Biscyclohexane-annulated diethyl dipyrrindicarboxylates: Observation of a dipyrrin form with absent visible absorption

Article abstract of DOI:10.1002/ejoc.201101729

A series of 2,3;7,8-biscyclohexano-fused diethyl 5-aryldipyrrin-1,9- dicarboxylates have been synthesized [aryl = phenyl, 3,4-dimethoxyphenyl and 2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecine-15-yl (m-benzo-15-crown-5)]. The obtain

Full text of DOI:10.1002/ejoc.201101729

FULL PAPER
DOI: 10.1002/ejoc.201101729
Biscyclohexane-Annulated Diethyl Dipyrrindicarboxylates: Observation of a
Dipyrrin Form with Absent Visible Absorption
Ksenia Tikhomirova,[a] Alexander Anisimov,[a] and Andrey Khoroshutin*[a]
Keywords: Conformation analysis / UV/Vis spectroscopy / Conjugation / N heterocycles / N ligands
A
series of 2,3;7,8-biscyclohexano-fused diethyl 5-aryl-
niques. Quantum chemical calculations have also been per-
formed, yielding two main conformations with proximate and
distant arrangements of the nitrogen atoms. In the latter con-
dipyrrin-1,9-dicarboxylates have been synthesized [aryl =
phenyl, 3,4-dimethoxyphenyl and 2,3,5,6,8,9,11,12-octahy-
dro-1,4,7,10,13-benzopentaoxacyclopentadecine-15-yl (m- formation the pyrrole–pyrrolene fragments were found to be
benzo-15-crown-5)]. The obtained compounds were shown
to exist in two equilibrium forms in solution, with one exhibit-
ing a visible absorption band (λabs 468–485 nm), and the
other, “vis-silent” form, having no absorption band in the vis-
ible region. The “vis-silent” form has been isolated and char-
acterized by NMR analysis, including two-dimensional tech-
nearly orthogonal, with no long chain of conjugation. Com-
putational results confirm the spectroscopic findings. Two
different forms were also characterized as their Mg2+ com-
plex and investigated by UV/Vis and by NMR spectroscopic
analyses and a clear difference was noted.
pose of this study was therefore the synthesis and investiga-
tion of new meso-substituted biscyclohexano-fused dipyrrin
derivatives, including a potentially ditopic receptor 2c, as
well as its “non-macrocyclic” and “π-neutral” analogues 2b
and 2a, respectively. In addition to being conformationally
restricted, dipyrrins of type 2 include an annelated cyclo-
hexane ring moiety that invokes high distortion of the por-
phyrins, which make them highly basic.[14] These features of
the studied substances could enhance their complexation
and/or sensor properties.
Introduction
Dipyrrins as their boron difluoride complexes
(BODIPY) have been known since the end of 1960s.[1] Their
high extinction coefficient and fluorescence properties facil-
itated their applications in areas such as, but not limited to,
biochemistry labeling[2] and laser dyes.[3] BODIPY com-
pounds have also been used as the signaling part of sensors
in a multitude of systems.[4,5] Metal-free dipyrrins were also
studied as ligands for prospective organometallic sys-
tems,[6,7] including those with potential sensor proper-
ties.[8–10]
Absorption and fluorescence properties of dipyrrins are
dependent on the type of substituents. These may enlarge
the π-system[9,11] leading to a redshift of the band maxima
in the optical spectra. Substituents may also turn on or off
various mechanisms for excited state relaxation.[4] For ex-
ample, meso-phenyl-substituted BODIPY dyes exhibit
much stronger fluorescence when a substituent is attached
at the pyrrole or the phenyl[12] ring that hinders rotation of
the phenyl group.
Because our group has been interested in the study of
brominated palladium tetrabenzoporphyrins,[13] we have ac-
cess to the key intermediate in their synthesis, i.e., tetra-
hydroisoindole carboxylic acid ester 1. This intermediate is
also a precursor of the corresponding dipyrrin.[6] The pur-
Recently, several accounts on cyclohexano-annulated
BODIPY compounds have appeared,[15] in addition to ear-
lier works devoted to bicyclo[2.2.2]octene derivatives by
Ono et al.[10] Garrido Montalblan et al. succeeded in the
synthesis of dipyrrin-1,10-dicarboxylate and its boron com-
plexes.[16] However, to the best of our knowledge, no report
on the sensing properties of dipyrrins themselves with the
cyclohexano-annulated pyrrole exist, with the exception of
a very recently released patent application by Cheprakov
and Vinogradov et al.[17]
[a] Chemistry Department, M. V. Lomonosov Moscow State
University,
Moscow 119991, Russia
Fax: +7-495-9328846
Results and Discussion
Starting dipyrromethanes were synthesized according to
the literature procedure.[18] Oxidation of dipyrromethanes
E-mail: khorosh@petrol.chem.msu.ru
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101729.
Eur. J. Org. Chem. 2012, 2201–2207
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K. Tikhomirova, A. Anisimov, A. Khoroshutin
FULL PAPER
to dipyrrins by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone amorphous substance that gave a colorless solution show-
(DDQ) proceeded smoothly for compounds with donor ing only the Set II signals (Figure 1, b). Conversely, ad-
aryl substituents in the meso positions. Compounds 4b and dition of trifluoroacetic acid to the solution followed by
4c were consumed in two hours to form pure 2b and 2c, its neutralization with triethylamine produced the spectrum
respectively (Scheme 1). However, 10% of the unsubstituted with only Set I (Figure 1, c).
phenyl compound 4a remained unaffected after three hours
1H NMR spectra of 2 had clear signals that could be
reaction; longer reaction times resulted in partial oxidation attributed to the aromatic rings and OCH2 groups accord-
of the cyclohexane ring (according to NMR and MALDI ing to their positions, multiplicity and integral intensities.
analyses). Thus, pure 2a was obtained after the residual 4a For example, OCH2CH3 quadruplets appeared at δ = 4.25
was removed from the reaction mixture by washing with and 4.37 ppm for the 2c Set II and Set I components.
hexane.
Crown ether signals in Set II were systematically shifted up-
field with respect to those of Set I. NMR analysis of the
phenyl region also reveal an upfield shift of H-16 and H-17
in Set II, whereas its H-14 resonance was placed 0.08 ppm
downfield compared to the same proton in Set I. NOESY
experiments (see the Supporting Information, Figures S6d
and S7a) confirmed these assignments.
Signals of methylene groups from the cyclohexane rings
(Figure 2) were assigned by COSY (see the Supporting In-
formation, Figures S4 and S5) experiments. They show
more pronounced differences; i.e., the Set II signals appear
at δ = 2.79, 1.66, 1.54–1.58 and 1.97 ppm, which are quite
close to those of dipyrromethane 4c (δ = 1.62–1.68, 2.13
and 2.77 ppm; see Figure 2). However, Set I features two
upfield shifted resonances at δ = 1.37 ppm (overlapping
with the CH3 of the ethoxycarbonyl) and 1.62 ppm,
whereas the signals at δ = 1.59 and 2.68 ppm, resembling
their analogues in the Set II, were found.
Scheme 1. The synthesis of dipyrrins.
1H NMR spectroscopic analysis of the pure samples of
2a–c revealed two sets of signals (denoted as “Set I” and
“Set II”, Figure 1) for each chemical group. In different ex-
periments, the isolated product contained these signals in
arbitrarily different integral intensity ratios. However, in the
case of 2c, this ratio reached ca. 7:1 after standing in CHCl3
solution.
Figure 2. 1H NMR spectra of the cyclohexane ring region of 2c.
(a) Isolated sample; (b) the Set II component; (c) after CF3COOH/
Et3N treatment (Set I); (d) the same region for 4c.
Both the optical spectra and the color of Set I and
Set II solutions were strikingly different (Figure 3). Set II,
the “colorless” solution, gave rise to only a UV band in
the UV/Vis spectrum, whereas Set I, the “red” component,
exhibited an intense band at 472 nm (CH3CN) in the visible
region. Addition of acid to the solutions caused the instant
emergence of a band at 524 nm. These values are close to
1
Figure 1. H NMR spectra of 2c in the CH2O region. (a) Isolated
sample (mixture of the sets); (b) after slow crystallization from hex-
anes (Set II); (c) after treatment with CF3COOH/Et3N (Set I).
We succeeded in isolating one of the components and those of protonated and nonprotonated dipyrrins.[19] Com-
1
1
obtained samples possessing just one set in their H NMR pounds 2a and 2b show the same H NMR (Table 1) and
spectra. Slow crystallization from hexane yielded a pink, UV/Vis (Table 2) features.
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Dipyrrin Conformations
The components differ in their physical properties; for
example, the Set II (colorless) component of all the studied
mixtures were better soluble in hexane than the Set I (red)
component. In contrast, solubility in acetonitrile showed
the opposite trend; for example, washing 2a with hexane
left primarily the “red” component (14:1 “red” to “color-
less” ratio, NMR), which gradually reverted to a 2:1 equi-
librium ratio in solution (the ratio for 2b was 5:1). The com-
ponents could also be discriminated by TLC (see the Exp.
Section). In addition, after being separated along one direc-
tion in 2D TLC, each spot gave rise to a second pair of the
same spots along the second direction.
Kinetics of the interconversion between 2cI (the Set I
component) and 2cII (Set II component) was measured at
300 K. The rate constant of the 2cI to 2cII reaction and
that of the reverse have been estimated to be 7ϫ10–6 and
1ϫ10–6 s–1, respectively (see the Supporting Information).
The data may be interpreted as the interconversion be-
tween two different conformations of each species; indeed,
such transformations have long been known[20] [Note: In
view of fast degenerate proton migration that causes a
switch between two double bond arrangements in the dipyr-
rin, we refer to these spatial transformations as conforma-
tional changes. Rotation about the meso-pyrrole single bond
in one arrangement becomes E–Z isomerization of the
meso-pyrrolene double bond in the other. Thus, we will use
the term “conformations”, meaning different positions of
the tetrahydroisoindole fragments with respect to each
other. The double bond arrangement also formally influ-
ences the name of the ring annelated to the pyrrole/pyr-
rolene moieties (“cylcohexano” or “cyclohexeno”). For the
sake of clarity we refer these rings to as “cyclohexano”.]
This interconversion has been the subject of theoretical cal-
culations.[21] Furthermore, a series of thorough studies of
biliverdin and its synthetic analogues have been performed
in relation to their helical structure.[22] Dipyrrinone deriva-
tives have been shown to form the E isomer depending on
the substituent at the carbon[23] or at the nitrogen atoms.[24]
The observed spectra possess two significant features.
First, the absence of the absorption band in the visible re-
gion for one of the conformers leads to the assumption that
there is no efficient overlap between pyrrolic chromophores
in the molecule. Second, the number of NMR signals in
both “colorless” and “red” conformers correspond to effec-
tively symmetrical structures, i.e., each group in the tetra-
Figure 3. UV/Vis spectra of 2c (in CH3CN) for samples exhibiting
Set I (dashed line) or Set II (solid line) resonances in the NMR,
and Set II after addition of excess HClO4 (dotted-dashed line).
1
Table 1. H NMR spectra of compounds 2a–c.
Component
1H NMR[a]
other
[d]
Ph[b]
OCH2
CH3
cyclo-C6
[c]
OCH2
2a
red
7.24-
7.27,
7.43-
7.45
7.28-
7.30,
7.34-
7.36
4.38,
1.41
–
2.68, 1.49–
1.52, 1.35,
1.58
(2:1)[e] colorless
4.24,
1.30
–
2.79, 1.66,
1.53–1.59,
1.96
2b
(5:1)
red
6.76,
6.82,
6.92
6.74,
6.82,
6.92
6.75,
6.79,
6.90
6.87,
6.73,
6.80
4.38,
1.41
3.85, 3.95
3.88, 3.79
2.70, 1.59,
1.39, 1.64
colorless
red
4.25,
1.31
2.80, 1.64,
1.58, 2.01
2c
(7:1)
4.37,
1.40
4.19, 4.09,
3.96, 3.90,
3.78–3.80
4.12, 4.03,
3.90, 3.86,
3.74–3.75
2.68, 1.59,
1.37, 1.62
colorless
4.25,
1.31
2.79, 1.66,
1.54–1.58,
1.97
[a] Signals of cyclohexane methylene groups were attributed by
COSY experiments (see the Supporting Information, Figures S1–
S5); those of the phenyl groups, CH3CH2 of ethoxycarbonyls, and
crown ether signals were assigned on the basis of their shapes, their
characteristic spin systems, and by relative integral intensities. [b]
Given in the order H-14, H-16, H-17 for 3-substituted rings. [c]
CH2 groups being closer to the phenyl ring are listed first; those
being equally close are listed in order R1 then R2. [d] In the order
H-4, H-5, H-6, H-7. [e] Equilibrium ratio (at room temperature) of
the components according to NMR integrals of OEt quadruplets.
1
hydroisoindole fragments yield only one type of H NMR
resonance.
To gain insight into the possible orientation of different
parts of the molecules, we performed geometry optimiza-
tion of unsubstituted dimethyl ester 2d at the HF/6-31(d)
level of theory. The results of the computation yields non-
symmetrical structures (see Figure 4, and the Supporting
Information Table S2). Both of these are featured by the in-
plane arrangement of the ester groups with respect to the
pyrrole and pyrrolene rings. The dipyrrin moiety in the cis-
syn conformation (Figure 4, a, and the Supporting Infor-
mation Table S2) is nearly flat and, therefore, has the ex-
pected overlap of a pyrrole–pyrrolene π-system. In contrast,
Table 2. UV/Vis spectra of compounds 2a–c.
Component
2a
2b
2c
mixture
(“set I”/“set II” ca 2:1)
mixture
(“set I”/“set II” ca 4:1)
mixture (“set I”
“set II” ca. 4:1)
colorless (“set II”)
264 (1),
468 (0.65)[a]
283 (1),
473 (0.10)
267 (1),
472 (0.44)
284
[a] Relative intensity.
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K. Tikhomirova, A. Anisimov, A. Khoroshutin
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the trans-anti conformation has a dihedral angle between have been reported by Dolphin et al.[27] X-ray data and
the planes of the pyrrole and pyrrolene rings equal to 69.9 ° spectroscopic studies in solution indicate that efficient over-
(Figure 4, b, and the Supporting Information Table S2), lap between the pyrrole and pyrrolene moieties exists. How-
precluding efficient overlap.
ever, an approximately 30° angle between the pyrrole–
pyrrolene planes is observed for the cis-anti conformation
due to steric hindrance caused by C- and N-bonded hydro-
gen atoms. Hindrance caused by the cyclohexane rings in
the compounds studied in this paper is much more severe,
which causes considerable deviation of the fragments from
the main dipyrrin plane and much slower interchange rate,
making the conformers visible in solution at room tempera-
ture.
The closest analogues of the studied substances are di-
pyrrins annulated to cyclic unsaturated[9] or bicyclic frag-
ments,[10] which have been used as precursors for new,
highly emissive fluorophores. However, no similar confor-
mational transformations have been mentioned.
UV/Vis and NMR Complexation Studies
We also performed some experiments to examine the re-
ceptor properties of 2cI and 2cII. The ratio of the compo-
nents gradually changes with time and/or possibly with the
addition of the analyte. Thus, only qualitative changes of
the UV/Vis spectra can be discussed. To test these proper-
ties, Mg2+ was chosen for two reasons: (1) Mg salts are
known to form complexes with both dipyrrin and crown
Figure 4. Results of geometry optimization at the HF/6-31(d) level
for the conformers of (a) 2dI, (b) 2dII (for details see the Support-
ing Information), and a corresponding schematic representation of
their degenerate spatial reorganization in (c) 2dI and (d) 2dII.
The effective symmetry observed for each conformation
can be the result of proton exchange. Despite the cis-syn
conformations being slightly twisted in solution,[25] such ex-
change is very efficient both in solution[20] and in the crys-
tal.[26] Effective symmetry for the trans-anti-2 demands that
the proton migration should be accompanied by substantial
rotation of each of the pyrrole–pyrrolene fragments (Fig-
ure 4).
The computational results are in accord with the NMR
experiments. NOESY spectra reveal clear cross-peaks be-
tween the signals of the NH-protons and the phenyl ortho-
protons for “colorless”, trans-anti conformer 2cII (see the
Supporting Information, Figure S6a). In addition, ortho-
protons are close to one of the methylene groups of the
cyclohexane ring, which is also manifested by the corre-
sponding cross-peak (see the Supporting Information, Fig-
ure S6c).
Furthermore, protons of the methylene groups in the
“red”, cis-syn form 2cI are in the anisotropy region of the
1
phenyl substituent, resulting in an upfield shift of their H
NMR signals. No such upfield resonances are observed for
2cII; its cyclohexane ring resonances are reasonably close
to those observed for 4c (see above).
The nearly orthogonal arrangement of the pyrrole–
pyrrolene fragments, and the absence of a visible absorption
in the studied compounds is a novel feature of dipyrrins
that has not been previously observed. For instance, three
Figure 5. Changes in the absorption spectra of 2c in CH3CN upon
addition of Mg(ClO4)2. (a) To the mixture of 2cI and 2cII ca. 2:1
(c = 2.5ϫ10–7 m): before addition of Mg(ClO4)2 (dashed line), after
addition of twofold excess of Mg(ClO4)2 (dotted-dashed line), after
addition of 300-fold excess of Mg(ClO4)2 (solid line). (b) 2cII: be-
fore addition of Mg(ClO4)2 (dotted line), after addition of ca. 50-
conformations (cis-syn, cis-anti and trans-anti) of dipyrrins fold excess of Mg(ClO4)2 (solid line).
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Dipyrrin Conformations
performed by using the Gaussian 03[29] program with a 6-31(d)
Hamiltonian. For details, see the Supporting Information.
ether fragments;[6,28] (2) we found that these salts do not
cause fast conformer interconversion.
Addition of magnesium perchlorate to solutions of 2cI
and 2cII in acetonitrile led to the following changes in the
visible region of the spectrum (Figure 5). An isosbestic
point was apparent at low concentration of the salt, indicat-
ing that the site of complexation and dipyrrin conformation
are invariable, i.e., the dye forms the only complex with the
metal ion. Absorption maximum is, as expected, shifted to
the red region. As the concentration of the salt was in-
creased, however, the development of new bands was seen
at even longer wavelengths. Under these conditions, the
isosbestic point disappears, indicating additional complex-
ation processes. In contast, no substantial change in the
UV/Vis spectrum of the 2cII solution was found upon ad-
dition of the Mg2+ salt.
General Procedure for the Preparation of the Dipyrromethanes: Al-
dehyde (1 equiv.), ethyl 4,5,6,7-tetrahydro-2H-isoindole-4-carboxyl-
ate 1 (2 equiv.) and tetrabutylammonium chloride (0.15 equiv.)
were dissolved in distilled CH2Cl2 and degassed with a stream of
Ar for 30 min. Freshly distilled BF3·Et2O (0.2 equiv.) was then
added by using a syringe and the solution was stirred under Ar at
room temperature for 6 h until no more starting aldehyde could be
detected by TLC analysis (TLC spots were visualized by 0.5% 2,4-
dinitrophenylhydrazine/2 m HCl, eluent CH2Cl2 or EtOAc). The re-
action mixture was washed with 10% aqueous Na2CO3, water, and
brine, and dried with Na2SO4. Removal of the solvent under vac-
uum gave a solid that was recrystallized from hexane or purified
by column chromatography.
Diethyl 3,3Ј-(Phenylmethylene)bis(4,5,6,7-tetrahydro-2H-isoindole-
1-carboxylate) (4a): Prepared according the general procedure with
NMR studies on the complex formation corroborate the
optical spectroscopic findings. They reveal that chemical
benzaldehyde (0.26 mL, 2.59 mmol, freshly distilled),
1 (1 g,
5.18 mmol), Bu4NCl (0.108 g, 0.39 mmol), and BF3·Et2O
shifts of the crown ether and phenyl protons of both 2cI (0.066 mL, 0.52 mmol) in CH2Cl2 (50 mL). Removal of the solvent
gave a light-orange solid. The product was recrystallized from hex-
ane, giving 4a (0.91 g, 74% yield) as a white solid; m.p. 158–159 °C.
1H NMR (CDCl3, 400 MHz): δ = 1.27 (t, J = 7.1 Hz, 6 H, -CH3),
1.63–1.70 [m, 8 H, -CH2(CH2)2CH2-], 2.17 [m, 4 H, -CH2(CH2)2-
CH2-], 2.77 [m, 4 H, -CH2(CH2)2CH2-], 4.19 (q, J = 7.1 Hz, 4 H,
-OCH2CH3), 5.39 (s, 1 H, CH), 7.08 (m, 2 H, ArH), 7.24–7.31 (m,
3 H, ArH), 8.59 (br. s, 2 H, NH) ppm. 13C NMR (CDCl3,
100 MHz): δ = 14.49, 21.26, 23.16, 23.33, 40.62, 59.71, 116.77,
119.75, 127.26, 128.20, 128.89, 129.22, 130.78, 139.04, 161.76 ppm.
LDI-TOF: m/z = 473.09 [M – H]+, 497.06 [M + Na]+, 513.00 [M
+ K]+. C29H34N2O4 (474.59): calcd. C 73.39, H 7.22, N 5.90; found
C 73.09, H 7.02, N 5.41.
and 2cII undergo a substantial change upon the addition
of Mg(ClO4)2. However, the change of the proton reso-
nances of the cyclohexane moiety indicates that only the
dipyrrin system of 2cI is involved in complex formation (see
the Supporting Information and Figure S9).
Conclusions
Representatives of 2,3;7,8-biscyclohexano-fused diethyl
5-aryldipyrrin-1,9-dicarboxylates have been synthesized,
namely, one possessing an unsubstituted phenyl group (Ph)
and derivatives substituted with π-donors. Two conforma-
tions have been observed for all three compounds in solu-
tion. The first, “regular” conformer exhibits a visible band
at ca. 470 nm, whereas the second, “vis-silent” form has no
visible band in the optical spectrum. Steric hindrance has
been shown to be responsible for disrupting the conjugation
in the “vis-silent” conformation. These forms have different
complexation behavior with respect to magnesium ions.
Diethyl
3,3Ј-[(3,4-Dimethoxyphenyl)methylene]bis(4,5,6,7-tetra-
hydro-2H-isoindole-1-carboxylate) (4b): Prepared according the ge-
neral procedure with 3,4-dimethoxybenzaldehyde (0.2144 g,
1.29 mmol),
1
(0.4986 g, 2.58 mmol), Bu4NCl (0.0528 g,
0.19 mmol), and BF3·Et2O (0.033 mL, 0.26 mmol) in CH2Cl2
(25 mL). Removal of the solvent gave an orange solid. The product
was purified by chromatography on silica gel (CH2Cl2); yield 0.64 g
(93%); m.p. 110–112 °C. 1H NMR (CDCl3, 400 MHz): δ = 1.28 (t,
J = 7.0 Hz, 6 H, -OCH2CH3), 1.63–1.72 [m, 8 H, -CH2(CH2)2CH2-
], 2.16 [m, 4 H, -CH2(CH2)2CH2-], 2.77 [m, 4 H, -CH2(CH2)2CH2-
], 3.77 (s, 3 H, -OCH3), 3.86 (s, 3 H, -OCH3), 4.20 (q, J = 7.0 Hz,
4 H, -OCH2CH3), 5.33 (s, 1 H, CH), 6.60 (m, 2 H, ArH), 6.79 (d, J
= 8.4 Hz, 1 H, ArH), 8.58 (br. s, 2 H, NH) ppm. 13C NMR (CDCl3,
100 MHz): δ = 14.52, 21.25, 23.14, 23.32, 40.58, 55.88, 55.93, 59.71,
111.47, 116.55, 119.62, 120.43, 129.31, 130.82, 131.15, 148.30,
149.32, 161.69 ppm. LDI-TOF: m/z = 533.04 [M – H]+, 557.01 [M
+ Na]+, 572.94 [M + K]+. C31H38N2O6 (534.64): calcd. C 69.64, H
7.16, N 5.24; found C 69.19, H 7.09, N 4.68.
Experimental Section
General: Unless otherwise noted, all materials were obtained from
commercial suppliers and used without further purification. 1H
(400 MHz, HMDS as internal standard) and 13C (100 MHz,
CDCl3 as internal standard) NMR spectra were obtained with a
Bruker Avance 400 instrument. 2D NOESY spectra were measured
with a Bruker Avance 600 spectrometer. CDCl3 was kept over dehy-
drated molecular sieves for at least 48 h prior to spectra acquisition.
Elemental analyses were performed by the Laboratory of Micro-
analysis of the Chemistry Department of Moscow State University.
Melting points were obtained with a capillary melting point appa-
ratus Mel-TempII in open-ended capillaries and are uncorrected.
The UV/Vis spectra were recorded with an Agilent-8453 spectro-
photometer. Laser desorption ionization (LDI-TOF) Mass spectra
were recorded with a Bruker Daltonics Autoflex II. Samples were
irradiated with a nitrogen laser (λ = 337 nm) and an accelerating
voltage of 19 kV. The theoretical calculation of intensity distribu-
tion of isotope peaks was performed using the program Iso-
topeViewer Version 1.0. Quantum mechanical calculations were
Diethyl 3,3Ј-(2,3,5,6,8,9,11,12-Octahydro-1,4,7,10,13-benzopenta-
oxacyclopentadecin-15-ylmethylene)bis(4,5,6,7-tetrahydro-2H-isoind-
ole-1-carboxylate) (4c): Prepared according the general procedure
with 2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclo-
pentadecine-15-carbaldehyde (0.3052 g, 1.03 mmol), 1 (0.3981 g,
2.06 mmol), Bu4NCl (0.0417 g, 0.15 mmol), and BF3·Et2O
(0.027 mL, 0.21 mmol) in CH2Cl2 (25 mL). Removal of the solvent
gave an orange solid. The product was purified by chromatography
on silica gel (EtOAc); yield 0.47 g (69%); m.p. 118–119 °C. 1H
NMR (CDCl3 , 400 MHz): δ = 1.30 (t, J = 7.1 Hz, 6 H,
-OCH2CH3), 1.62–1.68 [m, 8 H, -CH2(CH2)2CH2-], 2.13 [m, 4 H,
-CH2(CH2)2CH2-], 2.77 [m, 4 H, -CH2(CH2)2CH2-], 3.74 (m, 8 H,
Eur. J. Org. Chem. 2012, 2201–2207
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2205

K. Tikhomirova, A. Anisimov, A. Khoroshutin
FULL PAPER
-OCH2-), 3.86 (m, 2 H, -OCH2-), 3.90 (m, 2 H, -OCH2-), 4.03 (m,
7.1 Hz, -OCH2CH3 of 2bII), 1.41 [m, J = 7.1 Hz, -OCH2CH3 and
2 H, -OCH2-), 4.11 (m, 2 H, -OCH2-), 4.23 (q, J = 7.1 Hz, 4 H, -CH2(CH2)2CH2- of 2bI], 1.56–1.64 [m, -CH2(CH2)2CH2- of 2bI
-OCH2CH3), 5.28 (s, 1 H, CH), 6.60 (m, 2 H, ArH), 6.79 (d, J =
8.8 Hz, 1 H, ArH), 8.32 (br. s, 2 H, NH) ppm. 13C NMR (CDCl3,
100 MHz): δ = 14.52, 21.21, 23.08, 23.28, 40.51, 59.71, 68.54, 68.62,
and 2bII], 2.01 [m, -CH2(CH2)2CH2- of 2bII], 2.70 [m, -CH2-
(CH2)2CH2- of 2bI], 2.80 [m, -CH2(CH2)2CH2- of 2bII], 3.79 (s,
-OCH3 of 2bII), 3.85 (s, -OCH3 of 2bI), 3.88 (s, -OCH3 of 2bII),
69.05, 69.13, 70.18, 70.60, 113.76, 113.82, 116.54, 119.62, 121.15, 3.95 (s, -OCH3 of 2bI), 4.25 (q, J = 7.1 Hz, -OCH2CH3 of 2bII),
129.38, 130.65, 131.82, 147.90, 148.92, 161.63 ppm. LDI-TOF: m/z 4.38 (q, J = 7.1 Hz, -OCH2CH3 of 2bI), 6.74–6.76 (m, ArH of 2bI
= 663.14 [M – H]+, 687.13 [M + Na]+. C37H48N2O9 (664.79): calcd.
C 66.85, H 7.28, N 4.21; found C 66.61, H 7.15, N 4.10.
and 2bII), 6.81–6.83 (m, ArH of 2bI and 2bII), 6.92 (m, ArH of
2bI and 2bII), 8.71 (br. s, NH of 2bII), 12.73 (br. s, NH of 2bI) ppm.
13C NMR (CDCl3, 100 MHz): δ (mixture of isomers 2bI and 2bII
in a 7:1 ratio) = 14.33, 22.38, 23.07, 23.15, 24.36, 55.83, 56.05,
60.54, 111.12, 111.96, 121.84, 129.26, 132.97, 139.21, 139.48,
141.59, 144.21, 149.28, 149.86, 162.27 ppm (signals of isomer I only
observed in 13C NMR spectrum if the conformer ratio is higher
than 6:1). 13C NMR (CDCl3, 100 MHz): δ (mixture of isomers 2bI
and 2bII in a 3:1 ratio) = 14.33, 14.42, 21.92, 22.38, 22.93, 23.07,
23.15, 23.21, 23.34, 24.36, 55.79, 55.81, 55.83, 56.05, 59.81, 60.54,
110.08, 110.59, 111.12, 111.96, 116.09, 119.44, 119.69, 121.84,
128.77, 128.82, 129.26, 132.97, 133.16, 134.51, 139.21, 139.48,
141.59, 144.21, 148.86, 148.92, 149.28, 149.86, 161.69, 162.27 ppm.
Rf = 0.9 (2bI), 0.3 (2bII) (CHCl3, Al2O3). LDI-TOF: m/z = 533.04
[M + H]+, 555.01 [M + Na]+, 570.97 [M + K]+. C31H36N2O6
(532.63): calcd. C 69.90, H 6.81, N 5.26; found C 69.74, H 6.89, N
4.85.
General Method for the Preparation of the Dipyrrins: The dipyrro-
methane (1 equiv.) was dissolved in anhydrous THF and degassed
with a stream of Ar for 20 min. A solution of DDQ (2 equiv.) in
anhydrous THF was then added by using a syringe. The solution
was stirred under Ar at room temperature for 2 h, then the reaction
mixture was diluted with CH2Cl2, washed with 10 % aqueous
Na2SO3, 10% aqueous Na2CO3, water, and brine. The organic layer
was separated, dried with Na2SO4, and the solvents were evapo-
rated to dryness.
Ethyl 1-{[3-(Ethoxycarbonyl)-4,5,6,7-tetrahydro-2H-isoindol-1-yl]-
(phenyl)methylene}-4,5,6,7-tetrahydro-1H-isoindole-3-carboxylate
(2a): Compound 4a (0.100 g, 0.211 mmol) in THF (10 mL) and
DDQ (0.0958 g, 0.422 mmol) in THF (2 mL) were reacted accord-
ing to the general procedure for 3 h,giving mixture of 2aI, 2aII,
and the initial 4a in a ratio of 9:1:1 according to NMR spec-
troscopy. After washing with hexane (50 mL), a mixture of 2aI,
2aII (0.062 g, 0.13 mmol, 62%) was isolated as an orange powder.
1H NMR (CDCl3, 400 MHz): δ (mixture of isomers 2aI and 2aII
in a ratio of 12:1; integral intensities are given for the conformers
that were observed in more than 10:1 excess) = 1.35 [m, 4 H,
-CH2(CH2)2CH2- of 2aI], 1.41 (t, J = 7.0 Hz, 6 H, -OCH2CH3 of
2aI), 1.49–1.52 [m, 4 H, -CH2(CH2)2CH2- of 2aI], 1.52–1.59 [m, 4
H, -CH2(CH2)2CH2- of 2aI], 2.68 [m, 4 H, -CH2(CH2)2CH2- of
2aI], 4.38 (q, J = 7.0 Hz, 4 H, -OCH2CH3 of 2aI), 7.24–7.27 (m, 2
H, ArH of 2aI), 7.43–7.45 (m, 3 H, ArH of 2aI), 8.69 (br. s., 1 H*,
NH of 2aII), 13.06 (br. s, 1 H*, NH of 2aI) ppm. [* Integral inten-
sities of the exchanging protons make up 0.6–0.8 of the theoretical
value.] 1H NMR (CDCl3, 400 MHz): δ (mixture of isomers 2aI and
2aII in a ratio of 2:1) = 1.30 (t, J = 7.0 Hz, -OCH2CH3 of 2aII),
1.35 [m, -CH2(CH2)2CH2- of 2aI], 1.41 (t, J = 7.0 Hz, -OCH2CH3
of 2aI), 1.49–1.52 [m, -CH2(CH2)2CH2- of 2aI], 1.53–1.59 [m,
-CH2(CH2)2CH2- of 2aI and 2aII], 1.66 [m, -CH2(CH2)2CH2- of
2aII], 1.96 [m, -CH2(CH2)2CH2- of 2aII], 2.68 [m, -CH2-
(CH2)2CH2- of 2aI], 2.79 [m, -CH2(CH2)2CH2- of 2aII], 4.24 (q, J
= 7.0 Hz, -OCH2CH3 of 2aII), 4.38 (q, J = 7.0 Hz, -OCH2CH3 of
2aI), 7.24–7.27 (m, ArH of 2aI), 7.28–7.30 (m, ArH of 2aII), 7.34–
7.36 (m, ArH of 2aII), 7.43–7.45 (m, ArH of 2aI), 8.70 (br. s, NH
of 2aII), 13.02 (br. s, NH of 2aI) ppm. 13C NMR (CDCl3,
100 MHz): δ (mixture of isomers 2aI and 2aII in a 5:1 ratio) =
14.39, 14.49, 22.07, 22.37, 22.97, 23.09, 23.18, 23.23, 23.35, 24.25,
59.80, 60.56, 116.36, 119.75, 126.85, 128.43, 128.65, 128.71, 128.80,
129.06, 129.55, 132.66, 133.02, 137.05, 139.09, 139.62, 141.82,
142.01, 144.17, 161.55, 162.32 ppm. Rf = 0.74 (2aI), 0.26 (2aII)
(CHCl3/EtOH = 50:1; SiO2). LDI-TOF: m/z = 472.99 [M + H]+,
510.91 [M + K]+. C29H32N2O4 (472.24): calcd. C 73.70, H 6.83, N
5.93; found C 73.59, H 6.88, N 5.81.
Ethyl (1Z)-1-{[3-(Ethoxycarbonyl)-4,5,6,7-tetrahydro-2H-isoindol-1-
yl](2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclo-
pentadecin-15-yl)methylene}-4,5,6,7-tetrahydro-1H-isoindole-3-carb-
oxylate (2c): Compound 4c (0.300 g, 0.451 mmol) in THF (15 mL)
and DDQ (0.2048 g, 0.902 mmol) in THF (5 mL) were reacted ac-
cording to the general procedure. The product was recrystallized
1
from hexane to give 2c (0.2571 g, 86%) as a red powder. H NMR
(CDCl3, 400 MHz): δ (isomer 2cI) = 1.40 [m, J = 7.2 Hz, 10 H,
-OCH2CH3 and -CH2(CH2)2CH2-], 1.57–1.64 [m, 8 H, -CH2-
(CH2)2CH2-], 2.68 [m, 4 H, -CH2(CH2)2CH2-], 3.78–3.80 (m, 8 H,
-OCH2-), 3.90 (m, 2 H, -OCH2-), 3.96 (m, 2 H, -OCH2-), 4.09 (m,
2 H, -OCH2-), 4.19 (m, 2 H, -OCH2-), 4.37 (q, J = 7.2 Hz, 4 H,
-OCH2CH3), 6.75 (m, J = 1.8 Hz, 1 H, ArH), 6.79 (m, J1 = 1.8, J2
= 8.1 Hz, 1 H, ArH), 6.90 (m, J = 8.1 Hz, 1 H, ArH), 12.86 (br. s,
1
1 H*, NH) ppm. H NMR (CDCl3, 400 MHz): δ (isomer 2cII) =
1.31 (t, J = 7.1 Hz, 6 H, -OCH2CH3), 1.54–1.58 [m, 4 H,
-CH2(CH2)2CH2-], 1.66 [m, 4 H, -CH2(CH2)2CH2-], 1.97 [m, 4 H,
-CH2(CH2)2CH2-], 2.79 [m, 4 H, -CH2(CH2)2CH2-], 3.74–3.75 (m,
8 H, -OCH2-), 3.86 (m, 2 H, -OCH2-), 3.90 (m, 2 H, -OCH2-), 4.03
(m, 2 H, -OCH2-), 4.12 (m, 2 H, -OCH2-), 4.25 (q, J = 7.1 Hz, 4
H, -OCH2CH3), 6.73 (m, J1 = 1.7, J2 = 8.3 Hz, 1 H, ArH), 6.80
(d, J = 8.3 Hz, 1 H, ArH), 6.87 (m, J = 1.7 Hz, 1 H, ArH), 8.70
(br. s, 1 H*, NH) ppm. [* Integral intensities of the exchanging
protons make up 0.6–0.8 of the theoretical value.] 13C NMR
(CDCl3, 100 MHz): δ (mixture of isomers 2cI and 2cII in a ratio
of 8:1) = 14.42, 22.45, 23.14, 23.22, 24.50, 60.58, 68.34, 68.85,
69.31, 70.11, 70.13, 71.01, 113.14, 114.25, 122.20, 129.67, 133.06,
139.31, 139.64, 141.73, 144.16, 149.27, 149.96, 162.39 ppm (signals
of isomer I only observed in the 13C NMR spectrum if the con-
former ratio is higher than 6:1). 13C NMR (CDCl3, 100 MHz,
–20 °C): δ (isomer 2cII) = 14.37, 21.57, 22.74, 23.02, 23.33, 60.05,
67.82, 67.94, 68.96, 69.01, 69.74, 70.44, 111.53, 111.59, 115.71,
119.78, 119.85, 128.76, 128.99, 132.68, 134.51, 148.31, 148.82,
161.01 ppm. Rf = 0.44 (2cI), 0.3 (2cII) (CHCl3/EtOH = 15:1; SiO2).
LDI-TOF: m/z = 663.17 [M + H]+, 685.16 [M + Na]+, 701.11 [M
+ K]+. C37H46N2O9 (662.77): calcd. C 67.05, H 7.00, N 4.23; found
C 66.76, H 6.72, N 4.32.
Ethyl (1Z)-1-{(3,4-Dimethoxyphenyl)[3-(ethoxycarbonyl)-4,5,6,7-
tetrahydro-2H-isoindol-1-yl]methylene}-4,5,6,7-tetrahydro-1H-isoind-
ole-3-carboxylate (2b): Compound 4b (0.300 g, 0.561 mmol) in
THF (15 mL) and DDQ (0.2547 g, 1.122 mmol) in THF (5 mL)
were reacted according to the general procedure to give 2b
1
(0.2898 g, 97%) as a red powder. H NMR (CDCl3, 400 MHz): δ
Supporting Information (see footnote on the first page of this arti-
(mixture of isomers 2bI and 2bII in a ratio of 4:1) = 1.31 (t, J =
cle): 2D NMR spectra of the studied compounds; kinetic measure-
2206
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 2201–2207

Dipyrrin Conformations
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Acknowledgments
The authors acknowledge the support of this study from the Rus-
sian Foundation for Basic Research (grant number 09-03-00550a).
The authors are grateful to Dmitry Cheshkov, State Research Insti-
tute for Chemistry and Technology of Organoelement Compounds,
for kindly measuring the 2D NMR spectra. The authors would like
to thank Prof. Olga Fedorova and Dr. Yuri Fedorov, Institute of
Organoelement Compounds of the Russian Academy of Sciences
for encouraging and helpful discussions.
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Received: December 1, 2011
Published Online: February 29, 2012
Eur. J. Org. Chem. 2012, 2201–2207
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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