Twisted baskets

Article abstract of DOI:10.1002/chem.201406492

A preparative procedure for obtaining a pair of twisted molecular baskets, each comprising a chiral framework with either right ((P)-1_syn) or left ((M)-1_syn) sense of twist and six ester groups at the rim has been developed and optim

Full text of DOI:10.1002/chem.201406492

DOI: 10.1002/chem.201406492
Communication
&
Host–Guest Systems
Twisted Baskets
[a]
[a]
[a]
[a]
[b]
Keith Hermann, Yaowalak Pratumyot, Shane Polen, Alex M. Hardin, Erdin Dalkilic,
[b]
[a]
Arif Dastan, and Jovica D. Badji c´ *
model”, for predicting the complexation aptitude of C /D sym-
2
2
[6]
metric 1,1’-binapthyl corands toward racemic amino acids. In
Abstract: A preparative procedure for obtaining a pair of
twisted molecular baskets, each comprising a chiral frame-
work with either right ((P)-1 ) or left ((M)-1 ) sense of
[7]
this vein, the classical three-point interaction model or other
[8]
variants are employed to explain diastereoselectivity of drug–
syn
syn
[9]
receptor interactions as well as inclusion complexations.
Indeed, artificial hosts with an enforced cavity—cavitands—are
twist and six ester groups at the rim has been developed
and optimized. The racemic (P/M)-1syn can be obtained in
three synthetic steps from accessible starting materials.
The resolution of (P/M)-1syn is accomplished by its trans-
esterification with (1R,2S,5R)-(ꢀ)-menthol in the presence
[10]
also capable of resolving chiral compounds yet the experi-
mental outcome of such recognition events is difficult to ra-
[11]
tionalize let alone predict. As many naturally occurring mole-
cules, drugs, metabolites, chemical weapons and commodity
chemicals lack the rotation–reflection axis of symmetry, there
IV
P
m
of a Ti catalyst to give diastereomeric 8 and 8 . It was
P
m
found that dendritic-like cavitands 8 and 8 , in CD Cl ,
undergo self-inclusion ( H NMR spectroscopy) with a men-
thol moiety occupying the cavity of each host. Important-
2
2
[12]
1
exists a need to expand the scope of artificial chiral hosts to
[13]
selectively capture these substances
on the basis of their
size, shape and electronic structure, as well as learn more
about rules that govern the process of stereoselective recogni-
ly, the degree of inclusion of the menthol group was
1
(
H NMR spectroscopy) found to be greater in the case of
[8b,14]
P
m
tion.
Artificial chiral receptors could, furthermore, serve as
8
than 8 . Accordingly, it is suggested that different
P
m
counterparts to naturally occurring enzymes and antibodies for
rapidly and accurately reporting on the presence of stereoiso-
folding characteristic of 8 and 8 ought to affect the
physicochemical characteristics of the hosts to permit
their effective separation by column chromatography. The
[15]
meric substances in the environment and promoting their
[
16]
P
m
transformation.
In line with a need for developing cup-
absolute configuration of 8 /8 , encompassing right- and
left-handed “cups”, was determined with the exciton chir-
ality method and also verified in silico (DFT: B3LYP/TZVP).
Finally, the twisted baskets are strongly fluorescent due to
three naphthalene chromophores, having a high fluores-
[17]
[18]
shaped chiral hosts,
we recently described
a synthetic
method for the preparation of novel cavitands possessing
a nonfunctional hydrocarbon framework and twisted (Figure 1)
[17a,c,d]
inner space.
In particular, a tandem of cyclialkylation
P
m
reactions was promoted with strong acids to, via general-acid
catalysis, give rise to baskets (akin to (P/M)-1syn in Figure 1)
comprising six stereogenic centers of the same kind (R or S)
embedded in the host’s bicyclic platform. In this study, we fo-
cused on developing a preparative procedure for obtaining
a pair of functionalized baskets with right (P)-1syn and left (M)-
1syn twisted frameworks (Figure 1) to learn about the resolution
of such racemic host. Notably, chiral baskets of type 1 are C3
symmetric and modular cavitands, possessing: 1) six esters at
the rim for additional functionalization, 2) unique chiroptical
characteristics, 3) photochemically sensitizing sidewalls for
cence quantum yield within the rigid framework of 8 /8 .
[
1]
The resolution of chiral drugs and drug intermediates by frac-
[
2]
tional crystallization of diastereomeric salts represents a form
of stereoselective molecular recognition, allowing the produc-
[
3]
tion of enantiopure pharmaceuticals. In line with this useful
methodology, Newman and co-workers completed the separa-
tion of racemic (P/M)-hexahelicenes using a chiral derivative of
[
4]
fluorenone.
Allegedly, the aromatic fluorenone forms
[
19]
a
charge-transfer complex with inherently chiral hexa-
promoting photochirogenesis,
and 4) deep and twisted
[
5]
[20]
helicenes so that the attractive p–p interactions trigger the
aggregation and consequently the precipitation of the less-
soluble diastereomer. Furthermore, Cram and co-workers
developed a semi-empirical approach, “the tripodal binding
hydrophobic pockets
for discriminating chiral guests
[21]
(
Figure 1).
To obtain basket 1, we started with commercially available
-indanol and converted it into compound 2 (Scheme 1)
[22]
5
following already known procedures (Scheme S3 in the
[23]
[
a] K. Hermann, Y. Pratumyot, S. Polen, A. M. Hardin, Prof. J. D. Badjic´
Supporting Information).
The free-radical bromination of
Department of Chemistry and Biochemistry
The Ohio State University, 100 West 18th Avenue (USA)
E-mail: badjic.1@osu.edu
indane derivative 2 followed by, allegedly, E1 elimination of
the bromoalkane intermediate gave indene 3. Compound 3
was deprotonated with a strong base (nBuLi), at a low temper-
ature, to act as a nucleophile in promoting the substitution of
three iodine groups in 1,3,5-tris(iodomethyl)benzene. When
these reactants were combined in the proportion of 6:1 (with
[
b] E. Dalkilic, Prof. A. Dastan
Ataturk University, Faculty of Sciences, Erzurum (Turkey)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406492.
Chem. Eur. J. 2015, 21, 3550 – 3555
3550
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Communication
used menthol as solvent and ran
the reaction at an elevated tem-
perature (1808C) for a prolonged
[27]
period of time.
Importantly,
the chromatographic separation
P
m
of diastereomeric 8 /8
was
facile with each compound
having a distinct R value (R =
f
f
0
.37 and 0.50, Figure 2).
1
H NMR spectra (400 MHz,
P
m
CD Cl ) of isolated 8 and 8
2
2
(
Figure 3A) revealed a single set
of resonances corresponding to,
in each case, a C3 symmetric
compound; note that there was
1
no decoalescence of H NMR
signals at lower temperatures,
suggesting a rapid rotation of
the menthol moieties at the rim
(
from 273.0 to 233.0 K, Figur-
es S28 and S29 in the Support-
ing Information). First, we as-
signed proton resonances of
model compound 9 (Figure 3A
and B; for a full assignment see
Figures S17–S19 in the Support-
Figure 1. Energy-minimized structures (MMFFs) of twisted baskets (P)-1syn and (M)-1syn (top) and their chemical
structures (bottom).
1
1
ing Information) using its H- H
RRR
sixfold excess of indene 3), two diastereomeric products 4
4
/
COSY NMR correlations (Figures S17–S19) in addition to the re-
SSS
RRS SSR
[28]
and 4 /4 (Scheme S4 in the Supporting Information)
ported spectroscopic assignment for (1R,2S,5R)-(ꢀ)-menthol.
1
1
P
m
formed in a ratio of 1:4, respectively. The cyclialkylation of the
diastereomeric mixture of 4 was
Next, we recorded H- H COSY NMR spectra of 8 /8
then catalyzed with methanesul-
fonic acid to give (P/M)-1syn and
1anti (Scheme 1) in the same 1:4
proportion. On the basis of our
[
18]
earlier study, we reasoned that
this transformation is under ki-
netic control with homochiral
RRR SSS
4
/4 giving the desired (P/M)-
RRS SSR
1syn and heterochiral
4 /4
transforming into 1anti product;
to optimize the yield of cup-
shaped (P/M)-1 , we examined
syn
the stereoselectivity of the con-
version of 3 into 4, and describe
these efforts in Scheme S30 in
the Supporting Information.
To resolve racemic (P/M)-1syn,
we decided to investigate the
[
24]
transesterification of (P/M)-1syn
with (1R,2S,5R)-(ꢀ)-menthol in
the presence of titanium(IV) cat-
[
25]
[26]
alyst 7 (Figure 2). To ensure
complete transformation of
the hexaester reactant into steri-
a
P
m
cally hindered 8 and 8 , with six
Scheme 1. The synthesis of twisted basket (P/M)-1syn can be completed in several steps from simple starting
menthol groups at the rim, we material.
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Communication
at different times due to, in part,
their distinctive folding charac-
teristics with the more compact
P
8
being retained to a greater
degree.
To additionally probe the ab-
solute configuration of com-
P
m
pounds 8 /8 , encompassing
right- and left-handed “cups”
(
Figure 3), we used the exciton
chirality method (exciton cou-
pled circular dichroism spectros-
[29]
copy, ECCD).
In this regard,
three naphthalene chromo-
[
30]
phores
that constitute the
P
m
“
sides” of twisted baskets 8 /8
are embedded in the bicyclic
and chiral framework, formally
belonging to the second chiral
[31]
sphere.
Upon the absorption
of light at about 220 nm (naph-
1
thalene’s Bb transition in the
[30]
Platt’s notation),
each of the
juxtaposed naphthalenes should
P
m
in 8 /8 develop a strong electric
dipole transition moment posi-
tioned along the long axis of the
P
m
IV
2
Figure 2. The chromatographic separation (SiO , hexanes/diethyl ether=2:1) of 8 and 8 , obtained by a Ti -
[32]
aromatic ring (Figure 4A).
In
there
promoted transesterification of (P/M)-1syn, is facile, with each compound showing a separate band on a thin-layer
chromatographic plate. Spectroscopic analyses of the chromatographic fractions are in line with the top band
[
31]
line with the ECCD,
m
P
corresponding to 8 while the bottom one to 8 .
should be an exciton coupling
1
of the B transition dipole mo-
b
ments of the three degenerate
chromophores to give rise to a bisignate spectrum with the ro-
tational strengths and sign being a function of their interchro-
(
Figures S20–S27 in the Supporting Information) and assigned
all of the observed resonances to proton nuclei within these
diastereomeric compounds (Figure 3A and B, for a full assign-
[31]
mophoric distance (d) and orientation (W). In particular, the
sign of the couplet is based on the semi-empirical exciton chir-
ality rule which states: the negative chirality (the longer wave-
length component has De<0) is obtained when the chromo-
phores are positioned in space so that a counterclockwise rota-
tion of the front electric dipole moment by an acute angle
1
ment see Figures S20–S27). A correlation between H NMR
P
m
spectra of C symmetric 9 and C symmetric 8 /8 is apparent
yet we note an upfield shift of: 1) two doublets corresponding
to Ha/b nuclei, and 2) multiplet corresponding to the juxta-
2
3
posed H proton, all as a part of the menthol substituents (Fig-
m
[33]
ure 3A and B). To address the observation, we completed the
Monte Carlo conformational study (MMFFs force field, Spartan)
brings it onto the exciton axis in the back. Indeed, CD spec-
P
m
tra of diastereomeric baskets 8 /8 (Figure 4A) showed the ex-
pected ECCD couplets centered at 242 nm (p to p* transition,
P
m
of 8 /8 with the calculation suggesting a conformational dis-
tribution being predominantly populated (>95% on the basis
of the Boltzmann distribution at 298 K) with structures closely
[32]
Figure 4A) with two diastereomeric baskets having the op-
[21]
posite rotational strengths. Specifically, the positive Cotton
P
ꢀ1
ꢀ1
resembling those shown in Figure 3C. That is to say, the 8 dia-
effect (CE) at 234 nm (De =284m cm , blue spectrum in Fig-
1
stereomer places one of its menthol isopropyl groups in the
ure 4A) is accompanied with a negative CE at 251 nm (De =
2
ꢀ
1
ꢀ1
cavity of the cup-shaped and twisted framework so that H /Hb
ꢀ252m cm , blue spectrum in Figure 4A) to give the nega-
a
and H nuclei reside in the shielding region of the surrounding
tive sign of exciton chirality, which is consistent with the posi-
m
m
P
naphthalene rings. On the contrary, the 8 stereoisomer posi-
tion of exciton axis in 8 (Figure 4A). As the blue spectrum
P
tions an isopropyl group at top of the cavity (Figure 3C) with
H /H and H nuclei being less diamagnetically shielded with
(Figure 4A) corresponds to basket 8 isolated as the chromato-
graphic fraction with R =0.37, Figure 3), the CD assignment
a
b
m
f
1
1
the basket’s aromatics. It follows that the middle H NMR spec-
corroborates the absolute chirality obtained from the H NMR
m
trum in Figure 3A should correspond to 8 (R =0.50, Figure 2)
spectroscopic measurements (Figure 3). Furthermore, the large
f
having its H /Hb protons less diamagnetically shielded while
A value (jDe j + jDe j) of 535 indicates a through-space inter-
a
1
2
P
P
the bottom one correlates with 8 (R =0.37, Figure 2). Further-
action of the naphthalene chromophores in 8 with a contribu-
f
[35]
more, we surmise that the two diastereomers eluted on silica
tion from all three ECCD couplets
thereby following the
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Communication
1
m
P
Figure 3. A) H NMR spectra (400 MHz, CD
2
Cl
2
) of model compound 9 (top) and twisted baskets 8 (middle) and 8 (bottom). B) Chemical structures of 9 and
P
m
P
8
with selected protons labeled from H
a
to H
n
. C) Energy-minimized (MMFFs, Monte Carlo conformational search; 1000 steps) structures of 8 and 8 , repre-
senting the most abundant conformers (>95%) obtained in the calculation; note that CꢀH–p centroid distances are shown, while some hydrogen atoms are
removed for clarity.
[
31]
pair-wise additivity principle.
By comparison, model com-
tical sensors capable of reporting on the presence of minute
[38]
pound 9 exhibits a featureless CD spectrum (black spectrum in
Figure 4A) with low rotational strengths at all wavelengths.
Two menthol units are, in the absence of the bicyclic chiral
framework, exerting a negligible symmetry-breaking perturba-
quantities of chiral substances in the environment.
In conclusion, we have completed the preparation of
a novel cavitand comprising a twisted concave platform (P or
M) and six ester groups at the rim. This host is modular, that is,
prone to additional functionalization, with a deep and chiral
hydrophobic pocket made of three photochemically active
naphthalene rings. The functional and twisted baskets can
now be used for: 1) building chiroptical sensors capable of
reporting on the presence of minute quantities of chiral
[
31]
tion of the electronic states of 9. We also computed the UV/
Vis and CD spectra of energy-minimized (M)-1syn (Figure 4B)
using time-dependent density functional theory (TD-B3LYP/
[
34]
TZVP). Notably, there is a good agreement between the ob-
m
served positive exciton chirality of 8 centered at 242 nm and
the computed ECCD couplet of (M)-1 . The absolute configu-
ration of P/M twisted baskets is in this way confirmed and in
[39]
substances in the environment, 2) resolving useful drugs and
syn
[
13]
drug intermediates,
and/or 3) developing novel stereo-
[40]
line with the assignment from the ECCD method and NMR ex-
selective supramolecular catalysts.
P
m
periments. Finally, the emission spectra of diastereomeric 8 /8
are comparable (lexc. =280 nm, Figure S33 in the Supporting
Information). Importantly, the signal intensities for the baskets
(
I=282 a.u. at lem. =363 nm, Figure S33) are seventy times Acknowledgements
stronger than for the model compound 9 (I=4 a.u. at lem.
63 nm, Figure S33). Given that the absorption extinction coef-
=
3
This work was financially supported with funds obtained from
the National Science Foundation under CHE-1305179.
Computational support from the Ohio Supercomputer Center
is gratefully acknowledged. E.D. is grateful for the predoctoral
fellowship received from the Turkish National Science
Foundation (TUBITAK).
ficients (e, Figure 4) are at 280 nm three times greater for bas-
kets than for the model compound, we reason that naphtha-
lene chromophores ought to have a greater fluorescence
P
m [36]
quantum yield within the rigid framework of 8 /8 . Appa-
rently, twisted baskets are well suited
[
20a,37]
for building chirop-
3553
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
P
m
Figure 4. A) Absorption (UV/Vis, top left) and circular dichroism (CD, bottom left) spectra of 8 (blue, 2.5 mm), 8 (red, 2.5 mm) and 9 (black, 5.0 mm) in hexane
1
at 298 K. Energy-minimized structure of model compound 9 (MMFFs, Spartan) with the B
b
transition dipole moment along the naphthalene chromophore
top right). The negative exciton chirality characterizes twisted basket 8 since its three naphthalene chromophores (only two are shown) are positioned in
space so that a counterclockwise rotation of the front electric dipole moment (blue) by an acute angle brings it onto the exciton axis in the back (bottom
P
(
[
34]
right). B) Computed DFT: B3LYP/TZVP (black) UV/Vis (top) and CD (bottom) spectra of basket (M)-1syn; for comparison, UV/Vis (top) and CD (bottom) spectra
m
of 8 (red) are included. The blue sticks are computed electronic transitions that were subjected to Gaussian broadening (0.25 eV) and wavelength shift
(
ꢀ0.2 eV) for generating the computed spectra.
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Keywords: cavitands · encapsulation · host–guest systems ·
stereoselective recognition · supramolecular chirality
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