Aires, Buenos Aires 1428, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile photochemical preparation of 5-, 6-, and 7-substituted chroman-4-ones from aryl 3-methyl-
2-butenoate esters is described. The two-phase base-catalyzed method relies upon two consecutive
processes in one-pot reaction through a photo-Fries rearrangement and a based-catalyzed intramolecular
oxa-Michael addition to afford the desired products.
Received 26 May 2014
Revised 18 June 2014
Accepted 19 June 2014
Available online xxxx
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
photo-Fries rearrangement
oxa-Michael addition
Aryl esters
Two-phase reaction
Chroman-4-ones
The chroman-4-one (2,3-dihydro-4-oxo-4H-1-benzopyran) ring
system occupies an important position among oxygen heterocycles
and features in a wide variety of compounds of biological and
The reported methods for the synthesis of chromanones are
well documented and involve (i) condensation of phenols with
3,3-dimethyl acrylic acids or its derivatives (Friedel–Craft reaction
together with thermal Fries rearrangement); (ii) Claisen rearrange-
ment of propargyl ethers of phenols and, (iii) Knoevenagel conden-
sation of o-hydroxyphenones with aliphatic aldehydes and ketones
(Kabbe reaction).9 The last method perhaps is the most convenient
and practical procedure involving the base-catalyzed condensation
between a 2-hydroxyphenone and an aldehyde.
However, an alternative methodology scarcely considered is the
photo-Fries rearrangement reaction as a key step in the prepara-
tion of chroman-4-ones (see Scheme 1b).10 Recently, we have
reported a mild and convenient one-pot photochemical synthesis
of several chroman-4-one derivatives under a biphasic base catal-
ysis system in good to high yield.11 Since we are interested in the
application of the photo-Fries rearrangement reaction in organic
synthesis, in order to ascertain scope and limitations of the meth-
odology, we report the photochemical reaction of a series of p- and
m-substituted esters 2a–o under a one-pot base-mediated photo-
chemical reaction to afford a variety of substituted chroman-4-
one derivatives with good yield and noticeable regioselectivity.
The general synthetic approach and experimental conditions are
shown in Scheme 2.
medicinal interest.1 From
a synthetic viewpoint, substituted
chroman-4-ones are valued both as functional intermediates and
as targets in their own right.2 The chroman parent system has been
identified in natural products such as Sappanone B3 and robusta-
dial4 in addition to being a bioisostere for the hydantoin moiety.
Furthermore, fused-ring chromanones (coumarin and tetrahydro-
xanthone derivatives) are found in the cores of pseudobruceol I
and diversonol.5 Marine organisms are also a resource of molecules
with therapeutic value and puupehenone derivatives belong to this
category which is isolated from sponge Heteronema sp. and from
sponges mainly of the orders Verongida and Dictyoceratida.6
(À)-15-Oxopuupehenol is an example of a sesquiterpene and
phenolic fusioned moieties that include a benzopyranone scaffold
showing antimalarial activity.
Mono- and disubstituted chroman-4-one derivatives are
employed in medicine and can be used against a variety of biological
processes.7 Thus, 7,8-dichloro-4-chromanone oximes and novel 3-
benzylidene- and 3-benzyl-substituted chromanones can be used
to treat or assist in inhibiting a variety of diseases. Therefore, the vast
range of biological effects associated with this scaffold has resulted
in the chromanone ring system being considered as a privileged
structure.8 Some representative examples are shown in Scheme 1a.
The commercially available substituted phenols 1a–o were
allowed to react with 3-methyl-2-butenoate chloride in dry pyri-
dine at room temperature within 60 min. The molar ratio of the
acylating reagent to phenols was 1.1 to 1.2. After acidic work-up
and chromatographic purification esters 2a–o were obtained in
⇑
Corresponding author. Tel./fax: +54 1145763346.
E-mail address: smbonesi@qo.fcen.uba.ar (S.M. Bonesi).
http://dx.doi.org/10.1016/j.tetlet.2014.06.081
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Iguchi, D.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.06.081

2
D. Iguchi et al. / Tetrahedron Letters xxx (2014) xxx–xxx
(a) Chroman-4-one derivatives.
HO
O
OH
O
OH
O
O
(+)-Sappanone B
OH
OH
O
O
OH
O
O
OH
OH
OHC
MeO
OH
OH
O
H3C
(-)- 15-Oxopuupehenol
Diversonol
Robustadial
OH
(b) Retrosynthetic analysis.
O
OH
O
OH
O
R
R
R
R
O
O
Scheme 1. Natural compounds containing benzopyranone scaffold and proposed retro-synthesis.
Table 1
O
O
photo-Fries rearrangement reaction of p-substituted phenyl 3-methyl-2-butenoate
(ii)
R2 = H
esters (2a–i) in cyclohexane under basic catalysis (KOH 10% aq; biphasic system)a
OH
O
R1
3a - k
Entry
Substrate; R
Irradiation time (min)
Conv. %
Yieldb
%
O
(i)
R2
1
2
3
4
5
6
7
8
2a; Hc
2b; CH3
2b; CH3
60
90
120
90
115
105
105
60
60
90
125
100
90
70
95
100
95
94
79
80
100
98
95
100
92
3a; 100
3b; 86
3b; 95
3c; 71
3c; 95
3d; 40e
3d; 98g
3e; 100
3f; 100
3g; 96
3h; 90
3i; 94
R2
O
O
O
O
R2
(ii)
R1
1a - o
+
R1
d
R1 = H
2a - o
2c; Cl
R2
5l - o
2c; Cld
2d; NO2
4l - o
f
2d; NO2
2e; MeO
2f; PhO
2g; Ph
2h; t-Bu
2i; CO(CH2)3CH3
2j; CN
R2 = H
R1 = H
R1 = H (1a); CH3 (1b); Cl (1c); NO2 (1d); OMe (1e); OPh (1f);
Ph (1g); t-Bu (1h); CO(CH2)3CH3 (1i); CN (1j); COOCH3 (1k).
R2 = CH3O (1l); CH3 (1m); Cl (1n); NO2 (1o).
9
10
11
12
13
14
Scheme 2. Reagents and conditions: (i) 3-methyl-2-butenoyl chloride (1.1 equiv),
pyridine, 25 °C, 10–60 min; (ii) h
120 min, 25 °C, Ar.
m (254 nm), cyclohexane–KOH 10% aq (8:2), 60–
95
95
3j; 96
3k; 90
2k; COOCH3
120
a
Reaction conditions: 0.010 M of ester, excitation wavelength: 254 nm, quartz
excellent yields (>90%) and were fully characterized by means of
physical and spectroscopic methods (see Supplementary material).
The above acylation reactions of compounds 1a–o were also per-
formed in anhydrous benzene or dichloromethane in the presence
of dry pyridine. However, the yields dropped dramatically and the
reactions required longer time; moreover, reaction completion was
not achieved, recovering the starting material in significant extent.
vessel, degassed solvent (2.5 mL), under Ar, room temperature.
b
The yields are based on the conversion of the starting material.
Data taken from Ref. 11.
Standard conditions, organic solvent/benzene.
The p-nitrophenol was formed in 53% yield.
c
d
e
f
Standard conditions, organic solvent/benzene; base catalyst/Cs2CO3 (s).
g
The p-nitrophenol was formed in 2% yield.
Next, we embarked on
a systematic examination of the
yields (Table 1, entries 2, 8–11). Besides, substrates with electron-
withdrawing groups (chloro, methylketone, cyano or methyl car-
boxylate groups) in the phenyl ring afforded the corresponding
chroma-4-one in good yields (Table 1, entries 4, 12–14). Irradiation
of compound 2d under our optimized conditions afforded the
corresponding chroman-4-one 3d in low yield together with signif-
icant amount of 4-nitrophenol (Table 1, entry 6). This behavior is
due to a competitive photo solvolysis process of ester 2d. When
benzene is used instead of cyclohexane, the irradiation of
esters 2b, 2c, and 2d afforded the corresponding chroman-4-one
derivatives 3b, 3c, and 3d in good yields, respectively (Table 1,
entries 3, 5 and 7). It is interesting to point out that for ester 2d
the photochemical reaction was accomplished in benzene and in
the presence of Cs2CO3 as the base catalyst to improve the yield
of 3d and to diminish the formation of the 4-nitrophenol.
photochemical reaction of eight 4-substituted esters 2b–k bearing
electron-withdrawing and electron-donating groups in order to
explore the scope of the one-pot photochemical synthesis of chro-
man-4-one derivatives. Thus, the irradiation of esters 2b–k was
carried out with an excitation wavelength of 254 nm, in a biphasic
system (cyclohexane–KOH 10% aq) as the reaction medium, under
inert atmosphere (Ar) and room temperature. This optimized con-
dition was recently applied for the photochemical reaction of ester
2a that gratifyingly gave 2,2-dimethyl-chroman-4-one (3a) in
quantitative yield (see Table 1, entry 1).11 Therefore, we decided
to apply this optimized methodology to esters 2b–k.
As shown in Table 1, a variety of 4-substituted esters 2 were
employed as reaction substrates and the photochemical reaction
can afford the corresponding chroman-4-one derivatives in good
to excellent yields regardless of the different substitutions on the
aromatic ring. Clearly, substrate with an electron-donating group
on the aromatic ring gave a better yield than that of an electron-
withdrawing group on the aromatic ring when the organic solvent
of the biphasic system is cyclohexane. For example, substrates
with alkyl, aryl, methoxy, or phenoxy group, the yields of the cor-
responding chroman-4-ones were obtained in good to quantitative
Since our method showed that it is easy to obtain 6-substituted
chroman-4-one derivatives, we decided to extend this efficient and
optimized photochemical reaction to a series of m-substituted
esters 2l–o in order to prepare the corresponding 5- and 7-
substituted chroman-4-one derivatives. As is shown in Table 2,
the expected regioisomers 4 and 5 were obtained in good yields
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D. Iguchi et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
in a ca. 1:1 M ratio regardless of the electron-donating or electron
– withdrawing nature of the substituent attached to the aromatic
ring of esters 2 with the exception of the nitro group (2o). These
results can be rationalized taking into account the charge density
values of C-2 and C-6 that belong to the aromatic ring of esters
2l–o. Therefore, we have calculated the charge densities of all the
carbon atoms by AM1 Semi Empirical calculation of esters 2 (see
Supplementary material). Since both atom positions, viz. C-2 and
C-6, show similar values, it is expected that migration of the acyl
group would proceed to both positions with similar probability
during the photochemical process, which is the photo Fries step
that provides the o-hydroxy phenone intermediates. Then, in a sec-
ond step, the intramolecular oxa-Michael addition reaction takes
place efficiently under base-mediated catalysis onto both o-
hydroxy phenone intermediates (see Scheme 3 and Table 3). How-
ever, the charge density values at C-6 are slightly greater than
those values at C-2 and a nice correlation is observed between
these values and the respective yields of formation of chroman-
4-ones 4 and 5, respectively. It is also interesting to point out that
since charge density values at C-4 predict that the p-hydroxyphe-
none derivatives could be formed with similar probability as com-
pounds 4 and 5, in our experimental conditions this kind of
photoproduct was not detected, however.
Again, the nitro compound 2o shows a different behavior. The
chroman-4-one 4o was obtained in moderate yield with a high reg-
ioselectivity, along with significant amount of m-nitrophenol. The
use of Cs2CO3 as catalyst was beneficial to accomplish the photo-
chemical reaction, whereas, the use of KOH favored the formation
of m-nitrophenol in high yield, giving compound 4o in low yield
(see Table 2, entries 5 and 6). The regioselectivity observed for
ester 2o can be rationalized in terms of the charge density values.
Since in ester 2o C-6 shows a higher charge density than C-2, it is
expected that the acyl group will migrate preferentially to C-6 than
to C-2. Therefore, chroman-4-ones 4o and 5o will be obtained with
noticeable selectivity. Compound 4o was indeed obtained in mod-
erate yield as it was predicted by the Semi Empirical AM1 method,
while compound 5o was not formed.
O
4l - o
+
O
(a)
O
(a)
O
hν
R
(b)
basic
6
5
1
2
condition
O
3
R
4
5l - o
(b)
O
2l - o
R
o-hydroxyphenone
intermediates
Scheme 3. Formation of chroman-4-ones 4 and 5 under two-phase base-mediated
photochemical reaction.
Table 3
Charge density values of esters 2l–o calculated with the Semi Empirical AM1 methoda
Compd; R
Charge density values
C-1
C-2
C-3
C-4
C-5
C-6
2l; OCH3
2m; CH3
2n; Cl
0.085
0.055
0.063
0.051
À0.212
À0.136
À0.131
À0.077
0.093
À0.165
À0.141
À0.135
À0.081
À0.086
À0.112
À0.107
À0.124
À0.189
À0.162
À0.158
À0.120
À0.054
À0.048
À0.116
2o; NO2
a
Carbon atom numbering see Scheme 3.
We have also calculated the charge densities by the Semi
Empirical AM1 method for all the phenoxide ions of the o-hydroxy-
phenone intermediates formed after the photo-Fries rearrange-
ment has occurred. The relevant data of interest are the charge
densities belonging to the oxygen of the fenoxide ion and the
C-b of the
a,b-ethylenic moiety which are collected in Table 4.
The calculated data predict that the cyclization process could take
place efficiently and, in fact, this prediction is in agreement with
our experimental results.
With regard to mechanism, the findings herein and from previ-
ous studies10c,11 led us to surmise that these biphasic photoreac-
tions advance via 2-hydroxyphenone intermediates which is
formed during the photolysis of esters 2a–o and do not need to
be isolated. To rationalize the reaction mechanism we proposed
that the whole photochemical reaction of esters 2a–o takes place
actually in two consecutive steps: (i) the photochemical formation
of the rearrangement product (o-hydroxy phenone) and (ii) the
thermal intramolecular cyclization of the o-regioisomer to the
The success of the thermal cyclization reaction of esters 2a–o
can be ascribed to the polarization of the double bond, due to con-
jugation with the ketone group that determines the occurrence of
the nucleophilic attack exclusively at C-b of the double bond, in
agreement with the usual reactivity of
a,b-ethylenic carbonyl
compounds.12 Furthermore, the success of the intramolecular
oxa-Michael addition is improved in our experimental conditions
due to the formation of the phenoxide ion of the o-hydroxyphe-
none intermediate under basic catalysis. In this regard, the phen-
oxide ion becomes a better nucleophile than the hydroxy group
promoting the nucleophilic attack at C-b of the double bond
efficiently to afford chroman-4-one derivatives in high yields.
Table 4
Charge density values of phenoxide ion of the o-hydroxyphenone intermediates A, B,
and C, calculated with the Semi Empirical AM1 method
O
O
O
O
R
O
O
Table 2
photo-Fries rearrangement reaction of m-substituted phenyl 3-methyl-2-butenoate
esters (2l–o) in cyclohexane under basic catalysis (KOH 10% aq; biphasic system)a
R
R
A
B
C
Entry
Substrate; R
Irradiation time (min)
Conv. %
Yieldb
%
1
2
3
4
5
2l; OCH3
2m; CH3
2n; Cl
65
65
105
40
95
90
65
34
10
4m, 51; 5m, 42
4n, 50; 5n, 39
4o, 51; 5o, 41
4p, 59e; 5p, 0
4p, 30f; 5p, 0
R
Charge densities
B
A
C
c
2o; NO2
d
Ar–OÀ
C-b
Ar–OÀ
C-b
Ar–OÀ
C-b
2o; NO2
70
OCH3
OPh
t-Bu
CH3
H
À0.496
À0.478
À0.493
À0.495
À0.498
À0.485
À0.445
À0.103
À0.095
À0.103
À0.104
À0.105
À0.097
À0.078
À0.494
—
—
À0.490
À0.498
À0.484
À0.468
À0.103
—
—
À0.105
À0.105
À0.097
À0.005
À0.492
—
—
À0.499
À0.498
À0.487
À0.477
À0.105
—
—
À0.107
À0.105
À0.102
À0.085
a
Reaction conditions: 0.010 M of ester, excitation wavelength: 254 nm, quartz
vessel, degassed solvent (2.5 mL), under Ar, room temperature.
b
Yields are based on the conversion of the starting material.
Standard conditions, base catalyst/Cs2CO3 (s).
Standard conditions, solvent/benzene–KOH 10% aq.
m-nitrophenol was formed in 17.6% yield.
c
d
Cl
NO2
e
f
m-nitrophenol was formed in 70% yield.
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4
D. Iguchi et al. / Tetrahedron Letters xxx (2014) xxx–xxx
O
D. Iguchi thanks CONICET for doctoral scholarship (Doctorate
Program).
1
O
*
kd
O
O
Supplementary data
hν
R
R
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.
06.081.
Photo-Fries
rearrangement
kr
O
O
OH
O
References and notes
R
R
Organic layer
1. (a)Naturally Occuring Oxygen Ring Compounds; Dean, F. M., Ed.; Butherworths:
London, 1963; (b)Chromans and Tocopherols; Ellis, G. P., Lockhart, I. M., Eds.;
Wiley: New York, 1977; (c) Green, G.; Evans, J. M.; Vong, A. K. In Comprehensive
Heterocyclic Chemistry II; Mc Killop, A., Ed.; Pergamon: Oxford, 1996; Vol. 5, p
469.
2. (a) Hodgetts, K. J.; Maragkou, K. I.; Wallace, T. W.; Wootton, C. R. Tetrahedron
2001, 57, 6793–6804; (b) Fridin Saxim, M.; Pemberton, N.; Andersson, K.; Da,
S.; Dayrager, C.; Friberg, A.; Grotli, M.; Luthman, K. J. Org. Chem. 2009, 74, 2755–
2759.
Basic aqueous
layer
O
O
O
O
R
R
Intramolecular oxa-Michael
addition (Thermal process)
3. Takikawa, H.; Suzuki, K. Org. Lett. 2007, 9, 2713–2716.
4. Lipinski, C. A.; Aldinger, C. E.; Beyer, T. A.; Bordner, J.; Bussolotti, D. F.; Inskeep,
P. B.; Siegel, T. W. J. Med. Chem. 1992, 35, 2169–2177.
Scheme 4. Proposed reaction mechanism.
5. Butler, J. D.; Conrad, W. E.; Lodewyk, M. W.; Fettinger, J. C.; Tantillo, D. J.; Kurth,
M. J. Org. Lett. 2010, 12, 3410–3413.
6. Pritchard, R. G.; Sheldrake, H. M.; Taylor, I. Z.; Wallace, T. W. Tetrahedron Lett.
2008, 49, 4156–4159.
7. (a) Irsay, R. D. U.S. Patent N 3678170, 1972.; (b) MacDonald, J. E.; Hysell, M. K.;
Lui, Q.; Yu, D.; Ke, N.; Liu, G.; Li, H. Q. X.; Wong-Staal, F., WO N° 067451, 2008.
8. Meng, L.-G.; Liu, H.-F.; Wei, J.-L.; Gong, S.-N.; Xue, S. Tetrahedron Lett. 2010, 51,
1748–1750.
corresponding chroman-4-one through an intramolecular oxa-Mic-
heal addition reaction. A simplified mechanism for this reaction is
shown in Scheme 4, where kd means all the deactivation processes
(fluorescence emission and internal conversion) that compete with
the photochemical reaction kr.
Finally, we have measured the quantum yield (/r) of the bipha-
sic photochemical reaction of esters 2a–o using potassium ferriox-
alate as an actinometer.13 The /r values range between 0.30 and
0.50 indicating that the photoreaction is efficient and competes
with the deactivation processes of the singlet excited state,
namely, fluorescence emission and internal conversion.
In summary, we showed the scope and applicability of the one-
pot photochemical biphasic system for the preparation of 5-,
6-, and 7-substituted chroman-4-one derivatives starting from
p- and m-substituted aryl 3-methyl-2-butenoate esters. The esters
are easily prepared from commercially available substituted
phenols. With the application of this photochemical method, a ser-
ies of substituted chroman-4-one derivatives were prepared from
moderate to quantitative yields. Finally, this method exhibits
predictable regioselectivity and can be considered as a general
and wide useful and inexpensive methodology.
9. Friedel–Craft reaction: (a) Roy, O.; Loiseau, F.; Riahi, A.; Henin, F.; Muzart, J.
Tetrahedron 2003, 59, 9641; (b) Draper, R. W.; Hu, B.; Iyer, R. V.; Li, X.; Lu, Y.;
Rahman, M.; Vater, E. J. Tetrahedron 2000, 56, 1811; (c) Derrick, I. A. R.; Igbal,
M.; Livingstone, R.; McGreeny, B. J. J. Chem. Res. (S) 1999, 530; (d) Sebok, P.;
Jeko, J.; Timar, T.; Jaszberenyi, J. C. Heterocycles 1994, 38, 2099; (e) Amaral, A. C.
F.; Barnes, R. A. J. Heterocycl. Chem. 1992, 29, 1457; (f) Teixidor, P.; Camps, F.;
Messeguer, A. Heterocycles 1988, 27, 2459; (g) Timar, T.; Jaszberenyi, J. C. J.
Heterocycl. Chem. 1988, 25, 871; (h) Timar, T.; Hoszrafi, S.; Jaszberenyi, J. C.;
Kover, K. E.; Batta, G. Acta Chim. Hung. 1988, 125, 303; (i) Piccolo, O.; Fillippini,
L.; Tinucci, L.; Valoti, E.; Cittero, A. Tetrahedron 1986, 42, 885; (j) Tsukayama, M.
Bull. Chem. Soc. Jpn. 1975, 48, 80; Claisen rearrangement reaction (a)
Anjaneyulu, A. S. R.; Isaa, B. J. Chem. Soc., Perkin Trans. 1 1991, 2089; (b)
Ariamala, G.; Balasubramarnan, K. K. Tetrahedron 1989, 45, 309; (c) Rohatgi, B.
K.; Grupta, R. S.; Khanna, R. N. Indian J. Chem., Sect B 1981, 20, 505; (d)
Hlubucek, J.; Ritchie, E.; Taylor, W. C. Tetrahedron Lett. 1969, 17, 1369; Kabbe
condensation reaction (a) Kabbe, H. J.; Widdig, A. Angew. Chem., Int. Ed. 1982,
21, 247; (b) Paradkar, M. V.; Godbole, H. M.; Ranade, A. A.; Joseph, A. R. J. Chem.
Res. (S) 1998, 318.
10. (a) Miranda, M. A.; Primo, J.; Tormo, R. Tetrahedron 1987, 43, 2323; (b) Garcia,
H.; Iborra, S.; Primo, J.; Miranda, M. A. J. Org. Chem. 1986, 51, 4432; (c) Primo, J.;
Tormo, R.; Miranda, M. A. Heterocycles 1982, 19, 1819.
11. Samaniego López, C.; Erra-Balsells, R.; Bonesi, S. M. Tetrahedron Lett. 2010, 51,
4387–4390.
12. (a)Conjugate Addition Reactions in Organic Synthesis; Perlmutter, P., Ed.;
Pergamon: Oxford, 1992; p 126; (b) Baldwin, J. E.; Thomas, R. C.; Kruse, L. I.;
Silberman, L. J. Org. Chem. 1977, 42, 3846.
Acknowledgments
13. (a) Braslavsky, S. E.; Kuhn, H. J. Provisional List of Actinometers Comission III.3,
Photochemistry; IUPAC: Mulhein an der Ruhr, 1987; (b) Parker, A. C.
Photoluminiscence of Solutions with Applications to Photochemistry and
Analytical Chemistry; Elsevier: Amsterdam, New York, 1968.
The authors thank Universidad de Buenos Aires (X088 and
X372), CONICET (PIP0040 and PIP0155) for financial support. R.
Erra-Balsells and S. M. Bonesi are research members of CONICET.
Please cite this article in press as: Iguchi, D.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.06.081