Bis-, tris-, and tetrakis(squaraines) linked by stilbenoid scaffolds

Article abstract of DOI:10.1002/1099-0690(200107)2001:14<2757::aid-ejoc2757>3.0.co;2-2

The oligosquaraines 1-5, with stilbenoid scaffolds, were prepared by multistep syntheses in which the final steps consisted of condensation reactions between the semisquaric acid 21 and multiple resorcinols 12b-15b and 17b. The target compounds exhibit in

Full text of DOI:10.1002/1099-0690(200107)2001:14<2757::aid-ejoc2757>3.0.co;2-2

FULL PAPER
Bis-, Tris-, and Tetrakis(squaraines) Linked by Stilbenoid Scaffolds
Jürgen Gerold,[a] Uta Holzenkamp,[b] and Herbert Meier*[a]
Dedicated to Professor Leopold Horner on the occasion of his 90th birthday
Keywords: Conjugation / Donor-acceptor systems / Squaraines / UV/Vis spectroscopy
The oligosquaraines 1−5, with stilbenoid scaffolds, were pre-
depending on the conjugation in the stilbenoid scaffold.
pared by multistep syntheses in which the final steps con- Comparison with the monosquaraine 6 as a model compound
sisted of condensation reactions between the semisquaric
acid 21 and multiple resorcinols 12b−15b and 17b. The target
compounds exhibit intense (ε Ͼ 250000 L·mol−1·cm−1) and
sharp absorption bands with maxima between 687 to 778 nm,
reveals intramolecular interactions between different
donor−acceptor−donor moieties, which give rise to increased
absorption intensities.
Introduction
structures, and are conjugated through a stilbene and a 1,4-
distyrylbenzene unit, respectively. The tris(squaraine) 3 has
a tristyrylbenzene core and, thanks to the 1,3,5-trisubstitu-
tion, no possible linear conjugation pathway through the
central benzene ring. In contrast to 3, the tetrakis(squa-
raine) 4 is a conjugated system, although steric hindrance
prevents a completely planar arrangement. The central sat-
urated carbon atom in the tetrakis(squaraine) 5 blocks con-
jugation completely. As a simple model compound, we also
synthesized the monosquaraine 6. All of the squaraines pre-
pared in this work bear 4-(dialkylamino)phenyl groups with
two 2-hexyloctyl chains, to increase the solubility of the
compounds and strengthen the terminal donor activity.
Since their discovery by Treibs and Jacob in 1965, squa-
raines have attracted considerable attention.[1] They possess
semiconductor and photoconductor properties and have
been investigated for a variety of technological applications,
such as in xerographic photoreceptors, organic solar cells,
and optical recording media.[2] Squaraines show aggrega-
tion in the solid state and in LB films and have therefore
been extensively studied in the field of nonlinear optics.[3]
Their absorptions and emissions in the Vis and NIR re-
gions, combined with their narrow bands (∆ν1/2 Ͻ 1000
cmϪ1) and high molar absorption coefficients (εmax Ͼ 105
L·molϪ1·cmϪ1), also make them attractive in the area of
biolabeling.[4] Stilbenoid compounds, on the other hand,
have interesting photophysical and photochemical proper-
ties and are used in, for example, light-emitting diodes or
nonlinear optical devices.[5]
In previous reports we showed that the combination of
these two structural principles Ϫ squaraines and stilbenes
Ϫ resulted in NIR dyes with long-wavelength absorption
maxima of up to 900 nm and more.[6] The aim of this work
was to generate a new class of ‘‘multi-arm’’ squarylium
compounds with stilbenoid scaffolds Ϫ dyes with high chro-
mophore densities.
The synthetic procedure for the squaraines 1Ϫ6 is based
on the preparation of the corresponding stilbenoid resor-
cinols. The olefinic double bonds could easily be produced
by means of WittigϪHorner reactions. Scheme 2 summar-
izes the preparation of the phosphonates 7cϪ10c, which
were obtained by WohlϪZiegler brominations of the hydro-
carbons 7aϪ10a and Arbusov rearrangements. The bromo-
methyl compounds 7bϪ10b were treated in situ with triethyl
phosphite; the yields given in Scheme 2 refer to the trans-
formations a Ǟ c.
As shown in Scheme 3, the phosphonates 7cϪ10c served
as components for WittigϪHorner treatment with the alde-
hydes 11 and 16. Subsequent deprotection with boron trib-
romide afforded the resorcinols (12aϪ15a, 17a Ǟ 12bϪ15b,
17b). The total yields of these two steps amounted to
27Ϫ54%. The protection of the hydroxy groups proved to
be necessary, since free phenols give only very poor yields
in WittigϪHorner reactions.
Results and Discussion
Scheme 1 provides an overview of the two-, three-, and
fourfold squaraines 1Ϫ5 described here, together with the
model compound 6. The bis(squaraines) 1 and 2 have linear
The final reaction sequence for the preparation of the
squaraines 1Ϫ6 is illustrated in Scheme 4. The coupling
process applied to squaric acid was originally developed by
West and co-workers[7] and improved by Law, Whitten, et
al.[8] Squaric acid dichloride 18 reacted under
FriedelϪCrafts alkylation conditions with 1 equiv. of N,N-
[a]
Institute of Organic Chemistry, University of Mainz,
Duesbergweg 10Ϫ14, 55099 Mainz, Germany
Fax: (internat.) ϩ 49-(0)6131/392-5396
E-mail: hmeier@mail.uni-mainz.de
[b]
Present address: BASF AG,
67056 Ludwigshafen, Germany
Eur. J. Org. Chem. 2001, 2757Ϫ2763
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ193X/01/0714Ϫ2757 $ 17.50ϩ.50/0
2757

J. Gerold, U. Holzenkamp, H. Meier
FULL PAPER
Scheme 2. Preparation of the phosphonates 7cϪ10c; the corres-
ponding methyl compounds 7aϪ10a were brominated with NBS
and the crude bromo compounds 7bϪ10b heated to 160 °C with
excess triethyl phosphite in order to obtain the phosphonates
7cϪ10c
Scheme 3. Preparation of the multiple resorcinols 12bϪ15b and 17b
bis(2-hexyloctyl)aniline (19) to give compound 20, which
was hydrolyzed to the semisquaric acid 21 by treatment
with hydrochloric acid and trifluoroacetic acid. Two-, three-,
and fourfold condensation reactions between 21 and the
corresponding diols 12bϪ15b and 17b, in refluxing toluene/
1-butanol mixtures (3:1), afforded the desired squaraines
1Ϫ5 in reasonable yields. The water generated in the con-
densation reactions was removed by azeotropic distillation.
The model compound 6 was prepared in an analogous elec-
trophilic substitution (21 ϩ 22[9] Ǟ 6).
The isolated squaraines 1Ϫ5 are blue to violet, crystalline
compounds with metallic lusters. The monosquaraine 6
forms bronze, metal-like crystals. The compounds 1Ϫ6 are
Scheme 1. Target compounds: multiple squaraines 1Ϫ5 and model
compound 6
2758
Eur. J. Org. Chem. 2001, 2757Ϫ2763

Bis-, Tris-, and Tetrakis(squaraines) Linked by Stilbenoid Scaffolds
FULL PAPER
reasonably soluble in organic solvents such as chloroform, with the exception of 5. As an example, Figure 1 shows the
and could be completely characterized by their NMR and MALDI-TOF spectrum of 3, with the typical molecular ion
mass spectra.[10] Even 13C NMR spectra could be obtained, peak group [M ϩ n H]ϩ (n ϭ 0, 1, 2, 3,...), where n ϭ 2
represents the maximum.
We also performed condensation reactions between 13b,
14b, and 15b and a semisquaric acid[11] lacking the dial-
kylamino group present in 21. The reaction, carried out in
tributyl orthoformate as a water-removing agent, gave ex-
cellent yields (70Ϫ93%), but the resulting dark pigments
were totally insoluble in organic solvents, and so we re-
frained from their structural characterization.
The electronic properties of a series of nonconjugated
bis(squaraines) have already been described in the literat-
ure.[12] A cationic bis(squaraine) with extended conjugation
was described by Nakazumi et al.[13] and displayed a strong
pH dependence in its absorption spectra. Oligosquaraines
with molecular masses of about 2000 and some polysqua-
raines have been investigated by different groups.[14,15]
The electronic spectroscopic data (in chloroform) of the
oligosquaraines 1Ϫ5 and the model squaraine 6 are sum-
marized in Table 1. The absorption maxima vary from 674
to 778 nm, depending on the stilbenoid scaffold. Figure 2
shows a comparison of the absorption and emission of 1
and the model compound 6.
In both cases, the absorption consists of two close-lying
bands.[16] However, the absorption of the conjugated
bis(squaraine) 1,
a
donorϪacceptorϪdonorϪdonorϪ
Scheme 4. Preparation of the multiple squaraines 1Ϫ5 and the
model squaraine 6
acceptorϪdonor system, is bathochromically shifted by
104 nm relative to that of the donorϪacceptorϪdonor sys-
tem of model compound 6. The tetrakis(squaraine) 5, with
isolated chromophores, resembles that of 6 in its absorption
characteristics. The cross-conjugated tris(squaraine) 3, the
tetrakis(squaraine) 4 with sterically hindered conjugation,
and the linearly conjugated bis(squaraine) 2 exhibit increas-
ing red shifts, but do not reach the λmax value of 1. The
half-widths b of the absorption bands of the squaraines 1,
2, 3, and 5 are all in the range of 610 Ϯ 80 cmϪ1, but the
half-width of the band of 4, at 1334 cmϪ1, is more than
twice as large. Squaraine 4 is the only example in which
intramolecular steric interaction between squaraine moieties
is possible.
The intensities S of the absorption bands grow with in-
creasing numbers of squaraine units, but they do so more
than proportionally. The largest excess was found for the
Figure 1. MALDI-TOF spectrum of 3 in dithranol
Table 1. Absorption and fluorescence data of the multiple squaraines 1Ϫ5 and the model squaraine 6 in chloroform
Compound
Absorption
λmax
[nm]
Half-width
Intensity[a]
S
Intensity per
Emission
λmax
[nm]
εmax
b
squaraine unit
[108 cm2·molϪ1
]
[cmϪ1
]
[1011 cm·molϪ1
]
[1011 cm·molϪ1
]
1
2
3
4
5
6
778
724
693
707
687
674
2.58
4.56
8.11
3.93
7.23
2.70
552
689
532
1334
662
323
1.42
3.14
4.32
5.24
4.79
0.87
0.71
1.57
1.44
1.31
1.20
0.87
798
750
714
753
707
702
[a]
S ϭ b·εmax
Eur. J. Org. Chem. 2001, 2757Ϫ2763
2759

J. Gerold, U. Holzenkamp, H. Meier
FULL PAPER
Conclusions
Two-, three-, and fourfold squaraines 1Ϫ5, linked by stil-
benoid scaffolds, have been produced by condensation reac-
tions between the semisquaric acid 21 and the correspond-
ing resorcinols 12bϪ15b and 17b. Reasonably good solubil-
ity of the dyes in organic solvents such as chloroform was
guaranteed by the terminal bis(2-hexyloctyl)amino groups.
Depending on the conjugation of the systems Ϫ fully pre-
sent, sterically hindered, or totally absent Ϫ the absorption
bands exhibit maxima between 778 and 687 nm. The fluor-
escence bands are situated at the Vis/NIR border. Apart
from the high density of the chromophores both in the blue,
metal-like crystals and in solution, the absorption intensity
per squaraine unit in 2Ϫ5 Ϫ relative to that in the model
squaraine 6 Ϫ is increased thanks to intramolecular interac-
tions of the charge-transfer moieties.
Experimental Section
General Methods: The melting points were measured with a Büchi
melting point apparatus and are uncorrected. Ϫ The 1H and 13C
NMR spectra were recorded with Bruker AM 400 and AC 200
spectrometers with CDCl3 as solvent unless otherwise noted, using
TMS as internal standard. Ϫ The FD mass spectra were obtained
with a Finnigan Mat 95, the MALDI-TOF mass spectra with a
Micromass TOF spec E. A Beckman Acculab 4 spectrometer
served for the measurement of the IR spectra. Ϫ UV/Vis spectra
were taken with a Zeiss MCS 320/340, fluorescence spectra with a
PerkinϪElmer LS 50B spectrometer. Ϫ Silica gel (E. Merck 60,
70Ϫ230 mesh ASTM) was used for column chromatography. Ϫ
Elemental analyses were performed in the microanalytical laborat-
ory of the Institute of Organic Chemistry at the University of
Mainz, Germany.
Figure 2. Absorption and normalized emission spectra of the squa-
raines 1 and 6 in chloroform; the emission spectra are uncorrected,
concentration-independent, and also independent of the wave-
length of excitation
Preparation of the Phosphonates: The phosphonates 7c[6b] and 8c[17]
were prepared from the corresponding methyl compounds 7a and
8a by bromination with NBS and subsequent treatment with trie-
thyl phosphite according to the literature.
bis(squaraine) 2, the S value of which is more than three
times higher than that of the model compound 6. Obvi-
ously, there is intramolecular electronic interaction present
(between the different donorϪacceptorϪdonor moieties of
2Ϫ5), which enhances the absorption intensity.
The squaraines 1Ϫ6, when dissolved in chloroform, ex-
hibit weak fluorescence. The fluorescence band of 6 is a
mirror image of the absorption band (Figure 2). The emis-
sion at long wavelengths appears as a shoulder on the in-
tense band in the spectra of 2Ϫ6; only in 1 is such a shoul-
der not visible, perhaps because superimposed over by the
strong emission at 800 nm.
Diethyl 2,4,5-Tris[(diethoxyphosphoryl)methyl]benzylphosphonate
(9c): 1,2,4,5-Tetramethylbenzene (durene) (9a) (20.0 g, 150 mmol),
N-bromosuccinimide (NBS) (106.0 g, 600 mmol), and catalytic
amounts of AIBN (2.0 mg, 0.01 mmol) in 500 mL of dry tetrachlo-
romethane were heated under reflux and irradiation (500-W tung-
sten lamp) for 20 h. After cooling, the succinimide was filtered off
and washed with small amounts of tetrachloromethane. The or-
ganic phase was extracted with water and dried with anhydrous
Na2SO4. Addition of petroleum ether (40Ϫ70 °C) resulted in the
Interestingly, protonation by addition of trifluoroacetic precipitation of the crude tetrabromo compound, which was used
directly for the following step, in which it was heated in an excess
quantity of triethyl phosphite (124.0 g, 750 mmol) for 8 h at 160
°C. The generated bromoethane was distilled off during the reac-
tion and the excess triethyl phosphite was removed at the end.
acid to a chloroform solution of the squaraines affected
only the absorption behavior of the bis(squaraine) 1. Its
absorption maximum was blue-shifted, indicating that the
DϪAϪD character of the compound had decreased on
protonation. The maxima of the other squaraines were
slightly red-shifted, due to positive solvatochromism arising
from the enhanced polarity of the solvent. Solvatochro-
mism and lessening of the DϪAϪD character by protic
solvents cause opposite effects, so no quantitative conclu-
sions can be drawn.
˜
Overall yield 60.7 g (60%), colorless oil. Ϫ IR (NaCl): ν ϭ 2980
cmϪ1, 2920, 1500, 1470, 1440, 1385, 1230, 1160, 1090, 1025, 960,
1
880, 850, 790. Ϫ H NMR (CDCl3): δ ϭ 1.15 (t, 24 H, CH3), 3.28
[d, 2J(H,P) ϭ 19.7 Hz, 8 H, CH2P], 3.91 (m, 16 H, OCH2), 7.08 (s,
2 H, aromat. H). Ϫ 13C NMR (CDCl3): δ ϭ 16.3 (CH3), 30.7 (d, 1J
(C,P) ϭ 137.5 Hz, CH2P), 61.9 (OCH2), 130.0 (aromat. Cq), 134.3
(aromat. CH). Ϫ MS (FD): m/z (%) ϭ 678 (100) [Mϩ·]. Ϫ
2760
Eur. J. Org. Chem. 2001, 2757Ϫ2763

Bis-, Tris-, and Tetrakis(squaraines) Linked by Stilbenoid Scaffolds
C26H50O12P4 (678.6): calcd. C 46.02, H 7.43; found C 45.85, H ethane (DME) was added dropwise to a suspension of NaH (80%
FULL PAPER
7.36.
in paraffin, 0.32 g, 10.6 mmol) in the same solvent. After 15 min
of stirring at room temperature, a solution of 11 (0.6 g, 3.6 mmol)
in DME was added. The reaction mixture was heated under reflux
for 3 h and carefully quenched with methanol/water (1:1). The pre-
cipitated crystals were separated by filtration and washed with pet-
roleum ether (40Ϫ70 °C). Yield 0.5 g (87%), ochre crystals, m.p.
Diethyl 4-(Tris{4-[(diethoxyphosphoryl)methyl]phenyl}methyl)ben-
zylphosphonate (10c): Preparation analogous to that of 9c. Yield
85%, yellowish, resinous substance. Ϫ IR (KBr): ν ϭ 2990 cm
2960, 1630, 1500, 1440, 1410, 1390, 1360, 1240, 1160, 1100, 1020,
910, 860, 800. Ϫ 1H NMR (CDCl3): δ ϭ 1.19 (m, 24 H, CH3), 3.03
[d, 2J(H,P) ϭ 21.5 Hz, 8 H, CH2P], 3.90 (m, 16 H, OCH2), 7.01
(AAЈBBЈ, 16 H, aromat. H). Ϫ MS (FD): m/z (%) ϭ 920 (100)
[Mϩ·]. Ϫ C45H64O12P4 (920.9): calcd. C 58.69, H 7.01; found C
58.51, H 6.88.
Ϫ1
˜
,
241 °C. Ϫ IR (KBr): ν ϭ 2980 cmϪ1, 2920, 2820, 1580, 1440, 1415,
˜
1195, 1145, 1050, 940. Ϫ 1H NMR (CDCl3): δ ϭ 3.82 (s, 24 H,
OCH3), 6.41 (t, 4 H, aromat. H), 6.69 (d, 8 H, aromat. H), 7.01,
3
7.41 (AB, J ϭ 16.0 Hz, 8 H, olefin. H), 7.75 (s, 2 H, aromat. H,
central ring). Ϫ 13C NMR (CDCl3): δ ϭ 55.4 (OCH3), 100.3, 104.9
(aromat. CH), 125.3 (aromat. CH, central ring), 126.9, 131.7 (ol-
efin. CH), 135.6, 139.5, 161.2 (aromat. Cq). Ϫ MS (FD): m/z (%) ϭ
726 (100) [Mϩ·]. Ϫ C46H46O8 (726.3): calcd. C 76.01, H 6.38; found
C 76.07, H 6.26.
Preparation of the Stilbenoid Scaffolds
(E)-3,3Ј,5,5Ј-Tetramethoxystilbene (12a):[18] 3,5-Dimethoxybenzal-
dehyde (11)[19] (1.0 g, 6.0 mmol) and 7c (1.7 g, 6.0 mmol), dissolved
in 50 mL of dry DMF, were added dropwise to a solution of potas-
sium tert-butoxide (1.7 g, 15.2 mmol) in 50 mL of dry DMF. The
mixture was stirred at room temperature for 1 h and then poured
onto 100 g of crushed ice. A precipitate was formed; this was separ-
ated by filtration and recrystallized from n-hexane/acetone (10:1).
Yield 1.25 g (69%), colorless crystals, m.p. 126Ϫ128 °C (ref.:
5-[(E)-2-{2,4,5-Tris[(E)-2-(3,5-dihydroxyphenyl)ethenyl]phenyl}-
ethenyl]-1,3-benzenediol (14b): This preparation was performed ac-
cording to the procedure described for 12b. Yield 62%, ochre crys-
tals, m.p. Ͼ 300 °C. Ϫ IR (KBr): ν ϭ 3300 cmϪ1, 1580, 1145, 950.
˜
1
Ϫ H NMR ([D6]acetone): δ ϭ 6.32 (t, 4 H, aromat. H), 6.68 (d,
126Ϫ128 °C).[18] Ϫ H NMR (CDCl3): δ ϭ 3.81 (s, 12 H, OCH3),
6.39 (t, 2 H, aromat. H), 6.65 (d, 4 H, aromat. H), 7.00 (s, 2 H,
olefin. H).
1
8 H, aromat. H), 7.11, 7.53 (AB, 3J ϭ 16.0 Hz, 8 H, olefin. H),
7.96 (s, 2 H, aromat. H, central ring), 8.43 (s, 8 H, OH). Ϫ 13C
NMR (methanol-d4): δ ϭ 103.4, 106.2 (aromat. CH), 125.5
(aromat. CH, central ring), 126.8, 132.6 (olefin. CH), 136.6, 140.7,
159.7 (aromat. Cq). Ϫ MS (FD): m/z (%) ϭ 614 (100) [Mϩ·]. Ϫ
C38H30O8 (614.7): calcd. C 74.26, H 4.92; found C 73.99, H 5.01.
(E)-3,3Ј5,5Ј-Tetrahydroxystilbene (12b):[20] Compound 12a (1.0 g,
3.3 mmol) was dissolved in 100 mL of dry dichloromethane and
treated dropwise with boron tribromide (22 mL, 22.0 mmol, 1.0 
in n-hexane). The reaction mixture was heated under reflux for 7 h
and carefully quenched with water. The ochre crystals were separ-
ated by filtration and washed several times with water. Yield 0.48 g
1,3-Dimethoxy-5-{(E)-2-[4-(tris{4-[(E)-2-(3,5-dimethoxyphenyl)-
ethenyl]phenyl}methyl)phenyl]ethenyl}benzene (15a): This prepara-
tion was performed according to the procedure described for 14a.
The reaction time was 6 h. The crude product was dissolved in
acetone and precipitated by the addition of a mixture of petroleum
ether (40Ϫ70 °C)/diethyl ether (1:1). Yield 38%, ochre crystals, m.p.
(59%), m.p. Ͼ 300 °C (ref.: 320 °C).[20] Ϫ H NMR ([D6]acetone):
δ ϭ 4.96 (s, 4 H, OH), 6.22 (t, 2 H, aromat. H), 6.49 (d, 4 H,
aromat. H), 6.90 (s, 2 H, olefin. H). Ϫ MS (FD): m/z (%) ϭ 244
(100) [Mϩ·].
1
229 °C. Ϫ IR (KBr): ν ϭ 3000 cmϪ1, 2920, 2820, 1580, 1445, 1415,
˜
1
1195, 1140, 1055, 950, 840. Ϫ H NMR (CDCl3): δ ϭ 3.81 (s, 24
1,3,5-Tris[(E)-2-(3,5-dimethoxyphenyl)ethenyl]benzene (13a): This
preparation was performed according to the procedure described
for 12a, using 3 equiv. of compound 11 and with dry THF as solv-
ent in place of DMF. The reaction time was 3 h. Yield 88%, ochre
crystals, m.p. 173 °C. Ϫ IR (KBr): ν˜ ϭ 2950 cmϪ1, 2920, 2830,
H, OCH3), 6.39 (t, 4 H, aromat. H), 6.66 (d, 8 H, aromat. H), 6.99,
7.08 (AB, 3J ϭ 16.6 Hz, 8 H, olefin. H), 7.26, 7.40 (AAЈBBЈ, 16 H,
aromat. H). Ϫ 13C NMR (CDCl3): δ ϭ 55.3 (OCH3), 64.3 (Cq),
99.9, 104.5, 125.9, 131.2 (aromat. CH), 128.6, 128.7 (olefin. CH),
134.8, 139.3, 146.1, 160.9 (aromat. Cq). Ϫ MS (FD): m/z (%) ϭ
969 (100) [Mϩ·]. Ϫ C65H60O8 (969.2): calcd. C 80.55, H 6.24; found
C 80.33, H 6.20.
1
1590, 1450, 1200, 1150, 1065, 965. Ϫ H NMR (CDCl3): δ ϭ 3.83
(s, 18 H, OCH3), 6.41 (t, 3 H, aromat. H), 6.70 (d, 6 H, aromat.
H), 7.09 (s, 6 H, olefin. H), 7.50 (s, 3 H, aromat. H, central ring).
Ϫ
13C NMR (CDCl3): δ ϭ 55.3 (OCH3), 100.1, 104.6 (aromat.
5-{(E)-2-[4-(Tris{4-[(E)-2-(3,5-dihydroxyphenyl)ethenyl]phenyl}-
methyl)phenyl]ethenyl}-1,3-benzenediol (15b): Preparation analog-
ous to that of 12b. Yield 80%, ochre crystals, m.p. Ͼ 300 °C. Ϫ IR
(KBr): ν˜ ϭ 3400 cmϪ1, 1590, 1500, 1150, 960, 830. Ϫ 1H NMR
([D6]DMSO): δ ϭ 5.22 (s, 4 H, aromat. H), 5.49 (s, 8 H, aromat.
H), 6.09 (s, 8 H, olefin H), 6.26, 6.60 (AAЈBBЈ, 8 H, aromat. H),
8.37 (br. s, 8 H, OH). Ϫ 13C NMR ([D4]methanol): δ ϭ 65.7 (Cq),
103.0, 106.2, 126.8, 132.4 (aromat. CH), 128.9, 129.9 (olefin. CH),
136.5, 140.7, 147.3, 159.5 (aromat. Cq). Ϫ MS (FD): m/z (%) ϭ
857 (100) [Mϩ·]. Ϫ C57H44O8 (856.3): calcd. C 79.89, H 5.18; found
C 79.75, H 5.26.
CH), 124.1 (aromat. CH, central ring), 128.7, 129.2 (olefin. CH),
137.8, 139.2, 161.0 (aromat. Cq). Ϫ MS (FD): m/z (%) ϭ 564 (100)
[Mϩ·]. Ϫ C36H36O6 (564.7): calcd. C 76.57, H 6.43, found C 76.48,
H 6.42.
5-[(E)-2-{3,5-Bis[(E)-2-(3,5-dihydroxyphenyl)ethenyl]phenyl}-
ethenyl]-1,3-benzenediol (13b): Preparation analogous to that of
12b. The crude product was purified by column filtration (8 ϫ
10 cm, silica gel, toluene/acetone, 1:1). Yield 93%, ochre crystals,
m.p. Ͼ 300 °C. Ϫ IR (KBr): ν ϭ 3300 cmϪ1, 1605, 1580, 1330,
˜
1
1140, 1000, 955, 845. Ϫ H NMR ([D6]acetone): δ ϭ 6.32 (t, 3 H,
aromat. H), 6.62 (d, 6 H, aromat. H), 7.13, 7.24 (AB, 3J ϭ 16.6 Hz,
6 H, olefin. H), 7.69 (s, 3 H, aromat. H, central ring), 8.34 (s, 6 H,
OH). Ϫ 13C NMR ([D6]acetone): δ ϭ 103.2, 106.0 (aromat. CH),
124.7 (aromat. CH, central ring), 128.9, 130.3 (olefin. CH), 139.1,
140.3, 159.6 (aromat. Cq). Ϫ MS (FD): m/z (%) ϭ 480 (100) [Mϩ·].
Ϫ C30H24O6 (480.5): calcd. C 74.99, H 5.03; found C 74.75, H 5.12.
1-[(E)-2-{4-[(E)-2-(3,5-Dimethoxyphenyl)ethenyl]phenyl}ethenyl]-
3,5-dimethoxybenzene (17a): Terephthalaldehyde (16) (1.0 g,
7.5 mmol) and phosphonate 7c (4.3 g, 15.0 mmol), dissolved in
50 mL of dry DMF, were added dropwise to a solution of potas-
sium tert-butoxide (4.2 g, 38.0 mmol) in 100 mL of dry DMF. The
mixture was stirred at room temperature for 1 h and then poured
onto 100 g of crushed ice. A yellowish precipitate was formed; this
1,2,4,5-Tetrakis[(E)-2-(3,5-dimethoxyphenyl)ethenyl]benzene (14a):
Tetraphosphonate 9c (0.6 g, 0.9 mmol) in 100 mL of dry dimethoxy- was separated by filtration and washed with 100 mL of water. Yield
Eur. J. Org. Chem. 2001, 2757Ϫ2763 2761

J. Gerold, U. Holzenkamp, H. Meier
FULL PAPER
4.18 g (90%), pale yellow crystals, m.p. 162Ϫ165 °C. Ϫ IR (KBr): imposed). Ϫ MS (FD): m/z (%) ϭ 582 (100) [Mϩ·]. Ϫ C38H63NO3
ν ϭ 2950 cmϪ1, 1590, 1575, 1450, 1430, 1330, 1300, 1260, 1245, (581.9): calcd. C 78.43, H 10.91, N 2.41; found C 78.56, H 10.80,
˜
1
1065, 960, 940, 840, 820. Ϫ H NMR (CDCl3): δ ϭ 3.82 (s, 12 H, N 2.37.
OCH3), 6.39 (t, 2 H, aromat. H), 6.67 (d, 4 H, aromat. H), 7.06 (s,
General Procedure for the Preparation of the Squaraines 1؊6: The
4 H, olefin. H), 7.49 (s, 4 H, aromat. H, central ring). Ϫ 13C NMR
corresponding resorcinol (12bϪ15b, 17b, or 22) and the appropriate
(CDCl3): δ ϭ 55.4 (OCH3), 100.1, 104.6, 126.9 (aromat. CH),
amount of compound 21, dissolved in toluene/1-butanol (3:1), were
128.6, 128.8 (olefin. CH), 136.6, 139.4, 161.0 (aromat. Cq). Ϫ MS
refluxed for 4 h (unless otherwise stated). The progress of the reac-
(FD): m/z (%) ϭ 403 (100) [Mϩ·]. Ϫ C26H26O4 (402.5): calcd. C
tion was monitored by Vis/NIR spectroscopy every 30 min. Water
77.59, H 6.51; found C 77.60, H 6.57.
of condensation was removed by a filter containing anhydrous
NaSO4 between flask and reflux condenser. The reaction mixture
was finally allowed to cool to room temperature, the solvent was
removed, and the residue treated as described below.
5-[(E)-2-{4-[(E)-2-(3,5-Dihydroxyphenyl)ethenyl]phenyl}ethenyl]-1,3-
benzenediol (17b): Preparation analogous to that of 12b. The crude
product was purified by column filtration (8 ϫ 10 cm, silica gel,
acetone) and recrystallized from acetone/chloroform (1:1). Yield
Squaraine 1: The crude product was washed with n-hexane and hot
acetone. Yield 22%, metal-like, blue crystals, m.p. 220 °C (de-
comp.). Ϫ UV/Vis (CHCl3): λmax ϭ 778 nm/εmax ϭ 2.58·108
cm2·molϪ1, λmax ϭ 710 nm/εmax ϭ 1.20·108 cm2·molϪ1. Ϫ IR
˜
31%, light yellow crystals, m.p. Ͼ 300 °C. Ϫ IR (KBr): ν ϭ 3350
cmϪ1, 1600, 1580, 1340, 1140, 1005, 985, 960, 840. Ϫ 1H NMR
([D6]acetone): δ ϭ 6.30 (t, 2 H, aromat. H), 6.59 (d, 4 H, aromat.
H), 7.10 (s, 4 H, olefin. H), 7.56 (s, 4 H, aromat. H, central ring),
8.23 (br. s, 4 H, OH). Ϫ 13C NMR ([D6]acetone): δ ϭ 103.3, 106.1,
127.7 (aromat. CH), 128.8, 129.7 (olefin. CH), 137.7, 140.4, 159.6
(aromat. Cq). Ϫ MS (FD): m/z (%) ϭ 692 (100) [2Mϩ]; 346 (20)
[Mϩ·]. Ϫ C22H18O4 (346.4): calcd. C 76.29, H 5.24; found C 76.50,
H 5.20.
(KBr): ν ϭ 2920 cmϪ1, 1590, 1425, 1395, 1325, 1185, 1165, 1040,
˜
1
840, 780. Ϫ H NMR (CDCl3): δ ϭ 0.86 (t, 24 H, CH3), 1.25 (m,
80 H, CH2), 1.86 (m, 4 H, CH), 6.52 (s, 4 H, aromat. H), 6.81, 8.22
(AAЈBBЈ, 8 H, aromat. H), 7.05 (s, 2 H, olefin. H), 12.20 (s, 4 H,
OH). Ϫ 13C NMR (CDCl3): δ ϭ 13.7 (CH3), 22.6, 26.4, 29.6, 31.6,
31.7 (CH2), 37.0 (CH), 57.4 (NCH2), 107.8, 114.5, 134.9 (aromat.
CH), 109.7, 118.7, 148.5, 156.1, 163.0 (aromat. Cq), 132.8 (olefin.
CH), 175.8, 179.7, 182.9 (Cq, four-membered ring). Ϫ MS (FD):
m/z (%) ϭ 1376 (100) [M ϩ 4 H]ϩ·. Ϫ C90H134N2O8 (1372.1): calcd.
C 78.79, H 9.84, N 2.04; found C 78.40, H 9.70, N 2.33.
Synthesis of the Semisquaric Acid 21: 3,4-Dichloro-3-cyclobutene-
1,2-dione (squaric acid dichloride) (18) was prepared according to
the literature.[1b]
3-{4-[Bis(2-hexyloctyl)amino]phenyl}-4-chloro-3-cyclobutene-1,2-
dione (20): A solution of N,N-bis(2-hexyloctyl)aniline (19)[6d] (9.7 g,
20.0 mmol) and 3,4-dichloro-3-cyclobutene-1,2-dione (18) (3.0 g,
20.0 mmol) in 15 mL of dichloromethane was added dropwise to a
boiling suspension of aluminium trichloride (2.7 g, 20.0 mmol) in
30 mL of dichloromethane. The reaction mixture was stirred at re-
flux for 1 h, allowed to cool to room temperature, and then poured
into ice-cold water. The organic layer was separated, washed with
water, and dried with MgSO4. The solvent was removed, and the
residue purified by column chromatography [10 ϫ 35 cm, silica gel,
petroleum ether (40Ϫ70 °C)]. Yield 4.2 g (35%), red crystals, m.p.
58Ϫ62 °C. Ϫ IR (KBr): ν˜ ϭ 2940 cmϪ1, 2900, 2835, 1785, 1760,
1735, 1590, 1550, 1490, 1470, 1400, 1345, 1320, 1270, 1190, 1120,
1035, 830. Ϫ 1H NMR (CDCl3): δ ϭ 0.86 (t, 12 H, CH3), 1.24 (m,
40 H, CH2), 1.84 (m, 2 H, CH), 3.32 (d, 4 H, NCH2), 6.71, 8.08
(AAЈBBЈ, 4 H, aromat. H). Ϫ 13C NMR (CDCl3): δ ϭ 14.1 (CH3),
22.6, 26.4, 29.7, 31.5, 31.8 (CH2), 35.9 (CH), 56.7 (NCH2), 112.4,
131.5 (aromat. CH), 113.8, 153.4 (aromat. Cq), 171.4, 185.9, 189.9,
196.0 (Cq, four-membered ring). Ϫ MS (FD): m/z (%) ϭ 600 (100)
[Mϩ·]. Ϫ C38H62ClNO2 (600.4): calcd. C 76.02, H 10.41, N 2.33;
found C 75.66, H 10.49, N 2.28.
Squaraine 2: The crude product was washed with n-hexane, hot
methanol, and acetone. Yield 29%, bronze, metal-like crystals, m.p.
Ͼ 300 °C. Ϫ UV/Vis (CHCl3): λmax ϭ 724 nm/εmax ϭ 4.56·108
cm2·molϪ1, λmax ϭ 672 nm/εmax ϭ 1.51·108 cm2·molϪ1. Ϫ IR
(KBr): ν˜ ϭ 2920 cmϪ1, 2840, 1590, 1405, 1315, 1290, 1185, 830,
780. Ϫ 1H NMR (CDCl3): δ ϭ 0.86 (t, 24 H, CH3), 1.25 (m, 80 H,
CH2), 1.85 (m, 4 H, CH), 3.43 (d, 8 H, NCH2), 6.51 (s, 4 H, aromat.
3
H), 6.79 (d, 4 H, aromat. H), 6.94, 7.20 (AB, J ϭ 16.1 Hz, 4 H,
olefin. H), 7.50 (s, 4 H, aromat. H, central ring), 8.20 (d, 4 H,
aromat. H), 12.22 (s, 4 H, OH). Ϫ 13C NMR (CDCl3): δ ϭ 14.0
(CH3), 22.6, 26.4, 29.6, 31.6, 31.7 (CH2), 36.9 (CH), 57.3 (NCH2),
107.2, 114.4 (aromat. CH), 111.5, 118.4 (aromat. Cq), 127.8
(aromat. CH, central ring), 128.8, 133.5 (olefin. CH), 134.6
(aromat. CH), 137.2, 149.6, 155.9, 162.9 (aromat. Cq), 174.9, 179.5,
182.9 (Cq, four-membered ring). Ϫ MS (FD): m/z (%) ϭ 1477 (100)
[M ϩ 3 H]ϩ Ϫ C98H140N2O8 (1474.2): calcd. C 79.85, H 9.57, N
1.90; found C 79.83, H 9.58, N 2.00.
Squaraine 3: Reaction time 5 h, the crude product was washed with
cold acetone and purified by column filtration (8 ϫ 10 cm, silica
gel, chloroform). Yield 49%, violet, metal-like crystals, m.p. 130 °C.
Ϫ UV/Vis (CHCl3): λmax ϭ 693 nm/εmax ϭ 8.11·108 cm2·molϪ1
,
max ϭ 638 nm/εmax ϭ 2.26·108 cm2·molϪ1. Ϫ IR (KBr): ν ϭ 2920
˜
3-{4-[Bis(2-hexyloctyl)amino]phenyl}-4-hydroxy-3-cyclobutene-1,2-
dione (21): Trifluoroacetic acid (5 mL) and hydrochloric acid (2 ,
5 mL) were added to compound 20 (2.0 g, 3.3 mmol), dissolved in
20 mL of dichloromethane and 5 mL of acetone. The reaction mix-
λ
cmϪ1, 2850, 1590, 1405, 1310, 1185, 1035, 780. Ϫ 1H NMR
(CDCl3): δ ϭ 0.86 (t, 36 H, CH3), 1.25 (m, 120 H, CH2), 1.86 (m,
6 H, CH), 3.42 (m, 12 H, NCH2), 6.53 (s, 6 H, aromat. H), 6.78
ture was refluxed for 2 d. After evaporation of the solvent, yellow (d, 6 H, aromat. H), 6.93, 7.19 (AB, 3J ϭ 16.1 Hz, 6 H, olefin. H),
crystals remained, and these were washed several times with petro-
7.48 (s, 3 H, aromat. H, central ring), 8.18 (d, 6 H, aromat. H),
leum ether (40Ϫ70 °C). Yield 1.4 g (74%), yellow crystals, m.p. 12.22 (s, 6 H, OH). Ϫ 13C NMR (CDCl3): δ ϭ 14.0 (CH3), 22.6,
192Ϫ195 °C. Ϫ IR (KBr): ν˜ ϭ 3200 cmϪ1, 2950, 2920, 2850, 1750, 26.4, 29.6, 31.6, 31.7 (CH2), 36.9 (CH), 57.3 (NCH2), 107.3
1695, 1560, 1460, 1415, 1190, 1020. Ϫ 1H NMR ([D6]DMSO/ (aromat. CH), 111.6 (aromat. Cq), 114.4 (aromat. CH), 118.5
CDCl3, 3:1): δ ϭ 0.81 (t, 12 H, CH3), 1.19 (m, 40 H, CH2), 1.78
(m, 2 H, CH), 3.26 (d, 4 H, NCH2), 6.72, 7.78 (AAЈBBЈ, 4 H,
(aromat. Cq), 126.2 (aromat. CH, central ring), 129.0, 133.0 (olefin.
CH), 134.6 (aromat. CH), 137.4, 149.6, 155.8, 163.2 (aromat. Cq),
aromat. H). Ϫ 13C NMR ([D6]DMSO): δ ϭ 13.8 (CH3), 22.0, 25.7, 175.4, 180.3, 182.8 (Cq, four-membered ring). Ϫ MS (MALDI-
29.1, 30.7, 31.2 (CH2), 34.9 (CH), 55.1 (NCH2), 112.2, 128.0 TOF): m/z (%) ϭ 2174 (100) [(M ϩ 2H)ϩ·]. Ϫ C144H207N3O12
(aromat. CH), 116.0, 150.5 (aromat. Cq), 173.4 (aryl-substituted (2172.3): calcd. C 79.62, H 9.61, N 1.93; found C 79.44, H 9.70,
Cq, four-membered ring), 194.3 (3 Cq, four-membered ring, super-
N 2.11.
2762
Eur. J. Org. Chem. 2001, 2757Ϫ2763

Bis-, Tris-, and Tetrakis(squaraines) Linked by Stilbenoid Scaffolds
FULL PAPER
[2] [2a]
K. Y. Law, Chem. Rev. 1993, 93, 449Ϫ486 and references
Squaraine 4: The residue was washed with hot n-hexane, methanol,
and cold acetone, and was further purified by column filtration (8
ϫ 10 cm, silica gel, toluene/ethyl acetate, 30:1). Yield 20%, metal-
[2b]
therein. Ϫ
Photogr. Sci. Eng. 1983, 27, 5Ϫ9. Ϫ
R. O. Loutfy, C. K. Hsiao, P. M. Kazmaier,
[2c]
M. Emmelius, G. Paw-
lowski, H. W. Vollmann, Angew. Chem. 1989, 101, 1475Ϫ1502;
Angew. Chem. Int. Ed. Engl. 1989, 28, 1445Ϫ1471.
like, violet crystals, m.p. 143Ϫ145 °C. Ϫ UV/Vis (CHCl3): λmax
ϭ
707 nm/εmax ϭ 3.93·108 cm2·molϪ1, shoulder at 652 nm (ε ϭ
[3] [3a]
C.-T. Chen, S. R. Marder, L. T. Cheng, J. Chem. Soc.,
2.73·108 cm2·molϪ1). Ϫ IR (KBr): ν ϭ 2920 cmϪ1, 1590, 1400,
˜
[3b]
Chem. Commun. 1994, 259Ϫ260. Ϫ
Marder, L. T. Cheng, J. Am. Chem. Soc. 1994, 116, 3117Ϫ3118.
[3c] G. J. Ashwell, G. Gefferies, D. G. Hamilton, D. E. Lynch,
M. P. S. Roberts, G. S. Bahra, C. R. Brown, Nature 1995, 375,
C.-T. Chen, S. R.
1
1315, 1290, 1185, 1035, 780. Ϫ H NMR (CDCl3): δ ϭ 0.86 (t, 48
H, CH3), 1.25 (m, 160 H, CH2), 1.86 (m, 8 H, CH), 3.42 (d, 16 H,
NCH2), 6.57 (s, 8 H, aromat. H), 6.79 (d, 8 H, aromat. H), 6.95,
Ϫ
[3d]
3
385Ϫ388. Ϫ
332Ϫ342.
L. S. Pu, ACS Sympos. Ser. 1991, 455,
7.56 (AB, J ϭ 16.1 Hz, 8 H, olefin. H), 7.79 (s, 2 H, aromat. H,
central ring), 8.22 (d, 8 H, aromat. H), 12.24 (s, 8 H, OH). Ϫ 13C
NMR (CDCl3): δ ϭ 14.0 (CH3), 22.6, 26.4, 29.7, 31.6, 31.8 (CH2),
36.9 (CH), 57.3 (NCH2), 107.6, 114.4, 134.7 (aromat. CH), 130.7,
132.0 (olefin. CH), 111.9, 118.6, 149.7, 155.8, 163.3 (aromat. Cq),
175.5, 180.6, 182.9 (Cq, four-membered ring); the signals of the
carbon atoms of the central ring are not detectable. Ϫ MS
[4] [4a]
V. Ghazarossian, J. S. Pease, M. W. L. Ru, M. Laney, T.
[4b]
L. Tarnowski, Eur. Pat. Appl. EP 356173 A2, 1990. Ϫ
E.
Terpetschning, H. Szmacinski, A. Ozinskas, J. R. Lakowicz,
[4c]
Anal. Biochem. 1994, 217, 197Ϫ204. Ϫ
B. Oswald, L. Pat-
senker, J. Duschl, H. Szmacinski, O. Wolfbeis, E. Terpetschn-
ing, Bioconjugate Chem. 1999, 10, 925Ϫ931.
H. Meier, Angew. Chem. 1992, 104, 1425Ϫ1446; Angew. Chem.
Int. Ed. Engl. 1992, 31, 1399Ϫ1420, and references therein.
ϭ 2872 (100) [(M ϩ Ϫ
2H)ϩ·].
[5]
(MALDI-TOF): m/z (%)
C190H274N4O16 (2870.3): calcd. C 79.51, H 9.62, N 1.95; found C
78.88, H 9.72, N 2.08.
[6] [6a]
H. Meier, U. Dullweber, Tetrahedron Lett. 1996, 37,
1191Ϫ1194. Ϫ [6b] H. Meier, U. Dullweber, J. Org. Chem. 1997,
Squaraine 5: The crude product was washed with petroleum ether
(40Ϫ70 °C), hot methanol, and diethyl ether. Yield 35%, dark vi-
[6c]
62, 4821Ϫ4826. Ϫ
H. Meier, R. Petermann, U. Dullweber,
[6d]
J. Prakt. Chem. 1998, 340, 744Ϫ754. Ϫ
H. Meier, R.
olet crystals, m.p. 175 °C (decomp.). Ϫ UV/Vis (CHCl3): λmax
ϭ
Petermann, J. Gerold, Chem. Commun. 1999, 977Ϫ978.
L. A. Wendling, S. K. Koster, J. E. Murray, R. West, J. Org.
Chem. 1977, 42, 1126Ϫ1130.
687 nm/εmax ϭ 7.23·108 cm2·molϪ1, λmax ϭ 637 nm/εmax ϭ 2.96·108
[7]
cm2·molϪ1. Ϫ IR (KBr): ν ϭ 2920 cmϪ1, 2850, 1590, 1405, 1325,
˜
[8] [8a]
1295, 1185, 1035, 830, 780. Ϫ 1H NMR (CDCl3), 0.86 (t, 48 H,
CH3), 1.25 (m, 160 H, CH2), 1.87 (m, 8 H, CH), 3.42 (d, 16 H,
NCH2), 6.51 (s, 8 H, aromat. H), 6.79 (d, 8 H, aromat. H), 6.91,
7.22 (AB, 3J ϭ 15.8 Hz, 8 H, olefin. H), 7.23, 7.43 (AAЈBBЈ, 16 H,
aromat. H), 8.21 (d, 8 H, aromat. H), 12.22 (s, 8 H, OH). Ϫ MS
K. Y. Law, F. C. Bailey, Can. J. Chem. 1993, 71, 494Ϫ505.
[8b]
Ϫ
H. Chen, W. G. Herkstroeter, J. Perlstein, K. Y. Law, D.
G. Whitten, J. Phys. Chem. 1994, 98, 5138Ϫ5146. H. Chen, M.
S. Farahat, K. Y. Law, D. G. Whitten, J. Am. Chem. Soc. 1996,
118, 2584Ϫ2594.
[9]
E. Alonso, D. J. Ramon, M. Yus, J. Org. Chem. 1997, 62,
(MALDI-TOF): m/z (%)
ϭ 3115 (100) [(M ϩ Ϫ
2H)ϩ·].
417Ϫ421.
There are very few characterizations of squaraines by 13C
NMR and mass spectra in the literature; see for example ref.[6b]
M. W. Reed, D. J. Pollart, S. T. Perri, L. D. Foland, H. W.
Moore, J. Org. Chem. 1988, 53, 2477Ϫ2482.
K. Liang, M. S. Farahat, J. Perlstein, K. Y. Law, D. G. Whitten,
J. Am. Chem. Soc. 1997, 119, 830Ϫ831.
H. Nakazumi, K. Natsukawa, K. Nakai, K. Isagawa, Angew.
Chem. 1994, 106, 1021Ϫ1023; Angew. Chem. Int. Ed. Engl.
1994, 33, 1001Ϫ1004.
[10]
[11]
[12]
[13]
C209H288N4O16 (3112.6): calcd. C 80.65, H 9.33, N 1.80; found C
80.41, H 9.22, N 1.90.
Model Squaraine 6: The crude product was washed with hot meth-
anol and hot n-hexane. Yield 43%, bronze, metal-like crystals, m.p.
173 °C. Ϫ UV/Vis (CHCl3): λmax ϭ 674 nm/εmax ϭ 2.70·108
cm2·molϪ1
, λmax ϭ 626 nm/εmax ϭ . Ϫ
0.88·108 cm2·molϪ1
IR(KBr): ν ϭ 3400 cmϪ1, 2920, 2850, 1590, 1550, 1400, 1320, 1285,
1190, 1030, 1000, 960, 840, 780, 750, 690. Ϫ 1H NMR (CDCl3):
δ ϭ 0.86 (t, 12 H, CH3), 1.25 (m, 40 H, CH2), 1.86 (m, 2H, CH),
3.43 (d, 4 H, NCH2), 6.52 (s, 2 H, aromat. H), 6.80 (d, 2 H, aromat.
˜
[14]
C. R. Chenthamarakshan, A. Ajayaghosh, Chem. Mater. 1998,
10, 1657Ϫ1663.
[15] [15a] C. R. Chenthamarakshan, J. Eldo, A. Ajayaghosh, Macro-
3
H), 6.92, 7.25 (AB, J ϭ 16.6 Hz, 2 H, olefin. H), 7.37Ϫ7.51 (m, 5
[15b]
molecules 1999, 32, 251Ϫ257. Ϫ
A. Ajayaghosh, C. R.
H, aromat. H), 8.21 (d, 2 H, aromat. H), 12.23 (s, 2 H, OH). Ϫ
13C NMR (CDCl3): δ ϭ 14.1 (CH3), 22.6, 26.4, 29.6, 31.5, 31.8
(CH2), 36.9 (CH), 57.3 (NCH2), 107.1, 114.4 (aromat. CH), 111.3,
118.4 (aromat. Cq), 127.3 (aromat. CH), 127.8, 134.0 (olefin. CH),
128.8, 128.8 (aromat. CH), 134.6 (aromat. CH), 136.5, 149.8, 155.7,
163.0 (aromat. Cq), 175.2, 179.8, 182.9 (Cq, four-membered ring).
Ϫ MS (FD): m/z (%) ϭ 779 (100) [(M ϩ 3 H)ϩ·]. Ϫ C52H73NO4
(776.2): calcd. C 80.47, H 9.48, N 1.80; found C 80.23, H 9.52,
N 1.82.
Chenthamarakshan, S. Das, M. V. George, Chem. Mater. 1997,
[15c]
9, 644Ϫ646. Ϫ
D. E. Lynch, U. Geissler, I. R. Peterson,
M. Floersheimer, R. Terbrack, L. F. Chi, H. Fuchs, N. J. Calos,
B. Wood, C. H. L. Kennard, G. J. Langley, J. Chem. Soc., Per-
kin Trans. 2 1997, 827Ϫ832; D. E. Lynch, I. R. Peterson, M.
Floersheimer, D. Essing, L. F. Chi, H. Fuchs, N. J. Calos, B.
Wood, C. H. L. Kennard, G. J. Langley, J. Chem. Soc., Perkin
Trans. 2 1998, 779Ϫ784. Ϫ [15d] S. J. Pomfret, A. P. Monkman,
E. E. Havinga, Synth. Met. 1996, 78, 285Ϫ288.
See also: K. Y. Law in Organic Photochemistry (Ed.: V. Rama-
murthy, K. Schanze, Marcel Dekker), New York, 1997, p.
519Ϫ584 and references therein.
H. Meier, M. Lehmann, Angew. Chem. 1998, 110, 666Ϫ669;
Angew. Chem. Int. Ed. 1998, 110, 643Ϫ645.
See also: F. Eiden, C. Gerstlauer, Arch. Pharm. 1981, 314,
995Ϫ1006.
[16]
Acknowledgments
[17]
[18]
We are grateful to the Deutsche Forschungsgemeinschaft, the
Fonds der Chemischen Industrie, and the Materialwissen-
schaftliche Forschungszentrum der Universität Mainz for finan-
cial support.
[19]
[20]
Commercially available.
See also: M. Ali, K. Kondo, Y. Tsuda, Chem. Pharm. Bull.
1992, 40, 1130Ϫ1136.
[1] [1a]
A. Treibs, K. Jacob, Angew. Chem. 1965, 77, 680Ϫ681; An-
[1b]
gew. Chem. Int. Ed. Engl. 1965, 4, 694Ϫ695. Ϫ
Schmidt, Synthesis 1980, 961Ϫ994.
A. H.
Received March 7, 2001
[O01110]
Eur. J. Org. Chem. 2001, 2757Ϫ2763
2763