Synthesis, enantioseparation and photophysical properties of planar-chiral pillar[5]arene derivatives bearing fluorophore fragments

Article abstract of DOI:10.3762/bjoc.15.164

Planar chiral pillar[5]arene derivatives (P5A-DPA and P5A-Py) bearing bulky fluorophores were obtained in high yield by click reaction. The photophysical properties of both compounds were investigated in detail. P5A-DPA with two 9,10-diphenylanthracene (DPA) pigments grafted on the pillar[5]arene showed a high fluorescence quantum yield of 89.5%. This is comparable to the monomer DPA-6, while P5A-Py with two perylene (Py) pigments grafted on the pillar[5]arene showed a significantly reduced quantum yield of 46.4% vs 78.2% for the monomer Py-6. The oxygen-through-annulus rotation of the phenolic units was inhibited for both compounds due to the bulky chromophore introduced, and the resolution of the enantiomers was achieved due to the bulky size of the fluorophores. The absolute configuration of the enantiomers was determined by circular dichroism (CD) spectra. The solvent-induced aggregation behavior was investigated with the enantiopure P5A-DPA and P5A-Py. It was found that the CD signals were enhanced by aggregation. P5A-DPA showed aggregation-induced emission enhancement, while P5A-Py showed aggregation-induced emission quenching, accompanied by excimer emission when aggregating in water and THF mixed solution.

Full text of DOI:10.3762/bjoc.15.164

Synthesis, enantioseparation and photophysical properties
of planar-chiral pillar[5]arene derivatives bearing
fluorophore fragments
Guojuan Li‡, Chunying Fan‡, Guo Cheng, Wanhua Wu* and Cheng Yang*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 1601–1611.
Key Laboratory of Green Chemistry & Technology, College of
Chemistry and Healthy Food Evaluation Research Center, Sichuan
University, 29 Wangjiang Road, Chengdu, 610064, P. R. China
doi:10.3762/bjoc.15.164
Received: 30 April 2019
Accepted: 23 May 2019
Published: 18 July 2019
Email:
Wanhua Wu* - wuwanhua@scu.edu.cn; Cheng Yang* -
yangchengyc@scu.edu.cn
This article is part of the thematic issue "Novel macrocycles – and old
ones doing new tricks".
* Corresponding author ‡ Equal contributors
Guest Editor: W. Jiang
Keywords:
aggregation; circular dichroism; chirality; click chemistry; macrocycles;
pillar[5]arenes
© 2019 Li et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Planar chiral pillar[5]arene derivatives (P5A-DPA and P5A-Py) bearing bulky fluorophores were obtained in high yield by click
reaction. The photophysical properties of both compounds were investigated in detail. P5A-DPA with two 9,10-diphenyl-
anthracene (DPA) pigments grafted on the pillar[5]arene showed a high fluorescence quantum yield of 89.5%. This is comparable
to the monomer DPA-6, while P5A-Py with two perylene (Py) pigments grafted on the pillar[5]arene showed a significantly
reduced quantum yield of 46.4% vs 78.2% for the monomer Py-6. The oxygen-through-annulus rotation of the phenolic units was
inhibited for both compounds due to the bulky chromophore introduced, and the resolution of the enantiomers was achieved due to
the bulky size of the fluorophores. The absolute configuration of the enantiomers was determined by circular dichroism (CD) spec-
tra. The solvent-induced aggregation behavior was investigated with the enantiopure P5A-DPA and P5A-Py. It was found that the
CD signals were enhanced by aggregation. P5A-DPA showed aggregation-induced emission enhancement, while P5A-Py showed
aggregation-induced emission quenching, accompanied by excimer emission when aggregating in water and THF mixed solution.
Introduction
Planar-chiral compounds are structurally appealing and poten- typical examples of this type of compounds and have attracted
tially applicable in various functional materials such as chiral considerable attention due to their facile chemical synthesis and
discriminators [1,2], chiral polymers, supramolecular sensors versatile functionality [9] in recent years. Pillar[5]arenes [10],
[3] and chiral guest receptors [4,5]. Planar-chiral macrocyclic are cyclic pentamers composed of 1,4-dialkoxybenzene units
molecules are particularly interesting in the context of the connected through methylene bridges at the para-position and
host–guest complexation properties [6-8]. Pillararenes are have a unique symmetrical architecture with an overall cylin-
1601

Beilstein J. Org. Chem. 2019, 15, 1601–1611.
drical or pillar-like shape. By virtue of their rigid and symmetri- mophores possess unique photophysical properties and have
cal structures, as well as highly tunable functionality, the been widely used as triplet acceptor for TTA-based upconver-
studies on pillar[5]arenes have been developed rapidly in sion [34-36] and as energy donors in artificial light-harvesting
various areas, such as artificial transmembrane channels systems [37]. Thus, two pillar[5]arene derivatives P5A-DPA
[11,12], molecular complexation [13,14] and nonporous adap- and P5A-Py (Scheme 1), in which two Py and DPA units were
tive crystals [15-17]. One of the features of pillar[5]arenes that chemically grafted onto one of the phenolic units of
differs from the common macrocycles is the planar chirality re- pillar[5]arene, were designed and synthesized by click reaction
sulting from the different orientations of the alkoxy substitu- [38,39]. We supposed that the Py or DPA substituents on both
ents on the rims. Theoretically, eight conformers can be formed rims of the pillar[5]arenes will serve two purposes: Firstly, the
including diastereomeric ones: (Sp,Sp,Sp,Sp,Sp), (Rp,Sp,Sp,Sp,Sp), molecular sizes of Py and DPA are larger than the cavity size of
(Rp,Rp,Sp,Sp,Sp), (Rp,Sp,Rp,Sp,Sp) and their antipodal enantio- the pillar[5]arenes (ca. 5.5 Å), and the rotation of the units
mers: (Rp,Rp,Rp,Rp,Rp), (Sp,Rp,Rp,Rp,Rp), (Sp,Sp,Rp,Rp,Rp), should be inhibited generating a pair of Sp and Rp enantiomers.
(Sp,Rp,Sp,Rp,Rp). Among them, the enantiomeric per-Sp Secondly, the strong absorption in the visible range and high
(Sp,Sp,Sp,Sp,Sp) and per-Rp (Rp,Rp,Rp,Rp,Rp) conformers (abbre- fluorescence a quantum yield of the fluorophores will donate
viated as Sp and Rp, respectively), in which all of the alkoxy the pillar[5]arene derivatives novel photophysical properties.
substituents at both rims are oriented in the same direction, are
Results and Discussion
the most stable conformers due to the steric-hindrance effect of
the substituents in other conformers. In general, the Sp and Rp Syntheses of pillar[5]arene derivatives
conformers can interconvert from each other in solution through P5A-Py was synthesized by following our previously reported
the oxygen-through-annulus rotation along the methylene processes [34], and P5A-DPA was synthesized by the pro-
bridges [18], and the separation of these conformational isomers cesses illustrated in Scheme 1. The ethynyl-bearing
is impossible.
pillar[5]arene derivative P5A was obtained through the
co-cyclization of 4.0 equiv of 1,4-dimethoxybenzene with
Inhibiting the rotation of the phenolic units is prerequisite to 1.0 equiv of the hydroquinone derivative 7 and 5.0 equiv of
isolate the isomers. A complexation with a suitable guest mole- paraformaldehyde in the presence of BF3·OEt2 [40,41]. The
cule, such as a viologen derivative, can significantly slow down product was purified by silica gel column chromatography with
the rotation of the phenolic rings, and a conformational inter- hexane/ethyl acetate 10:1 as the eluent. The first fraction was
conversion can only occur when the guest molecule decom- permethylated pillar[5]arene, and P5A was collected as the
plexed from the cavity [19-21]. Several strategies have been second fraction in 45% yield. The subsequent reaction of com-
established to block the interconversion of the Sp and Rp pound P5A with the azide-substituted DPA derivative DPA-6
conformers: Implanting a guest moiety in the pillar[5]arene was carried out in the presence of CuSO4·5H2O (2.0 equiv) and
cavity by rotaxanation or introducing a fused side ring into one sodium ascorbate (4.0 equiv) for 24 h in DMF at 65 °C giving
of the phenolic rings [22,23]. Covalently connected bulky sub- P5A-DPA in almost quantitative conversion (Scheme 1). The
stituents installed on both rims of the pillar[n]arene provided a crude product was separated from CuSO4 and sodium ascor-
more straightforward method to inhibit the rotation [24-26]. In bate through extraction with Et2O/water. Further purification of
per-cyclohexylmethyl-substituted pillar[5]arene the rotation of the crude product was carried out by silica gel column
the phenolic units is blocked and enabled the chiral separation chromatography (dichloromethane/methanol 30:1) to afford
of the Sp and Rp enantiomers [25]. Also installing bulky substit- P5A-DPA in 72% yield. The structure of P5A-DPA was char-
uents onto only one phenolic ring was found to be effective to acterized by 1H NMR, 13C NMR and high-resolution mass
inhibit the rotation of the units [27-29].
spectrometry (HRMS, Supporting Information File 1, Figures
S19–S21).
Pillar[5]arene itself shows only moderate absorption and weak
fluorescence in the UV region, and the chemical introduction of Photophysical properties
chromophores onto the rims of pillar[5]arene has been applied The photophysical properties were investigted by UV–vis
to allow its use as receptors for molecular sensing or biological absorption and fluorescence spectroscopy, as well as through
applications [30-33]. It occurred to us that connecting fluoro- fluorescence decay measurements. As shown in Figure 1a, the
phores with strong absorptions in the visible range and with a UV–vis spectrum of P5A-DPA showed two main absorption
high fluorescence quantum yield are beneficial for the applica- bands at 270–320 nm and 350–420 nm, respectively, which
tion of pillar[5]arene in these fields. Perylene (Py) and 9,10- were assigned to the absorption bands of the pillar[5]arene
diphenylanthracene (DPA) are well known for their desirable moiety and the π→π* transition of the DPA fragment, respec-
absorption and high fluorescence quantum yield. These chro- tively. The absorption spectra of P5A-DPA and P5A-Py are
1602

Beilstein J. Org. Chem. 2019, 15, 1601–1611.
Scheme 1: Preparation of A1/A2-difunctionalized pillar[5]arenes (P5A-DPA and P5A-Py) by click reactions. Reagents and conditions: i) phenyl-
boronic acid, Pd(PPh3)4, K2CO3, CsF, toluene/THF/H2O, 120 °C, yield: 82%; ii) N-bromosuccinimide, chloroform, 60 °C, yield: 87%; iii) (4-hydroxy-
phenyl)boronic acid, Pd(PPh3)4, K2CO3, CsF, toluene/THF/H2O, reflux, 8 h, yield: 70%; iv) K2CO3, 1,5-dibromopentane, acetone, 80 °C, yield: 67%;
v) NaN3, DMF, yield: 84%; vi) paraformaldehyde, BF3·OEt2, 1,2-dichloroethane, yield: 14%; vii) CuSO4·5H2O, sodium ascorbate, DMF, yield: 72%.
1603

Beilstein J. Org. Chem. 2019, 15, 1601–1611.
Figure 1: UV–vis absorption (a) and fluorescence emission spectra (b) of Py-6, P5A-Py (λex = 420 nm) and DPA-6, P5A-DPA (λex = 375 nm) in
CHCl3, c = 1.0 × 10−5 M, 25 °C.
almost the result of the addition of the spectra of pillar[5]arene units in one macrocyclic host, the fluorescent quantum yield
with DPA-6 and Py-6. The molar extinction coefficient at the was only slightly decreased. This should be mainly due to the
longer wavelength for P5A-DPA was almost double that steric hindrance of the 9- and 10-phenyl groups, which inhibit-
of DPA-6, e.g., ε = 23100 M cm−1 for P5A-DPA vs ed the π–π stacking of the anthracene core in DPA.
12200 M cm−1 for DPA-6 at 397 nm, indicating that there is no
strong interaction between the two DPA units in P5A-DPA at The fluorescent lifetimes of P5A-Py and P5A-DPA were com-
the ground state. Similar results were observed for P5A-Py pared with Py-6 and DPA-6. As shown in Figure 2, the lifetime
except for a slight bathochromic shift of the absorption com- of P5A-Py is 3.4 ns which is shorter than that of Py-6 (4.4 ns),
pared with Py-6.
demonstrating that grafting two Py units in close proximity in
one host, opened a new route for nonradiative decay. This phe-
The fluorescence spectrum of P5A-DPA is very similar to that nomenon further verified the occurrence of π–π stacking of the
of DPA-6 (Figure 1b). P5A-DPA showed an intense fluores- Py fragments in P5A-Py. The lifetime of P5A-DPA was 4.8 ns,
cence in the visible light region, peaked at 420 nm with a which is very similar to DPA-6 (5.0 ns) and the non-substituted
shoulder at 437 nm and a fluorescence quantum yield as high as DPA (5.3 ns) [42]. The fact that grafting DPA units in one host
89.5%, which is very close to that of DPA-6 (ΦF = 92.1%). For did not influence the fluorescent quantum yield together with
P5A-Py, however, it showed a slightly red-shifted emission the appealing host–guest properties make P5A-DPA an ideal
compared with Py-6, with the emission peaks at 456 nm and candidate for applications as acceptor for triplet–triplet annihi-
483 nm and a shoulder at 516 nm (Table 1). The quantum yield lation upconversion.
was significantly decreased to 46.4% compared with 78.2% for
Py-6. We ascribed the decreased fluorescence of P5A-Py to the Optical properties
π–π stacking of the Py units caused by the high local concentra- Due to the fact that the Py and DPA units are too large to enter
tion of perylene. For P5A-DPA, which also bears two DPA the cavity of the pillar[5]arene, we expected that the oxygen-
Table 1: Photophysical parameters of the synthesized monomers Py-6, DPA-6 and the planar-chiral hosts P5A-Py, P5A-DPA.a
λabs / nm
εb
λem / nm
ΦFc
/ nsd
4.75
4.95
3.36
4.40
P5A-DPA
DPA-6
P5A-Py
Py-6
295, 376, 397
376, 397
295, 419, 446
418, 445
3.04, 2.50, 2.31
1.30, 1.22
2.39, 4.53, 5.88
2.31, 2.99
420, 437
420, 435
456, 483, 516
451, 480, 514
89.5
92.1
46.4d
78.2d
aIn CHCl3 at 1.0 × 10−5 M. bMolar extinction coefficient at the absorption maxima (104 M−1 cm−1). cFluorescence quantum yields estimated by a rela-
tive method using DPA (ΦF = 95% in ethanol) as the standard. dPerylene (ΦF = 98% in n-hexane) as the standard. dFluorescence lifetimes.
1604

Beilstein J. Org. Chem. 2019, 15, 1601–1611.
Figure 2: (a) Fluorescence decay curves of Py-6 and P5A-Py at 450 nm and (b) fluorescence decay curves of DPA-6 and P5A-DPA at 420 nm,
excited at 390 nm nanosecond LED, c = 1.0 × 10−5 M in CHCl3, 25 °C.
through-annulus rotation is blocked in P5A-DPA and P5A-Py, chiral column to confirm the enantiomeric purity and to check if
giving rise to two pairs of enantiomeric pillar[5]arene deriva- racemization of the enantiomers takes place in solution at room
tives. The racemic mixtures of P5A-DPA and P5A-Py were re- temperature. In both cases, only the original peak was detected
spectively separated by chiral high-performance liquid chroma- and the peak for the antipodal enantiomer was not detected, in-
tography (HPLC). As shown in Figure 3a, injection of a P5A- dicating that no racemization of the enantiomer of P5A-DPA
DPA solution onto a DAICEL CHIRALPAK IA chiral HPLC takes place at room temperature (Figure 3b and c). P5A-Py
column afforded two well-separated peaks of almost equal areas showed a similar phenomenon (Figure S22a,b and c). These
at 8.8 min and 10.0 min, respectively, demonstrating that P5A- results demonstrated that the bulky substituents DPA or pery-
DPA was the racemic mixture of Rp and Sp configuration. The lene effectively prevent the ring rotations, and therefore making
two fractions were collected separately and re-injected into the the separation of Rp and Sp conformers possible.
Figure 3: (a) Chiral HPLC traces of P5A-DPA, (b), (c) the first and second fractions of P5A-DPA, detected by UV at 295 nm. Conditions: column,
DAICEL CHIRALPAK IA; mobile phase: hexane/dichloromethane 35:65; flow rate = 4.0 mL/min; temperature 25 °C). (d) CD and (e) UV–vis spectra
of racemic P5A-DPA, the first and second fractions (10 μM) in CHCl3 at 25 °C.
1605

Beilstein J. Org. Chem. 2019, 15, 1601–1611.
Figure 3d and e show the circular dichroism (CD) and UV–vis THF solutions. Taking the Rp conformers for example, the
spectra of each fraction for P5A-DPA. The CD signals of both aggregation of P5A-DPA is apparently divided into two stages
fractions observed at 270–320 nm are perfect mirror images, (Figure 4). By adding 50% water to THF, the UV–vis absorp-
thus confirming that the two fractions contain a pair of enantio- tion wavelength of P5A-DPA did not change but the CD
mers. The positive Cotton effect observed at ca. 310 nm was signals were obviously enhanced. This is most probably due to
assigned to the Rp configured enantiomer whereas the negative the formation of aggregates and the rotation of the phenolic
Cotton effect at ca. 310 nm was assigned to the corresponding units in pillar[5]arene host was inhibited in the aggregates. The
Sp configured isomer, based on calculations as well as single- aggregation behavior was further investigated by dynamic light
crystal X-ray diffraction published recently [20]. Thus, the scattering (DLS) analyses, and revealed the formation of nano-
absolute configuration of the first fraction was assigned to be Sp particles with 615 nm in average diameter after the addition of
and the second fraction was assigned to the Rp isomer. Unex- 50% water (Figure 4d). Increasing the amount of water to 70%
pectedly, no induced CD signals of DPA units at 360–420 nm or 90% led to a bathochromic shift of the UV–vis absorption
were observed though P5A-DPA showed a strong absorption and a raise of the baseline, indicating particles of larger size
(with ε > 104 M−1 cm−1, Figure 3e) in this region. The same is were formed. The CD signals continuously increased accompa-
true for P5A-Py (Supporting Information File 1, Figure S22d nied with a red-shifting of the wavelength, thus demonstrating
and e). We attributed the absence of induced CD signals to the that the DPA pigments started to aggregate in solutions with a
long distances of the fluorophores to the chiral center and there- water content of more than 50%. The π–π stacking of the
fore chirality transfer is non-effective [43].
anthracene core in DPA is responsible for the red-shift of the
absorption. For P5A-Py, however, adding water to the THF
Supramolecular assembly usually leads to different photophysi- solution led to a continuous red-shifting of the UV–vis spectra
cal properties than homogeneous solutions. We have demon- and increasing of CD signal, which is reasonable, as the planar
strated that solvents play a critical role in chiral recognition and structure of Py tends more to get aggregated in water by π–π
chiral photoreactions [44-51]. We therefore further investigated stacking. Aggregates with an average diameter of 531 nm are
the solvent-induced aggregation behaviors of the chirally pure formed in 60% water (Figure S23, Supporting Information
conformers by adding different proportions of water into their File 1) [52]. Exciton coupling CD (ECCD) signals were not ob-
Figure 4: (a) CD, (b) UV–vis and (c) fluorescence spectra of the RP5A-DPA (20 μM) in THF and THF/H2O solvent mixtures; inset: the normalized fluo-
rescence spectrum. (d) DLS measurements of the aggregated RP5A-DPA obtained from the mixture of THF/H2O 1:1 (v/v).
1606

Beilstein J. Org. Chem. 2019, 15, 1601–1611.
Experimental
served for both compounds, even when adding more than 90%
water, demonstrating that the chromophores DPA or Py units General materials and methods
were not asymmetrically oriented in the aggregates. All reagents and chemicals used in synthesis were analytically
pure and used as received without further purification. 1H NMR
Interestingly, by adding water into the THF solution of P5A- and 13C NMR spectra were recorded at room temperature on a
DPA, the fluorescence intensity firstly increased by 3 times at Bruker AMX–400 spectrometer (operating at 400 MHz for
50% water and then decreased when the water content was in- 1H NMR and 100 MHz for 13C NMR) in CDCl3 with TMS as
creased to 70% (Figure 4c). We attributed the enhanced emis- an internal standard. Due to the poor solubility, NMR spectra of
sion of P5A-DPA in the presence of less than 50% water to an P5A-DPA were recorded at room temperature on a Bruker
inhibited rotation of the 9,10-phenyl groups in the DPA units, AMX-600 spectrometer (operating at 600 MHz for 1H NMR
which reduced the inactivation of the excited state through non- and 151 MHz 13C NMR) in CDCl3. High-resolution mass spec-
radiative transition [53]. However, when increasing the amount tra (HRMS) were measured using a Waters-Q-TOF Premiers
of water, the fluorescence starts to decrease due to the π–π (ESI) apparatus. UV–vis spectra were obtained on a JASCO
stacking of DPA, which led to aggregation-caused quenching v-650 spectrometer. Fluorescence spectra and fluorescence life-
(ACQ), as was demonstrated by UV–vis absorption and CD time decay measurements were recorded on a HORIBA Fluo-
studies. For compound P5A-Py, however, the fluorescence in- roMax–4 (TCSPC) spectrofluorimeter. Circular dichroism spec-
tensity continuously decreased when adding water into the THF tra were measured on a JASCO J-1500 spectrometer using a
solutions, and at 70% water content, a new emission peak at quartz cuvette of 1 cm path length installed in a Unisoku cryo-
540 nm was observed which was assigned to the excimer emis- stat. Dynamic light scattering (DLS) studies were done on a
sion of Py (Figure S23, Supporting Information File 1) [54].
Zetasizer Nano ZS90 instrument. A preparative Chiralpak IA
column was used for the separation of enantiomers.
Conclusion
Two new planar chiral macrocyclic hosts P5A-DPA and P5A- Synthesis and characterization of P5A-DPA
Py were synthesized by grafting two fluorophore pigments The synthesis and characterization data of P5A-Py have been
(DPA or Py) on pillar[5]arene through CuAAC “click” reaction. reported elsewhere [34].
The new macrocyclic compounds showed strong absorptions in
the visible range and high fluorescence quantum yield, e.g., Synthesis of 2: In a similar manner as described previously
P5A-DPA showed absorption at 350–410 nm and a quantum [55]. An aqueous solution of K2CO3 (5.4 g, 39.0 mmol) and
yield of 89.5% which is comparable to the monomer DPA-6. CsF (3.7 g, 24.3 mmol) was added to the solution of compound
On the other hand, P5A-Py with two perylene (Py) pigments 1 (5.0 g, 19.5 mmol) and phenylboronic acid (3.6 g, 25.4 mmol)
grafted on the pillar[5]arene showed intense absorption at in the mixed solvent of toluene (40 mL), tetrahydrofuran
385–460 nm and a quantum yield of 46.4%. The rotation of (60 mL) and water (10 mL). After bubbling argon through the
phenolic units was inhibited by introducing two bulky fluoro- mixture for 15 min, Pd(PPh3)4 (300 mg, 0.3 mmol) was added,
phores in one of the phenolic units of pillar[5]arene, and the en- and the mixture was stirred and refluxed for 8 h. Then, the reac-
antiomers were isolated by chiral HPLC. The absolute configu- tion mixture was extracted with dichloromethane, and the
rations of each fraction were determined by circular dichroism organic layer was dried over anhydrous Na2SO4. The solution
(CD) spectroscopy. The first fraction showing negative CD was concentrated under reduced pressure to obtain the crude
signals was assigned to the Sp conformer and the second frac- product which was further purified by chromatography (silica
tion showing positive CD signals was assigned to the Rp gel, dichloromethane/petroleum ether 30:1) to afford com-
conformer. Aggregation behaviors of the enantiopure chiral pound 2 as white solid (4.8 g, 82.0%). Mp 156 °C; the 1H NMR
pillar[5]arene derivatives were investigated by adding water spectrum of 2 is shown in Figure S1, Supporting Information
into the THF solution. P5A-DPA and P5A-Py showed an en- File 1. 1H NMR (400 MHz, CDCl3) 8.49 (s, 1H), 8.04 (d, J =
hancement of the CD signals by adding water due to restriction 8.5 Hz, 2H), 7.67 (d, J = 8.8 Hz, 2H), 7.60–7.51 (m, 3H),
of the conformers’ rotations by aggregation. P5A-DPA showed 7.47–7.42 (m, 4H), 7.36–7.34 (m, 2H). The 13C NMR spec-
a fluorescence enhancement when less than 50% of water was trum of 2 is shown in Figure S2, Supporting Information File 1.
added, while P5A-Py showed emission quenching and excimer 13C NMR (100 MHz, CDCl3) 138.76, 137.01, 130.33, 131.22,
emission when adding water into the THF solution. This work 130.18, 128.34, 128.30, 127.42, 126.81, 126.52, 125.30, 125.07.
presented a new strategy for achieving versatile planar chiral The electrospray ionization mass spectrum of 2 is shown in
hosts, and these hosts will have potential applications in various Figure S3, Supporting Information File 1. HRESIMS (m/z):
fields such as supramolecular sensing, host–guest recognition [M]+ calcd for [C20H14]+, 254.1096, found, 254.1125;
and triplet–triplet annihilation upconversion.
[M + Na]+ calcd for [C20H14Na]+, 277.0993; found, 277.1268.
1607

Beilstein J. Org. Chem. 2019, 15, 1601–1611.
Synthesis of 3: In a Schlenk tube under argon, compound 2 Information File 1. 13C NMR (100 MHz, CDCl3) 158.46,
(2.5 g, 10.0 mmol) and N-bromobutanimide (2.1 g, 12.0 mmol) 139.15, 136.97, 136.95, 132.41, 131.36, 131.07, 130.23, 129.93,
were dissolved in chloroform (30.0 mL). The mixture was 128.43, 127.47, 127.08, 126.98, 124.99, 124.93, 114.40, 67.70,
heated at 60 °C for 2 h and then the solvent evaporated under 33.71, 32.59, 28.62, 25.01. The electrospray ionization mass
reduced pressure. Afterwards, dichloromethane and water were spectrum of 4 is shown in Figure S12, Supporting Information
added, and the organic layer was collected and dried over an- File 1. HRESIMS (m/z): [M]+ calcd for [C31H27BrO]+,
hydrous Na2SO4. The crude product was purified by column 494.1245; found, 494.1265; [M + H]+ calcd for [C31H28BrO]+,
chromatography using dichloromethane/petroleum ether 20:1 as 495.1324; found, 495.1264.
the eluent to yield 3 as white solid (2.8 g, 87%). Mp
154–155 °C. The 1H NMR spectrum of 3 is shown in Figure S4, Synthesis of DPA-6: To a solution of compound 5 (400.0 mg,
Supporting Information File 1. 1H NMR (400 MHz, CDCl3) 0.81 mmol) in DMF (15 mL) was added NaN3 (510.3 mg,
8.61 (d, J = 8.9 Hz, 2H), 7.64 (d, J = 8.7 Hz, 2H), 7.61–7.53 (m, 8.1 mmol). The mixture was stirred at 45 °C for 4 h (monitored
5H), 7.42–7.33 (m, 4H). The 13C NMR spectrum of 3 is shown by TLC until complete consumption of the starting material).
in Figure S5, Supporting Information File 1. 13C NMR The reaction mixture was poured into water and the aqueous
(100 MHz, CDCl3) 138.38, 137.79, 131.12, 131.02, 130.22, layer was extracted with ether (3 × 10 mL). The combined
128.43, 127.83, 127.73, 127.47, 127.38, 126.93, 125.52, 122.71. organic layers were dried over Na2SO4 and concentrated in
The electrospray ionization mass spectrum of 3 is shown in vacuo. The residue was purified by column chromatography on
Figure S6, Supporting Information File 1. HRESIMS (m/z): silica-gel (eluent: dichloromethane/petroleum ether 1:1) to give
[M]+ calcd for [C20H13Br]+, 332.0201; found, 332.1679.
compound DPA-6 as pale yellow solid (310.8 mg, 84%). Mp
140.2 °C. The 1H NMR spectrum of DPA-6 is shown in Figure
Synthesis of 4: Compound 4 was prepared by a similar proce- S13, Supporting Information File 1. 1H NMR (400 MHz,
dure as that used for 2, but with 3 (1.5 g, 70%) as the starting CDCl3) 7.81–7.74 (m, 2H), 7.72 (d, J = 7.4 Hz, 2H), 7.65–7.53
material. The 1H NMR spectrum of 4 is shown in Figure S7, (m, 3H), 7.53–7.46 (m, 2H), 7.44–7.31 (m, 6H), 7.17–7.11 (m,
Supporting Information File 1. Mp 245–247 °C; 1H NMR 2H), 4.14 (t, J = 6.3 Hz, 2H), 3.39 (t, J = 6.7 Hz, 2H), 2.00–1.89
(400 MHz, CDCl3) 7.77–7.72 (m, 2H), 7.71–7.65 (m, 2H), (m, 2H), 1.78 (d, J = 13.3 Hz, 2H), 1.72–1.63 (m, 2H). The
7.63–7.57 (m, 2H), 7.57–7.52 (m, 1H), 7.48–7.46 (m, 2H), 13C NMR spectrum of DPA-6 is shown in Figure S14, Support-
7.37–7.30 (m, 6H), 7.09–7.04 (m, 2H). The 13C NMR spec- ing Information File 1. 13C NMR (100 MHz, CDCl3) 158.48,
trum of 4 is shown in Figure S8, Supporting Information File 1. 139.16, 136.97, 132.43, 131.37, 131.07, 130.25, 129.95, 128.45,
13C NMR (100 MHz, CDCl3) 154.97, 139.08, 136.98, 136.75, 127.48, 127.10, 127.05, 125.01, 124.95, 114.40, 67.69, 51.46,
132.57, 131.31, 130.18, 129.89, 129.66, 128.39, 127.44, 126.97, 29.01, 28.78, 23.57. The electrospray ionization mass spectrum
126.94, 124.97, 124.94, 120.78, 115.35, 115.27. The electro- of DPA-6 is shown in Figure S15, Supporting Information
spray ionization mass spectrum of 4 is shown in Figure S9, File 1. HRESIMS (m/z): [M]+ calcd for [C31H27N3O]+),
Supporting Information File 1. HRESIMS (m/z): [M − H]− 457.2154; found, 457.2150; [M + H]+ calcd for [C31H28N3O]+,
calcd for [C26H17O]−, 345.1279; found, 345.1277.
458.2232; found, 458.2213.
Synthesis of 5: Under nitrogen atmosphere, a mixture of com- Synthesis of P5A: Analogous as previously described [56]. To
pound 4 (800.0 mg, 2.3 mmol), K2CO3 (952.2 mg, 6.9 mmol) a solution of 1, 4-dimethoxybenzene (1.7 g, 1.2 mmol) and 7
and 1,5-dibromopentane (1.1 g, 4.6 mmol) was dissolved in (55.8 mg, 0.3 mmol) in 1,2-dichloroethane (150 mL), paraform-
30 mL dry acetone and the mixture was heated under reflux for aldehyde (271.5 mg, 3.0 mmol) was added under nitrogen at-
30 h. After cooling, the solvents were removed under reduced mosphere. Then, boron trifluoride diethyl etherate (120 μL) was
pressure and the residue was purified by column chromatogra- added to the solution and the mixture was stirred at room tem-
phy on silica gel (100–200 mesh) using a mixture dichloro- perature for 1 h. Water (100 mL) was added to quench the reac-
methane/petroleum ether 1:1 as the eluent to give compound 5 tion. The mixture was filtered and the solvent was removed.
in 67% yield, 760.8 mg. Mp 155.6–161.1 °C. The 1H NMR The residue was dissolved in dichloromethane. The organic
spectrum of 5 is shown in Figure S10, Supporting Information layer was dried over anhydrous Na2SO4 and the solvent was
File 1. 1H NMR (400 MHz, CDCl3) 7.78–7.72 (m, 2H), evaporated to afford the crude product, which was isolated by
7.71–7.66 (m, 2H), 7.58 (d, J = 15.4 Hz, 3H), 7.49–7.45 (m, flash column chromatography using ethyl acetate/petroleum
2H), 7.34 (d, J = 13.6 Hz, 6H), 7.12 (d, J = 8.7 Hz, 2H), 4.12 (t, ether 1:5 (v/v) as the eluent to give P5A as white solid
J = 6.3 Hz, 2H), 3.50 (t, J = 6.8 Hz, 2H), 2.02 (d, J = 14.7 Hz, (108.8 mg, 14%). The 1H NMR spectrum of P5A is shown in
2H), 1.96–1.88 (m, 2H), 1.73 (d, J = 8.5 Hz, 2H). The Figure S16, Supporting Information File 1. 1H NMR (400 MHz,
13C NMR spectrum of 5 is shown in Figure S11, Supporting CDCl3) 6.82 (s, 2H), 6.80 (s, 2H), 6.79 (s, 2H), 6.76 (s, 2H),
1608

Beilstein J. Org. Chem. 2019, 15, 1601–1611.
6.73 (s, 2H), 4.49 (d, J = 2.3 Hz, 4H), 3.80–3.76 (m, 10H), velopment Program of China (No. 2017YFA0505903), and
3.70–3.64 (m, 24H), 2.02 (t, J = 2.3 Hz, 2H). The 13C NMR Science & Technology Department of Sichuan Province
spectrum of P5A is shown in Figure S17, Supporting Informa- (2019YJ0090, 2019YJ0160, 2017SZ0021). Comprehensive
tion File 1. 13C NMR (100 MHz, CDCl3) 150.71, 150.67, Training Platform of Specialized Laboratory, College of Chem-
150.64, 150.57, 149.33, 129.09, 128.37, 128.31, 128.03, 127.85, istry and Prof. Peng Wu of Analytical &Testing Center,
115.74, 114.23, 113.96, 113.86, 113.71, 78.87, 74.72, 56.36, Sichuan University for lifetime measurement are greatly appre-
55.83, 55.79, 55.64, 29.69, 29.61. The electrospray ionization ciated.
mass spectrum of P5A is shown in Figure S18, Supporting
ORCID® iDs
Information File 1. HRESIMS (m/z): [M + Na]+ calcd for
[C49H50NaO10]+, 821.3302; found, 821.3292.
Guojuan Li - https://orcid.org/0000-0003-0910-2663
Chunying Fan - https://orcid.org/0000-0001-8884-6067
Wanhua Wu - https://orcid.org/0000-0002-8969-1148
Cheng Yang - https://orcid.org/0000-0002-2049-1324
Synthesis of P5A-DPA: A mixture of compound P5A
(20.0 mg, 0.02 mmol), DPA-6 (47.2 mg, 0.1 mmol),
CuSO4·5H2O (12.0 mg, 0.05 mmol), and sodium ascorbate
References
(19.0 mg, 0.1 mmol) in DMF (5 mL) was refluxed under N2 at-
1. Ôi, S.; Miyano, S. Chem. Lett. 1992, 21, 987–990.
doi:10.1246/cl.1992.987
mosphere for 48 hours. The solvent was removed by rotary
evaporation and the residue was purified by chromatography on
silica gel (dichloromethane/ethyl acetate 30:1) to yield a brown
solid (30.0 mg, 72%). The 1H NMR spectrum of P5A-DPA is
shown in Figure S19, Supporting Information File 1. 1H NMR
(600 MHz, CDCl3) 7.71 (m, J = 10.6 Hz, 8H), 7.57 (m, J =
7.2 Hz, 6H), 7.51–7.44 (m, 4H), 7.34 (m, J = 9.9 Hz, 12H),
7.22–6.75 (m, 14H), 4.07–3.39 (m, 40H), 2.97 (m, 4H),
2.14–1.59 (m, 14H). The 13C NMR spectrum of P5A-DPA is
shown in Figure S20, Supporting Information File 1. 13C NMR
(151 MHz, CDCl3) 171.17, 150.96, 139.08, 137.03, 136.89,
132.39, 131.33, 130.22, 129.96, 128.45, 127.50, 127.06, 126.92,
125.01, 124.98, 114.33, 113.82, 99.99, 67.49, 60.42, 58.48,
56.23, 55.72, 53.41, 31.95, 31.46, 30.21, 29.72, 29.68, 29.41,
29.38, 29.01, 29.00, 22.71, 21.07, 18.44, 14.22. The electro-
spray ionization mass spectrum of P5A-DPA is shown in
Figure S21, Supporting Information File 1. HRESIMS (m/z):
[M + Na]+ calcd for [C111H104N6NaO12]+, 1736.7643; found,
1736.7744; [M + K]+ calcd for [C111H104N6KO12]+,
1753.1813; found, 1753.7510.
2. Hattori, T.; Harada, N.; Oi, S.; Abe, H.; Miyano, S.
Tetrahedron: Asymmetry 1995, 6, 1043–1046.
doi:10.1016/0957-4166(95)00120-e
3. Fiesel, R.; Huber, J.; Scherf, U. Angew. Chem., Int. Ed. Engl. 1996, 35,
2111–2113. doi:10.1002/anie.199621111
4. Fiesel, R.; Huber, J.; Apel, U.; Enkelmann, V.; Hentschke, R.;
Scherf, U.; Cabrera, K. Macromol. Chem. Phys. 1997, 198,
2623–2650. doi:10.1002/macp.1997.021980901
5. Katoono, R.; Kawai, H.; Fujiwara, K.; Suzuki, T. Tetrahedron Lett.
2004, 45, 8455–8459. doi:10.1016/j.tetlet.2004.09.115
6. Wei, X.; Wu, W.; Matsushita, R.; Yan, Z.; Zhou, D.; Chruma, J. J.;
Nishijima, M.; Fukuhara, G.; Mori, T.; Inoue, Y.; Yang, C.
J. Am. Chem. Soc. 2018, 140, 3959–3974. doi:10.1021/jacs.7b12085
7. Rao, M.; Kanagaraj, K.; Fan, C.; Ji, J.; Xiao, C.; Wei, X.; Wu, W.;
Yang, C. Org. Lett. 2018, 20, 1680–1683.
doi:10.1021/acs.orglett.8b00520
8. Rao, M.; Wu, W.; Yang, C. Molecules 2019, 24, No. 1502.
doi:10.3390/molecules24081502
9. Ogoshi, T.; Yamafuji, D.; Kotera, D.; Aoki, T.; Fujinami, S.;
Yamagishi, T.-a. J. Org. Chem. 2012, 77, 11146–11152.
doi:10.1021/jo302283n
10.Ogoshi, T.; Kanai, S.; Fujinami, S.; Yamagishi, T.-a.; Nakamoto, Y.
J. Am. Chem. Soc. 2008, 130, 5022–5023. doi:10.1021/ja711260m
11.Si, W.; Chen, L.; Hu, X.-B.; Tang, G.; Chen, Z.; Hou, J.-L.; Li, Z.-T.
Angew. Chem., Int. Ed. 2011, 50, 12564–12568.
Supporting Information
doi:10.1002/anie.201106857
12.Hu, X.-B.; Chen, Z.; Tang, G.; Hou, J.-L.; Li, Z.-T. J. Am. Chem. Soc.
2012, 134, 8384–8387. doi:10.1021/ja302292c
13.Li, H.; Chen, D.-X.; Sun, Y.-L.; Zheng, Y. B.; Tan, L.-L.; Weiss, P. S.;
Yang, Y.-W. J. Am. Chem. Soc. 2013, 135, 1570–1576.
doi:10.1021/ja3115168
14.Wei, P.; Li, D.; Shi, B.; Wang, Q.; Huang, F. Chem. Commun. 2015, 51,
15169–15172. doi:10.1039/c5cc06682d
Supporting Information File 1
Characterization spectra of all compounds, chiral HPLC
traces of P5A-Py, CD and UV–vis spectra of the two
fractions P5A-Py and the aggregation behaviors of
P5A-Py.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-164-S1.pdf]
15.Jie, K.; Zhou, Y.; Li, E.; Huang, F. Acc. Chem. Res. 2018, 51,
2064–2072. doi:10.1021/acs.accounts.8b00255
16.Jie, K.; Zhou, Y.; Li, E.; Zhao, R.; Huang, F. Angew. Chem., Int. Ed.
2018, 57, 12845–12849. doi:10.1002/anie.201808998
17.Li, E.; Zhou, Y.; Zhao, R.; Jie, K.; Huang, F. Angew. Chem., Int. Ed.
2019, 58, 3981–3985. doi:10.1002/anie.201900140
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (Nos. 21402129, 21871194, 21572142,
21372165 and 21321061), the National Key Research and De-
1609

Beilstein J. Org. Chem. 2019, 15, 1601–1611.
18.Ogoshi, T.; Kitajima, K.; Aoki, T.; Yamagishi, T.-a.; Nakamoto, Y.
J. Phys. Chem. Lett. 2010, 1, 817–821. doi:10.1021/jz900437r
19.Gui, J.-C.; Yan, Z.-Q.; Peng, Y.; Yi, J.-G.; Zhou, D.-Y.; Su, D.;
Zhong, Z.-H.; Gao, G.-W.; Wu, W.-H.; Yang, C. Chin. Chem. Lett.
2016, 27, 1017–1021. doi:10.1016/j.cclet.2016.04.021
20.Yao, J.; Wu, W.; Liang, W.; Feng, Y.; Zhou, D.; Chruma, J. J.;
Fukuhara, G.; Mori, T.; Inoue, Y.; Yang, C. Angew. Chem., Int. Ed.
2017, 56, 6869–6873. doi:10.1002/anie.201702542
21.Lv, Y.; Xiao, C.; Yang, C. New J. Chem. 2018, 42, 19357–19359.
doi:10.1039/c8nj04802a
42.Xu, K.; Zhao, J.; Escudero, D.; Mahmood, Z.; Jacquemin, D.
J. Phys. Chem. C 2015, 119, 23801–23812.
doi:10.1021/acs.jpcc.5b05325
43.Ogoshi, T.; Yamafuji, D.; Yamagishi, T.-a.; Brouwer, A. M.
Chem. Commun. 2013, 49, 5468–5470. doi:10.1039/c3cc42612b
44.Yan, Z.; Huang, Q.; Liang, W.; Yu, X.; Zhou, D.; Wu, W.; Chruma, J. J.;
Yang, C. Org. Lett. 2017, 19, 898–901.
doi:10.1021/acs.orglett.7b00057
45.Dai, L.; Wu, W.; Liang, W.; Chen, W.; Yu, X.; Ji, J.; Xiao, C.; Yang, C.
Chem. Commun. 2018, 54, 2643–2646. doi:10.1039/c8cc00840j
46.Yi, J.; Liang, W.; Wei, X.; Yao, J.; Yan, Z.; Su, D.; Zhong, Z.; Gao, G.;
Wu, W.; Yang, C. Chin. Chem. Lett. 2018, 29, 87–90.
doi:10.1016/j.cclet.2017.05.004
22.Ogoshi, T.; Yamafuji, D.; Aoki, T.; Kitajima, K.; Yamagishi, T.-a.;
Hayashi, Y.; Kawauchi, S. Chem. – Eur. J. 2012, 18, 7493–7500.
doi:10.1002/chem.201200122
23.Kitajima, K.; Ogoshi, T.; Yamagishi, T.-a. Chem. Commun. 2014, 50,
2925–2927. doi:10.1039/c3cc49794a
47.Alagesan, M.; Kanagaraj, K.; Wan, S.; Sun, H.; Su, D.; Zhong, Z.;
Zhou, D.; Wu, W.; Gao, G.; Zhang, H.; Yang, C.
24.Ogoshi, T.; Kitajima, K.; Aoki, T.; Fujinami, S.; Yamagishi, T.-a.;
Nakamoto, Y. J. Org. Chem. 2010, 75, 3268–3273.
doi:10.1021/jo100273n
25.Ogoshi, T.; Masaki, K.; Shiga, R.; Kitajima, K.; Yamagishi, T.-a.
Org. Lett. 2011, 13, 1264–1266. doi:10.1021/ol200062j
26.Nierengarten, I.; Buffet, K.; Holler, M.; Vincent, S. P.;
Nierengarten, J.-F. Tetrahedron Lett. 2013, 54, 2398–2402.
doi:10.1016/j.tetlet.2013.02.100
J. Photochem. Photobiol., A 2016, 331, 95–101.
doi:10.1016/j.jphotochem.2015.10.023
48.Wei, X.; Yu, X.; Zhang, Y.; Liang, W.; Ji, J.; Yao, J.; Rao, M.; Wu, W.;
Yang, C. J. Photochem. Photobiol., A 2019, 371, 374–381.
doi:10.1016/j.jphotochem.2018.11.038
49.Yang, C.; Wang, Q.; Yamauchi, M.; Yao, J.; Zhou, D.; Nishijima, M.;
Fukuhara, G.; Mori, T.; Liu, Y.; Inoue, Y. Photochem. Photobiol. Sci.
2014, 13, 190–198. doi:10.1039/c3pp50255d
27.Ogoshi, T.; Yamafuji, D.; Akutsu, T.; Naito, M.; Yamagishi, T.-a.
Chem. Commun. 2013, 49, 8782–8784. doi:10.1039/c3cc44672g
28.Strutt, N. L.; Schneebeli, S. T.; Stoddart, J. F. Supramol. Chem. 2013,
25, 596–608. doi:10.1080/10610278.2013.822973
29.Strutt, N. L.; Fairen-Jimenez, D.; Iehl, J.; Lalonde, M. B.; Snurr, R. Q.;
Farha, O. K.; Hupp, J. T.; Stoddart, J. F. J. Am. Chem. Soc. 2012, 134,
17436–17439. doi:10.1021/ja3082523
30.Mastai, Y. Chem. Soc. Rev. 2009, 38, 772–780.
doi:10.1039/b812587m
31.Wu, X.; Gao, L.; Hu, X.-Y.; Wang, L. Chem. Rec. 2016, 16, 1216–1227.
doi:10.1002/tcr.201500265
50.Yao, J.; Yan, Z.; Ji, J.; Wu, W.; Yang, C.; Nishijima, M.; Fukuhara, G.;
Mori, T.; Inoue, Y. J. Am. Chem. Soc. 2014, 136, 6916–6919.
doi:10.1021/ja5032908
51.Yang, C.; Inoue, Y. Chem. Soc. Rev. 2014, 43, 4123–4143.
doi:10.1039/c3cs60339c
52.Wu, J.; Liang, W.; Niu, T.; Wu, W.; Zhou, D.; Fan, C.; Ji, J.; Gao, G.;
Men, J.; Yang, Y.; Yang, C. Chem. Commun. 2018, 54, 9206–9209.
doi:10.1039/c8cc03660h
53.Leung, N. L. C.; Xie, N.; Yuan, W.; Liu, Y.; Wu, Q.; Peng, Q.; Miao, Q.;
Lam, J. W. Y.; Tang, B. Z. Chem. – Eur. J. 2014, 20, 15349–15353.
doi:10.1002/chem.201403811
32.Feng, W.; Jin, M.; Yang, K.; Pei, Y.; Pei, Z. Chem. Commun. 2018, 54,
13626–13640. doi:10.1039/c8cc08252a
33.Hu, X.-Y.; Gao, L.; Mosel, S.; Ehlers, M.; Zellermann, E.; Jiang, H.;
Knauer, S. K.; Wang, L.; Schmuck, C. Small 2018, 14, No. 1803952.
doi:10.1002/smll.201803952
54.Basu, B. J.; Thirumurugan, A.; Dinesh, A. R.; Anandan, C.;
Rajam, K. S. Sens. Actuators, B 2005, 104, 15–22.
doi:10.1016/j.snb.2004.04.092
55.Zhong, F.; Zhao, J. Dyes Pigm. 2017, 136, 909–918.
doi:10.1016/j.dyepig.2016.09.057
34.Fan, C.; Wu, W.; Chruma, J. J.; Zhao, J.; Yang, C. J. Am. Chem. Soc.
2016, 138, 15405–15412. doi:10.1021/jacs.6b07946
35.Xu, W.; Liang, W.; Wu, W.; Fan, C.; Rao, M.; Su, D.; Zhong, Z.;
Yang, C. Chem. – Eur. J. 2018, 24, 16677–16685.
doi:10.1002/chem.201804001
56.Wu, X.; Zhang, Y.; Lu, Y.; Pang, S.; Yang, K.; Tian, Z.; Pei, Y.; Qu, Y.;
Wang, F.; Pei, Z. J. Mater. Chem. B 2017, 5, 3483–3487.
doi:10.1039/c7tb00752c
36.Ogawa, T.; Yanai, N.; Monguzzi, A.; Kimizuka, N. Sci. Rep. 2015, 5,
No. 10882. doi:10.1038/srep10882
37.Peng, H.-Q.; Chen, Y.-Z.; Zhao, Y.; Yang, Q.-Z.; Wu, L.-Z.;
Tung, C.-H.; Zhang, L.-P.; Tong, Q.-X. Angew. Chem., Int. Ed. 2012,
51, 2088–2092. doi:10.1002/anie.201107723
38.Yu, G.; Zhang, Z.; Han, C.; Xue, M.; Zhou, Q.; Huang, F.
Chem. Commun. 2012, 48, 2958–2960. doi:10.1039/c2cc00125j
39.Nierengarten, I.; Guerra, S.; Holler, M.; Nierengarten, J. F.;
Deschenaux, R. Chem. Commun. 2012, 48, 8072–8074.
doi:10.1039/c2cc33746k
40.Cao, D.; Kou, Y.; Liang, J.; Chen, Z.; Wang, L.; Meier, H.
Angew. Chem., Int. Ed. 2009, 48, 9721–9723.
doi:10.1002/anie.200904765
41.Liu, L.; Cao, D.; Jin, Y.; Tao, H.; Kou, Y.; Meier, H. Org. Biomol. Chem.
2011, 9, 7007–7010. doi:10.1039/c1ob05871a
1610

Beilstein J. Org. Chem. 2019, 15, 1601–1611.
License and Terms
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/4.0). Please note
that the reuse, redistribution and reproduction in particular
requires that the authors and source are credited.
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(https://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.15.164
1611