Base-induced photorearrangements from 3-styrylfurans to 2-methylnaphthalenes

Article abstract of DOI:10.1016/j.tetlet.2011.10.141

Irradiation of 3-(4-substituted styryl)furans in basic media yielded a series of 7-substituted-2-methylnaphthalenes, including methoxy, isopropyl, ethyl, methyl, fluoro, and cyano substituents. This base-induced photorearrangement involves cis-trans photoisomerization, 6e photocyclization, base-induced elimination, and a Norrish Type I photoreaction and is a novel method to synthesize unsymmetrical 2,7-disubstituted naphthalenes.

Full text of DOI:10.1016/j.tetlet.2011.10.141

Tetrahedron Letters 52 (2011) 7199–7201
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Base-induced photorearrangements from 3-styrylfurans
to 2-methylnaphthalenes
⇑
Jinn-Hsuan Ho , Tunng-Hsien Lee, Chia-Kai Lo, Chao-Li Chuang
Department of Chemical Engineering, National Taiwan University of Science and Technology, Sec. 4, No. 43, Keelung Road, Taipei 10607, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Irradiation of 3-(4-substituted styryl)furans in basic media yielded a series of 7-substituted-2-methyl-
naphthalenes, including methoxy, isopropyl, ethyl, methyl, fluoro, and cyano substituents. This base-
induced photorearrangement involves cis–trans photoisomerization, 6e photocyclization, base-induced
elimination, and a Norrish Type I photoreaction and is a novel method to synthesize unsymmetrical
2,7-disubstituted naphthalenes.
Received 17 August 2011
Revised 20 October 2011
Accepted 26 October 2011
Available online 31 October 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Base-induced photorearrangement
Photocyclization
3-Styrylfurans
2-Methylnaphthalenes
The reactive intermediates of the photocyclization of stilbenes,
trans-4a,4b-dihydrophenanthrenes,1 can undergo a variety of reac-
tions, such as oxidative photocyclization,2 eliminative photocycli-
zation,3 hydrogen shift reactions,4 and rearrangement reactions.5
Styrylfurans are analogs of stilbene and also undergo numerous
photoreactions.6 2-Styrylfurans can undergo the photocyclization
to give trans-9a,9b-dihydronaphtho[2,1-b]furans DHNF intermedi-
ates (Scheme 1). These intermediates can not only react with
iodine or oxygen to give naphtho[2,1-b]furans,7 but can also under-
go photorearrangement to give benzofurans6e–g with proper sub-
stituents on carbon 8 of DHNF. In Ho’s studies of acid-catalyzed
photorearrangements, acids promoted the acid-catalyzed [1,9]H
shift to transform DHNF into DHNF0. Subsequently, DHNF0 could
react to yield four different types of products depending on the
substituents and solvent conditions used. These reactions involved
the benzene ring (blue part) of DHNF0, including ring-opening reac-
tions to give type A and B products, hydrolysis of the vinyl ether to
give a type C product, or a [1,3]H shift to give a type D product.
Unlike 2-styrylfurans, 3-styrylfurans are much less studied. In
our preliminary research, irradiation of 3-styrylfurans in acid med-
ia gave no photorearrangement products except for 3-(4-
methoxystyryl)furan,5a but these reactions did yield a series of 2-
methylnaphthalenes in basic media (Scheme 2). According to our
expectations, the photocyclization intermediates of 3-styrylfurans,
trans-9a,9b-dihydronaphtho[1,2-b]furans DHNF00, may undergo a
base-induced elimination reaction to break the furan ring (red
part) and to generate the naphthalene ring. Then, a deformylation
occurred to give 2-methylnaphthalenes. Herein, we would like to
report this base-induced photorearrangement of 3-styrylfurans.
R
O
(a)
R
trans-2-styrylfurans
O
A
O
hv
R
R
(b)
(c)
O
O
B
C
O
R
O
O
DHNF'
cis-2-styrylfurans
acid-
hv
catalyzed
R
[1,9]H shift
8
H
H
(d)
O
O
DHNF
D
(a) R = Alkyl, F, Cl in CH2Cl2 or R = Alkoxy in C6H6
(b) R = Alkoxy in CH2Cl2 or R = F in 0.5 M HCl/CH3CN
(c) R = OCH3 in 0.5 M HCl/CH3CN
(d) R = F, Cl, Br in 0.5 M HCl/CH3CN
⇑
Corresponding author. Tel.: +886 2 2737 6642; fax: +886 2 2737 6644.
E-mail address: jhho@mail.ntust.edu.tw (J.-H. Ho).
Scheme 1. Overview of the acid-catalyzed photorearrangement of 2-styrylfurans.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.10.141

7200
J.-H. Ho et al. / Tetrahedron Letters 52 (2011) 7199–7201
Table 2
R
O
Photoreactions of 3-styrylfurans 1aꢀ1h in C6H6/0.02 M EtOH/0.023 M DBU
H
R
R
hv
R
O
R
2-methyl-
naphthalene
hv 300nm
trans-3-styrylfurans
DHNF''
DBU/EtOH
in C6H6, 6hr
O
1a-1h
2a-2g
Scheme 2. Base-induced photorearrangement of trans-3-styrylfurans.
R
En.
React.
Substituent
Conver. (%)
Product.
Yield (%)
R
(a) or (b)
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
OMe
i-Pr
Et
Me
H
F
CN
NO2
100
100
100
100
100
100
100
28
2a
2b
2c
2d
2e
2f
41
61
46
58
42
35
X
O
X= Cl or Br
cis/trans-1a-1h
R = OMe(a), i-Pr(b), Et(c), Me(d), H(e), F(f), CN(g), NO2(h)
15 (52)a
0
Scheme 3. Syntheses of 3-styrylfurans 1aꢀ1h. Reactions and conditions: (a)
Wittig–Horner reaction: (1) POEt3, (2) CH3ONa, 3-fural; (b) Wittig reaction: (1)
PPh3, (2) K2CO3, 3-fural.
1g
1h
2g
—
a
Irradiation of 1g in CH3OH/H2O (19:1) with 0.09 M KOH for 3 h resulted in a
reaction with 76% conversion and 52% yield.
The reactants, 3-(4-substituted styryl)furans 1aꢀ1h, were syn-
thesized from 4-substituted benzyl halides and 3-fural (Scheme
3) by the Wittig or Wittig–Horner reaction.8 The general synthetic
procedures are described in the Supplementary data.
base-induced photorearrangement to give the corresponding
7-substituted-2-methylnaphthalenes 2aꢀ2g in 61ꢀ15% yields.
Compared with the moderate yields of 1aꢀ1e, the yield of 3-(4-
cyanostyryl)furan 1g was low (15%). The electron transfer between
DBU and 1g may occur and result in an anion radical on 1g, and this
anion radical can be stabilized by the cyano group. Therefore, the
photorearrangement of compound 1g may be retarded. When we
irradiated 1g in CH3OH/H2O (19:1) with 0.09 M KOH, the yield of
2g was 52%. This result indicates that DBU indeed affects the pho-
torearrangement. Reactant 1h with the nitro group gave no photo-
rearrangement product, possibly because the nitro group retards
the formation of the photocyclization intermediate DHNF00.12
We have proposed a possible mechanism for this base-induced
photorearrangement of 3-styrylfurans 1, as shown in Scheme 4.
cis-3-Styrylfurans 1 can undergo 6e photocyclization to the inter-
mediate DHNF00, and then the elimination reaction occurs between
carbons 9a and 9b of DHNF00 to produce the intermediate N1. Sub-
sequently, N1 can transform into intermediate N3 via protonation
and tautomerization. Finally, a Norrish Type I photoreaction13 oc-
curs to give product 2. To confirm the existence of intermediate
N3, reactant 1d was irradiated with sodium borohydride in KOH/
methanol, and the reductive product 3d was obtained in a 31%
yield (Scheme 5).
The optimal irradiation conditions for 0.005 M 3-(4-methylsty-
ryl)furan 1d are shown in Table 1. First, neutral solvents, including
methanol, acetonitrile, and benzene, were used (entries 1ꢀ3), and
no rearrangement product 2d was observed. When we used meth-
anol/water (19:1) with 0.09 M KOH (entry 4), product 2d was
obtained in a 40% yield, and some minor methanol-addition prod-
ucts9 were also found. To prevent this side reaction, a powder KOH/
acetonitrile solvent was used instead of methanol, and product 2d
was obtained in a 41% yield (entry 5). Because of the poor solubility
of KOH in acetonitrile, we replaced this base with DBU (1,8-diaza-
bicyclo[5.4.0]undec-7-ene), and the yields of 2d in DBU/acetoni-
trile solvent were low (ꢁ20%) (entries 6 and 7). These low yields
may have been the result of the fact that acetonitrile is a good sol-
vent for electron transfer reactions between stilbene-type com-
pounds and amines.10 The better electron transfer ability of
acetonitrile has been reported in the literature.11 Finally, we used
benzene and DBU (entries 8ꢀ10), and we obtained product 2d with
acceptable yields, 56–58%.
The results of the irradiation of 1aꢀ1h in DBU/EtOH/benzene
are shown in Table 2. Most reactants, 1aꢀ1g, could undergo the
Table 1
Photoreactions of 3-(4-methylstyryl)furan 1d under different conditions
2
O
1
O
O
9
H
3
hv
hv
R
R
R
R
R
9b
hv 300nm
condition
and time
8
7
9a
4
5
6DHNF''
O
2d
1d
N1
N2
cis-1
O
H
HO
R
R
En. Conditions
Time (h) Conver. (%) Yield (%)
1
2
3
4
5
6
7
8
9
10
CH3OH
CH3CN
C6H6
3
3
3
3
6
6
6
6
6
6
38
2
19
93
100
100
100
100
100
100
0
0
0
40
41
20
20
56
58
20
2
N3
NaBH4
CH3OH/H2O (19:1) + 0.09 M KOH
CH3CN/powder KOH
CH3CN/0.023 M DBU
CH3CN/0.13 M H2O/0.023 M DBU
C6H6/0.023 M DBU
C6H6/0.02 M EtOH/0.023 M DBU
C6H6/C2H5OH (9:1) + 0.023 M DBU
OH
3
Scheme 4. Mechanism of the formation of 2-methylnaphthalene 2 and 2-naphth-
2-yl-ethanol 3.

J.-H. Ho et al. / Tetrahedron Letters 52 (2011) 7199–7201
7201
Supplementary data
OH
hv 300nm
+
1d
KOH, NaBH4
CH3OH/H2O
Supplementary data (experimental procedures for the synthe-
ses and the irradiations of 1a–1h, and spectral data and 1H and
13C NMR spectra of products 2a–2g and 3d) associated with this
article can be found, in the online version, at doi:10.1016/
j.tetlet.2011.10.141.
2d (17%)
3d (31%)
Scheme 5. Reduction of the intermediate N3 with NaBH4.
In the elimination reaction of DHNF00 that yields N1, the oxygen
atom O1 on carbon 9b of DHNF00 can be regarded as a leaving
group. This concept, using the heteroatom of a heterocycle as a
leaving group in the 6e photocyclization intermediate, has been
shown in some cases, such as the photorearrangements of 3,30-
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base, and its conjugate acid has a pKa value around 10. For the 3-
styrylfurans, the enthenolate part of intermediate N1 has a similar
pKa value of its conjugate acid.18 Therefore, we believe that the
enthenolate could be a leaving group in the elimination reaction.
In addition, the aromatization of the naphthalene ring would also
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tion of 1d in DBU/benzene solution at 40 °C in the dark was com-
pleted. There was no photorearrangement product, and only 60% of
the reactant would be recovered. This may be the reason that the
yield of 2d when using irradiation is limited to approximately
60%. Our attempts to further improve the yields by changing the
solvent, base, wavelength, and temperature all failed. One possible
explanation for this decomposition may be that 3-styrylfurans
have a more acidic hydrogen on the furan ring,19 which may lead
to unexpected and complicated side reactions.
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Many synthetic methods for the production of substituted
naphthalenes have been developed.20 Several 2,7-disubstituted
naphthalenes have been synthesized successfully, for example,
2,7-dimethylnaphthalene,21
2-cyano-7-methylnaphthalene,22
2-methoxy-7-nitronaphthalene,23 and 2-methoxy-7-methylnaph-
thalene.24 Most methods involve elaborate synthetic routes and
yield only specific unsymmetrical 2,7-disubstituted naphthalenes,
and are not used for the production of a series of substituents.
Some of those reaction conditions are harsh or environmentally
unfriendly, such as the usage of strong reagents, very low temper-
atures, or transition metal catalysts. In some cases, two isomers are
obtained. Compared with these methods, our approach has some
merits: (1) it can synthesize a series of 7-substituted-2-methyl-
naphthalenes without yielding other isomers; (2) 3-styrylfurans
can be prepared from symmetrical commercial chemicals; and
(3) the reaction conditions, including those of the Wittig reactions
and photorearrangements, are mild and environmentally friendly.
In summary, we have successfully synthesized a series of
7-substituted-2-methylnaphthalenes 2a–2g by the irradiation of
3-(4-substituted styryl)furans 1a–1g in basic media. This photore-
arrangement involves three major steps: 6e photocyclization, base-
induced elimination, and a Norrish Type I photoreaction. The key
step is the base-induced elimination reaction of the DHNF00 inter-
mediate, which breaks the furan ring to generate a naphthalene
instead of a phenanthrenoid.
Acknowledgment
23. Bianchi, L.; Dell’Erba, C.; Maccagno, M.; Petrillo, G.; Rizzato, E.; Sancassan, F.;
Severi, E.; Tavani, C. J. Org. Chem. 2005, 70, 8734.
24. Kelly, T. R.; Silva, R. A.; De Silva, H.; Jasmin, S.; Zhao, Y. J. Am. Chem. Soc. 2000,
122, 6935.
We are grateful to the National Science Council of the Republic
of China (Taiwan) for financial support.