Photooxygenations of 1-naphthols: An environmentally friendly access to 1,4-naphthoquinones

Article abstract of DOI:10.1016/j.tet.2005.11.021

Dye sensitized photooxygenations of 1-naphthols were investigated with soluble and solid-supported sensitizers and moderate to excellent yields of the corresponding 1,4-naphthoquinones were achieved in relatively short irradiation times. The mild and environmentally friendly reaction conditions made this application particularly attractive for 'Green Photochemistry'. Consequently, the photooxygenation of 1,5-dihydroxynaphthalene was studied with non-concentrated and moderately concentrated sunlight and 5-hydroxy-1,4- naphthoquinone (Juglone) was obtained in yields up to 71%.

Full text of DOI:10.1016/j.tet.2005.11.021

Tetrahedron 62 (2006) 1467–1473
Photooxygenations of 1-naphthols: an environmentally
friendly access to 1,4-naphthoquinones
Oliver Suchard,a Ronan Kane,b Bernard J. Roe,b Elmar Zimmermann,c Christian Jung,d
b,
Prashant A. Waske,a Jochen Mattaya, and Michael Oelgemoller
*
*
¨
a
¨
¨
¨
Organische Chemie I, Fakultat fur Chemie, Universitat Bielefeld, Postfach 10 01 31, D-33501 Bielefeld, Germany
bSchool of Chemical Sciences and National Institute for Cellular Biotechnology, Dublin City University, Glasnevin, Dublin 9, Ireland
c
¨ ¨ ¨ ¨
Institut fur Organische Chemie, Universitat zu Koln, Greinstr. 4, D-50939 Koln, Germany
d
¨
¨
¨
Deutsches Zentrum fur Luft- und Raumfahrt e.V. (DLR), Solarforschung, Linder Hohe, D-51147 Koln, Germany
Received 9 September 2005; revised 26 October 2005; accepted 10 November 2005
Available online 1 December 2005
Abstract—Dye sensitized photooxygenations of 1-naphthols were investigated with soluble and solid-supported sensitizers and moderate to
excellent yields of the corresponding 1,4-naphthoquinones were achieved in relatively short irradiation times. The mild and environmentally
friendly reaction conditions made this application particularly attractive for ‘Green Photochemistry’. Consequently, the photooxygenation of
1,5-dihydroxynaphthalene was studied with non-concentrated and moderately concentrated sunlight and 5-hydroxy-1,4-naphthoquinone
(Juglone) was obtained in yields up to 71%.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
can serve as a versatile alternative and various
examples involving 1-naphthols have been reported in the
literature.5–7
Naphthoquinone derivatives based on 5-hydroxy-1,
4-naphthoquinone (Juglone, 1) represent an important
class of natural products.1 Additionally, Juglone serves as
a valuable building block for the synthesis of biologically
active quinonoid compounds (Chart 1),2 and was thus
selected by us as starting material for our ongoing
photoacylation study.3 Most commonly, Juglone is
synthesized from the cheap and commercially available
1,5-dihydroxynaphthalene by oxidation, but many of these
thermal pathways suffer from severe disadvantages con-
cerning yield, selectivity, sustainability, scale-up or repro-
ducibility, respectively.4 Dye sensitized photooxygenations
Due to the favorable absorption of most dyes within the
visible spectrum, photooxygenation reactions have been
subjected to concentrated sunlight and served as model
systems for environmentally friendly and benign ‘Green
Photochemistry’.8–10 Recently, we have briefly reported on
solar photooxygenations to Juglone using novel holographic
mirror elements.8b In this publication, we would like to
present a comprehensive study on solar and artificial
light induced photooxygenations of 1-naphthols and
1,5-dihydroxynaphthalene in particular.
O
OH
O
O
O
OH
OH
O
OH
O
Pr
OH
O
O
OH
O
O
HO
HO
O
O
OH
O
O
O
O
O
O
Juglomycin A
Urdamycinone B
Juglone (1)
Frenolicin B
(+)-Nocardione A
Chart 1. Quinonoid natural products synthesized from Juglone 1 (for more examples see Ref. 2).
Keywords: Quinones; Photochemistry; Photooxygenations; Green chemistry.
*
Corresponding authors. Tel.: C49 521 106 2072; fax: C49 521 106 6417 (J.M.); tel.: C353 1 700 5312; fax: C353 1 700 5503 (M.O.);
e-mail addresses: mattay@uni-bielefeld.de; michael.oelgemoeller@dcu.ie
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.11.021

1468
O. Suchard et al. / Tetrahedron 62 (2006) 1467–1473
2. Results and discussion
was followed by GC or TLC analysis. After 5 h, Juglone
was isolated via column chromatography using chloroform
as eluent or, more conveniently, via continuous extraction
with n-hexane in a Soxhlet extractor.
2.1. Experiments with artificial light
To find suitable reaction conditions for the solar chemical
campaign, we have launched a detailed laboratory study
with artificial light and selected the photooxygenation of
1,5-dihydroxynaphthalene 2 to Juglone 1 as model system
(Scheme 1). A major disadvantage of the literature
procedures was the usage of the hazardous solvents
dichloromethane, acetonitrile or methanol, respectively,5
which needed to be replaced for a solar ‘outdoor’
application. In order to simplify the work-up procedure,
we furthermore examined the usage of solid-supported
In methanol, methylene blue was found to be the most
effective sensitizer and the desired 1 was isolated in a
reasonable yield of 51%. As would be expected for
heterogeneous conditions, the yields for the solid-supported
sensitizers were slightly lower with 43% (MBIE) and 32%
(RBMF), respectively. With Sensitoxw, the characteristic
orange color of Juglone became clearly visible after a
relatively short irradiation time. After 5 h, TLC analysis
showed no sign of sensitizer leaching. In contrast to the
literature,12 significant leaching was, however, observed for
methylene blue on ion exchange resin as noticeable from the
green color of the final reaction mixture. Similar preferences
and yields were obtained when ethanol was used as solvent.
Likewise, the photooxygenation proceeded satisfactory in
isopropanol and 2 was isolated in yields of 33–58%. The
best Juglone yields of 71 and 68% were obtained using
acetone as solvent and rose bengal or Sensitoxw as sensitizer
(entries 17 and 19), respectively. In comparison to the
irradiations in alcohols, methylene blue gave a somewhat
lower yield of 48%. Due to the limited solubility of the diol
2 in pure dichloromethane, photosensitized oxygenations
were conducted alternatively in a 9:1 mixture with
methanol, and Juglone was formed in yields of 28–51%.
Surprisingly, the product yields did not improve as would be
expected from the longer 1O2 lifetime in this solvent
mixture.13 Enhanced photobleaching of the dye and
photodecomposition of 1 due to the rather harsh radiation
emitted from the medium-pressure mercury lamp,5b
in combination with the formation of acid from the
halogenated solvent, might explain this unexpected drop.
sensitizers, in particular Sensitoxw (rose bengal on Merry-
11
field resin; RBMF
)
and methylene blue on ion exchange
resin (MBIE).12 Both materials can be easily removed by
filtration and are, in principal, reusable.
OH
O
O
hυ, O
2
sensitizer / solvent
OH
OH
2
1
Scheme 1. Photooxygenation of 1,5-dihydroxynaphthalene 2.
Following a standardized procedure, a 0.01 M solution of
1,5-dihydroxynaphthalene 2 was irradiated with a 150 W
medium-pressure mercury lamp in the presence of a
sensitizer while a gentle stream of oxygen was passed
through the solution (Table 1). The progress of the reaction
Table 1. Experimental data for the photooxygenations of 2 with artificial
light (150 W medium-pressure mercury lamp)
In the absence of sensitizer, 1 was formed in only small
amounts of 2–3% in all alcoholic solvents and in the
dichloromethane/methanol mixture. Solely the irradiation
of 2 in pure acetone furnished Juglone in a significant
yield of 8% (entry 20),14 and we tentatively postulate a
type-I photooxidation for its formation as known for
phenols.5d
Entry
Sens.a
Solvent
Time (h)
1 (%)
1
2
3
4
5
6
7
8
MB
RB
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
EtOH
EtOH
EtOH
EtOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
Acetone
Acetone
Acetone
Acetone
Acetone
CH2Cl2/MeOHc
CH2Cl2/MeOHc
CH2Cl2/MeOHc
CH2Cl2/MeOHc
CH2Cl2/MeOHc
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
51
34
43
32
2
54
37
47
32
2
58
38
46
33
2
48
71
41
68
8
MBIE
RBMF
b
—
MB
RB
MBIE
RBMF
A scale-up of the photooxygenation to 1 was furthermore
examined in acetone with Sensitoxw, and the concentration
of 2 was stepwise increased in 0.01 mol/l intervals. Up to a
concentration of 2 of 0.05 mol/l, complete conversions were
achieved within 5 h and 1 was isolated in yields of 65–70%.
At higher concentrations, prolonged irradiation times up to
10 h were required but 1 was still isolated in good yields of
63–68%. At a diol concentration of 0.1 mol/l, the reaction
was stopped after 10 h. At this stage, GC analysis showed a
conversion of ca. 80%. After work-up, Juglone was obtained
in 55% yield (69% based on conversion).
9
b
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
—
MB
RB
MBIE
RBMF
b
—
MB
RB
MBIE
RBMF
b
—
MB
51
34
43
28
3
RB
MBIE
RBMF
The photooxygenation protocol was additionally applied to
1-naphthol 3a, 1-acetoxy-5-hydroxynaphthalene 3b and
5-acetamido-1-hydroxynaphthalene 3c (Scheme 2;
Table 2), respectively. 1-Naphthol 3a readily gave 53% of
1,4-naphthoquinone 4a when irradiated with a medium-
pressure mercury lamp in acetone and in the presence of
Sensitoxw. In line with the literature,15b irradiation of 3b
b
—
a Sensitizers: methylene blue (MB), rose bengal (RB), methylene blue on
ion exchange resin (MBIE), rose bengal on Merryfield resin (Sensitoxw,
RBMF).
b Without sensitizer.
c CH2Cl2/MeOH (9:1).

O. Suchard et al. / Tetrahedron 62 (2006) 1467–1473
1469
under identical conditions furnished Juglone 1 in 68% yield.
Obviously, the acetate-group is cleaved during the course of
the reaction. In contrast, the related amide-linked compound
3c readily gave the corresponding quinone 4c in a good
yield of 61%.15a
was somewhat lower with 78% when air was used as oxygen
source (entry 10). Noteworthy, this procedure represents the
so far best synthetic pathway to Juglone.4
2.2. Solar chemical experiments
OH
O
In July and August 2005, we have conducted a series of
solar chemical experiments at Dublin City University
(latitude 538230N, 68150W, 50 m above sea level). Due to
the volatility and flammability of the solvent acetone, we
have selected the less hazardous isopropanol for our
campaign. Following this strategy, various solutions of 2
were exposed in a Schlenck-flask equipped with a cold
finger and a reflux condenser to direct sunlight while the
solution was purged with a gentle stream of air. All
experiments went smoothly and gave satisfactory results in
reasonable periods of time without any noticeable side-
products (Table 4). The first run (I) was performed with
soluble rose bengal under ideal solar conditions and Juglone
was isolated in a moderate yield of 39% after 3.5 h of
illumination. A somewhat lower yield of 30% of 1 was
obtained when the reaction was repeated for 6.5 h during a
partly sunny period (II). With soluble methylene blue as
sensitizer (III), Juglone became available in 44% yield after
5.5 h of partly sunny weather. Likewise, Sensitoxw was
tested as a heterogeneous sensitizer (IV). Within 1⁄2 h, the
orange color of 1 became clearly visible and further
intensified with progressing illumination. Due to the limited
distribution of the solid sensitizer within the reaction
mixture, 1 was obtained in just 19% yield after 6.5 h of
perfect weather conditions. Noteworthy, all reactions
described above could have been driven easily to high
conversions with longer illumination times. Thus, the
preliminary results obtained clearly indicate that the solar
photosensitized oxygenation of 1,5-dihydroxynaphthalene
opens a promising and environmentally friendly pathway
to Juglone.
hυ, O2
Sensitox(R) / acetone
R
R
O
3
4
Scheme 2. Photooxygenations of 3.
Table 2. Experimental data for the photooxygenations of 3 with artificial
light (150 W medium-pressure mercury lamp)
Entry
R
Sens.a
Solvent
Time (h)
4 (%)
1
2
3
H (3a)
OAc (3b)
NHAc (3c)
RBMF
RBMF
RBMF
Acetone
Acetone
Acetone
5
5
5
53 (4a)
68 (1)
61 (4c)
a Sensitizer: rose bengal on Merryfield resin (Sensitoxw, RBMF).
Due to the absorption of Juglone within the emission spectra
of the medium-pressure mercury lamp,16 we have con-
ducted a series of experiments using a pair of 500 W
halogen lamps (Table 3). Since solution purging with pure
oxygen is furthermore problematic for industrial appli-
cations, we have examined its replacement with compressed
air. Almost all experiments were run in non-hazardous
isopropanol. Irradiations with pure oxygen readily furnished
Juglone in yields of 25–70% after 5 h. Since the given set-up
did not allow an even distribution of the solid-supported
sensitizers within the reaction mixture, the experiments
involving Sensitoxw and methylene blue on ion exchanger
resin (MBIE) showed significantly lower conversions and
yields. With compressed air, prolonged irradiation times of
10 h were required but the desired 1 was still obtained in fair
to high yields of 21–71%. For laboratory purposes, we have
furthermore modified the conditions reported by Cossy and
Belotti for photooxygenations of 8-hydroxyquinolines.7b
Irradiation in a 9:1 mixture of dichloromethane and
methanol for 2 h and in the presence of TTP as sensitizer
yielded 1 in an excellent yield of 88% (entry 5). The yield
Table 4. Experimental data for the solar photooxygenation reactions of 2
with non-concentrated sunlight
Experiment
I
II
III
IV
Date
Scale
12.07.2005 13.07.2005
25.07.2005
08.08.2005
2 (g)
0.56
0.56
0.56
0.56
Sens. (g)a
Solvent
V (ml)
0.05 (RB)
i-PrOH
350
0.05 (RB)
i-PrOH
350
0.05 (MB)
i-PrOH
350
0.4 (RBMF
i-PrOH
350
)
Table 3. Experimental data for the photooxygenations of 2 with artificial
light (2!500 W halogen lamps)
Time
ISTb
14:15–
17:45
3.5
11:45–
18:15
6.5
10:45–
16:15
5.5
10:15–16:45
Entry
Sens.a
Solvent
Gas
Time (h)
1 (%)
1
2
3
4
5
6
7
8
9
10
MB
RB
MBIE
RBMF
TPPb
MB
i-PrOH
i-PrOH
i-PrOH
i-PrOH
O2
O2
O2
O2
5
5
5
5
69
70
34
25
88
71
55
45
21
78
Total (h)
Weather
Yield 1 (%) 39
6.5
Sunny
Partly sunny Partly sunny Sunny
30 44 19
CH2Cl2/MeOHc O2
2
a Sensitizers: rose bengal (RB), methylene blue (MB), rose bengal on
Merryfield resin (Sensitoxw, RBMF).
i-PrOH
i-PrOH
i-PrOH
i-PrOH
Air
Air
Air
Air
10
10
10
10
3
b Irish summer time.
RB
MBIE
RBMF
TPPb
Further solar chemical experiments were performed with
moderately concentrated sunlight at the solar chemical
facility of the German Aerospace Center (DLR) close to
Cologne/Germany (latitude 508510N, 78070E, 70 m above
sea level).17 A parabolic trough collector designed for
laboratory-scale (!500 ml) applications and equipped with
CH2Cl2/MeOHc Air
a Sensitizers: methylene blue (MB), rose bengal (RB), methylene blue on
ion exchange resin (MBIE), rose bengal on Merryfield resin (Sensitoxw,
RBMF), tetraphenylporphine (TPP).
b TPP insoluble in i-PrOH.
c CH2Cl2/MeOH (9:1).

1470
O. Suchard et al. / Tetrahedron 62 (2006) 1467–1473
Table 5. Experimental data for the solar photooxygenation reactions of 2
with moderately concentrated sunlight
an aluminum mirror (aperture: 41!36 cm) was selected
(Fig. 1).18 The reactor offers a geometric concentration
factor of about 18 suns, but its efficiency is reduced in
practice due to optical losses. Tracking of the sun is
performed manually for the elevation and the azimuth every
15 min. The reaction mixture is pumped through the jacket
of a Liebig condenser (diameter: 2.4 cm), which is placed in
the focal line of the concentrator. Cooling water is passed
through the inner tube of the condenser. Oxygen is added
via a simple Y-connector, which limited its homogeneous
distribution within the absorber tube.
Experiment
V
VI
VII
Date
Scale
15.08.2003
09.09.2003
11.09.2003
2 (g)
0.5
1.0
1.0
0.1
MeOH
100
Rose bengal (g)
Solvent
V (ml)
0.05
i-PrOH
100
0.01
i-PrOH
100
Time
CESTa
Total (h)
14:20–16:50
2.5
13:45–17:15
3.5
2.5
10:15–14:45
4.5b
—
Effective (h)c
Weather
⁄
3
2
Sunny
Mostly sunny
Partly sunny
Photons (mol)d
Total
0.26
0.07
0.21
0.16
88
69 (78)g
0.16
—
86
46 (54)g
Effectivee
Conversion (%)f
Yield 1 (%)
O95
71 (75)g
a Central European summer time.
b Stopped due to rainfall.
c Time until conversion reaches an almost constant value.
d Estimated amount of photons collected between 500–600 nm.18
e Estimated amount of photons (500–600 nm) for effective illumination
time.
f
Conversion of 2 as determined by GC analysis (vs tetradecane).
g Yield based on conversion of 2.
100
direct normal irradiance
2
1
700
600
500
400
300
200
100
0
80
60
40
20
0
n
Figure 1. Laboratory-scale parabolic trough reactor during the solar
photooxygenation of 1,5-dihydroxynaphthalene 2 (the red color of the
sensitizer rose bengal can be clearly seen).
I
In August and September 2003, three laboratory-scale
experiments were conducted, and the progress of each
reaction was followed by GC analysis versus tetradecane as
internal standard. Due to its favorable solar sensitization
efficiency and overall stability,8d rose bengal was chosen as
sensitizer. The experimental details and results from the
solar chemical studies are summarized in Table 5.
00:00
00:30
01:00
01:30
02:00
02:30
Illumination time [h]
Figure 2. Direct normal irradiance (In) and product composition versus
illumination time for the photooxygenation of 1,5-dihydroxynaphthalene 2
(Experiment VI).
The first run (V) was performed during a sunny period with
0.5 g of diol 2 and 0.05 g of rose bengal in 100 ml of
isopropanol. The starting material was readily consumed
and already after 40 min, GC analysis revealed complete
conversion (O95%). During that time the reactor collected
0.07 mol of photons in the important absorption range of
rose bengal between 500–600 nm.19 After work-up, the
desired product 1 was obtained in 71% yield. For the second
experiment (VI) under mostly sunny conditions, the amount
of diol 2 was doubled to 1.0 g and after 2.5 h, GC analysis
showed a constant value for Juglone 1 of 74% (Fig. 2).
The collector received 0.16 mol of photons in the range of
500–600 nm,19 slightly more than double the amount as
during the first experiment. After a total illumination period
of 3.5 h, Juglone was obtained in a good yield of 69%
(78% based on conversion). For the final experiment (VII),
the solvent was replaced by methanol. Due to the less
favorable weather, the illumination time needed to be
extended. After 4.5 h of partly sunny conditions, the
reaction was stopped at 86% conversion due to beginning
rainfall. At this stage the reactor had collected 0.16 mol of
photons between 500–600 nm.19 After work-up, Juglone
was isolated in a moderate yield of 46% (54% based on
conversion).
3. Conclusion
In conclusion, photooxygenations of 1-naphthols to the
corresponding 1,4-naphthoquinones can serve as a useful
and environmentally friendly alternative to existing thermal
processes. The solar chemical reaction of the cheap and
commercially available 1,5-dihydroxynaphthalene can be
easily performed with non-concentrated or concentrated

O. Suchard et al. / Tetrahedron 62 (2006) 1467–1473
1471
sunlight, and yields the valuable intermediate Juglone.
Thus, a realization of Giacomo Ciamician’s spectacular
vision of ‘the photochemistry of the future’ (presented at
the International Congress of Applied Chemistry in New
York in 1912)20 seems feasible.
mp: 152 8C (lit.:5a 151–154 8C). 1H NMR (400 MHz,
CDCl3): dZ6.94 (s, 2H), 7.27 (dd, 1H, JZ2.2, 7.5 Hz),
7.60–7.65 (m, 2H), 11.90 ppm (s, 1H, OH). 13C NMR
(100 MHz, CDCl3): dZ114.0, 118.1, 123.5, 130.8, 135.5,
137.6, 138.6, 160.6, 183.2, 189.3 ppm. MS (EI, 70 eV):
m/zZ174 (MC, 100%), 146, 118, 90, 63, 39. IR (KBr): nZ
3400, 3058, 1662, 1641, 1590, 1448, 1289, 1225, 1151,
1098, 1081, 863, 827, 762, 703 cmK1. UV/vis (n-hexane):
lmaxZ247.8, 318.0, 427.8 nm. Anal. Calcd for C10H6O3: C
68.97, H 3.47. Found: C 68.25, H 3.70.
4. Experimental
4.1. General methods
Melting points were measured on a Bu¨chi B-540 apparatus
and are uncorrected. 1H and 13C NMR spectra were
recorded on a Bruker Avance 400 spectrometer (400 and
100 MHz, respectively) using the solvent residual peak as
internal standard. Chemical shifts (d) are given in ppm,
coupling constants (J) in Hz. MS spectra were recorded on a
Finnigan MAT 8230 (EI) spectrometer. IR spectra were
recorded as KBr discs on a Perkin-Elmer 298 infrared
spectrophotometer, UV/vis spectra on a Perkin-Elmer
Lambda 7 spectrophotometer using n-hexane (Janssen
Chimica, spectrophotometric grade) as solvent. For
combustion analysis a Heraeus CHN-O-Rapid Elemental
Analyzer was used. GC analysis was performed on a
Shimadzu GC-14A or a Hewlett-Packard GC 5890 Series II.
A Hanau TQ-150 medium-pressure mercury lamp (150 W)
or Armley 500 W halogen lamps (2!500 W) and immer-
sion well reactors (lO280 nm) were used for irradiation
experiments. TLC was carried out on Merck Kieselgel 60
F254, column chromatography on silica gel (Macherey and
Nagel) 230–240 mesh using chloroform or a 19:1 mixture of
chloroform and methanol. 1,5-Dihydroxynaphthalene 2 was
purified according to a modified procedure of Johnson and
co-workers.21 1-Acetoxy-5-hydroxynaphthalene 3b was
synthesized as reported by Becher et al.,22 5-acetamido-1-
hydroxynaphthalene 3c via a method described by Jindal
and co-workers.23 Sensitoxw was prepared with chloro-
methylated styrene–divinylbenzene copolymer (50–100
mesh, 1% cross-linked) according to Schapp et al.,11
methylene blue on ion exchange resin (Lewatit SC 104 or
MonoPlus SP 112) according to Williams and co-workers.12
Solvents and reagents were commercially available and
were used without further purification.
4.2.1.2. 1,4-Naphthoquinone (4a). Isolated by column
chromatography using chloroform as eluent. Yellow-
brownish solid, mp: 128 8C (lit.:24 128.5 8C). 1H NMR
(400 MHz, CDCl3): dZ6.96 (s, 2H), 7.74 (m, 2H),
8.06 ppm (m, 2H). 13C NMR (100 MHz, CDCl3): dZ
126.5, 132.0, 134.1, 138.8, 185.2 ppm. MS (EI, 70 eV):
m/zZ158 (MC, 100%), 130, 102, 76, 62, 50, 40. IR (KBr):
nZ1660, 1587, 1331, 1302, 1146, 1115, 1059, 863,
771 cmK1. UV/vis (n-hexane): lmaxZ240.2, 245.3,
328.3 nm. Anal. Calcd for C10H6O2: C 75.94, H 3.82.
Found: C 75.55, H 3.91.
4.2.1.3. 5-Acetamido-1,4-naphthoquinone (4c). Iso-
lated by column chromatography using chloroform as
eluent. Yellow solid, mp: 170 8C (lit.:14a 172 8C). 1H
NMR (400 MHz, CDCl3): dZ2.29 (s, 3H), 6.90 (d, 1H,
JZ10 Hz), 6.94 (d, 1H, JZ10 Hz), 7.72 (dd, 1H, JZ
8.4 Hz), 7.81 (dd, 1H, JZ1.2, 8.4 Hz), 9.07 (dd, 1H, JZ1.2,
8.4 Hz), 11.85 (s, 1H, NH). 13C NMR (100 MHz, CDCl3):
dZ25.8, 116.1, 122.1, 126.2, 132.3, 135.9, 138.1, 140.1,
141.5, 170.1, 184.7, 189.3 ppm. MS (EI, 70 eV): m/zZ215
(MC), 173 (100%), 145, 117, 101, 91, 63, 43. IR (KBr): nZ
3477, 3211, 1707, 1666, 1646, 1609, 1580, 1496, 1408,
1302, 1264, 1159, 833, 766, 723 cmK1. Anal. Calcd for
C12H9N1O3: C 66.97, H 4.22, N 6.51. Found: C 66.51, H
4.39, N 6.70.
4.2.2. General procedure (halogen lamps). Five hundred
and forty milligrams (3.5 mmol) of 1,5-dihydroxynaphtha-
lene 2 were dissolved in 350 ml of solvent. The sensitizer
was added (MB: 50 mg; RB: 50 mg; MBIE: 400 mg; RBMF
:
400 mg; TPP: 20 mg) and the solution was irradiated
(2!500 W halogen lamps) in a Schlenck-flask equipped
with a cold finger and a reflux condenser for 2–10 h at room
temperature while purging with a gentle stream of oxygen
or air. Evaporated solvent was frequently refilled. The
progress of the reaction was monitored by TLC analysis.
The reaction mixture was filtrated the solvent removed in
vacuum, and the residue was purified by column
chromatography (SiO2, CHCl3) or by extraction in a
Soxhlet extractor with n-hexane. Experimental details are
given in Table 3.
4.2. Irradiation and illumination experiments
4.2.1. Irradiations with artificial light. General pro-
cedure (medium-pressure mercury lamp). The naphthol
(1.5 mmol) was dissolved in 150 ml of solvent. The
sensitizer was added (MB: 10 mg; RB: 20 mg; MBIE:
400 mg; RBMF: 100 mg) and the solution was irradiated
with a Hanau TQ-150 medium-pressure mercury lamp
(150 W) for 5 h at room temperature while purging with a
gentle stream of oxygen. Evaporated solvent was frequently
refilled. The progress of the reaction was monitored by TLC
or GC analysis. The reaction mixture was filtrated, the
solvent removed in vacuum, and the residue was purified by
column chromatography on silica or by extraction in a
Soxhlet extractor with n-hexane. Experimental details are
given in Tables 1 and 2.
4.2.3. Illuminations with sunlight. General procedure
(non-concentrated sunlight). Five hundred and forty
milligrams (3.5 mmol) of 1,5-dihydroxynaphthalene 2
were dissolved in 350 ml of isopropanol. The sensitizer
was added (RB: 50 mg; MB: 50 mg; RBMF: 400 mg) and the
solution was exposed to direct sunlight in a Schlenck-flask
equipped with a cold finger and a reflux condenser for
3.5–6.5 h while purging with a gentle stream of air.
Evaporated isopropanol was frequently refilled and
4.2.1.1. 5-Hydroxy-1,4-naphthoquinone (Juglone, 1).
Isolated by Soxhlet extraction with n-hexane. Orange solid,

1472
O. Suchard et al. / Tetrahedron 62 (2006) 1467–1473
4. For overviews on thermal preparations of Juglone, see: (a)
Wakamatsu, T.; Nishi, T.; Ohnuma, T.; Ban, Y. Synth.
Commun. 1984, 14, 1167. (b) Crouse, D. J.; Wheeler, M. M.;
Goemann, M.; Tobin, P. S.; Basu, S. K.; Wheeler, D. M. S.
J. Org. Chem. 1981, 46, 1814. (c) Grundmann, C. Synthesis
1977, 644.
the progress of the reaction was monitored by TLC analysis.
The reaction mixture was filtrated, the solvent removed
in vacuum, and the residue was purified by column
chromatography (SiO2, CHCl3) or by extraction in a Soxhlet
extractor with n-hexane. Experimental details are given
in Table 4.
5. (a) Wurm, G.; Geres, U. Arch. Pharm. (Weinheim) 1985, 318,
931. (b) Durchstein, H.-J.; Wurm, G. Arch. Pharm.
(Weinheim) 1984, 317, 809. (c) Wurm, G.; Geres, U.;
Schmidt, H. Dtsch. Apoth. Ztg. 1980, 120, 2045. (d)
Griffiths, J.; Chu, K.-Y.; Hawkins, C. J. Chem. Soc., Chem.
Commun. 1976, 676.
4.2.4. General procedure (concentrated sunlight).
1,5-Dihydroxynaphthalene 2 was dissolved in 100 ml of
solvent. Rose bengal was added and the solution was
exposed to moderately concentrated sunlight in a parabolic
trough reactor for 2.5–4.5 h while purging with a gentle
stream of oxygen. The progress of each reaction was
monitored by GC analysis versus tetradecane as internal
standard. The solvent was removed in vacuum, and the
residue was purified by column chromatography (SiO2,
CHCl3/MeOHZ19:1). Experimental details are given in
Table 5.
6. (a) Takaguchi, Y.; Yanagimoto, Y.; Fujima, S.; Tsuboi, S.
Chem. Lett. 2005, 33, 1142. (b) Jensen, A. W.; Daniels, C.
J. Org. Chem. 2003, 68, 207. (c) Chawla, H. M.; Kaul, K.;
Kaul, M. Indian J. Chem., Sect. B 1993, 32, 733.
7. (a) Amarasekara, A. S. Synth. Commun. 1999, 29, 3063. (b)
Cossy, J.; Belotti, D. Tetrahedron Lett. 2001, 42, 4329.
¨
8. (a) Oelgemoller, M.; Jung, C.; Ortner, J.; Mattay, J.; Schiel, C.;
¨
Zimmermann, E. The Spectrum 2005, 18, 28. (b) Oelgemoller,
M.; Jung, C.; Ortner, J.; Mattay, J.; Zimmermann, E. Green
Chem. 2005, 7, 35. (c) Pohlmann, B.; Scharf, H.-D.; Jarolimek,
U.; Mauermann, P. Sol. Energy 1997, 61, 159. (d) Esser, P.;
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Acknowledgements
This research project was financially supported by the
Arbeitsgemeinschaft Solar Nordrhein-Westfalen (Themen-
feld 3: Solare Chemie und Solare Materialuntersuchungen)
and Dublin City University (Research Alliance Fund). The
authors would like to thank Prof. Axel G. Griesbeck for
valuable discussions and Damien McGuirk for technical
assistance.
¨
1994, 33, 2009. (e) Oelgemoller, M.; Jung, C.; Ortner, J.;
Mattay, J.; Schiel, C.; Zimmermann, E. Proceedings of the
2004 International Solar Energy Conference, Portland, OR,
USA, July 11–14, 2004; ASME, 2004; ISEC2004-65021.
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Albini, A.; Fagnoni, M. Green Chem. 2004, 6, 1. (b) Mattay, J.
Chem. unserer Zeit 2002, 36, 98. (c) Albini, A.; Fagnoni, M.;
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