Calix[4]pyrrole-based heteroditopic ion-pair receptor that displays anion-modulated, cation-binding behavior

Article abstract of DOI:10.1002/chem.201202777

A new ditopic ion-pair receptor 1 was designed, synthesized, and characterized. Detailed binding studies served to confirm that this receptor binds fluoride and chloride ions (studied as their tetraalkylammonium salts) and forms stable 1:1 complexes in CDCl₃. Treatment of the halide-ion complexes of 1 with Group I and II metal ions (Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, and Ca²⁺; studied as their perchlorate salts in CD₃CN) revealed unique interactions that were found to depend on both the choice of the added cation and the precomplexed anion. In the case of the fluoride complex [1?F]^- (preformed as the tetrabutylammonium (TBA⁺) complex), little evidence of interaction with the K⁺ ion was seen. In contrast, when this same complex (i.e., [1?F]^- as the TBA⁺ salt) was treated with the Li⁺ or Na⁺ ions, complete decomplexation of the receptor-bound fluoride ion was observed. In sharp contrast to what was seen with Li⁺, Na⁺, and K⁺, treating complex [1?F]^- with the Cs⁺ ion gave rise to a stable, receptor-bound ion-pair complex [C?1?F] that contains the Cs ⁺ ion complexed within the cup-like cavity of the calix[4]pyrrole, which in turn was stabilized in its cone conformation. Different complexation behavior was observed in the case of the chloride complex [1?Cl] ^-. In this case, no appreciable interaction was observed with Na ⁺ or K⁺. In addition, treating [1?Cl]^- with Li⁺ produces a tightly hydrated dimeric ion-pair complex [1?LiCl(H₂O)]₂ in which two Li⁺ ions are bound to the crown moiety of the two receptors. In analogy to what was seen in the case of [1?F]^-, exposure of [1?Cl]^- to the Cs⁺ ion gives rise to an ion-pair complex [C?1?Cl] in which the cation is bound within the cup of the calix[4]pyrrole. Different complexation modes were also observed when the binding of the fluoride ion was studied by using the tetramethylammonium and tetraethylammonium salts.

Full text of DOI:10.1002/chem.201202777

FULL PAPER
DOI: 10.1002/chem.201202777
Calix[4]pyrrole-Based Heteroditopic Ion-Pair Receptor That Displays
Anion-Modulated, Cation-Binding Behavior
In-Won Park,[a] Jaeduk Yoo,[a] Suman Adhikari,[a] Jung Su Park,[b]
Jonathan L. Sessler,*[b] and Chang-Hee Lee*[a]
Abstract: A new ditopic ion-pair recep-
tor 1 was designed, synthesized, and
characterized. Detailed binding studies
served to confirm that this receptor
binds fluoride and chloride ions (stud-
ied as their tetraalkylammonium salts)
and forms stable 1:1 complexes in
CDCl3. Treatment of the halide-ion
complexes of 1 with Group I and II
contrast, when this same complex (i.e.,
[1·F]À as the TBA+ salt) was treated
with the Li+ or Na+ ions, complete de-
complexation of the receptor-bound
fluoride ion was observed. In sharp
contrast to what was seen with Li+,
Na+, and K+, treating complex [1·F]À
with the Cs+ ion gave rise to a stable,
ent complexation behavior was ob-
served in the case of the chloride com-
plex [1·Cl]À. In this case, no apprecia-
ble interaction was observed with Na+
or K+. In addition, treating [1·Cl]À
with Li+ produces a tightly hydrated
dimeric ion-pair complex [1·LiCl-
AHCTUNGTRENNUNG
(H2O)]2 in which two Li+ ions are
receptor-bound
ion-pair
complex
bound to the crown moiety of the two
receptors. In analogy to what was seen
in the case of [1·F]À, exposure of
[1·Cl]À to the Cs+ ion gives rise to an
ion-pair complex [Cs·1·Cl] in which the
cation is bound within the cup of the
calix[4]pyrrole. Different complexation
modes were also observed when the
binding of the fluoride ion was studied
by using the tetramethylammonium
and tetraethylammonium salts.
metal ions (Li+, Na+, K+, Cs+, Mg2+
,
[Cs·1·F] that contains the Cs+ ion com-
plexed within the cup-like cavity of the
calix[4]pyrrole, which in turn was stabi-
lized in its cone conformation. Differ-
and Ca2+; studied as their perchlorate
salts in CD3CN) revealed unique inter-
actions that were found to depend on
both the choice of the added cation
and the precomplexed anion. In the
case of the fluoride complex [1·F]À
(preformed as the tetrabutylammonium
(TBA+) complex), little evidence of in-
teraction with the K+ ion was seen. In
Keywords: anion recognition · cal-
ix[4]pyrroles · cation modulation ·
ditopic receptor · ion pairs · recep-
tors
Introduction
ether and an amide functionality.[9,10] However, to the best
of our knowledge, only a limited number of systems contain-
ing spatially separated anion- and cation-recognition motifs
have been reported. Such systems are of interest because
they might allow for a fine-tuning of the recognition proper-
ties by appropriate placement of the ion binding sites within
suitably preorganized scaffolds. In the limit, this might allow
for the specific binding of a particular ion-pair species in
preference to ostensibly similar salts combinations. Howev-
er, the design features that regulate the specifics of ion-pair
binding remain poorly understood. Recently, we reported a
calix[4]pyrrole-based ion-pair receptor that displayed three
different ion-pair binding modes with Cs+-ion salts, namely
solvent bridged, contact, and host separated (shared).[11,12]
Herein, we report a new ion-pair receptor, specifically a cal-
ix[4]pyrrole core that is covalently linked to a m-dibenzo-
[26]crown-8 subunit through phenyl spacers.[13] This system,
receptor 1, displays ion-pair binding features that are very
different from what has been observed so far. Specifically,
receptor 1 was found to act as an anion-modulated, cation-
selective ion-pair receptor in CDCl3 and CD3CN, displaying
selectivity for specific cation and anion combinations within
a series of closely related salts of alkali and alkaline earth
The design, synthesis, and application of ditopic receptors,
molecular systems that can bind both anionic and cationic
guests, represents a current challenge in supramolecular
chemistry. These receptor systems are particularly attractive
because of their potential applications in various fields in-
cluding salt solubilization,[1] extraction,[2] trans-membrane
ion-transport agents,[3] and as recognition elements in che-
mosensors[4] or logic gates.[5] To date, a number of systems
containing two different binding sites within a single molec-
ular framework have been reported.[6–8] Most of these sys-
tems possess heterotopic binding domains, such as a crown-
[a] I.-W. Park, Dr. J. Yoo, Dr. S. Adhikari, Prof. C.-H. Lee
Department of Chemistry, Kangwon National University
Chun Cheon 200-701 (Korea)
Fax : (+82)33-253-7582
E-mail: chhlee@kangwon.ac.kr
[b] J. S. Park, Prof. J. L. Sessler
Department of Chemistry and Biochemistry
University of Texas at Austin
Austin TX 78712-0165 (USA)
E-mail: sessler@mail.utexas.edu
1
metals. As detailed below, H NMR spectroscopic studies re-
vealed that receptor 1 binds the fluoride and chloride ions
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202777.
Chem. Eur. J. 2012, 18, 15073 – 15078
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
15073

(as the corresponding tetraalkylammonium salts) strongly
and forms stable 1:1 complexes in CDCl3 and CD3CN. Treat-
ment of the resulting anion-bound complexes with Group I
or II metal ions (Li+, Na+, K+, Cs+, Mg2+, and Ca2+ as
their perchlorate salts in CD3CN) gave rise to spectral fea-
tures that were found to depend on both the choice of the
cation and the nature of the initially complexed anion. Re-
ceptor 1 also interacts with tetramethylammonium fluoride
(TMAF) and tetraethylammonium fluoride (TEAF) in
CDCl3. It forms a contact ion pair with TMAF in which the
ammonium ion is bound to the crown moiety, but stabilizes
a receptor-stabilized ion-pair complex in the case of the os-
tensibly similar TEAF salt. Presumably, this reflects the fact
that the relatively smaller cation (tetramethylammonium vs.
tetraethylammonium) is bound more strongly by the crown-
ether moiety.
Figure 1. Single-crystal X-ray structures of a) free host 1 (solvent is not
shown, the elipsodal probability is 50%) and b) CsF complex of receptor
1 ([Cs·1·F]), and view of the 3D-packing arrangement. Note that the
cesium cation resides within the cup of the calix[4]pyrrole and half of the
ether moiety, which adopts a cone-like conformation.
Results and Discussion
The synthesis of the receptor 1 was accomplished as shown
in Scheme 1. The ditosylated triethylene glycol 2 was react-
ed with 3,5-dihydroxy acetophenone to afford the corre-
sponding macrocyclic bisketone 3 in 26% yield.[14]
role moiety of the free host 1 adopts a partial cone confor-
mation (Figure 1a). In the solid state, on the other hand, the
CsF complex of the receptor 1 was found to exist in the
form of a receptor-shared ion-pair complex (Figure 1b). The
fluoride ion is tightly held inside the cavity, while the caesi-
um cation is mainly bound within the calix[4]pyrrole “cup”
of the cone that is formed upon anion binding and interacts
with two oxygen atoms on the crown moiety. The net result
is a receptor-separated ion-pair complex [Cs·1·F].
The single-crystal structure of the LiCl complex of the re-
ceptor 1 reveals completely different structural features. As
shown in Figure 2, two molecules of the ClÀ-bound complex
formed with receptor 1 and LiCl associate to form a dimeric
structure that is connected by two Li+ ions, each of which is
bound by four oxygen atoms. One water molecule was
bound between Li+ and ClÀ by the separated distance of
+
À
À
4.7 ꢁ (the O Cl distance was 3.11 ꢁ and the O Li dis-
Scheme 1. Synthesis of receptor 1.
Reaction of 3 with pyrrole and TFA at 608C produced
the corresponding dipyyromethane 4 in 49% yield.[15] Lewis
acid catalyzed condensation of 4 with acetone afforded the
desired receptor 1 in low yield (5%). Attempted condensa-
tion with other acids, such as BF3, also gave the desired
product 1, albeit in the same low yield. On the other hand,
because each of the steps could be carried out easily, usea-
ble quantities of material were readily obtained. The struc-
ture of receptor 1 was confirmed by standard spectroscopic
means and by single-crystal X-ray diffraction analysis, as
shown in Figure 1. The structure reveals that the calix[4]pyr-
Figure 2. Single-crystal X-ray structures of dimeric complex of receptor 1
([1·LiClACTHNUTRGNE(NUG H2O)]2). Note that the two crown moieties of chloride-bound re-
ceptors hold two lithium ions to form a dimeric hydrated ion-pair com-
plex.
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Chem. Eur. J. 2012, 18, 15073 – 15078

Calix[4]pyrrole-Based Heteroditopic Ion-Pair Receptor
FULL PAPER
tance was 1.92 ꢁ). Although complicating the overall ar-
rangements of the crown moiety, this system is best descri-
bed as a water-separated dimeric ion-pair complex [{1·LiCl-
tons, appeared at d=12.64 ppm as a doublet (JHF =42 Hz).
These seemingly unusual spectroscopic findings can be ex-
plained by assuming the concomitant formation of two dif-
ferent ion-pair complexes, namely 5 and 6, as shown in
Scheme 2. The complex formed at an early stage of the titra-
tion is thought to be the contact ion-pair complex 6 in which
the TEAF binds to the cavity. This complex is not thermo-
dynamically stable and, thus, converts to the more stable re-
ceptor-shared ion-pair complex 5 in which the ammonium
counter cation interacts with the crown-ether moiety.
ACHTUNGTRENNUNG
to bind ion-pair salts in CDCl3 were investigated by
1H NMR spectroscopy. Significant chemical-shift changes
were observed when the receptor 1 was subjected to titra-
tion with the fluoride ion (as its tetrabutylammonium salt;
see the Supporting Information, Figure S9). In the course of
À
reaching saturation, the signals of the pyrrole N H protons
move significantly downfield, while the signals of the b-pyr-
In this last complex 5, the cation interacts with the cavity-
bound anion. In complex 6, produced early on in the titra-
À
rolic and Ar
H protons move slightly upfield. These spectral
tion, the N H resonance is observed at d=11.72 ppm, a
chemical-shift value that is consistent with the presence of
changes are interpreted in terms of conformational locking.
However, they could reflect the presence of a weak anion–p
interaction as well. The protons on the crown-ether subunits
were also shifted to slightly lower field, an effect that is like-
wise ascribed to conformational locking. On the basis of this
analysis, we conclude that the receptor-separated ion-pair
complex 5 (Scheme 2) is being formed. The addition of
À
pyrrole N H–anion hydrogen bonds that are weakened
owing to anion–cation interactions involving the centrally
À
cobound cation. Upon conversion to 5, the N H signals in
question move to lower field, finally appearing at d=
12.64 ppm, as noted above. Such spectral changes are consis-
tent with the formation of a species characterized by strong
À
N H–anion interactions, as would be expected for the re-
ceptor-shared ion-pair complex 5.
1H NMR titrations of receptor 1 with TMAF resulted in
spectral changes similar to those of the addition of TEAF.
However, unlike TEAF, spectral features corresponding to a
small quantity of contact ion pair (i.e., 6) remained, even
after the addition of an excess (10 equiv) of TMAF. These
observations provide support for the notion that the rela-
tively smaller tetramethylammonium ion, in contrast to the
tetraethylammonium ion, allows for better interactions with
the crown moiety. Such findings are consistent with the ex-
pectation that this smaller cation can be better accommodat-
ed by the crown-ether moiety. Relative to what was seen
with TEAF, this favors formation of 6 over 5, although the
last is still the dominant species by the time ten equivalents
of FÀ have been added to the solution (as inferred from the
integration data). Since 1H NMR-spectroscopic titrations
with various ammonium salts proved consistent with the for-
mation of contact ion-pair complexes, we also examined the
cation-recognition properties of the preformed supramolec-
ular anion–host ensembles [1·X]À (X=FÀ or ClÀ; as the
TBA+ salt). These preformed anion complexes were treated
with various Group I metal ions. First, the fluoride-bound
complex [1·F]À was treated with various Group I metal ions,
as well as with Ca2+ and Mg2+ (as their respective perchlo-
rate salts). Owing to limited solubility of the salts in chloro-
form, [D3]acetonitrile was used as the solvent. Upon addi-
tion of the Cs+ ion (as its perchlorate salt) to the ensemble
[1·F]À, the signals corresponding to the fluoride-bound pyr-
Scheme 2. Proposed binding modes of receptor 1 with tetraalkylammoni-
um fluoride in CDCl3. Receptor-shared ion-pair complex 5 and contact
ion-pair complex 6.
À
À
À
other anions (ClÀ, CH3COOÀ, NO3 , H2PO4 , or HSO4 , all
as their respective TBA+ salts) was also found to induce
similar changes in the chemical shifts of receptor 1, as re-
corded in CDCl3. Particularly large chemical-shift changes
À
in the pyrrole N H signal were observed when either chlor-
ide or acetate ions were added. In contrast, relatively
modest changes were observed in the case of dihydrogen
phosphate, hydrogen sulfate, and nitrate ions. These obser-
vations lead to the conclusion that these last anions are not
bound strongly to receptor 1 under these solution-phase
conditions.
Downfield shifts in the signals corresponding to the pro-
tons on the crown-ether moiety were also observed, findings
that are consistent with interactions with the counter cati-
ons. To further assess the possible effects of the counter
1
À
cation, H NMR-spectroscopic titrations of receptor 1 with
role N H protons shifted slightly upfield, while those of the
several alkylammonium salts, including TEAF and TMAF,
were carried out (see the Supporting Information, Figur-
es S10 and 11). Upon titration with TEAF, two new signals,
crown-ether moiety remained unchanged (see the Support-
ing Information, Figure S12). A similar trend was observed
upon the addition of the K+ ion; however, in this case, the
À
À
corresponding to the anion-bound pyrrole N H, appeared
at d=12.35 and 11.72 ppm. However, these signals disap-
signal of the pyrrole N H protons remained unchanged.
Upon cation addition, the signal of the pyrrole b-protons
were shifted to slightly lower field as compared to the corre-
sponding signals for the complex [1·F]À.
peared upon addition of excess TEAF, and a new signal,
corresponding to the typical fluoride-ion-bound N H pro-
À
Chem. Eur. J. 2012, 18, 15073 – 15078
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15075

J. L. Sessler, C.-H. Lee et al.
The results above are most easily rationalized by the Cs+
being bound to the cup of the cone-shaped calix[4]pyrrole
through cation–p interactions. Concurrent fluoride-ion bind-
the complex. In contrast, the addition of Li+, K+, Ca2+, and
Mg2+, (all as the corresponding perchlorate salts) induced
only partial decomplextion of the bound chloride complex.
The decomplexation was greater in the case of Ca2+ and
Mg2+ than with Li+ and K+, as inferred from the relative in-
À
ing by the pyrrole N H protons results in the formation of a
receptor-shared ion-pair complex [Cs·1·F]. The K+ ion is ex-
pected to stabilize weaker cation–p interactions; thus, it was
expected to form a complex that is less well ordered, but
one in which both the anion and cation are associated with
the receptor through the cup of the calixpyrrole rather than
through the crown ether (i.e., [K·1·F]). Support for this con-
clusion stems from the observation of broadened but split
À
tegrals of the N H signals at about d=11.3 and 8.22 ppm
(bound and unbound forms, respectively). A summary of
the experimental findings and the proposed interactions of
the prebound chloride-ion complex with cations of alkali
and alkaline earth metals is shown in Scheme 4.
À
N H signals at about d=12.78 ppm (J=234 Hz), which is in
analogy to what was seen for complex 5 discussed above.
Interestingly, the use of other cations, including Li+, Na+,
Ca2+, and Mg2+, serves to engender decomplexation of the
bound fluoride ion, as inferred from crown-ether signals
that resemble those of the uncomplexed form. The presence
À
of N H resonances at about d=8.24 ppm, as seen in the ab-
sence of an added salt, indicated that coulombic interactions
between these latter cations and the fluoride ion under
these solution-phase conditions is stronger than those pro-
À À
vided by the four N H F hydrogen bonding motifs. A sum-
mary of the results observed with the fluoride ion is present-
ed in Scheme 3. It was further observed that when the re-
Scheme 4. Schematic representation of the proposed binding modes of
anion complex [1·Cl]À (TBA+ salt) that were observed in the presence of
various metal ions in acetonitrile. The metal ions were used in the form
of their respective perchlorate salts. M1+ =K+ and Li+; M2+ =Na+,
Mg2+, Ca2+
.
Conclusion
We have demonstrated that the benzocrown-ether-capped
ion-pair receptor 1, which contains both cation and anion
binding sites within a single overall framework, supports the
formation of two different ion-pair complexes that were ob-
served depending on the nature of the cation. The receptor–
fluoride complex [1·F]À forms a receptor-shared ion-pair
complex in which the Cs+ ion is bound to the cup of the cal-
ix[4]pyrrole, as inferred from spectroscopic studies carried
out in [D3]acetonitrile. Decomplexation of the bound fluo-
Scheme 3. Schematic representation of the proposed binding modes of
anion complex [1·F]À (TBA+ salt) that were observed in the presence of
various metal ions in acetonitrile. The metal ions were used in the form
of their respective perchlorate salts. M+ =Li+, Na+, Mg2+, Ca2+
.
ride ion was observed when NaClO4, LiClO4, Ca
ACHTUNGTRENNUNG(ClO4)2,
and Mg
AHCTUNGTRENNUNG
ceptor–chloride complex [1·Cl]À (TBA+ salt) was treated
with various alkaline-metal ions (as their perchlorate salts),
different changes were seen than in the case of the corre-
sponding fluoride-ion complex. When the preformed com-
plex [1·Cl]À was treated with excess Cs+ ions, a typical up-
[D3]acetonitrile (TBA+ salt). The receptor-bound chloride
complex [1·Cl]À displayed different cation-binding proper-
ties than the analogous fluoride-ion complex [1·F]À. In this
case, addition of CsClO4 resulted in the formation of a cup-
bound ion-pair complex, as well as a crown-ether-bound
contact ion-pair complex. While complete decomplextion
was observed upon addition of NaClO4, only partial decom-
plextion of the bound chloride was achieved upon treatment
with K+, Ca2+, and Mg2+. Thus, the present results serve to
underscore the emerging impression that the ion-pair-recog-
nition properties of appropriately designed ditopic receptors
can be highly specific. Moreover, the results presented
À
field shift of the hydrogen-bonded pyrrole N H proton res-
onances and downfield shifts in the crown-ether signals
were observed (see the Supporting Information, Figure S13).
These results are consistent with one Cs+ ion being bound
within the cup of the calix[4]pyrrole (present in its anion-
stabilized cone conformation). Addition of Na+ ion resulted
in complete decomplextion of the bound chloride ion from
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Calix[4]pyrrole-Based Heteroditopic Ion-Pair Receptor
FULL PAPER
6H); 13C NMR (150 MHz, CDCl3): d=159.3, 151.6, 138.8, 136.8, 106.9,
104.7, 102.7, 100.3, 70.5, 69.8, 67.0, 45.4, 34.9, 32.7, 29.7, 29.1, 27.0 ppm;
MALDI-TOF: m/z calcd for C50H60N4O8 +H: 845.44 ; found: 845.48
[M+H]+.
herein lead us to suggest that these binding properties can
be specifically tuned through appropriate synthetic modifi-
cations to the receptor framework, and that the nature of
the cobound cation and anion plays a major role in defining
the structure of the resulting ion-pair complex. These find-
ings have important implications for the design of new ion-
recognition systems.
Acknowledgements
This work was supported by NRF funded by the Ministry of Education,
Science and Technology (MEST; grant 2009–0087013). The Central In-
strumentation Facility at KNU and BK21 program are acknowledged for
support. The work in Austin was funded by the US Department of
Energy Office of Basic Energy Sciences (grant 10651589 to J.L.S.).
Thanks are also expressed to the WCU (World Class University) pro-
gram (R32–2010–000–10217–0) administered through the National Re-
search Foundation of Korea and funded by MEST, which made study
leaves in Korea possible for J.L.S.
Experimental Section
1H NMR spectra were recorded on 400 MHz NMR spectrometers by
using TMS as the internal standard. Chemical shifts are reported in parts
per million (ppm). When peak multiplicities are given, the following ab-
breviations are used: s, singlet; br s, broad singlet; d, doublet; t, triplet;
m, multiplet. 13C NMR spectra were proton decoupled and recorded on
150 and 100 MHz NMR spectrometers by using TMS as the internal
standard. All other chemicals and solvents were purchased from commer-
cial sources and were used as such, unless otherwise noted. Column chro-
matography was performed over silica gel. ITC titration was performed
by using HPLC grade acetonitrile.
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Synthesis and spectral characterization of bis(5-
ACHTUNGTRENNUNG
yl)ethyl)-1,3-phenylene)-[26]crown-8 (4): Bisketone
3
0.71 mmol), pyrrole (7 mL), and TFA (0.526 mL, 7.1 mmol) were mixed
and heated at reflux (ca. 608C) for 24 h. After this time, the reaction was
deemed complete and the reaction mixture was cooled to room tempera-
ture. At this point, CH2Cl2 (5 mL) and TEA (1 mL) were added and the
resulting mixture was extracted with CH2Cl2 (3ꢂ50 mL) and dried over
anhydrous MgSO4. The solvent was removed under reduced pressure and
the residue was purified by column chromatography (silica gel; CH2Cl2/
EtOAc=2:1) to yield compound 4 as a brown, gummy thick liquid
(0.266 g, 49%). 1H NMR (400 MHz, CDCl3): d=7.77 ( br s, 4H), 6.61–
6.59 (m, 4H), 6.33 (t, 2H, J=2.5 Hz), 6.30 (d, J=2.5 Hz, 4H), 6.14–6.12
(m, 4H), 5.97–5.96 (m, 4H), 3.97 (t, 8H, J=2.8 Hz), 3.77 (t, 8H, J=
5.0 Hz), 3.68 (s, 8H), 1.97 ppm (s, 6H); 13C NMR (100 MHz, CDCl3): d=
159.7, 149.6, 137.1, 116.8, 108.2, 106.9, 106.1, 99.6, 70.8, 69.6, 67.3, 44.9,
28.8 ppm; MALDI-TOF: m/z calcd for C44H52N4O8 +3H: 767.38; found:
767.42 [M+3H]3+
.
Synthesis and spectral characterization of the bis(1,3-phenylene)-
[26]crown-8-capped calix[4]pyrrole (1): Compound 4 (0.33 g, 0.43 mmol),
acetone (50 mL), and BiACTHNUTRGNEUNG(NO3)3·5H2O (0.075 g, 25% mol) were mixed
and the reaction mixture was stirred at room temperature for 4 h. After
filtration of the reaction mixture, a saturated solution of KOH (10 mL)
was added to the filtrate, which was extracted with CH2Cl2 (3ꢂ50 mL).
The solvent was removed under reduced pressure and the residue was
purified by column chromatography (silica gel; CH2Cl2/EtOAc=1:1) to
yield compound 1 as a white solid (0.018 g, 5%).1H NMR (300 MHz,
CDCl3): d=8.18 (br s, 4H), 6.39 (s, 2H), 6.13 (d, J=2.14 Hz, 4H), 5.98
(t, 4H, J=2.7 Hz), 5.88 (t, 4H, J=2.7 Hz), 3.99–3.94 (m, 8H), 3.83–3.82
(m, 4H), 3.69–3.61 (m, 12H), 1.89 (s, 6H), 1.65 (s, 6H), 1.51 ppm (s,
Chem. Eur. J. 2012, 18, 15073 – 15078
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15077

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Received: August 2, 2012
Published online: October 2, 2012
15078
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 15073 – 15078