Staudinger ligation towards cyclodextrin dimers in aqueous/organic media. Synthesis, conformations and guest-encapsulation ability

Article abstract of DOI:10.3762/bjoc.10.73

β-Cyclodextrin (β-CD) dimers have been prepared using the bioorthogonal Staudinger ligation for the first time. In addition to a known linker, methyl 2-(diphenylphosphanyl)terephthalate, a doubly active linker was specifically developed that enabled conne

Full text of DOI:10.3762/bjoc.10.73

Staudinger ligation towards cyclodextrin dimers in
aqueous/organic media. Synthesis, conformations
and guest-encapsulation ability
Malamatenia D. Manouilidou, Yannis G. Lazarou, Irene M. Mavridis
and Konstantina Yannakopoulou*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 774–783.
Institute of Advanced Materials, Physicochemical Processes,
Nanotechnology & Microsystems, National Center for Scientific
Research “Demokritos”, Terma Patriarchou Gregoriou & Neapoleos,
Aghia Paraskevi Attikis, 15310 Greece. Tel. +30210 6503796, Fax:
doi:10.3762/bjoc.10.73
Received: 18 October 2013
Accepted: 10 March 2014
Published: 03 April 2014
+30 210 6511766
Email:
Associate Editor: H. Ritter
Konstantina Yannakopoulou* - dyanna@chem.demokritos.gr
©
2014 Manouilidou et al; licensee Beilstein-Institut.
*
Corresponding author
License and terms: see end of document.
Keywords:
calculations; conformations; cyclodextrin; dimer; inclusion; PM3;
Staudinger ligation
Abstract
β-Cyclodextrin (β-CD) dimers have been prepared using the bioorthogonal Staudinger ligation for the first time. In addition to a
known linker, methyl 2-(diphenylphosphanyl)terephthalate, a doubly active linker was specifically developed that enabled connec-
tion of two β-CD units in a single step and in aqueous/organic media, under mild conditions and with good yields. A three-carbon
spacer between the β-CD torus and the azido group was required for facile dimer formation. The products, as studied by NMR spec-
troscopy, were found to adopt closed conformations by intramolecular self-inclusion. On the other hand, association via intermolec-
ular binding was also observed in aqueous solution, confirmed by DOSY NMR experiments. Despite self-inclusion, the β-CD cavi-
ties were capable of guest encapsulation, as shown by titration experiments: the binding constant with 1-adamantylamine was
similar to that of natural β-CD. Theoretical calculations for isolated molecules (PM3 level of theory) and in the presence of solvent
[
water, PM3(COSMO)] as well as DFT calculations suggested that the compounds prefer to adopt conformations which bring the
phenyl groups either inside the β-CD cavity (inclusion) or over its narrow side (vicinal). Thus, Staudinger ligation could be the
method of choice for linking CDs exhibiting (i) ease of preparation in aqueous media, in short steps, under mild conditions and in
good yields, (ii) satisfactory aqueous solubility and independent binding capacity of the cavities.
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Introduction
The Staudinger reaction [1] is a classical method for the prepar- potent drug solubilizers and transporters through biological
ation of amines from phosphines and azides [2,3]. The inter- barriers with increasingly important applications [11]. A
mediate, a phosphaza ylide (1, Scheme 1a) is readily formed specific category of modified CDs are the CD oligomers, com-
with loss of nitrogen gas, while subsequent hydrolysis yields the pounds with two or three CD macrocycles linked together at
corresponding amine and phosphine oxide. A variant of this different positions (2–2’, 3–3’, 3–2’, 6–2’ etc) via selected
reaction is the Staudinger ligation [4], a bioorthogonal reaction spacers. The multicavity structures can, in principle, be
that has become an important tool of chemical biology in the precisely tailored to fit specific guest molecules. The oligomers
last decade [5,6]. Introduced by Bertozzi and co-workers, the are reported to display improved to significantly enhanced
concept of ligation was based on the design of an intramolec- binding capacity as well as superior molecular recognition
ular electrophilic trap, such as the ester carbonyl in methyl ability [12-14], compared to the natural CDs. Suitable design
terephthalate 2 (Scheme 1b), that upon encounter of an azide has resulted in better hosts for applications as sensors and cata-
(
e.g. PhN3) can capture the nucleophilic nitrogen of the lysts [15], hosts of photoactive ligands [16,17], enzyme mimics
resulting phosphaza ylide (3). After methanol loss and hydrol- [18,19], among others, although the synthesis was frequently
ysis the reaction proceeds to amide bond formation with challenging and elaborate. On another approach, the increased
concomitant phosphine oxidation and formation of 4 in aqueous molecular size of oligomers may be advantageous for drug
environment. The ligation proceeds via a cyclic intermediate formulations due to foreseen increased circulation time and
(
Scheme 1b) which has been isolated and its X-ray structure EPR (Enhanced Permeation and Retention) effect [20], and the
solved in the case of reaction between benzyl azide and ability to carry increased payload, compared to natural CDs.
2
-(diphenylphosphanyl)benzoic acid methyl ester [7].
Only few examples of CD oligomers have been studied as hosts
to drugs [21]. On a practical point of view, on the other hand,
The advantages of the Staudinger ligation and its “traceless” not many reactions have been efficiently applied to produce CD
variant [6,8] are the employment of the azido group, a moiety oligomers. These include the well-known copper-catalyzed
orthogonal to naturally existing functional groups and readily azide–alkyne cyclization (CuAAC, “click” reaction) between an
introduced into many and different “soft” substrates, the mild azido-CD derivative and an alkyne linker [19,22-24] [22], or
reaction conditions, the high yields of the products, and the vice versa, the metal catalyzed reactions between propargyl-
absence of a catalyst. Numerous applications of linker 2 CDs and aryl dihalides such as the Sonogashira and Glaser–Hay
involving labeling of biomolecules with recognition sites in couplings [16,23], the classical formation of ester [12,17],
aqueous media in vitro and on live cells and animals in vivo amide [25,26] or imide [27] bonds between CDs or amino CDs
have been demonstrated [6].
and acid- or anhydride-linkers, and finally urea/thiourea bonds
28,29]. However, the obvious advantages of the Staudinger
[
Cyclodextrins (CDs, Scheme 1c), are cyclic oligomers of ligation (high reaction rates, absence of a catalyst, aqueous
glucopyranose that act as hosts to hydrophobic molecules in environment, rigid spacer/linker), have not been explored so far
aqueous environment [9,10]. CDs have been recognized as toward formation of CD dimers.
Scheme 1: (a) Staudinger reaction (b) Staudinger ligation, (c) the cyclodextrin structure with glucopyranose unit numbering: n = 7, β-CD; H3 and H5
atoms are located inside the cavity.
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For any realistic application of CD oligomers, especially for from methyl 2-aminoterephthalate via a sequence of diazotiza-
drug encapsulation, some key requirements can be set: (i) ease tion–iodination–phosphanylation reactions [4] (Scheme 2a).
of preparation, (ii) efficient purification, (iii) aqueous solubility
of the oligomer, (iv) structural characterization regarding con- The new, doubly phosphanylated dimethyl terephthalate linker
formation and dynamics, intra- and intermolecular interactions, 3 was prepared from p-xylene via consequtive iodination–oxi-
self-inclusion and aggregation in water, and ultimately (iv) dation–esterification reactions [32-34], followed by the final
availability of the individual CD cavities for molecular inclu- phosphanylation step that afforded linker 3 in excellent yield
sion. The later has become of critical importance since it has (Scheme 2b). Compounds 2 and 3 each displayed one signal in
been shown recently [30] that depending on the linker the 31P NMR spectrum at −3.5 and −3.8 ppm (Supporting Infor-
connecting the CD moieties, self-inclusion occurs in water, mation File 1, Figure S1). Compound 3 showed a single
frequently associated with inversion of one glucopyranose unit, methoxy group in the 1H NMR spectrum (3.57 ppm) and a
that may totally incapacitate the cavity and annihilate the single carbonyl signal in the 13C NMR spectrum (166.8 ppm),
prospective utility of the oligomer as a multivalent host. These reflecting the compound’s high molecular symmetry. Both
important findings explain the inconsistent or unexpected linkers were stored under argon at −20 °C to minimize oxi-
behavior of CD dimers in the past [31] and call for re-evalua- dation, although best results were obtained when used freshly
tion of binding constants of dimers determined by various prepared.
research groups.
Staudinger ligation using 2 and mono-[6-(3-azidopropylamino)-
In the present work, Staudinger ligation is used for the first time 6-deoxy]-β-CD (Scheme 3) in dimethylformamide/water (15:1)
as an efficient, water compatible strategy toward CD dimer gave monomer 4 in excellent yield (95%); in acetonitrile/water
preparation. In addition to linker 2 (Scheme 2a), the new double (2:1, v/v) the yield was nearly half (48%).
arylphosphine methyl ester linker 3 has been developed
(
Scheme 2b) that enabled facile homodimer formation in a Bertozzi and co-workers have studied the mechanism of the
single step, in organic/aqueous medium. As an indispensable Staudinger ligation of arylphosphine ester linkers such as 2 with
part of this approach, the full NMR spectroscopic characteriza- various alkyl azide substrates [7]. They have shown that the
tion combined with theoretical calculations, revealed the full ligation is a second order process: polar protic solvents and
capacity of the cavities for molecular inclusion despite the electron-donating substituents on the aryphosphine accelerate
dynamic equilibria between closed and open conformations of the reaction, while the type of the ester (methyl, aryl) does not
the products in water.
have an effect on the rate. The actual intermediate is a five-
membered phosphanazolone [7] (Scheme 1b). Double ligation
to form dimer 6 was optimized taking into consideration the
Results and Discussion
Synthesis – The Staudinger ligation via linker 2 (Scheme 2) was mechanistic aspects of the prototype reaction [7]. Thus 3 and
initially explored to produce the corresponding functional mono-[6-(3-azidopropylamino)-6-deoxy]-β-CD were mixed in
monomer 4 and thus the β-CD dimer 5. Linker 2 was prepared an NMR tube at 60 °C with dry CDCl3 and dry DMF-d7 where
Scheme 2: (a) i) HCl, NaNO2/H2O, then KI/H2O, 58%, ii) Ph2PH, Pd(OAc)2, Et3N, MeOH, 48%; (b) i) CH3COOH, H2SO4, CCl4, I2, IO3, 85 °C, 4 h,
exclusion of light, 70%; ii) C5H5N, H2O, KMnO4, reflux, 24 h, exclusion of light, 55%; iii) KOH in water (10% w/v), KMnO4, reflux, 4 h, exclusion of
light, 32% iv) MeOH, H2SO4, reflux, 5 h, 98% v) dry THF, dry DMF, Et3N, Pd(CH3COO)2, Ph2PH, 70 °C, 12 h, 98%.
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Beilstein J. Org. Chem. 2014, 10, 774–783.
Scheme 3: Staudinger ligation reactions: (a) Preparation of 4 from mono[6-(3-azidopropylamino)-6-deoxy]-β-CD and 2 (DMF/H2O, 15:1, v/v, 40 °C,
12 h, 95%) and then of dimer 5 from 4 and mono(6-15N-amino-6-deoxy)-β-CD (HATU, DIPEA, dry DMF, 10%); (b) One-step facile preparation of
homodimer 6 from mono-[6-(3-azidopropylamino)-6-deoxy]-β-CD and the divalent linker 3 (DMF/CHCl3/H2O, 48 h, 62%).
both reactants were completely soluble and the evolution of the of 4 with mono(6-15N-amino-6-deoxy)-β-CD under typical
3
1P NMR signals was monitored with time. The formation of an amide coupling conditions provided the desired dimer 5
intermediate, most likely a phosphaza ylide was evidenced by a (Scheme 3) but only in 10% yield. This product displayed one
signal at 19.8 ppm [7] that emerged following addition of the signal at 35.2 ppm in the 31P NMR spectrum and one at
azido-β-CD in the CDCl3/DMF-d7 solution; however complete 141.6 ppm in the 15N NMR spectrum, the latter considerably
formation of the product required 48 h. Apparently steric factors deshielded compared to that of the starting mono(6-15N-amino-
influence the progress of the reaction, also considering the five- 6-deoxy)-β-CD at 61.7 ppm, thus confirming the structure of
membered ring intermediate (Scheme 1b) required for the liga- dimer 5. However, while the excellent yield of the Staudinger
tion to proceed. Addition of deuterium oxide resulted in hydrol- ligation step to 4 showed its applicability and suitability for
ysis of the intermediate and disappearance of the 19.8 ppm peak cyclodextrin substrates, the poor yield of CD-dimer 5 suggested
within ~4 h while the emergence of a peak at 35.4 ppm vali- the presence of impediments in the introduction of a second
dated the formation of dimer 6. The above confirmed DMF as β-CD moiety. Examination of the 3D structure of 4 revealed
the optimal solvent. In DMSO and CH3CN, numerous 31P that the phenyl moieties impose steric restrictions toward the
signals upon dissolution of 2 or 3 were observed evidently due approach of mono(6-15N-amino-6-deoxy)-β-CD. The sugges-
to secondary reactions leading to a lower yield for the ligation tion of hindrance was supported by the fact that Staudinger liga-
step.
tion using 2 and 3 although very successful with mono[6-(3-
azidopropylamino)-6-deoxy]-β-CD, where the azido group is
Monomer 4 and dimer 6 displayed a single 31P NMR signal at connected to the β-CD moiety via the flexible aminopropyl
5.2 ppm and the correct mass in MALDI–TOF MS. Reaction spacer, failed with mono(6-azido-6-deoxy)-β-CD. Therefore, if
3
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Beilstein J. Org. Chem. 2014, 10, 774–783.
steric problems are circumvented using a suitable spacer β-CD cavity as well as the primary side H5, H6,6’ and OH6
between the CD macrocycle and the azido group, the process (and not with the secondary side OH2, OH3), indicating that in
becomes very attractive: The reaction conditions are quite unde- 6 (much more than in 4) the phenyl moieties are located over
manding and mild, the purification of the products is straight- the narrow opening of the β-CD moiety at close distance. In
forward and efficient and the yields are good to excellent. Thus both 4 and 6, dipolar interactions with cavity proton H3 did not
the procedure could become the method of choice for a variety seem to develop, as indicated by the assignments of the signals,
of CD substrates to form derivatives and either homo- or thus ruling out the self-inclusion in DMSO. The above show
heterodimers.
that configurations with phenyl groups over the narrow β-CD
opening, as observed in DMSO, evolve in two limiting ways in
Conformations of Staudinger products and guest binding in water: either open up entirely exposing the phenyl moieties to a
aqueous solution as derived from NMR spectroscopy. – The neighboring cavity of another molecule, or close in by self-
study of the conformations in water is essential for the evalua- inclusion in their own cavity.
tion of the compounds’ efficiency toward guest inclusion. The
1
H NMR spectrum of monomer 4 and dimer 5 showed quite In order to evaluate the extent of intermolecular interactions in
disperse signals (Supporting Information File 1, Figure S2), dimer 6 in D2O, 2D diffusion ordered spectroscopy (DOSY)
anticipated due to the lack of molecular symmetry, while the was used. The diffusion coefficient D6, of 6 (1 mM, Supporting
spectrum of 6 (Figure S2) was simpler reflecting a rather Information File 1, Figure S6), was found 1.7 × 10−11 m2/s
symmetrical structure. The 2D ROESY NMR spectra of 4 and 5 while in the presence of four equivalents of 1-adamantylamine
suggested formation of self-inclusion complexes via the phenyl hydrochloride (ada) it increased to D(6/ada) = 2.4 × 10−11 m2/s,
groups of the linker, since strong dipolar interactions between suggesting a faster motion of the complex 6–ada in the solu-
the narrow side protons (H5, H6,6’, dispersed over a wide range tion, presumably due to the breaking apart of intermolecularly
of frequencies) and the phenyl groups were observed (the associated dimers. Comparing the above values with those of
terephthalate moiety did not participate) (Figure S2). Self-inclu- natural β-CD (0.5 mM [38], Dβ-CD = 3.29 ± 0.07 × 10−10 m2/s)
sion could arise intramolecularly by rotation of the amino- a ratio of Dβ-CD/D6 ≈ 19 is obtained, much larger than the
propyl spacers in order to effect insertion of the phenyl groups corresponding molecular weight ratio, FWβ-CD/FW6 ≈ 0.38,
in the narrow side of the cavity, but also intermolecularly, as while D(6/ada)/ D6 ≈ 1.3, revealing the important effect of inter-
indirectly evidenced by the strong concentration dependence of molecular attractive forces on the translational motion of the
the chemical shifts in the 1H NMR spectrum in D2O. Likewise, dimer. The inclusion of ada evidently helped to reduce these
the 2D ROESY spectrum of 6 in D2O (Supporting Information forces but aggregation was not totally prevented. It is known
File 1, Figure S3) showed clearly that the phenyl groups devel- that β-CD forms aggregates in aqueous solution as shown by
oped strong through-space interactions with the cavity protons DLS [39] and DOSY measurements at different concentrations
H3 (all signals bundled together in an apparent quartet) and [38,40,41]; it is therefore reasonable to assume that a part of the
weaker with H5, and H6,6’ (signals grouped together, as well). aggregation of the dimer is due to the β-CD moieties. Finally,
The indicated self-inclusion could take place intermoleculary given that in any host–guest solution in the fast exchange
from the wider β-CD side as well as intramolecularly from the regime in the NMR time scale,
narrow side. Such self-inclusion has been observed and taken
advantage for selective catalysis in recently published cyclodex- Dobs = Dbound fbound + Dfree (1 − fbound) [42] (f = mole fraction)
trin-phosphanes [35-37]. Moreover, some of the observed inter-
actions could arise from the proximity of the phenyl and β-CD the observed Dada ≈ 6.6·10−11 m2/s, suggests that the guest
moieties in the dimeric structure. In order to improve our under- diffuses at a rate close to that of free ada, because (i) there is an
standing and separate the water-induced inclusion configura- excess of it in the solution deliberately added to maximize the
tions from the ones imposed by the bulkiness of the molecules, complex concentration and (ii) the binding constant is moder-
the 2D ROESY spectra of 4 and 6 in DMSO-d6 were examined. ately strong, therefore an average diffusion coefficient is
The spectra revealed that the phenyl protons indeed develop observed.
ROE interactions with the primary side β-CD protons H5 and
H6.H6’, as well as with all the aminopropyl chain protons in Cavity availability for inclusion complexation. – The above
monomer 4 (Figure S4). The above suggest that the phenyl results suggest that the phenyl groups might be serious competi-
groups of the linker prefer to linger over the narrow β-CD tors to any incoming guest molecule. To test the accessibility of
opening. Likewise, dimer 6 in DMSO-d6 (Supporting Informa- the cavities and the usability of the products as molecular
tion File 1, Figure S5) displayed through space interactions carriers, titration of 4 and 6 with ada in D2O were carried out.
between the phenyl protons and the H1, H2, H4 external to the The corresponding plots (Figure 1) revealed that one equivalent
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Beilstein J. Org. Chem. 2014, 10, 774–783.
of ada for monomer 4 and two equivalents of ada for dimer 6 in aqueous solution by self-inclusion, are accessible by external
were required to saturate the chemical shifts of the cavity guest molecules while the strength of binding does not seem to
proton, H3, suggesting full binding capacity of all available decrease by the presence of the linker moieties, compared to the
cavities. When the chemical shift changes of H3 [Δδ(CD-Η3] of parent β-CD. In addition to closed configurations of 4 and 6 in
6
were plotted vs ½ concentration of the titrant (i.e. per cavity) water formed by intramolecular phenyl inclusion, open configu-
(
Figure 1), the resulting curve nearly coincided with that of the rations are additionally present, which promote intermolecular
curve of monomer 4. The striking similarity of the induced aggregation and can account for the slow Brownian motion of
chemical shift displacements of CD-Η3 for 4 and 6, reveal a the dimer in the aqueous solution. The fact that in DMSO the
similar mode of inclusion, simultaneous and independent for linkers prefer to reside over the cavity indicates that intermedi-
each cavity of 6 and subsequently very similar association ate conformations, between extended and self-included may
constants. Indeed, non-linear fitting of the observed shifts to a exist in water as well.
suitable equation for 1:1 binding in the fast exchange regime
[
43] showed that the association constants, as logK, are in the Computational results
order of ~4.2 i.e. comparable with those reported for the binding Quantum mechanical calculations were carried out for mono-
of β-CD alone with the same guest [44]. Therefore the strength mer 4 and dimer 6 at the PM3 level of theory for isolated mole-
of the binding did not reveal severe competition from the phen- cules, as well as in the presence of solvent (water) at the
yl groups of the spacers.
PM3(COSMO) level of theory in order to assess solvation
effects. For critical configurations of 4, the PM3 results were
compared against those calculated by DFT at the B3P86/6-
3
1G(d',p') level of theory. The average structural deviation of
the PM3 geometries from those derived at the B3P86/6-
1G(d',p') level was reasonably small, 0.015 Å for bond lengths,
whereas for bond and dihedral angles the average deviation was
.1 and 7.2 degrees, respectively. Several initial geometries
3
2
with a varying degree of phenyl groups’ orientation and prox-
imity to β-CD (stemming mainly from the torsional flexibility
of the aminopropylamino–spacer moiety) were fully optimized
at the PM3(COSMO) and PM3 level of theory. The calculated
geometries were sorted out into three limiting configurations: i)
open, in which the phenyl rings are positioned on the exterior of
β-CD, ii) vicinal, in which two phenyl rings are close to the pri-
mary side rim of β-CD and iii) inclusion, in which one phenyl
ring penetrates inside the β-CD cavity. A gauche–trans (gt)
arrangement of the C5-C6OH moieties in 4 was found to
disfavor the inclusion configuration by more than 10 kJ/mol,
due to the subsequent contraction of the primary entrance of the
β-CD cavity, compared to a gauche–gauche (gg) arrangement.
The energies of PM3(COSMO) as well as PM3 for isolated
Figure 1: 1H NMR chemical shift change (Δδ) of CD cavity Η3 signal
of compounds titrated with 1-adamantylamine·HCl (ada) in D2O
(500 MHz, 298 K): a) monomer 4 (1 mM, filled squares, solid line) and
b) dimer 6 (1 mM) (empty squares, dotted line) plotted per cavity vs ½
concentrations of ada.
The through-space interactions between the phenyl protons and molecules, for the various configurations of 4 (Figure 2) span a
those of β-CD in 6 and 4 (Supporting Information File 1, Figure range of 25 kJ/mol, with the inclusion case being the most ther-
S3) clearly changed in the presence of ada (Supporting Infor- mochemically favorable, closely followed by the vicinal case.
mation File 1, Figure S7). Specifically, spatial proximity of The PM3(COSMO) energies of the corresponding configura-
phenyl groups with H6 (and not β-CD-H3, as clearly assigned tions for dimer 6 (Figure 3) span a greater range, 65 kJ/mol,
from the HSQC spectra) and H5, H6’, is evident from the 2D with a mixed inclusion/vicinal configuration (Figure 3c) being
ROESY spectra, suggesting that upon entrance of ada from the the most thermochemically stable. However, configurations
wider side, the phenyl moieties are lifted over the narrow with phenyl groups immersed in both β-CD cavities (Figure 3d)
opening. In both molecules ada is inserted with the amino are the least stable by PM3(COSMO) (in the presence of water),
group protruding from the wider β-CD side (Figure S7).
although highly favorable by PM3 (in the absence of solvent).
This was attributed to insufficient hydration of the primary
In summary, NMR experiments have shown that the com- hydroxy groups due to steric crowding around the primary sides
pounds, although they form intra- and intermolecular complexes of the two β-CD tori. The DFT energies for critical configura-
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Beilstein J. Org. Chem. 2014, 10, 774–783.
Figure 2: The most stable conformations of 4 at the PM3(COSMO) level of theory: (a) open, (b) vicinal, and (c) inclusion conformation.
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Beilstein J. Org. Chem. 2014, 10, 774–783.
Figure 3: Typical conformations of 6: (a) open conformation, (b) vicinal, (c) inclusion/vicinal and (d) double inclusion conformation.
tions of 4 span a range of 17 kJ/mol, comparable to the range of open configuration was explored at the PM3(COSMO) level of
PM3 energies. Thus, semiempirical as well as DFT calculations theory. A variety of bimolecular arrangements was considered,
suggest that both compounds prefer to adopt conformations by also taking into account the possibility of phenyl inclusion in
with the phenyl groups either inside the CD cavity (inclusion) either side of β-CD. For an all-gg conformation of C5-C6OH in
or over the C6 area (vicinal) of comparable energies, therefore 4, phenyl inclusion via the primary side was more thermochem-
they may easily interconvert.
ically favored by ca. 40 kJ/mol, whereas for an all-gt con-
formation, inclusion via the secondary side was more favored
The energetics of intermolecular inclusion of a phenyl group by ca. 45 kJ/mol, attributed to the contraction of the primary
inside the β-CD cavity for a pair of monomers 4 having the side opening. The geometries for the most stable arrangements
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Beilstein J. Org. Chem. 2014, 10, 774–783.
for a pair of 4, are shown in Supporting Information File 1,
Supporting Information
Figure S8.
The Supporting Information provides full experimental
procedures and detailed analytical data for the synthesis of
all compounds; additional 2D NMR spectra; the
computational procedure and a table of intramolecular
distances for the three limiting conformations of 4 and 6
and for a pair of 4. Theoretical geometries of
intermolecular dimers of 4 and of the complex 4/ada are
also shown.
The shortest distances between hydrogen atoms of the closest
phenyl ring and each kind of glucose hydrogen atom of the
β-CD for the most stable configuration of each limiting case are
shown in Supporting Information File 1, Table S1. In the inclu-
sion structures of 4 and 6 the calculated shortest distances of the
inserted phenyl moiety were found only with the β-CD cavity
H3, H5 and H6,6'. In the vicinal lowest-energy configurations,
the corresponding calculated shortest distances of both phenyl
groups were found only with the primary side protons H5 and
H6,6’ and not with H3, in agreement with NMR data in DMSO.
Finally, in the open structures the shortest distances observed
were with the cavity exterior H1, H2 and H4. The above are in
line with the experimental findings in the 2D ROESY NMR
data in D2O. Intermolecular arrangements (Supporting Informa-
Supporting Information File 1
Experimental, analytical and computational data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-73-S1.pdf]
tion File 1, Figure S8), verified by the experimentally observed Acknowledgements
aggregation, may also incorporate phenyl group inclusion CycloLab SA (Budapest) is thanked for a gift of mono(6-15N-
complexes with similarly short distances between phenyl hydro- amino-6-deoxy)-β-cyclodextrin. Partial funding by the Marie
gens with H3, H5 and H6,6', shown in Table S1 (Supporting Curie Initial Training Networks (FP7-People-ITN-2008, Project
Information File 1) and in line with the NMR data.
No 237962 "CYCLON") is gratefully acknowledged. A scholar-
ship to M.D.M. by NCSR “Demokritos” is gratefully acknowl-
The accommodation of the protonated form of edged. We also thank Mr. A. R. L. Marouvo Gonçalves of our
-adamantylmine inside the cavity of monomer 4 was also group for preparing NDTAM·HCl. Istituto di Ricerche
1
explored at the PM3(COSMO) level of theory. A variety of Chimiche e Biochimiche "G. Ronzoni" Milano, Italy, is also
initial complex geometries was considered consisting of vicinal acknowledged for the MALDI–TOF measurements.
as well as of inclusion configurations for 4. The resultant opti-
mized geometries reveal that ada is encapsulated inside the References
β-CD cavity by its hydrophobic alkyl side leaving the proto-
nated amine moiety well outside the cavity. Its accommodation
into the inclusion configuration proceeds by a push of the phen-
yl ring. Moreover, the PM3(COSMO) level of theory suggests
that ada accommodation by 4 is energetically favorable by ca.
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Scriven, E. F. V.; Turnbull, K. Chem. Rev. 1988, 88, 297–368.
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Conclusion
The present work demonstrates the application of the
Staudinger ligation reaction for the first time to form β-CD
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