Synthesis of the novel conjugated ω,ω′-diaryl/heteroaryl hexatriene system with the central double bond in a heteroaromatic ring: photochemical transformations of 2,3-divinylfuran derivatives

Article abstract of DOI:10.1016/j.tet.2006.05.034

New β,β′-aryl/heteroaryl 2,3-divinylfuran derivatives (9a-d) in which a hexatriene system is a part of heteroaromatic ring have been synthesized and their photochemical properties were investigated. The primary process observed was the isomerization to trans,trans-isomers 9a-d followed by photochemical rearrangement of the furan ring giving the phototransposition products (I-IV). Stilbenes (20, 21) and phenanthrenes (22, 25, and 26), formed as secondary products from the competitive intermolecular cycloadditions, were also observed.

Full text of DOI:10.1016/j.tet.2006.05.034

Tetrahedron 62 (2006) 7396–7407
Synthesis of the novel conjugated u,u0-diaryl/heteroaryl
hexatriene system with the central double bond in a
heteroaromatic ring: photochemical transformations
of 2,3-divinylfuran derivatives
a
Irena Skoric, Ivana Flegar, Zeljko Marinic and Marija Sindler-Kulyk
a
b
a,
ˇ
ˇ
ˇ
*
´
´
aDepartment of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb,
Marulicꢀev trg 19, 10000 Zagreb, Croatia
bNMR Center, Rudjer Bosꢁkovicꢀ Institute, Bijenicꢁka cesta 54, 10000 Zagreb, Croatia
Received 23 December 2005; revised 3 May 2006; accepted 11 May 2006
Available online 5 June 2006
Abstract—New b,b0-aryl/heteroaryl 2,3-divinylfuran derivatives (9a–d) in which a hexatriene system is a part of heteroaromatic ring have
been synthesized and their photochemical properties were investigated. The primary process observed was the isomerization to trans,trans-
isomers 9a–d followed by photochemical rearrangement of the furan ring giving the phototransposition products (I–IV). Stilbenes (20, 21)
and phenanthrenes (22, 25, and 26), formed as secondary products from the competitive intermolecular cycloadditions, were also observed.
Ó 2006 Elsevier Ltd. All rights reserved.
R
1. Introduction
R
O
In our first paper about a photochemical approach to hetero-
polycyclic compounds from hexatriene systems in which the
central double bond is placed in a benzene ring (1), we de-
scribed the furan derivative of o-divinylbenzene (2a) and
its transformation into the fused bicyclo[3.2.1]octadiene
structure 3 (Fig. 1).1
R'
R'
1a: R = heteroaryl; R' = H
b: R = R' = heteroaryl
c: R = aryl; R' = H
2a: R = H; R' = vinyl
b: R = CH3; R' = vinyl
c: R = Br; R' = vinyl
d: R = R' = vinyl
e: R = vinyl; R' = H
R
The investigation was extended to many modified furan,2–9
pyrrole,10–16 and sydnone17,18 analogues (1a,b; 2b–e), where
their photochemical behavior was examined. We have
observed that the photochemistry of furan derivatives of
o-divinylbenzene1,2,4,6,7,9 resultsina[2+2] cycloadditionand
formation of intramolecular cycloadducts 3 or 4 (Fig. 1),
obtained via 1,6- or 1,4-biradical ring closure, respectively
(Scheme 1). In the case of b-(3-substituted-2-furyl)-o-
divinylbenzenes (2b–d),7 the 1,4-biradical ring closure
leading to benzobicyclo[2.1.1]hexene derivatives (4) was
preferred due to steric reasons. The same benzobicyclo-
[2.1.1]hexene structure was obtained on irradiation of
b-aryl o-divinylbenzenes (1c).19–31
O
R''
O
3: R'' = H, CH3, Ph
4: R = CH3, Br, vinyl
Figure 1.
On irradiation of compound 2d (unsaturated system with the
vinyl group on benzene and on the furan ring) the benzobicy-
clo[2.1.1]hexene structure 5 was isolated.7 The furobicy-
clo[2.1.1]hexene derivative 6 (Fig. 2), formed as a result of
thevinyl-furangroupparticipatinginacycloaddition,wasnot
isolated. If the cycloadduct 6 was formed, further [p2+s2]
cycloaddition19 and formation of tricyclic structure 7 via the
excited state of the styryl group would be expected. The
tricyclic structure 8 was not obtained due to the nonabsorp-
tion of the vinyl-furan derivative 5 under the experimental
Keywords: Oxygen heterocycles; Pericyclic reaction; Photochemistry;
Rearrangement; Synthesis.
* Corresponding author. Tel.: +385 1 4597246; fax: +385 1 4597250;
e-mail: marija.sindler@fkit.hr
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.05.034

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7397
2. Results and discussion
R
R
R
Introduction of the second aromatic moiety to the b-position
of the vinyl group of 2e to give compounds 9a–d could result
in intramolecular complexation9 and formation of the
bicyclic structures (Scheme 2). In such a way the access
to a variety of differently functionalized heteropolycyclic
compounds could be enabled.
hν
O
1,6-ring closure
R
O
O
hν
1,4-ring closure
R
O
R
O
O
O
2
If we consider the 2,3-divinylfurans 9 as heteroaromatic
analogues of disubstituted o-divinylbenzenes (1b),9 we
might expect the formation of several furobicyclo structures
(10–13) on irradiation of 9. The furobicyclo[2.1.1]hexene
(10) and furobicyclo[3.1.0]hexene (11, 110) derivativesmight
be formed by initial [2+2] cycloaddition via 1,4-biradicals
14 and/or 140 (Fig. 4), followed by ring closure.
4 (R = H)
R
3 (R = H)
Scheme 1.
In the formation of products 10, the aromatic character of
the furan ring is pronounced while in the case of 11/110,
through the participation of two of the furan p electrons
and vinylcyclopropane–cyclopentene rearrangement,43,44
the diene character of the furan ring is prevailing. If one of
the b-substituents is a furan ring (9c,d), then 1,6-biradical
ring closure1,2,4,6,7,9 may occur leading to furobicyclo-
[3.2.1]octadiene structures (12, 120) by the mechanism
described therein. If the head-to-head [2+2] cycloaddition
took place, the furobicyclo[2.2.0]hexene 13 would be
formed (Scheme 2).
O
O
5
6
O
O
8
7
Novel b,b0-aryl/heteroaryl-substituted 2,3-divinylfurans 9a–
d, were prepared by a series of reactions according to Scheme
3. The corresponding cis- and trans-aryl/heteroaryl-2-vinyl-
furans 16 are prepared bya Wittig reaction from aldehydes 15
in moderate-to-good yields (47–82%). To reduce the number
of isomers of the final products 9a–d, the isomers 16a–c were
separated by column chromatography on silica gel. The
trans-16b and trans-16c were transformed to trans-17a and
trans-17b, respectively, by formylation with n-butyl lithium
and DMF (w50% yield). The trans-16a was converted to 9e,f
(Scheme 3) in three steps giving a mixture of cis,trans- and
trans,trans-isomers. After column chromatography, the
trans,trans-9e and trans,trans-9f were debrominated to
trans,trans-9b and trans,trans-9c, respectively. By using a
Wittig reaction with formyl derivatives trans-17a,b, the cor-
responding triphenylphosphonium salts, 9a–d were prepared
as mixtures of cis,trans- and trans,trans- isomers (65–95%).
All isolated compounds were identified and characterized
spectroscopically. The ratios of the isomers were determined
from the NMR spectra and by GC–MS measurements.
Figure 2.
conditions. On irradiation of 2e (hexatriene system in which
the central double bond is a part of only a furan ring) mostly
high-molecular-weight material was obtained.7
In order to get a deeper understanding of the photochemical
behavior of the hexatriene system with the central double
bond in a heteroaromatic ring, we describe herein for the first
time the synthesisand photochemistryof b,b0-aryl/heteroaryl
2,3-divinylfurans 9 (Fig. 3), compounds with carefully
chosen aryl/heteroaryl substituents were used. There are
examples of b,b0-disubstituted 2,3-divinylfurans32–42 in the
literature but to the best of our knowledge, there are no exam-
ples with aryl/heteroaryl substituents.
CH3
4f
3
Diaryl-substituted 2,3-divinylfurans 9a,b (Fig. 3) have very
similar patterns in their respective H NMR. Generally, all
3β(et)
1
5f
O
O
O
2
cis,trans- and trans,trans-9 derivatives show ethylenic dou-
blets at 6.6–7.1 ppm. Within this region for all trans,trans-9
derivatives considerable shifts of one of the ethylenic dou-
blets to higher fields are observed compared to the other
three ethylenic protons. The 4f proton of the furan ring A
of all trans,trans-9 is found at 6.6–6.7 ppm while the 4f
proton of the furan ring B (9c,d) is shifted by 0.2 ppm
to the higher field and is well recognizable.
2α(et)
9a
9b
CH3
O
O
O
9c
9d
Compounds 9a–d are the starting materials for irradiation
experiments and show strong absorption maxima at
Figure 3.

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I. Skoricꢀ et al. / Tetrahedron 62 (2006) 7396–7407
7398
R
R'
O
O
O
O
12
12'
hν
hν
R' = 2-furyl
R = 2-furyl
R
R
R
hν
R = 2-furyl or aryl
hν
O
O
O
R'
R'
R'
10
13
9a: R = R' = Ph
R, R' = aryl or 2-furyl
b: R = p-tolyl; R' = Ph
c: R = 2-furyl; R' = Ph
d: R = p-tolyl; R' = 2-furyl
hν
R
R
R'
O
O
R'
11
11'
R, R' = aryl or 2-furyl
Scheme 2. Hypothetic photochemical products of intramolecular [2+2] cycloaddition of ethylenic bonds.
R
comparison to the corresponding aryl analogues (trans,
trans-9a, trans,trans-9b) due to the conjugation from the
oxygen lone pair through the furan ring. The ethanolic solu-
tion of cis,trans-9a (UV concentration) was irradiated and
the process was followed by UV measurements (Fig. 5).
After 15 s, the shift of the absorption maximum to the longer
wavelength (359 nm) with concomitant formation of another
maximum at 275 nm was observed. The new maxima (275
and 359 nm) correspond to the trans,trans-9 isomer. These
maxima decrease on further irradiation as it is comparable
with the results of the pure trans,trans-9a irradiations
(Fig. 6).
R'
R
O
O
R'
14
Figure 4. Possible 1,4-biradicals.
14'
350–360 nm with similar molar absorption coefficients. The
furan derivatives trans,trans-9c and trans,trans-9d demon-
strate bathochromic shifts of 5 and 6 nm, respectively, in
R
R
R'CH2P+Ph3Br-
R'
NaOEt/EtOH
CHO
O
O
16a: R = CH3; R' = Ph (82%)
b: R = Br; R' = Ph (82%)
c: R = Br; R' = 2-furyl (47%)
15a: R = CH3
b: R = Br
column
chromatography
trans-16a
trans-16b
trans-16c
1) NBS/CCl4
1) n-BuLi/Et2O
2) PPh3/C6H6
2) DMF
3) a: p-Tol-CHO; NaOEt/EtOH
b: 2-furancarbaldehyde; NaOEt/EtOH
R
R
CHO
RCH2P+Ph3Br-
NaOEt/EtOH
Ph
R'
trans-17a: R' = Ph (59%)
R'
Br
O
O
O
9a: R = R' = Ph (83%)
9e: R = p-tolyl (58%)
f: R = 2-furyl (39%)
b: R = p-tolyl; R' = Ph (95%)
c: R = 2-furyl; R' = Ph (75%)
d: R = p-tolyl; R' = 2-furyl (65%)
trans-17b: R' = 2-furyl (52%)
column
column
chromatography
chromatography
trans,trans-9a-d
trans,trans-9e,f
1) n-BuLi/Et2O; 2) H2O
Scheme 3.

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I. Skoricꢀ et al. / Tetrahedron 62 (2006) 7396–7407
7399
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
15''
60''
240''
960''
0'';
1,0
0,8
30'';
120'';
480'';
cis,trans-9b
0,6
%
trans,trans-9b
phototransposition
products of 9b
0,4
0,2
0,0
0
5
10
15
20
25
30
35
Time / min
Figure 7.
200
250
300
350
400
λ / nm
trans,trans-9b
Figure 5.
phototransposition products of 9b
80
70
60
50
stilbene derivatives 20,21
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
0''
15''
30''
60''
120''
240''
480''
%
40
30
20
10
0
1
2
3
4
5
6
7
8
Time / hour
Figure 8.
appeared and become more intensive on further irradiation.
The H NMR spectra of all photomixtures showed neither
1
signals for the aliphatic protons of the furobicyclic structures
(10–13) nor cyclobutane dimeric structures as in the case of
irradiation experiments of b,b0-disubstituted o-divinylbenz-
enes.9 In a lower magnetic field, the signals of new aromatic
protons at 7.5–8.0 ppm have been observed along with new
ethylenic protons at 6.5–7.0 ppm, having characteristic cou-
pling constants for cis- and trans-isomers. In the case of the
methyl derivatives (9b and 9d) several new methyl signals
appear.
200
250
300
350
400
λ / nm
Figure 6.
Compounds 9a–d with concentrations of 10ꢀ3 M were irra-
diated in benzene solutions under anaerobic conditions in
Rayonet reactor at 350 nm. The reaction course of all these
substituted divinylfurans 9 was followed by GC–MS mea-
surements and the results were similar. In the first 15 min
of irradiation, only the isomerization of cis,trans- to trans,
trans-isomers 9 was detected (Fig. 7; 9b presented).
When the irradiation of 9a–d was performed at higher
concentrations (w10ꢀ1 M), the appearance of stilbenes and
styrylfurans (20, 21; Scheme 4) was observed by GC–MS
already after 30 min. On further irradiation, the signals of
20 and 21 increase faster than at lower concentrations and
on prolonged irradiation time, regardless of concentration,
they are the only detectable compounds besides phenan-
threnes (22, 25, and 26) and a high amount of tarry material.
The naphthofurans 23 and 24 and furobenzofuran 27 are not
detected in the reaction mixture, although their correspond-
ing precursors 20 and 21 are found, and this is not a surpris-
ing fact. Electrocyclization reactions of styrylfurans4 are
not efficient reactions and compete with the formation of
high-molecular-weight products.
On further irradiation for 30 min (Fig. 7), the new signals
appeared having similar retention times with molecular
ions identical to the starting compounds. The ratios of the
sums of these new signals (see Section 4) in the experiments
with 9a–d to the signals of the corresponding starting com-
pounds trans,trans-9 were ca. 1:4. After 1 h of irradiation,
the ratio of the signals remained the same (Fig. 8) and did
not change on further irradiation. After 2 h of irradiation,
besides already described signals, the signals, which corre-
spond to stilbenes and styrylfurans (20, 21; Scheme 4)

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7400
R'
R
R
O
O
O
R
hν
hν
II (18a-c)
(R')R
R(R')
O
R'
R'
R'
R'
9a: R = R' = Ph
O
O
b: R = p-tolyl; R' = Ph
c: R = 2-furyl; R' = Ph
d: R = p-tolyl; R' = 2-furyl
(I)
O
O
III
(R)R'
R(R')
n
IV
19
hν
R
R
R(R')
R(R')
O
R'
20
R'
13
21
hν
hν
CH3
CH3
O
O
O
O
CH3
CH3
22 (3%)
23
24
25 (2-5%)
26 (4%)
27
Scheme 4.
R'
From the NMR spectra of the photomixtures, combined with
the GC data, one can conclude that the signals observed in
GC–MS correspond to the phototransposition products,
the constitutional isomers I–IV of distyrylfurans 9a–d
(Scheme 4).
II
III
R
O
IV
a
b
R
R
R'
To confirm this assumption one of these constitutional iso-
mers, the substituted 2,5-divinylfuran II (18a–c; Scheme
4) was prepared by independent synthesis (Section 4). The
retention times of synthesized 18a–c, as mixtures of config-
urational isomers (cis,cis-, cis,trans-, and trans,trans-18),
were compared to the signals of the photomixtures of irradi-
ation experiments of 9a–c. Only the signal for trans,trans-
2,5-divinylfuran derivative II (18a–c) coincides to one of
the signals of the irradiation mixture of I (9a–c). After addi-
tion of trans,trans-2,5-divinylfuran II to the photomixture
the coincident signal increases. It is not unexpected that cis,
cis- or cis,trans-isomers II have not been found in the photo-
mixture because they undergo fast cis–trans isomerization
and that is confirmed by independent irradiation of II. The
remaining GC-signals in the photomixture of 9, having
similar retention times and the same molecular ions, could
correspond to the structures III and/or IV.45
a
b
R
R'
R'
R'
a
O
IIA
O
a
O
IIIA
b
b
I
(9a-d)
hν
hν
hν
R'
R
R
a
b
R'
R
a
O
II
O
III
O
b
IA
(18a-d)
R = -CH=CH-Ar(Het)
R' = -CH=CH-Ar'(Het')
Scheme 5. Schematic representation of the possible phototransposition
products.
R
R
R'
The formation of phototransposition furan derivatives
(I–IV) can be explained by formal [2+2] cycloaddition of
the p systems in the furan ring and rearrangements of the
obtained epoxy-furan derivatives (IA–IIIA; Scheme 5).46–48
The anticipated dienic character of the furan ring might be
a reason for their formation and mutual rearrangement.
This process of cycloaddition is prevailing due to the more
convenient and rigid conformation of furan double bonds
compared to ethylenic p system (Scheme 6).
O
O
O
O
R'
R
R
R'
R'
The existence of compounds 20 on irradiation of 9 can
be explained by thermal cleavage of the intra- (13) or
Scheme 6. Possible conformations of trans,trans-9 derivatives.

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7401
intermolecular cycloadducts (19), formed by intra- or inter-
molecular [2+2] cycloaddition of the ethylenic bonds
(Scheme 4). Since in the irradiation mixture of unsymmetri-
cally substituted 2,3-divinylfuran derivatives 9b–d besides
20 (Scheme 4) the symmetrically substituted stilbene deriv-
atives 21 were found; it is concluded that the stilbene deriv-
atives were derived from the intermolecular cycloadduct 19,
although the formation of 20 and the intramolecular process
via unstable 13 cannot be excluded. Moreover, on irradiation
of 2,5-divinylfuran derivatives 18a–c no phototransposition
products were detected showing that II (18a–c) does not un-
dergo an intramolecular furan rearrangement process to the
constitutional isomers (Scheme 5). Instead, they polymerize
by intermolecular cycloaddition reactions of ethylenic bonds
(19). Even though no evidence of a cyclobutane structure was
found in the 1H NMR spectra, we are sure of its formation due
to the detection of all combination of ethene derivatives 20
and 21 in GC–MS measurements. Only some of their corre-
sponding phenanthrenes (22, 25, and 26) are the isolated
products from the photoreaction mixture after prolonged
irradiation and complete conversion of the starting
compounds 9. The phenanthrene derivatives are formed
by electrocyclization reaction of stilbenes via oxidation of
dihydrophenanthrenes, as a result of imperfect anaerobic
reaction conditions.49
Gas Chromatograph–Varian Saturn 2200) equipped with
FactorFour Capillary Column VF-5ms. Irradiations were
performed in a Pyrexvessel in benzene solutions in a Rayonet
˚
reactor equipped with RPR 3500 A lamps. All irradiation
experiments were carried out in deoxygenated solutions by
bubbling a stream of argon prior to irradiation. Melting points
were obtained using an Original Kofler Mikroheitztisch
apparatus (Reichert, Wien) and are uncorrected. Elemental
analyses were carried out on Perkin–Elmer, Series II,
CHNS Analyzer 2400. Silica gel (Merck 0.063–0.2 mm)
was used for chromatographic purifications. Thin-layer chro-
matography (TLC) was performed on Merck precoated silica
gel 60 F254 plates. Solvents were purified by distillation.
Boiling range of petroleum ether, used for chromatographic
separation, was 40–70 ꢁC.
Furan-2-carbaldehyde was obtained from a commercial
source. Benzyltriphenylphosphonium bromide, p-methyl-
benzyltriphenylphosphonium bromide, and 2-furylmethyl-
triphenylphosphonium bromide were synthesized from the
corresponding bromides50 and triphenylphosphine in benz-
ene solution.
3-Methyl-2-furancarbaldehyde (15a)51 was prepared by ox-
idation of 3-methyl-2-furfurylalcohol52 that was obtained
from 3-methyl-2-furoate53 with LiAlH4. 3-Bromo-2-furan-
carbaldehyde (15b) was prepared from 2-furancarboxylic
acid according to a described procedure.54 Compound 17a
was prepared as described in the literature.7
3. Conclusion
The conjugated hexatriene systems with a central double
bond placed in a furan ring (9a–d) have been studied. They
behave completely different from the hexatriene system
with the central double bond incorporated in benzene ring
(1) under the same reaction conditions. Compared to dihetero-
aryl systems (1b) in which the intramolecular cycloaddition
and cis–trans isomerization were the competitive processes it
is obvious that in this system the primary process is cis–trans
isomerization to trans,trans-isomers. Because of dienic char-
acter of the furan ring and prevailing conformation of the
furan double bonds, the excitation of the hexatriene causes
the [2+2] cycloaddition within the furan ring followed by
rearrangement, rather than the [2+2] cycloaddition of
ethylenic bonds. The photochemical process within the furan
ring is competitive with the intermolecular cycloaddition
processes and leads to the formation of phototransposition
(I–IV) and high-molecular-weight products, respectively.
The phototransposition process is confirmed by independent
synthesis of one of the representatives (18a–c).
4.2. Preparation of 16a and 16c
Starting compounds 16a and 16c were prepared from benz-
yltriphenylphosphonium bromide and the corresponding
aldehydes, 3-methyl-2-furancarbaldehyde (15a) and 3-
bromo-2-furancarbaldehyde (15b), respectively. Starting
compound 3-bromo-2-(2-phenylethenyl)furan (16b) was
obtained according to the described procedure.7
A solution of sodium ethoxide (0.12 g, 5.2 mmol in 10 mL
ethanol) was added dropwise to a stirred solution of benzyl-
triphenylphosphonium bromide (1.69 g, 4.0 mmol) and 3-
methyl-2-furancarbaldehyde (15a) (4.0 mmol) in absolute
ethanol (100 mL). Stirring was continued under a stream of
nitrogen for 1 day at room temperature. After removal of the
solvent, the residue was worked up with water and benzene.
The benzene extracts were dried with MgSO4 and concen-
trated. The crude reaction mixture was purified and the
isomers of 16a were separated by repeated column chroma-
tography on silica gel using petroleum ether/diethyl ether
(0–3%) mixture as eluent. The first fractions yielded trans-
isomer and the last fractions yielded cis-isomer. Starting
compound 16c was prepared also by Wittig reaction from
2-furylmethyltriphenylphosphonium bromide and 3-bromo-
2-furancarbaldehyde (15b). Characterization data of the new
compounds 16a and 16c are given below.
4. Experimental
4.1. General
The 1H and 13C NMR spectra were recorded on a Bruker AV-
600 Spectrometer at 300 or 600 MHz and 75 or 150 MHz,
respectively. All NMR spectra were measured in CDCl3
using tetramethylsilane as reference. The assignment of the
signals is based on 2D CH-correlation and 2D HH-COSY,
LRCOSY, and NOESY experiments. UV spectra were mea-
sured on a Varian Cary 50 UV–vis Spectrophotometer. IR
spectra were recorded on Perkin–Elmer Spectrum One.
Mass spectra were obtained on a GC–MS (Varian CP-3800
4.2.1. 3-Methyl-2-(2-phenylethenyl)furan (16a). Yield
82.0%; according to H NMR spectroscopy, a mixture of
45% cis and 55% trans isomers was obtained.
1
trans-16a: Rf 0.61 (petroleum ether); colorless crystals; mp
50–51 ꢁC; UV (EtOH) lmax (log 3) 340 (4.37, sh), 322

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7402
1
(4.50), 310 (4.43, sh) nm; H NMR (300 MHz, CDCl3) dH
7.47 (d, J¼7.8 Hz, 2H), 7.33 (m, 3H), 7.22 (t, J¼7.2 Hz,
1H), 6.97 (d, J¼16.5 Hz, H-et, 1H), 6.89 (d, J¼16.5 Hz,
H-et, 1H), 6.27 (s, H-4f, 1H), 2.12 (s, CH3, 3H); 13C NMR
(CDCl3) dC 148.70 (s), 141.20 (d), 137.33 (s), 128.52 (2d),
127.11 (d), 126.03 (2d), 125.38 (d), 118.60 (s), 114.57 (d),
114.08 (d), 10.00 (q); MS (EI) m/z (%) 184 (M+, 100), 141
(10), 115 (5); HRMS (EI) Calcd for C13H12O: 184.088266.
Found: 184.093546.
the separation of the starting compound from the first frac-
tions, the last fractions yielded 2-trans-(2-furylethenyl)-
furan-3-carbaldehyde (17b) in 52% yield. Compound 17b
was confirmed by GC–MS measurements before the next
reaction step, the preparation of 9d.
4.4. Preparation of 9a–d
Starting compounds 9a–c were prepared by Wittig reaction
from the 2-trans-(2-phenylethenyl)furan-3-carbaldehyde
(trans-17a) and corresponding triphenylphosphonium salts,
benzyltriphenylphosphonium bromide, p-methylbenzyltri-
phenylphosphonium bromide, and 2-furylmethyltriphenyl-
phosphonium bromide, respectively. To a stirred solution of
the corresponding triphenylphosphonium salts (1.9 mmol)
and the 2-trans-(2-phenylethenyl)furan-3-carbaldehyde
(trans-17a) (0.350 g, 1.8 mmol) in absolute ethanol
(50 mL), a solution of sodium ethoxide (0.067 g, 2.9 mmol in
10 mL ethanol) was added dropwise. Stirring was continued
under a stream of nitrogen for 1 day at room temperature.
After removal of the solvent, water was added to the residue
and extracted with benzene. The benzene extracts were dried
and concentrated. The crude reaction mixture was purified
and the isomers of products 9a–c were separated by repeated
column chromatography on silica gel using petroleum ether
and petroleum ether/diethyl ether (0–5%) mixture as eluent.
Starting compound 9d was prepared similarly by Wittig
reaction from the 2-trans-[2-(2-furyl)ethenyl]furan-3-
carbaldehyde (trans-17b) and p-tolyltriphenylphosphonium
bromide. The first fractions yielded cis,trans-isomer and
the last fractions yielded trans,trans isomers. Characteriza-
tion data of the new compounds 9a–d are given below.
cis-16a: Rf 0.55 (petroleum ether); colorless oil; UV (EtOH)
lmax (log 3) 325 (3.83, sh), 317 (4.16), 311 (4.18) nm; H
1
NMR (300 MHz, CDCl3) dH 7.43 (d, J¼7.5 Hz, 1H), 7.23–
7.35 (m, 4H-ar), 7.21 (d, J¼1.8 Hz, 1H, H-5f), 6.44 (d,
J¼12.3 Hz, 1H, H-et), 6.28 (d, J¼12.3 Hz, 1H, H-et), 6.20
(d, J¼1.8 Hz, 1H, H-4f), 1.98 ppm (s, 3H, CH3); 13C NMR
(CDCl3) dC 147.76 (s), 140.88 (d), 137.31 (s), 128.81 (2d),
127.68 (2d), 126.91 (2d), 120.08 (s), 115.36 (d), 113.45
(d), 10.37 (q); MS (EI) m/z (%) 184 (M+, 100), 141 (25),
115 (10); Anal. Calcd for C13H12O: C, 84.75; H, 6.57. Found:
C, 84.68; H, 6.67.
4.2.2. 3-Bromo-2-(2-furylethenyl)furan (16c). Yield
47.0%; according to H NMR spectroscopy, a mixture of
34% cis and 66% trans isomers was obtained.
1
trans-16c: Rf 0.61 (petroleum ether); colorless crystals; mp
45–46 ꢁC; UV (EtOH) lmax (log 3) 337 (4.27), 319 (4.46),
312 (4.40, sh) nm; H NMR (300 MHz, CDCl3) dH 7.46
1
(m, 1H, H-1), 7.35 (d, J¼1.8 Hz, 1H, H-5), 6.89 (ABq,
J¼16.1 Hz, 2H, H-et), 6.49 (d, J¼1.8 Hz, 1H, H-4), 6.45
(m, 1H, H-2), 6.40 (d, J¼3.2 Hz, 1H, H-3); 13C NMR
(CDCl3) dC 152.71 (s), 149.81 (s), 142.56 (d), 142.02 (d),
116.36 (d), 115.20 (d), 111.98 (d), 111.87 (d), 109.71 (d),
99.14 (s); MS (EI) m/z (%) 238/240 (M+, 100), 159 (10),
131 (70).
4.4.1. 2-(2-Phenylethenyl)-3-(2-phenylethenyl)furan
(9a). Yield 83.2%; according to 1H NMR spectroscopy,
a mixture of 29% cis,trans- and 71% trans,trans isomers
was obtained.
cis-16c: Rf 0.52 (petroleum ether); colorless oil; UV (EtOH)
lmax (log 3) 321 (3.80, sh), 315 (4.09), 311 (4.14) nm; H
1
cis,trans-9a: Rf 0.40 (petroleum ether); yellow-green crys-
tals; mp 85–86 ꢁC; UV (EtOH) lmax (log 3) 346 (4.46), 279
(4.02), 235 (4.02) nm; 1H NMR (600 MHz, CDCl3) dH 7.44
(d, J¼7.2 Hz, 2H, H-ar), 7.28–7.36 (m, 6H), 7.21–7.26 (m,
2H), 7.20 (d, J¼1.8 Hz, 1H, H-5f), 7.05 (d, J¼16.2 Hz, 1H,
H(et)-2b), 6.97 (d, J¼16.2 Hz, 1H, H(et)-2a), 6.62 (d,
J¼12.0 Hz, 1H, H(et)-3b), 6.53 (d, J¼12.0 Hz, 1H, H(et)-
3a), 6.11 (d, J¼1.8 Hz, 1H, H-4f); 13C NMR (CDCl3) dC
150.65 (s, C-2), 141.37 (d, C-5f), 137.40 (s), 136.99 (s),
130.08 (d, C-3b), 128.77 (2d), 128.56 (2d), 128.07 (2d),
127.54 (d), 127.43 (d), 127.13 (d), 126.32 (2d), 119.99 (s,
C-3), 119.46 (d, C-3a), 114.37 (d, C-2a), 111.60 (d, C-4f);
IR (evaporated film from CHCl3) 2985, 2908, 2853, 1580,
1500, 965, 750 cmꢀ1; MS (EI) m/z (%) 272 (M+, 100), 243
(7), 115 (6).
NMR (600 MHz, CDCl3) dH 7.46 (d, J¼1.9 Hz, 1H, H-5),
7.43 (m, 1H, H-1), 7.10 (d, J¼3.4 Hz, 1H, H-3), 6.52 (d,
J¼1.9 Hz, 1H, H-4), 6.46 (m, 1H, H-2), 6.37 (d,
J¼13.4 Hz, 1H, H-et), 6.19 (d, J¼13.4 Hz, 1H, H-et); 13C
NMR (CDCl3) dC 151.58 (s), 148.77 (s), 141.94 (d),
141.51 (d), 115.75 (d), 114.44 (d), 111.43 (d), 110.74 (d),
109.52 (d), 100.92 (s); MS (EI) m/z (%) 238/240 (M+, 75),
159 (10), 131 (100); Anal. Calcd for C10H7BrO2: C, 82.42;
H, 5.38. Found: C, 82.20; H, 5.45.
4.3. Preparation of 17b
To a stirred solution of trans-16c (3.0 mmol) in anhydrous
diethyl ether (40 mL) cooled to ꢀ70 ꢁC, n-butyl lithium
(3.3 mmol, 1.6 M hexane solution) was added under a stream
of nitrogen over 30 min. After additional stirring for 30 min,
anhydrous N,N-dimethylformamide (6.6 mmol, 0.48 mL)
was added. After 1 h at that temperature, the mixture was
allowed gradually towarm up to 0 ꢁC, over 3 h. Dilute hydro-
chloric acid (1.5 mL, 7 mol/L) was added and the layers were
separated. After extraction, the combined organic phase was
dried over MgSO4. The reaction mixture was purified and
separated by column chromatography on silica gel using
petroleum ether/diethyl ether (0–10%) as the eluent. After
trans,trans-9a: Rf 0.38 (petroleum ether); yellow crystals;
mp 165–166 ꢁC; UV (EtOH) lmax (log 3) 376 (4.36, sh),
359 (4.51), 343 (4.41, sh), 279 (4.29), 236 (3.91) nm; H
1
NMR (600 MHz, CDCl3) dH 7.50–7.53 (m, 4H), 7.35–7.39
(m, 5H), 7.24–7.28 (m, 2H), 7.13 (d, J¼16.2 Hz, 1H,
H(et)-3a), 7.12 (d, J¼16.2 Hz, 1H, H(et)-2a), 7.09 (d,
J¼16.2 Hz, 1H, H(et)-2b), 6.84 (d, J¼16.2 Hz, 1H, H(et)-
3b), 6.68 (d, J¼1.8 Hz, 1H, H-4f); 13C NMR (CDCl3) dC
150.00 (s, C-2), 142.40 (d, C-5f), 137.27 (s), 136.96 (s),

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7403
128.80 (d, C-3b), 128.60 (2d), 128.57 (2d), 128.35 (d),
128.21 (d), 127.59 (d), 127.40 (d), 127.36 (d), 126.33 (2d),
126.12 (2d), 121.99 (s, C-3), 117.78 (d, C-3a), 113.69 (d,
C-2a), 108.69 (d, C-4f); IR (evaporated film from CHCl3)
trans,trans-9c: Rf 0.43 (petroleum ether); yellow crystals; mp
110 ꢁC; UV (EtOH) lmax (log 3) 383 (4.38, sh), 364 (4.49),
346 (4.40, sh), 292 (4.34), 280 (4.30, sh) nm; H NMR
1
(600 MHz, CDCl3) dH 7.53 (d, J¼7.6 Hz, 2H), 7.41 (d,
Jw1 Hz, 1H), 7.36 (m, 2H), 7.26 (t, J¼7.6 Hz, 2H), 7.11 (d,
J¼16.1 Hz, 1H), 7.07 (d, J¼16.1 Hz, 1H), 7.05 (d,
J¼15.9 Hz, 1H), 6.62 (d, J¼15.9 Hz, 1H), 6.61 (d, Jw1 Hz,
1H), 6.43 (m, 1H), 6.32 (d, J¼3.1 Hz, 1H); 13C NMR
(CDCl3) dC 152.83 (s), 149.76 (s), 142.06 (d), 141.53 (d),
136.62 (s), 128.22 (2d), 127.20 (d), 127.04 (d), 125.96
(2d), 121.41 (s), 116.19 (d), 116.03 (d), 113.33 (d), 111.20
(d), 108.03 (d), 107.68 (d); IR (evaporated film from
CHCl3) 2915, 2824, 1608, 1500, 1440, 965, 750 cmꢀ1; MS
(EI) m/z (%) 262 (M+, 100), 233 (10), 115 (7); Anal. Calcd
for C18H14O2: C, 82.42; H, 5.38. Found: C, 82.73; H, 5.09.
2973, 2915, 2849, 1583, 1500, 1433, 950, 750, 683 cmꢀ1
;
MS (EI) m/z (%) 272 (M+, 100), 243 (8), 115 (5); Anal. Calcd
for C20H16O: C, 88.20; H, 5.92. Found: C, 88.53; H, 5.65.
4.4.2. 2-(2-Phenylethenyl)-3-[2-(4-methylphenyl)ethe-
nyl]furan (9b). Yield 95.1%; according to H NMR spec-
1
troscopy, a mixture of 35% cis,trans- and 65% trans,trans-
isomers was obtained.
cis,trans-9b: Rf 0.43 (petroleum ether); yellow-green crys-
tals; mp 124 ꢁC; UV (EtOH) lmax (log 3) 346 (4.48), 284
(4.04), 235 (4.04) nm; 1H NMR (600 MHz, CDCl3) dH 7.49
(d, J¼7.8 Hz, 2H), 7.30–7.36 (m, 5H), 7.14 (d, J¼12.6 Hz,
1H), 7.13 (d, J¼7.8 Hz, 2H), 7.08 (d, J¼16.2 Hz, 1H), 6.89
(d, J¼12.6 Hz, 1H), 6.73 (d, J¼16.2 Hz, 1H), 6.37 (s, 1H,
H-4f), 2.32 (s, 3H, CH3); 13C NMR (CDCl3) dC 152.31 (s),
141.23 (d), 137.65 (s), 136.34 (s), 133.29 (s), 129.02 (2d),
128.39 (d), 128.26 (2d), 128.06 (d), 127.45 (d), 126.08
(2d), 126.00 (2d), 123.10 (s, C-3), 113.90 (d), 112.89 (d),
112.58 (d), 20.80 (q); IR (evaporated film from CHCl3)
3000, 2915, 2849, 1583, 1500, 950, 750 cmꢀ1; MS (EI) m/z
(%) 286 (M+, 100).
4.5.2. 2-(2-Furylethenyl)-3-[2-(4-methylphenyl)ethenyl]-
furan (9d). Yield 64.8%.
trans,trans-9d: Rf 0.40 (petroleum ether); yellow crystals;
mp 117 ꢁC; UV (EtOH) lmax (log 3) 385 (4.41), 366 (4.50),
347 (4.36, sh), 289 (4.32), 280 (4.28, sh), 249 (4.03), 241
(4.06) nm; 1H NMR (600 MHz, CDCl3) dH 7.41 (d,
J¼7.9 Hz, 2H), 7.39 (d, Jw1 Hz, 2H), 7.34 (d, J¼1.3 Hz,
1H), 7.16 (d, J¼7.9 Hz, 2H), 7.07 (d, J¼16.1 Hz, 1H), 7.03
(d, J¼15.8 Hz, 1H), 6.84 (d, J¼15.8 Hz, 1H), 6.80 (d,
J¼16.1 Hz, 1H), 6.66 (d, J¼1.3 Hz, 1H), 6.43 (m, 1H),
6.35 (d, J¼3.2 Hz, 1H); 13C NMR (CDCl3) dC 152.72 (s),
149.23 (s), 141.96 (d), 141.73 (d), 136.86 (s), 134.17 (s),
128.90 (2d), 128.36 (d), 125.68 (2d), 121.96 (s), 116.44
(d), 114.32 (d), 111.81 (d), 111.40 (d), 108.68 (d), 108.36
(d), 20.73 (q); IR (evaporated film from CHCl3) 3049,
2923, 2851, 1596, 1495, 1446, 961, 741 cmꢀ1; MS (EI) m/z
(%) 276 (M+, 100), 219 (2), 115 (2); Anal. Calcd for
C19H16O2: C, 82.58; H, 5.84. Found: C, 82.28; H, 5.49.
trans,trans-9b: Rf 0.39 (petroleum ether); yellow crystals;
mp 134 ꢁC; UV (EtOH) lmax (log 3) 378 (4.38, sh), 360
(4.51), 344 (4.41, sh), 284 (4.36), 278 (4.34, sh), 235
(3.99) nm; 1H NMR (600 MHz, CDCl3) dH 7.53 (d,
J¼7.8 Hz, 2H), 7.41 (d, J¼7.8 Hz, 2H), 7.37 (m, 4H), 7.18
(d, J¼7.8 Hz, 2H), 7.13 (d, J¼16.0 Hz, 1H), 7.10 (d,
J¼16.0 Hz, 1H), 7.09 (d, J¼16.0 Hz, 1H), 6.83 (d,
J¼16.0 Hz, 1H), 6.69 (d, J¼1.6 Hz, 1H, H-4f), 2.37 (s, 3H,
CH3); 13C NMR (CDCl3) dC 149.41 (s), 141.98 (d), 136.91
(s), 136.69 (s), 134.16 (s), 128.92 (2d), 128.47 (d), 128.22
(2d), 127.15 (d), 126.84 (d), 125.94 (2d), 125.67 (2d),
122.00 (s), 116.45 (d), 113.45 (d), 108.35 (d), 20.74 (q); IR
(evaporated film from CHCl3) 3000, 2907, 2849, 1583,
1500, 1432, 965, 750 cmꢀ1; MS (EI) m/z (%) 286 (M+,
100); Anal. Calcd for C21H18O: C, 88.08; H, 6.34. Found:
C, 88.35; H, 6.19.
4.6. Preparation of 9e and 9f
To a solution of trans-3-methyl-2-(2-phenylethenyl)furan
(trans-16a) (0.197 g, 1.0 mmol) in CCl4 (20 mL) were added
freshly crystallized N-bromosuccinimide (NBS) (0.430 g,
2.2 mmol) and a few mg (5–6 mg) of AIBN. The mixture
was heatedunder refluxuntil the NBS was consumed (usually
6–7 h). The reaction mixturewas cooled to room temperature
and filtered to remove the succinimide and concentrated un-
der reduced pressure to give a brown oil of the corresponding
trans-dibromide. The oil was dissolved in benzene (10 mL)
and triphenylphosphine (0.317 g, 1.1 mmol) was added and
the solution was stirred overnight. The precipitated phospho-
nium salt was filtered and subjected to a Wittig reaction with
the corresponding aldehydes, p-tolylaldehyde (0.144 g,
1.0 mmol) and furan-2-carbaldehyde (0.096 g, 1.0 mmol) in
a similar manner as described above for 9a–d. After column
chromatography on silica gel with petroleum ether/diethyl
ether (0–3%) mixture as eluent, a 57.8% yield of 9e and
39% yield of 9f were obtained for three reaction steps. In
the first fractions cis,trans-isomers 9e,f were isolated and in
the last fractions trans,trans-isomers 9e,f were isolated.
4.5. Photochemical isomerization of the mixture of
isomers of 9c,d into trans,trans-9c,d
A mixture of cis,trans- and trans,trans-isomers of 9c,d
(w1:1) in benzene (7.7 mM) was purged with argon for
15 min and irradiated at 350 nm in a Rayonet reactor in a Py-
rex tube for 30 min. The photochemical isomerization from
cis,trans-isomers of 9c,d to trans,trans-isomers of 9c,d was
followed by GC–MS measurements with time. After 15 min,
the reaction mixture contained 91% of the trans,trans-iso-
mers of 9c,d. The solvent was removed in vacuum and the
oily residue chromatographed on silica gel column using
petroleum ether to get pure trans,trans-9c,d in the last
fractions.
4.5.1. 2-(2-Phenylethenyl)-3-(2-furylethenyl)furan (9c).
Yield 75.3%; according to 1H NMR spectroscopy, a mixture
of 27% cis,trans- and 73% trans,trans-isomers was
obtained.
4.6.1. 5-Bromo-2-(2-phenylethenyl)-3-[2-(4-methyl-
phenyl)ethenyl]furan (9e). Yield 57.8%; according to H
NMR spectroscopy, a mixture of 38% cis,trans-isomers
and 62% trans,trans-isomers were obtained.
1

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7404
cis,trans-9e: Too small a quantity to be analyzed completely;
Rf 0.62 (petroleum ether/diethyl ether 19:1); 1H NMR
(600 MHz, CDCl3) dH 7.38 (d, J¼7.9 Hz, 2H), 7.27–7.36
(m, 3H), 7.17 (d, J¼7.9 Hz, 2H), 7.08 (d, J¼7.9 Hz, 2H),
6.98 (d, J¼15.9 Hz, 1H, H-et), 6.82 (d, J¼15.9 Hz, 1H, H-
et), 6.56 (d, J¼12.0 Hz, 1H, H-et), 6.34 (d, J¼12.0 Hz,
1H, H-et), 6.04 (s, 1H), 2.30 (s, 3H, CH3); MS (EI) m/z
(%) 364/366 (M+, 100), 285 (20), 257 (30), 115 (20).
1.6 M hexane solution) was added under a stream of nitrogen
over 30 min. After additional stirring for 30 min, the mixture
was allowed to gradually warm up to room temperature, over
4 h. To a stirred reaction mixture, water (1.0 mmol, 0.02 mL)
was added and the solution was stirred overnight. Dilute hy-
drochloric acid (0.3 mL, 7 mol/L) was added and the layers
were separated. After extraction, the combined organic
phases were dried over MgSO4. The reaction mixturewas pu-
rified and separated by column chromatography on silica gel
using petroleum ether/diethyl ether (0–2%) as eluent. After
the separation of the starting compound from the first frac-
tions, the last fractions yielded 2-(2-phenylethenyl)-3-[2-
(4-methylphenyl)ethenyl]furan (trans,trans-9b) in a 71.3%
yield or 2-(2-phenylethenyl)-3-[2-(2-furyl)ethenyl)ethenyl]-
furan (trans,trans-9c) in a 65.5% yield.
trans,trans-9e: Rf 0.57 (petroleum ether/diethyl ether 19:1);
yellow-green crystals; mp 98–100 ꢁC; UV (EtOH) lmax
(log 3) 380 (4.19, sh), 364 (4.33), 347 (4.24, sh), 286
(4.23), 237 (3.85) nm; 1H NMR (600 MHz, CDCl3) dH 7.47
(d, J¼7.9 Hz, 2H), 7.33 (m, 4H), 7.22 (t, J¼7.9 Hz, 1H),
7.13 (d, J¼7.9 Hz, 2H), 7.02 (d, J¼16.0 Hz, 1H, H-et),
6.96 (d, J¼16.0 Hz, 1H, H-et), 6.94 (d, J¼16.0 Hz, 1H,
H-et), 6.72 (d, J¼16.0 Hz, 1H, H-et), 6.56 (s, 1H), 2.32 (s,
3H, CH3); 13C NMR (CDCl3) dC 151.61 (s), 137.81 (s),
136.85 (s), 134.21 (s), 129.86 (d), 129.49 (2d), 128.78 (2d),
127.90 (d), 127.71 (d), 126.51 (2d), 126.29 (2d), 124.37
(s), 123.38 (s), 115.77 (d), 112.81 (d), 110.54 (d), 21.31
(q); IR (evaporated film from CHCl3) 2932, 2873, 1600,
1500, 1456, 956, 940, 750 cmꢀ1; MS (EI) m/z (%) 364/366
(M+, 100), 285 (15), 257 (30), 115 (15); Anal. Calcd for
C21H17BrO: C, 69.05; H, 4.69. Found: C, 69.43; H, 4.33.
4.9. Preparation of 18a–c
Starting compounds 18a–c were prepared in three steps by
a Wittig reaction and Vilsmeier formylation. In the first
step, 2-styrylfuran was prepared by Wittig reaction from
benzyltriphenylphosphonium bromide and freshly distilled
furan-2-carbaldehyde. To a stirred solution of benzyltri-
phenylphosphonium bromide (8.29 g, 20.0 mmol) and
furan-2-carbaldehyde (1.70 g, 18.0 mmol) in absolute etha-
nol (100 mL), a solution of sodium ethoxide (0.615 g,
27.0 mmol in 10 mL ethanol) was added dropwise. Stirring
was continued under a stream of nitrogen for 1 day at room
temperature. After removal of the solvent, the residue was
worked up with water and benzene. The benzene extracts
were dried and concentrated. The crude reaction mixture
was purified and the pure mixture of isomers of 2-styrylfuran
(81.8%) was separated by column chromatography on silica
gel using petroleum ether as eluent. Vilsmeier formylation
was carried out from 2-styrylfuran (1.26 g, 7.4 mmol) dis-
solved in dimethylformamide (1.71 mL, 22.0 mmol). After
being stirred atw12 ꢁC for 15 min, phosphorus oxychloride
(1.14 g, 7.4 mmol) was added and the reaction mixture was
allowed gradually to warm up to room temperature and
stirred for 4 days. The reaction mixture was decomposed
by the continuous addition (with cooling) of 15% sodium hy-
droxide solution and the product was worked up with diethyl
ether. The diethyl ether extracts were washed with water.
After removal of the solvent, the crude reaction mixture of
2-formyl-5-styrylfuran (as a mixture of cis- and trans-
isomer) was used in a Wittig reaction to prepare 2,5-distyryl-
furan derivatives 18a–c. To a stirred solution of formyl
derivative (0.285 g, 1.4 mmol) and the phosphonium salts
(1.6 mmol), benzyltriphenylphosphonium bromide, p-meth-
ylbenzyltriphenylphosphonium bromide, or 2-furylmethyl-
triphenylphosphonium bromide, respectively, in absolute
ethanol (100 mL) a solution of sodium ethoxide (0.05 g,
2.2 mmol in 10 mL ethanol) was added dropwise. Stirring
was continued under a stream of nitrogen for 1 day at room
temperature. After removal of the solvent, the residue was
worked up with water and benzene. The benzene extracts
were dried and concentrated. The crude reaction mixture
was purified and the mixture of four isomers of products
18a–c were isolated by column chromatography on silica
gel using petroleum ether/diethyl ether (0–3%) mixture as
eluent. After the photochemical isomerization of the mixture
of four isomers of 18a–c (see the procedure for isolating
trans,trans-9f) into trans,trans-isomers of 18a–c. The
4.7. Photochemical isomerization of the mixture of
isomers of 9f into trans,trans-9f
A mixture of cis,trans- and trans,trans-isomers of 9f (w2:3)
in benzene (9.5 mM) was purged with argon for 15 min and
irradiated at 350 nm in a Rayonet reactor in a Pyrex tube.
The photochemical isomerization from cis,trans-isomers
of 9c,d to trans,trans-isomers of 9f was followed by
GC–MS measurements with time. After 15 min, the reaction
mixture contained 95% of the trans,trans-isomers of 9f. The
solvent was removed in vacuum and the oily residue was
chromatographed on silica gel column using petroleum ether
to get pure trans,trans-9f from the last fractions.
4.7.1. 5-Bromo-2-(2-phenylethenyl)-3-[2-(2-furyl)ethe-
nyl]furan (trans,trans-9f). Rf 0.59 (petroleum ether/diethyl
ether 19:1); yellow crystals; mp 95 ꢁC; UV (EtOH) lmax
(log 3) 385 (4.38), 367 (4.47), 351 (4.36, sh), 298 (4.38),
288 (4.36), 239 (3.80) nm; 1H NMR (600 MHz, CDCl3) dH
7.50 (dd, J¼7.5, 1.6 Hz, 2H), 7.41 (d, J¼1.5 Hz, 1H), 7.35
(dt, J¼7.5, 1.6 Hz, 2H), 7.25 (m, 1H), 7.07 (d, J¼16.0 Hz,
1H), 6.98 (d, J¼16.0 Hz, 1H), 6.96 (d, J¼16.2 Hz, 1H),
6.56 (d, J¼16.2 Hz, 1H), 6.53 (s, 1H), 6.43 (dd, J¼3.3,
1.5 Hz, 1H), 6.33 (d, J¼3.3 Hz, 1H); 13C NMR (CDCl3) dC
148.15 (s), 147.18 (s), 137.57 (d), 132.03 (s), 124.02 (2d),
123.18 (d), 123.14 (d), 121.79 (2d), 119.19 (s), 118.77 (s),
112.66 (d), 110.51 (d), 107.94 (d), 107.07 (d), 105.46 (d),
104.14 (d); IR (evaporated film from CHCl3) 3044, 2922,
2840, 1595, 1508, 1494, 949, 928, 771 cmꢀ1; MS (EI) m/z
(%) 340/342 (M+, 100), 261 (5), 233 (5); Anal. Calcd for
C18H13BrO2: C, 63.36; H, 3.84. Found: C, 63.73; H, 3.53.
4.8. Preparation of 9b,c from 9e,f
To a stirred solution of trans,trans-9e or trans,trans-9f
(0.9 mmol), respectively, in anhydrous diethyl ether
(40 mL) cooled to ꢀ70 ꢁC, n-butyl lithium (1.0 mmol,

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I. Skoricꢀ et al. / Tetrahedron 62 (2006) 7396–7407
7405
solvent was removed in vacuum and the oily residue was
chromatographed on silica gel column using petroleum ether
to isolate pure trans,trans-18a–c. Characterization data of
the new compounds trans,trans-18b and trans,trans-18c
are given below.55
Compound 9a: 0 h: 7.530, M+ 272, 23%, cis,trans-9a; 8.380,
M+ 272, 77%, trans,trans-9a; after 1 h: 7.020, M+ 272, 5%,
III/IV; 7.970, M+ 272, 4%, III/IV; 8.120, M+ 272, 4%, III/
IV; 8.380, M+ 272, 80%, trans,trans-9a; 8.880, M+ 272, 7%,
trans,trans-18a; after 2 h: 4.690, M+ 180, 2%; 5.390, M+
180, 1%; 7.020, M+ 272, 4%, III/IV; 7.970, M+ 272, 4%,
III/IV; 8.120, M+ 272, 4%, III/IV; 8.380, M+ 272, 77%, trans,
trans-9a; 8.880, M+ 272, 8%, trans,trans-18a; after 4 h: 4.690,
M+ 180, 21%; 5.390, M+ 180, 6%; 7.020, M+ 272, 3%, III/IV;
7.970, M+ 272, 3%, III/IV; 8.120, M+ 272, 4%, III/IV; 8.380,
M+ 272, 58%, trans,trans-9a; 8.880, M+ 272, 5%, trans,trans-
18a; after 8 h: 4.690, M+ 180, 44%; 4.750, M+ 178, 5%, 25;
5.390, M+ 180, 27%; 7.020, M+ 272, 1%, III/IV; 7.970, M+
272, 1%, III/IV; 8.120, M+ 272, 1%, III/IV; 8.380, M+ 272,
19%, trans,trans-9a; 8.880, M+ 272, 2%, trans,trans-18a.
4.9.1. 2-(2-Phenylethenyl)-5-[2-(4-methylphenyl)ethe-
nyl]furan (trans,trans-18b). Yield 58.7%; Rf 0.63 (petro-
leum ether/CH2Cl2 9:1); yellow-green crystals; mp 91–
92 ꢁC; UV (EtOH) lmax (log 3) 386 (4.50), 281 (4.33), 276
1
(4.34) nm; H NMR (600 MHz, CDCl3) dH 7.49 (d, J¼
7.6 Hz, 2H), 7.39 (d, J¼8.0 Hz, 2H), 7.34 (t, J¼7.6 Hz,
2H), 7.24 (t, J¼7.6 Hz, 2H), 7.16 (d, J¼8.0 Hz, 1H), 7.11
(d, J¼16.2 Hz, 1H), 7.10 (d, J¼16.1 Hz, 1H), 6.87 (d, J¼
16.2 Hz, 1H), 6.83 (d, J¼16.1 Hz, 1H), 6.37 (d, J¼3.4 Hz,
1H), 6.34 (d, J¼3.4 Hz, 1H), 2.35 (s, 3H, CH3); 13C NMR
(CDCl3) dC 152.75 (s), 152.37 (s), 137.10 (s), 136.67 (s),
133.84 (s), 128.98 (2d), 128.24 (2d), 127.07 (d), 126.89
(d), 126.64 (d), 125.88 (2d), 125.84 (2d), 115.80 (d),
114.88 (d), 110.79 (d), 110.33 (d), 20.80 (q); IR (evaporated
film from CHCl3) 2981, 2925, 1592, 1507, 1442, 1017, 956,
782, 750 cmꢀ1; MS (EI) m/z (%) 286 (M+, 100); Anal. Calcd
for C21H18O: C, 88.08; H, 6.34. Found: C, 88.27; H, 6.18.
Compound 9b: 0 h: 7.860, M+ 286, 33%, cis,trans-9b; 8.950,
M+ 286, 67%, trans,trans-9b; after 1 h: 7.380, M+ 286, 4%,
III/IV; 7.840, M+ 286, 5%, III/IV; 8.740, M+ 286, 4%, III/
IV; 8.950, M+ 286, 80%, trans,trans-9b; 9.270, M+ 286, 7%,
trans,trans-18b; after 2 h: 4.550, M+ 208, 2%; 4.690, M+
180, 3%; 5.390, M+ 180, 1%; 7.380, M+ 286, 3%, III/IV;
7.840, M+ 286, 4%, III/IV; 8.740, M+ 286, 6%, III/IV;
8.950, M+ 286, 75%, trans,trans-9b; 9.270, M+ 286, 6%,
trans,trans-18b; after 4 h: 4.550, M+ 208, 8%; 4.690, M+
180, 13%; 5.110, M+ 194, 10%; 5.380, M+ 194, 1%; 5.390,
M+ 180, 3%; 7.380, M+ 286, 2%, III/IV; 7.840, M+ 286,
3%, III/IV; 8.740, M+ 286, 3%, III/IV; 8.950, M+ 286, 53%,
trans,trans-9b; 9.270, M+ 286, 4%, trans,trans-18b; after
8 h: 4.550, M+ 208, 17%; 4.690, M+ 180, 31%; 5.110, M+
194, 12%; 5.310, M+ 192, 3%, 22; 5.380, M+ 194, 3%; 5.390,
M+ 180, 7%; 7.840, M+ 286, 1%, III/IV; 8.740, M+ 286, 1%,
III/IV; 8.950, M+ 286, 23%, trans,trans-9b; 9.270, M+ 286,
2%, trans,trans-18b.
4.9.2. 2-(2-Furylethenyl)-5-(2-phenylethenyl)furan
(trans,trans-18c). Yield 64.2%; Rf 0.61 (petroleum ether/
CH2Cl2 9:1); yellow crystals; mp 87 ꢁC; UV (EtOH) lmax
(log 3) 383 (4.52), 283 (4.35), 274 (4.34) nm; 1H NMR
(600 MHz, CDCl3) dH 7.44 (d, J¼7.6 Hz, 2H), 7.35 (d,
J¼1.4 Hz, 1H), 7.31 (t, J¼7.6 Hz, 2H), 7.21 (m, 1H), 7.06
(d, J¼16.1 Hz, 1H), 6.88 (d, J¼16.1 Hz, 1H), 6.82 (d,
J¼16.1 Hz, 1H), 6.75 (d, J¼16.1 Hz, 1H), 6.39 (m, 1H),
6.33 (d, J¼3.4 Hz, 2H), 6.31 (d, J¼3.4 Hz, 1H); 13C NMR
(CDCl3) dC 152.65 (s), 152.49 (s), 152.26 (s), 141.75
(d), 136.60 (s), 128.22 (d), 127.10 (2d), 126.78 (d), 125.89
(d), 125.86 (d), 115.69 (d), 114.53 (d), 114.08 (d), 111.36
(d), 110.97 (d), 110.90 (d), 108.49 (d); IR (evaporated film
from CHCl3) 2932, 2874, 1508, 956, 815, 782 cmꢀ1; MS
(EI) m/z (%) 262 (M+, 100); Anal. Calcd for C18H14O2: C,
82.42; H, 5.38. Found: C, 82.29; H, 5.53.
Compound 9c: 0 h: 6.560, M+ 262, 41%, cis,trans-9c; 7.450,
M+ 262, 59%, trans,trans-9c; after 1 h: 6.330, M+ 262, 4%,
III/IV; 6.920, M+ 262, 4%, III/IV; 7.170, M+ 262, 3%, III/
IV; 7.450, M+ 262, 82%, trans,trans-9c; 8.030, M+ 262, 7%,
trans,trans-18c; after 2 h: 3.650, M+ 160, 1%; 4.080, M+
170, 3%; 4.600, M+ 170, 1%; 4.690, M+ 180, 2%; 6.330, M+
262, 4%, III/IV; 6.920, M+ 262, 3%, III/IV; 7.170, M+ 262,
2%, III/IV; 7.450, M+ 262, 78%, trans,trans-9c; 8.030, M+
262, 6%, trans,trans-18c; after 4 h: 3.650, M+ 160, 6%;
4.080, M+ 170, 17%; 4.600, M+ 170, 6%; 4.690, M+ 180,
4%; 6.330, M+ 262, 2%, III/IV; 6.920, M+ 262, 3%, III/IV;
7.170, M+ 262, 1%, III/IV; 7.450, M+ 262, 57%, trans,trans-
9c; 8.030, M+ 262, 4%, trans,trans-18c; after 8 h: 3.650, M+
160, 13%; 4.080, M+ 170, 31%; 4.600, M+ 170, 19%; 4.690,
M+ 180, 11%; 4.750, M+ 178, 2%, 25; 6.330, M+ 262,
1%, III/IV; 6.920, M+ 262, 1%, III/IV; 7.450, M+ 262, 21%,
trans,trans-9c; 8.030, M+ 262, 1%, trans,trans-18c.
4.10. Irradiation experiments
A mixture of cis,trans- and trans,trans-isomers of 9a–d in
benzene (3.3 mM) was purged with argon for 30 min and
irradiated at 350 nm in a Rayonet reactor in a Pyrex tube.
The reaction course was followed by GC–MS. The GC–MS
analysis of the photomixtures was performed on a Varian
CP-3800 Gas Chromatograph–Varian Saturn 2200 equipped
with FactorFour Capillary Column VF-5ms, 30 mꢂ0.25 mm
ID; GC operating conditions for all experiments: column
temperature programed from 110 to 300 ꢁC (3 min isother-
mal) at a rate of 33 ꢁC/min; carrier gas: helium; flow rate:
1 mL/min; injector temperature: 300 ꢁC; volume injected:
5 mL.
Compound 9d: 0 h: 7.100, M+ 276, 17%, cis,trans-9d; 7.880,
M+ 276, 83%, trans,trans-9d; after 1 h: 6.850, M+ 276, 5%, II/
III/IV; 7.290, M+ 276, 3%, II/III/IV; 7.480, M+ 276, 5%, II/
III/IV; 7.880, M+ 276, 81%, trans,trans-9d; 8.410, M+ 276,
6%, II/III/IV; after 2 h: 3.650, M+ 160, 1%; 4.330, M+ 184,
2%; 4.550, M+ 208, 4%; 6.850, M+ 276, 5%, II/III/IV; 7.290,
M+ 276, 4%, II/III/IV; 7.480, M+ 276, 5%, II/III/IV; 7.880,
M+ 276, 75%, trans,trans-9d; 8.410, M+ 276, 4%, II/III/IV;
after 4 h: 3.650, M+ 160, 2%; 4.330, M+ 184, 6%; 4.550, M+
208, 24%; 6.850, M+ 276, 3%, II/III/IV; 7.290, M+ 276, 3%,
According to chromatograms, the following data for irradi-
ation of 2,3-divinylfuran derivatives 9a–d were obtained
[irrad. time/h, tR./min, M+, %, compd; 2-[2-(2-
furyl)ethenyl]furan (M+ 160), 2-styrylfuran (M+ 170),
stilbene (M+ 180), 2-[2-(4-methylphenyl)ethenyl]furan
(M+ 184), 4-methylstilbene (M+ 194), 4,40-dimethylstilbene
(M+ 208)]:

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I. Skoricꢀ et al. / Tetrahedron 62 (2006) 7396–7407
7406
II/III/IV; 7.480, M+ 276, 3%, II/III/IV; 7.880, M+ 276, 55%,
trans,trans-9d; 8.410, M+ 276, 4%, II/III/IV; after 8 h: 3.650,
M+ 160, 8%; 4.330, M+ 184, 17%; 4.500, M+ 206, 4%, 26;
4.550, M+ 208, 40%; 6.850, M+ 276, 1%, II/III/IV; 7.290,
M+ 276, 1%, II/III/IV; 7.880, M+ 276, 27%, trans,trans-9d;
8.410, M+ 276, 2%, II/III/IV.
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Visnjevac, A.; Kojic-Prodic, B.; Sindler-Kulyk, M. Tetrahedron
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After complete conversion (8–10 h) of the starting material
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residue was chromatographed on a silica gel column using
petroleum ether/diethyl ether (2–50%). The only obtained
products (besides high amount of tarry material) were small
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This work was supported by grants from the Ministry of
Science, Education, and Sports of the Republic of Croatia
(grant nos. 0125004 and 0098059).
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