FULL PAPER

DOI: 10.1002/ejoc.201301681

Limits of the Inversion Phenomenon in Triazolyl-Substituted β-CyclodextrinDimers

Jonathan Potier,[a,b] Stéphane Menuel,[a,b] Nathalie Azaroual,[a,c] Eric Monflier,[a,b] andFrédéric Hapiot*[a,b]

Keywords: Host-guest systems / Cyclodextrins / Macromolecules / Conformation analysis / Structure elucidation

Six different β-cyclodextrin (β-CD) dimers have been synthe-sized by copper-catalyzed azide alkyne cycloaddition. Thenature of the spacer connecting the two β-CDs has been var-ied in length and hydrophilicity. For each dimer, a detailedNMR study was carried out in D2O. It was found that,whereas β-CD dimers having short spacer exhibited only oneconformation, β-CD dimers having long and/or hydrophobic

Introduction

Despite the huge amount of work already dedicated tocyclodextrins (CDs),[1] these truncated cone-shaped mo-lecules continue to fascinate researchers in many differentareas such as drug release,[2] cosmetics,[2] the food indus-try,[3] sensing,[4] biomimetics,[5] chiral enantiomer separa-tion,[6] and catalysis.[7] The recognition ability of these α-d-glucopyranose-based cyclic molecules towards organic mo-lecules mainly explains this fascination. From linear alkylchains to more bulky aromatic moieties, α-, β- and γ-CDsare able to host numerous substrates in water, provided thatthe guests display hydrophobic fragments that are stericallycompatible with the CD cavity. For decades the idea thatnative (unmodified) CDs are rigid structures has fueled re-search into their molecular recognition properties.[8] Thecase of the native β-CD is especially illustrative. Its recogni-tion ability is essentially rationalized by the greater rigidityconferred by the participating intramolecular hydrogenbonds. Thus, many different substrates can be includedwithin the native β-CD cavity. To widen the scope of CDsas molecular hosts, chemical modifications of their struc-ture has been developed. A wide range of substituents hasthus been covalently grafted on either the CD primary orsecondary face.[9] Nevertheless, the resulting structural

[a] Université Lille Nord de France,59000 Lille, France

[b] UArtois, Unité de Catalyse et de Chimie du Solide (UCCS),Faculté des Sciences Jean Perrin,Rue Jean Souvraz, SP 18, 62307 Lens Cedex, FranceE-mail: frederic.hapiot@univ-artois.frhttp://www.uccs.univ-artois.fr/frederichapiot.htm

[c] UDSL, EA 4481, Faculté de Pharmacie, Université de Lille 2,BP 83, 59006 Lille, FranceSupporting information for this article is available on theWWW under http://dx.doi.org/10.1002/ejoc.201301681.

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spacers showed two different conformations. Indeed, the lat-ter underwent an inversion process leading to self-inclusionof the spacer in one of the two CD cavities. The behavior ofthe six β-CD dimers in water has thus been rationalized andthis allowed the proportion of “free” cavities actually avail-able for molecular recognition to be determined.

alteration is not without consequence. At best, the water-solubility of the CD host is enhanced and the associationconstant outmatches that measured with the native counter-part. In other cases, addition of substituents at random onthe CD structure may disrupt the intramolecular hydrogenbond network, resulting in an enhanced solubility in waterwithout affecting the molecular recognition properties. Forexample, the water-solubility of the randomly methylated β-CD (RAME-β-CD, substitution degree: 1.8 methyl groupper glucopyranose unit) is � 500 gL–1 at 25 °C,[10] which isfar higher than that of native β-CD (18.8 gL at 25 °C).[11]

Yet, native β-CD and RAME-β-CD display similar associa-tion constants regarding guest molecules.[12] However,grafting a substituent on the CD structure does not alwayslead to better intrinsic properties. Two key phenomena mayhinder the inclusion of a guest within the cavity of substi-tuted CDs. First, if the hydrophobic character of the sub-stituent is high and its structure can fit the CD cavity, thena self-inclusion process may occur. As such, we recentlyshowed that the recognition ability of chemically modifiedCDs can be dramatically altered by flexible hydrophobicsubstituents that are capable of entering the CD cavity.[13]

In this case, the molecular recognition of guests remainspossible but is significantly hampered. Once the CD hasbeen substituted, another phenomenon can also alter therecognition process. In 2004, Kano et al. revealed thatdouble full rotation of the substituted glucopyranose unitcan occur, leading to a CD conformation in which the cav-ity is fully occupied by the substituent,[14] thus demonstrat-ing that the CD cavity is not as rigid as first anticipated. Inline with this observation, Liu et al. reported in 2008 thatthe flipping phenomenon can also take place with tetrakis-(permethyl-β-CD)-modified zinc(II) porphyrins.[15] Simi-larly, in 2010, Harada et al. revealed that CD dimers were

J. Potier, S. Menuel, N. Azaroual, E. Monflier, F. HapiotFULL PAPERalso involved. In their study, an alkyl altro-α-CD dimer wasconverted into the pseudo[1]rotaxane dimer through tum-bling of the altropyranose unit of altro-α-CD in D2O.[16]

They also showed that the tumbling process originated fromthe breakage of the hydrogen bond network, which consti-tuted the main energetic barrier for the conformationalchange of the alkyl altro-α-CD dimer. Their study was ex-tended to [2]rotaxane and [3]rotaxane.[17] Whereas [3]rotax-ane resisted reorientation of its altro-α-CD stopper groupdue to insufficient space, the altro-α-CD stopper of [2]-rotaxane tumbled, resulting in conversion into pseudo[2]-rotaxane in D2O. Recently, we demonstrated that triazolyl-substituted CD dimers were also subjected to inversion phe-nomena. More precisely, the recognition ability of water-soluble β-CD-based ditopic receptors linked together by abis(triazolyl)dimethoxybenzene spacer strongly dependedupon the location of the substituents on the phenyl ring.[18]

Because of the inversion phenomenon, the para-isomerproved to be inappropriate for the recognition of long alkyl-chain substrates because only one out of two CD cavitieswas available. On the other hand, the bulkier meta- andortho-isomers disfavored the inversion phenomenon, re-sulting in an enhanced recognition ability. Similar observa-tions on the inversion process were made very recently byDésiré et al. on β-CD glycerol dimers.[19]

Considering the above observations, we wanted to ratio-nalize the behavior of triazolyl-substituted CDs when incor-porated in a dimer structure. To this end, we investigated aseries of β-CD-triazolyl-containing dimers to evaluate(i) the role of the triazolyl group, (ii) the impact of the bulkof the spacer, (iii) the role of spacer flexibility, and (iv) theinfluence of spacer hydrophobicity on the inversion phe-nomenon. To do so, six different β-CD dimers having link-ers of different nature were synthesized and their behaviorin water was observed by using 1D and 2D NMR spectro-scopic analyses. Note that the mechanism of inversion re-garding the CD is beyond the scope of this study.

Results and Discussion

Synthesis

Various synthetic approaches have been developed to ac-cess six β-CD dimers in which the β-CD residues arebridged by linkers of different length, hydrophobicity andflexibility. Synthesis of 1 was realized in dimethyl sulfoxide(DMSO) at 30 °C by copper-catalyzed azide alkyne cyclo-addition (CuAAC) using mono-azido β-CD and dipropynylether as reactants (Scheme 1) and the CuSO4·5H2O/Na-as-corbate couple as a catalyst. After stirring for 15 h, thecrude product was precipitated in acetone. The Cu-catalystwas removed by complexation with ammonia and subse-quent chromatography on a silica column. After evapora-tion of the water–ammonia mixture, the product was sub-mitted to an additional chromatography column using aCH3CN/H2O mixture (8:2 v/v) to purify the β-CD dimer(1). Dimer 1 was obtained in 80–85 % yield as a white pow-der.

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Scheme 1. Synthesis of CD dimer 1.

Dimer 2 was synthesized by following the same experi-mental procedure from meta-diethynylbenzene and mono-azido β-CD (Scheme 2). After workup, 2 was isolated as awhite powder in 80–85% yield. This compound consistedof two β-CDs linked together by a rigid meta-bis(triazolyl)-phenyl spacer. By following the same experimental pro-cedure, dimer 3 was easily attainable from para-diethyn-ylbenzene as a reactant. Changing from meta to para geom-etry on the aromatic phenyl ring altered neither the reactiontime nor the yield.

Scheme 2. Synthesis of CD dimers 2 and 3.

Unsymmetrical dimers required a multistep strategy. Thelinker had to be prepared first before the coupling reactionwith the β-CD residues was possible. To do so, 4-iodo-phenol and trimethylsilylethyne were reacted in pyrrolidineat 60 °C for 5 h under nitrogen in the presence of CuI and[PdCl2(PPh3)2] (Scheme 3). Liquid-liquid extraction withNaHCO3-saturated water and ethyl acetate gave a crudeproduct that was subsequently purified by chromatographyon silica column using CH2Cl2 as eluent. 4-(Trimethylsilyl-ethynyl)phenol (4) was isolated as a white powder (96%yield), which was then reacted with propargyl bromide inCH3CN under reflux overnight.

Inversion Phenomenon in β-Cyclodextrin Dimers

Scheme 3. Synthesis of CD dimer 6.

After liquid-liquid extraction, the crude product waspurified by chromatography to give the para-disubstitutedbenzene derivative 5 as a white powder in 75 % yield. Notethat during the course of the reaction the alkyne fragmentwas completely deprotected, thus avoiding the need for anadditional deprotection step. Compound 5 was then sub-jected to CuAAC with mono-azido-β-CD under the experi-mental conditions described above, leading to dimer 6 in80–85% yield.

More flexible β-CD dimers were prepared according tothe following procedure. Pentaethylene glycol and propargylbromide were mixed in CH3CN in the presence of Cs2CO3

and heated to reflux overnight (Scheme 4). Dialkynyl com-pound 7 was obtained in poor yield (14 %) after liquid-li-quid extraction and chromatography on the silica column(CH2Cl2/EtOAc, 7:3 v/v). Subsequent reaction of 7 with2 equiv. mono-azido-β-CD under CuAAC conditions pro-duced dimer 8 in 80–85 % yield.

Similarly, 1-chloro-2-[2-(2-chloroethoxy)ethoxy]ethanereacted overnight with 4 under basic conditions at reflux inCH3CN (Scheme 5). The crude product was first purifiedby liquid-liquid extraction with NaHCO3-saturated waterand ethyl acetate.

Subsequent chromatography on a silica column (CH2Cl2/heptane, 50:50 to 100:0 v/v) gave dialkynyl derivative 9 as apale-yellow powder in 37% yield. As noted above for thesynthesis of 5, the alkyne was completely deprotected dur-ing the course of the reaction. Compound 9 and mono-azido-β-CD reacted in DMSO under CuAAC conditions togive dimer 10 (80–85 %) after purification.

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Scheme 4. Synthesis of CD dimer 8.

Scheme 5. Synthesis of CD dimer 10.

NMR Measurements

We sought to clarify the key parameters that control theinversion phenomenon by a careful examination of the in-fluence of the structure of the linker on the dimer confor-mation. In this context, the synthesized β-CDs dimers weresubmitted to 1D and 2D NMR experiments in D2O at25 °C. Three different groups of protons served as probesto evaluate the structural modification of the β-CD dimersin D2O, namely the triazolyl protons, the linker protons,and the inner CD protons (H-3 and H-5). The 1H NMR

J. Potier, S. Menuel, N. Azaroual, E. Monflier, F. HapiotFULL PAPER

Figure 1. 1H NMR spectrum of dimer 1 in D2O at 25 °C.

spectrum of 1 is displayed in Figure 1. Only one singletcould be detected in the triazolyl proton region (ca. 8 ppm)indicative of a single dimer structure. However, the numer-ous signals observed between 4.5 and 5.0 ppm were clearlyillustrative of the magnetic inequivalence of the H-6 protonsand suggested that the CD was not as rigid as first antici-pated. It seemed that the CD attempted to rotate but theshortness of the linker and the hindrance of the bulky CDstructure prevented the inversion phenomenon in this case.A 2D NOESY experiment confirmed the absence of inter-action between the linker and the CD cavities. Indeed, nocross-peaks could be detected between the triazolyl protonsand the inner CD protons. Additionally, no nuclear Over-hauser effect (NOE) could be observed between the triaz-olyl protons H-8 and either the β-CD protons or the gluco-

Figure 2. 1H NMR spectrum of dimer 2 in D2O at 25 °C.

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pyranoside methylene group (H-6 protons) attached to thetriazolyl cycle. Accordingly, 100 % of dimer 1 was in a sym-metrical conformation.

The 1H NMR spectrum of 2 is displayed in Figure 2.Among the several signals detected in the 7–8.5 ppm region,only one corresponded to triazolyl protons (δ = 8.30 ppm),with the others being attributed to aromatic protons. Fromthe triazolyl singlet, we deduced that all the triazolyl pro-tons were in the same chemical environment. To determinewhether the two CD cavities were unaffected or reversed,a 2D NOESY experiment was performed. The absence ofcorrelations clearly indicated that the β-CD cavities of 2remained untouched. Thus, the cavities contained neitherthe triazolyl moieties nor the phenyl ring. Accordingly, asobserved for 1, dimer 2 adopted a symmetrical conforma-

Inversion Phenomenon in β-Cyclodextrin Dimers

tion. However, here again, the inequivalent resonances de-tected for the CH2–N pattern in the 4.50–5.00 ppm rangewere indicative of local disorder around the substitutedglucopyranoside unit.

Under the same conditions, 3 behaved in a similar wayto 2. In addition to the singlet at δ = 8.30 ppm characteristicof the triazolyl protons (Figure 3), the singlet at δ =7.83 ppm reinforced the idea of a symmetrical structure be-cause the four aromatic protons were equivalent. Hereagain, no cross-peak between aromatic and CD protonscould be detected in the 2D NOESY spectrum of 3 in D2O.The inequivalent resonances detected for the methyleneprotons linking the β-CD and the triazolyl cycle still sug-gested that the substituted glucopyranose unit was ratherflexible and possessed some degrees of freedom.

Figure 3. 1H NMR spectrum of dimer 3 in D2O at 25 °C.

Figure 4. 1H NMR spectrum of dimer 6 in D2O at 25 °C (NR: nonreversed; R: reversed; RI: reversed and included).

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The 1H NMR spectrum of the unsymmetrical dimer 6displayed more intricate resonance patterns (Figure 4). Inthe 8.2–6.8 ppm region, both singlets and doublets were de-tected. Using 1H-13C HSQC and HMBC, the most de-shielded singlets [δ = 8.18 (H-8) and 8.04 (H-12) ppm] wereattributed to triazolyl protons in a dimer conformation forwhich both CD cavities were unaffected by the inversionphenomenon. The existence of a nonreversed conformationwas confirmed by the singlet at δ = 5.21 ppm, indicative oftwo magnetically equivalent H-11NR protons (as observedfor H-9 in dimer 1). An NOE revealed dipolar interactionsbetween H-8NR and both H-6NR and H-9NR (henceforth,the following subscript abbreviations will be used: NR: non-reversed; R: reversed; RI: reversed and included). Con-versely, the singlets at δ = 7.90 and 7.24 ppm were attrib-

J. Potier, S. Menuel, N. Azaroual, E. Monflier, F. HapiotFULL PAPERuted by 1H-13C HSQC and HMBC to the triazolyl protonsH-8R and H-12RI, respectively, in a reversed conformation.Additionally, cross-peaks could be detected in the NOESYspectrum between H-12RI and the inner CD protons H-3and H-5 (see the Supporting Information). Aromatic pro-tons were also illustrative of two different populations ofdimers because two sets of doublets were detected in the7.7–6.8 ppm region. Whereas the doublets at 7.69 (H-9NR)and 7.11 (H-10NR) ppm did not show any correlations withthe CD protons in the NOESY spectrum, the doublets at7.58 (H-9RI) and 6.88 (H-10RI) ppm exhibited intense cross-peaks with the inner CD protons H-3 and H-5, indicativeof the inclusion of the phenyl moiety within the CD cavity.Dipolar contacts between H-10RI and H-12RI were illustra-tive of the distorted structure of the spacer within the CDcavity. Moreover, the two doublets at δ = 5.27 and 5.38 ppmwere characteristic of two magnetically inequivalent methyl-ene protons H-11R in a dimer structure in which one CDout of two was reversed. Protons H-11R became inequiva-lent, probably due to the chiral environment exhibited bythe close CD cavity and the torsional stress of the moleculerequired to bring the aromatic cycle into the CD cavity.Note that the significant difference in chemical shifts Δδbetween H-5R or H-5NR and H-5R indirectly confirmed thepresence of the inversed conformation. The relative pro-portions of symmetrical and unsymmetrical conformationswere calculated from integration of the two singlets at δ =8.18 and 7.90 ppm; thus, 23% symmetrical (nonreversed)and 77% unsymmetrical (partly reversed) dimer were iden-tified.

Dimer 8 also displayed two different conformations inD2O. In fact, three triazolyl protons were detected in the8.0–7.8 ppm region (Figure 5). The most deshielded signalat δ = 7.97 ppm was identified as triazolyl protons H-8NR

in a symmetrical conformation. As observed above for 6,NOE experiments revealed dipolar interactions between

Figure 5. 1H NMR spectrum of dimer 8 in D2O at 25 °C (NR: nonreversed; R: reversed).

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H-8NR and both H-6NR and H-9NR. The existence of twoadditional peaks unambiguously demonstrated that oneCD in two was able to undergo the inversion phenomenon(Figure 5). Only one additional peak would have been ob-served for 100 % symmetrical conformations with both theirCD reversed. The existence of a fully reversed stable confor-mation of dimer 8 in D2O could be ruled out at this stagebecause of the equal value of integrations of the two reso-nances at δ = 7.93 and 7.87 ppm, indicative of two differenttriazolyl protons in an unsymmetrical conformation forwhich one CD was reversed and the second nonreversed. Ifpresent, the fully reversed conformation existed in D2O ina very small proportion that was below the detection limitsof the NMR measurements. The absence of NOE betweenH-8NR or H-8R and the CD protons clearly indicated thatthe ethoxylated fragment was included in the reversed CDcavity. The ratio of symmetrical and unsymmetrical confor-mations was given by integration of the triazolyl signals as62% symmetric vs. 38 % unsymmetrical conformations.

The 1H NMR spectrum of dimer 10 was interpreted bydetailed analysis of the aromatic and triazolyl resonances(Figure 6). Three distinct signals were identified for the tria-zolyl protons by 1H-13C HSQC and HMBC (see the Sup-porting Information). Interestingly, the proportion of non-reversed conformations steadily decreased with time. Thepeak at δ = 7.98 ppm was assigned to two triazolyl protonsin a nonreversed, symmetrical structure (H-8NR). The sing-let at δ = 7.76 ppm was identified as a triazolyl proton (H-8RI) included in a CD cavity (NOE with H-6RI and innerCD protons H-3 and H-5). The third singlet at δ =7.69 ppm was assigned to a triazolyl proton (H-8R) that wasnot included in the CD cavity (NOE with H-6R). The exis-tence of two different populations of dimers was corrobo-rated by the six doublets observed between 7.5 and6.60 ppm, two of which were overlapped (δ = 7.40 ppm).The doublets at δ = 7.44 and 6.84 ppm were attributed to

Inversion Phenomenon in β-Cyclodextrin Dimers

Figure 6. 1H NMR spectrum of dimer 10 in D2O at 25 °C (NR: nonreversed; R: reversed; RI: reversed and included).

aromatic protons (H-9NR and H-10NR) in a nonreversedarchitecture (NOE with H-8NR and H-9NR, respectively).One of the doublets at δ = 7.39 ppm and the doublet at δ= 7.08 ppm were attributed to aromatic protons (H-9R andH-10R) not included in the CD cavity of a dimer for whichat least one of the CD cavity tumbled. Finally, the remain-ing doublets (7.40 and 6.67 ppm) were assigned to aromaticprotons (H-9RI and H-10RI) included in the CD cavity. Inthis case, 43 % symmetrical vs. 57% unsymmetrical confor-mations were identified (Figure 6). Dipolar interactionswere detected between H-8RI, H-9RI and H-10RI and theCD protons H-3 and H-5. In contrast to H-11R and H-11NR (singlets), the two H-11RI protons were magneticallyinequivalent due to their proximity to the CD cavity, asclearly shown in the HSQC spectrum (see the SupportingInformation). Accordingly, two different populations havebeen detected for dimer 10, with 43 % symmetrical and 57%unsymmetrical conformations (Figure 6).

Discussion

Based on the above analysis, several conclusions couldbe drawn. First, the analysis of 1, 2 and 3 highlighted oneof the main features guiding the inversion phenomenon inCD dimers, namely the spacer length. Each of the com-pounds displayed 100% symmetrical conformations, thusdemonstrating that the inversion process was greatly disfa-vored. The higher flexibility of the spacer of 1 did not leadto higher percentage of reversed dimer, suggesting that theinversion process was not correlated to the spacer rigiditybut rather to the distance between the CDs. A completetumbling was inconceivable because of the shortness of thetriazolyl–methyloxymethyl–triazolyl or triazolyl–phenyl–tri-azolyl spacer. Actually, the distance between the nitrogenatoms of the N–C6 fragments connected to the CDs hasbeen estimated to be 8 Å for 1 and 9 Å for 2. Knowingthat the β-CD cavity is approximately 8 Å deep, a stable

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conformation could not be obtained by inclusion of thespacer within the CD cavity. Even though the spacer couldenter the CD cavity by the secondary face, the resultingclose proximity of the two CDs precluded the existence ofa stable reversed conformation. Thus, one of the CDs at-tempted to tumble but the short spacer and the bulk of theCD prevented it from doing so. Although less congestedthan 2, dimer 3 also yielded 100 % symmetrical conforma-tion. However, the local disorder detected around the sub-stituted glucopyranoside unit of these dimers (multiple res-onances detected for the CH2–N pattern) clearly indicatedthat a partial tumbling of the CD remained possible. Dimer6 showed that the limit of the inversion process could beeasily overcome because an additional flexible group (CH2–O) between the phenyl ring and one of the triazolyl cyclesallowed for the inversion process to take place. The distancebetween the nitrogen atoms of the N–C6 fragments con-nected to the CDs was estimated to be 18 Å in this case,which is far greater than the depth of the CD cavity(ca. 8 Å). Symmetrical and unsymmetrical conformationswere then observed, with the unsymmetrical conformationpredominating (77 vs. 23 %). Upon the 360° rotation, H-9RI, H-10RI, H-11RI and H-12RI were included within theCD cavity. The higher stability of the latter regarding the“free cavities” dimer probably resulted from a conjunctionof two parameters. First, the hydrophobic spacer had a nat-ural tendency to enter the hydrophobic CD cavity (hydro-phobic effect). Second, hydrogen bonds between the pri-mary face of the nonreversed CD could interact with thesecondary face of the reversed CD.

The analysis of dimer 8 brought additional informationon the stability of CD dimers in water. In this case, thesymmetrical “free cavities” dimer predominated (62 vs.38%). Although the polyethoxylated spacer was more flexi-ble and longer than the spacer of dimers 1, 2, 3 and 6, itsmore balanced hydrophobic/hydrophilic character disfa-vored the inversion process. Thus, as expected, the chemical

J. Potier, S. Menuel, N. Azaroual, E. Monflier, F. HapiotFULL PAPERnature of the spacer was also of importance for the controlof the inversion process. Dimer 10 confirmed the NMRspectroscopy results obtained with 6, although the relativeproportions of symmetrical and unsymmetrical conforma-tions were much more balanced for 10 (43 vs. 57%, respec-tively). One of the CD groups underwent the inversion phe-nomenon, whereas the cavity of the second remained“free”. Among the three different groups constituting thespacer (triazolyl, phenyl, and ethoxylated chain), the CDcavity encapsulated the more hydrophobic phenyl moiety,as clearly shown by NOESY experiments.

From the above considerations, it was possible to estab-lish the essential criteria to be respected to avoid the inver-sion phenomenon occurring in CD dimers. First, the in-clusion of a spacer within a CD cavity depended more uponits length than upon its rigidity. Second, the length of thespacer should be as short as possible. Third, high spacerhydrophilicity was required to favor the displacement of theequilibrium towards conformations in which the CDs have“free” cavities. Note that although the triazolyl group mightinitiate the inversion process, it did not especially stabilizethe reversed conformation once the inversion process hadtaken place.

Conclusions

The present study sought to address the scope of the in-version phenomenon on triazolyl-substituted β-CDs. It wasshown that the proportion of CDs undergoing a 360° rota-tion mainly depended upon the nature of the spacer. Largeproportions of dimers for which only one out of two CDcavities was available have been observed for β-CDs linkedtogether by long and/or hydrophobic spacers, thus limitingthe application of such CD dimers in aqueous media. Con-versely, short spacers proved to be appropriate to ensure theexistence in water of CD dimers having “free” cavities. Assuch, we unambiguously demonstrated that CD dimerscomposed of two β-CDs linked together by a short triazo-lyl–methyloxymethyl–triazolyl or triazolyl–phenyl–triazolylspacer displayed 100 % symmetrical conformation in water.This is of crucial importance for catalytic systems in whichmultivalency and cooperativity are crucial.[20] More gen-erally, any system based on β-CD dimers for which molecu-lar recognition processes are involved can benefit from theresults of this study. Our current investigations are focusedon the use of these “free cavities” CD dimers in aqueouscatalysis.

Experimental SectionGeneral Remarks: All chemicals were purchased from Acros orAldrich Chemicals in their highest purity. All solvents were used assupplied without further purification. Distilled water was used inall experiments. Column chromatography was carried out by usingthe “flash” method with 40 μm silica. Analytical thin-layerchromatography (TLC) was performed on E. Merck aluminum-backed silica gel (Silica Gel F254). Compounds were identified withUV light (254 nm) and/or by staining with a solution of sulfuric

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acid in methanol. NMR spectra were recorded either with a BrukerDRX300 spectrometer operating at 300 MHz for 1H nuclei and at75 MHz for 13C nuclei, or with a Bruker Avance spectrometer op-erating at 500 MHz for 1H and at 126 MHz for 13C nuclei with aTXI probe at 295 K with a mixing time of 500 ms. CDCl3 (99.50%isotopic purity), [D6]DMSO (99.80% isotopic purity) and D2O(99.92% isotopic purity) were purchased from Euriso-Top. Massspectra were recorded with a MALDI-TOF/TOF Bruker DaltonicsUltraflex II spectrometer in positive reflectron mode with 2,5-DHBas the matrix. The distances between CDs within the CD-dimerswere estimated by using Avogadro, version 1.1.0 (Force field:MMFF94s, Geometry Optimization by 500 steps with steepest de-scent algorithm).

(4-Hydroxyphenyl)(trimethylsilyl)ethyne (4): A solution of 4-iodo-phenol (1.52 g, 6.9 mmol) and copper iodide (66 mg, 0.35 mmol,0.05 equiv.) in pyrrolidine (100 mL) was treated with bis(triphenyl-phosphine)palladium(II) dichloride (500 mg, 0.69 mmol, 0.1 equiv.)The solution was degassed under nitrogen, (trimethylsilyl)ethyne(1.01 g, 10.3 mmol, 1.5 equiv.) was added, and the mixture wasstirred under nitrogen for 5 h at 60 °C. The crude product was ex-tracted with ethyl acetate and washed with a saturated aqueoussolution of sodium hydrogen carbonate. After evaporation of ethylacetate, the product was purified by chromatography (CH2Cl2) togive 4 (1.26 g, 96%) as a white powder. Rf = 0.48 (CH2Cl2). 1HNMR (300 MHz, [D6]DMSO, 25 °C): δ = 9.93 (s, 1 H, OH), 7.26[d, J = 8.7 Hz, 2 H, CH(5)], 6.73 [d, J = 8.7 Hz, 2 H, CH(6)], 0.19[s, 9 H, CH3(1)] ppm. 13C NMR (75 MHz, [D6]DMSO, 25 °C): δ =158.2 (C4), 133.3 (C5), 115.6 (C6), 112.4 (C7), 106.08 (C3), 91.5(C2), 0.1 (C1) ppm.

1-Ethynyl-4-(prop-2-yn-1-yloxy)benzene (5): Compound 4 (700 mg,3.6 mmol) and Cs2CO3 (1.76 g, 5.4 mmol, 1.5 equiv.) were dissolvedin acetonitrile (50 mL) and the solution was degassed under nitro-gen. Propargyl bromide (80 wt.-% in toluene, 523 mg, 4.4 mmol)was added and the resulting mixture was stirred overnight undernitrogen at 85 °C. The crude product was extracted with ethyl acet-ate and washed with a saturated aqueous solution of sodium hydro-gen carbonate. After evaporation of ethyl acetate, the product waspurified by chromatography (heptane/CH2Cl2, 5:5 to 0:5) to give 5(420 mg, 75%) as a white solid. Rf = 0.52 (CH2Cl2). 1H NMR(300 MHz, [D6]DMSO, 25 °C): δ = 7.41 [d, J = 8.7 Hz, 2 H,CH(6)], 6.96 [d, J = 8.7 Hz, 2 H, CH(5)], 4.81 [d, J = 1.8 Hz, 2 H,CH2(3)], 4.02 [s, 1 H, CH(9)], 3.58 [t, J = 1.8 Hz, 1 H, CH(1)] ppm.13C NMR (75 MHz, [D6]DMSO, 25 °C): δ = 157.9 (C4), 133.6(C6), 115.6 (C5), 114.9 (C7), 83.8 (C8), 79.9 (C9), 79.4 (C2), 79.0(C1), 55.9 (C3) ppm.

4,7,10,13,16,19-Hexaoxodocosa-1,21-diyne (7): Pentaethylene glycol(300 mg, 1.3 mmol) and Cs2CO3 (1.26 g, 3.9 mmol) were dissolvedin acetonitrile (30 mL) and degassed under nitrogen. Propargylbromide (80 wt.-% in toluene, 345 mg, 2.9 mmol) was added andthe resulting mixture was stirred overnight under nitrogen at 85 °C.The crude product was extracted with ethyl acetate and washedwith saturated aqueous sodium hydrogen carbonate. After evapora-tion of ethyl acetate, the product was purified by chromatography(ethyl acetate/CH2Cl2, 7:3) to give 7 (59 mg, 14%) as a white pow-der. Rf = 0.53 (EtOAc/CH2Cl2, 7:3). 1H NMR (300 MHz, CDCl3,25 °C): δ = 4.20 [d, J = 2.4 Hz, 4 H, CH2(3)], 3.65–3.69 [m, 20 H,CH2(4 and 5)], 2.42 [t, J = 2.4 Hz, 2 H, CH(1)] ppm. 13C NMR(75 MHz, CDCl3, 25 °C): δ = 58.5 (C3), 69.2 (C2), 70.6–70.5 (C4and C5), 74.6 (C1), 83.7 (C2), 114.3 (C6), 114.6 (C5), 133.6 (C4),159.2 (C3) ppm.

Bis[2-(4-ethynylphenoxy)ethoxy]ethane (9): Bis(2-chloroethyl) ether(984 mg, 5.25 mmol), 4 (2.0 g, 10.5 mmol, 2 equiv.) and cesium

Inversion Phenomenon in β-Cyclodextrin Dimers

carbonate (6.8 g, 21 mmol, 4 equiv.) were dissolved in acetonitrile(100 mL) and the resulting mixture was stirred overnight under ni-trogen at 85 °C. The crude product was extracted with ethyl acetateand washed with saturated aqueous sodium hydrogen carbonate.After evaporation of ethyl acetate, the product was purified bychromatography (heptane/dichloromethane, 5:5 to 0:5) to give 9(950 mg, 37%) as a pale-yellow powder. Rf = 0.52 (CH2Cl2). 1HNMR (300 MHz, CDCl3, 25 °C): δ = 7.40 [d, J = 8.7 Hz, 2 H,CH(4)], 6.83 [d, J = 8.7 Hz, 2 H, CH(5)], 4.11 [t, J = 4.5 Hz, 4 H,CH2(7)], 3.85 [t, J = 4.5 Hz, 4 H, CH2(8)], 2.99 [s, 2 H, CH(1)],3.74 [s, 4 H, CH2(9)] ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ= 159.2 (C3), 133.6 (C4), 114.6 (C5), 114.3 (C6), 83.7 (C2), 75.9(C1), 71.0 (C9), 69.6 (C8), 67.4 (C7) ppm.

General Procedure for Copper-Catalyzed Azide Alkyne Cycload-dition (CuAAC): Mono-6-azido-β-CD (2 equiv.) and hydrated cop-per sulfate (4 equiv.) were added to a solution of dialkynyl deriva-tive (1 equiv.) in DMSO (50 mL). After subsequent dropwise ad-dition of a freshly prepared solution of sodium ascorbate (8 equiv.)dissolved in H2O (10 mL), the solution was stirred at room temp.for 24 h. The crude product was then precipitated with acetone togive a green solid. After filtration, the residue was purified by col-umn chromatography (water/ammoniac, 80:20) to remove the cop-per salts. After solvent evaporation, the product was precipitationin a water/acetone mixture. The solid was filtered and purified bychromatography (CH3CN/H2O, 7:3) to give the pure dimer (80–85% yield) as a white powder.

Bis[1-(6A-deoxy-β-D-cyclodextrin)-1H-1,2,3-triazol-4-ylmethyl]Ether (1): 1H NMR (500 MHz, D2O, 25 °C): δ = 7.99 [s, 2 H,CH(8)], between 4.85 and 5.10 (m, 14 H, H-1), 4.54–4.93 (m, 4 H,H-6 substituted subunit), 4.65 [s, 4 H, CH2(9)], 4.11 (t, J = 9 Hz,2 H, H-5 substituted subunit), 3.97–3.35 (m, 74 H, H-2, H-3, H-4,H-5, H-6), 3.07 (d, J = 12 Hz, 2 H, H-6), 2.73 (d, J = 12 Hz, 2 H,H-6) ppm. 13C NMR (126 MHz, D2O, 25 °C): δ = 127.3 (C8), 104–100 (C1), 84–80 (C4, C5), 75–70 (C2, C3), 64.6 (C9), 63–60 (C6nonsubstituted subunit and C6 substituted subunit), 51.7 (C6 sub-stituted subunit) ppm. HRMS: calcd. for [C90H144N6O69 + Na]+

2435.78; found 2435.62.

1,3-Bis[1-(6A-deoxy-β-D-cyclodextrin)-1H-1,2,3-triazol-4-yl]benz-ene (2): 1H NMR (500 MHz, D2O, 25 °C): δ = 8.32 [s, 2 H, CH(8)],8.14 [s, 1 H, CH(13)], 7.73 [d, J = 8 Hz, 2 H, CH(11)], 7.53 [t, J =8 Hz, 1 H, CH(12)], 5.13–4.81 (m, 14 H, H-1), 5.02–4.56 (m, 4 H,H-6 substituted subunit), 4.08 (t, J = 9 Hz, 2 H, H-5 substitutedsubunit), 4.00–3.20 (m, 74 H, H-2, H-3, H-4, H-5, H-6), 3.00 (d, J

= 12 Hz, 2 H, H-6), 2.79 (d, J = 12 Hz, 2 H, H-6) ppm. 13C NMR(126 MHz, D2O, 25 °C): δ = 130.5 (C12), 124.3 (C11), 123.1 (C8),121.9 (C13), 102–98 (C1), 83–79 (C4, C5), 75–70 (C2, C3), 61–58(C6 nonsubstituted subunit and C6 substituted subunit), 51.6 (C6substituted subunit) ppm. HRMS: calcd. for [C94H144N6O68 +Na]+ 2468.79; found 2468.52.

1,4-Bis[1-(6A-deoxy-β-D-cyclodextrin)-1H-1,2,3-triazol-4-yl]benz-ene (3): 1H NMR (500 MHz, D2O, 25 °C): δ = 8.30 [s, 2 H, CH(8)],7.83 [s, 4 H, CH(9)], 5.13–4.81 (m, 14 H, H-1), 5.00–4.55 (m, 4 H,H-6 substituted subunit), 4.08 (t, J = 9 Hz, 2 H, H-5 substitutedsubunit), 4.02–3.20 (m, 74 H, H-2, H-3, H-4, H-5, H-6), 3.00 (d, J

= 12 Hz, 2 H, H-6), 2.74 (d, J = 12 Hz, 2 H, H-6) ppm. 13C NMR(126 MHz, D2O, 25 °C): δ = 126.4 (C Ar), 123.2 (C8), 105–100(C1), 84–80 (C4, C5), 75–70 (C2, C3), 62–58 (C6 nonsubstitutedsubunit and C6 substituted subunit), 51.4 (C6 substituted sub-unit) ppm. HRMS: calcd. for [C94H144N6O68 + Na]+ 2468.79;found 2468.79.

1-(6A-Deoxy-β-D-cyclodextrin)-4-(4-{[1-(6A-deoxy-β-D-cyclodex-trin)-1H-1,2,3-triazol-4-yl]methoxy}phenyl)-1H-1,2,3-triazol (6): 1H

Eur. J. Org. Chem. 2014, 1547–1556 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org 1555

NMR (500 MHz, D2O, 25 °C): δ = 7.91 [s, 1 H, CH(8NR)], 8.05 [s,1 H, CH(12NR)], 7.91 [s, 1 H, CH(8R)], 7.69 [d, J = 8 Hz, 2 H,CH(9NR)], 7.58 [d, J = 8 Hz, 2 H, CH(9R)], 7.25 [s, 1 H, CH(12RI)],7.11 [d, J = 8 Hz, 2 H, CH(10NR)], 6.88 [d, J = 8 Hz, 2 H,CH(10R)], 5.40 [d, J = 14 Hz, 1 H, 1/2 CH2(11R)], 5.29 [d, J =14 Hz, 1 H, 1/2 CH2(11R)], 5.23 [s, 14 H, CH2(11NR)], 5.11–4.81(m, 14 H, H-1), 4.68–5.06 (m, 2 H, H6R), 4.95–4.54 (m, 4 H,H6NR), 4.67–4.39 (m, 2 H, H6RI), 4.15 (t, J = 9 Hz, 1 H, H5R

substituted subunit), 4.11–2.61 (m, 159 H, H-2, H-3, H-4, H-5, H-6) ppm. 13C NMR (126 MHz, D2O, 25 °C): δ = 128–125 (C9),125.0 (C12), 122.5 (C8), 118.6 (C10), 115.9 (C10), 104–100 (C1),84–80 (C4, C5), 75–70 (C2, C3), 62.6 (C11), 61–58 (C6 nonsubsti-tuted subunit and C6 substituted subunit), 52.1 (C6 substitutedsubunit) ppm. HRMS: calcd. for [C95H146N6O69 + Na]+ 2498.80;found 2497.98.

1,18-Bis[1-(6A-deoxy-β-D-cyclodextrin)-1H-1,2,3-triazol-4-yl]-2,5,8,11,14,17-hexaoxaoctadecane (8): 1H NMR (500 MHz,D2O, 25 °C): δ = 7.97 [s, 2 H, CH(8NR)], 7.93 and 7.88 [s, 2 H,CH(8R)], 5.11–4.85 (m, 14 H, H-1), 4.96–4.62 (m, 2 H, H6RI), 4.52–4.97 (m, 2 H, H6R), 4.54–4.93 (m, 4 H, H6NR), 4.61 [m, 8 H,CH2(9)], 4.11 (t, J = 9 Hz, 2 H, H5NR), between 3.98–3.22 (m, 155H, H-2, H-3, H-4, H-5, H-6), 3.06 (d, J = 12 Hz, 2 H, H-6), 2.71(d, J = 12 Hz, 2 H, H-6) ppm. 13C NMR (126 MHz, D2O, 25 °C):δ = 126–123 (C8), 104–100 (C1), 84–80 (C4, C5), 75–70 (C2, C3),72–68 (C10, C11, C12, C13, C14), 63.7 (C9), 61–58 (C6 nonsubsti-tuted subunit and C6 substituted subunit), 52.4 (C6 substitutedsubunit) ppm. HRMS: calcd. for [C100H164N6O74 + Na]+ 2656.92;found 2656.16.

1,2-Bis(2-{4-[1-(6A-deoxy-β-D-cyclodextrin)-1H-1,2,3-triazol-4-yl]phenoxyl}ethoxy)ethane (10): 1H NMR (500 MHz, D2O, 25 °C):δ = 7.98 [s, 2 H, CH(8NR)], 7.75 [s, 1 H, CH(8RI)], 7.69 [s, 1 H,CH(8R)], 7.44 [d, J = 8 Hz, 4 H, CH(9NR)], 7.40 [d, J = 8 Hz, 2 H,CH(9RI)], 7.39 [d, J = 8 Hz, 2 H, CH(9R)], 7.08 [d, J = 8 Hz, 2 H,CH(10R)], 6.84 [d, J = 8 Hz, 4 H, CH(10NR)], 6.67 [d, J = 8 Hz, 2H, CH(10RI)], 5.14–4.78 (m, 14 H, H-1), 5.02–4.45 (m, 2 H, H6R),4.9 (m, 2 H, H6RI), 4.88–4.39 (m, 4 H, H6NR), 4.3 [br. s, 2 H,CH2(11R)], 4.15 (m, 1 H, H5RI), 4.02 [br. s, 4 H, CH2(11NR)], be-tween 2.58 and 4.00 (m, 181 H, H-2, H-3, H-4, H-5, H-6, H-11RI,H-12, H-13) ppm. 13C NMR (126 MHz, D2O, 25 °C): δ = 128–126(C9), 122–118 (C8), 117–113 (C10), 104–101 (C1), 85–80 (C4, C5),75–70 (C2, C3), 70–67 (C11, C12), 61–58 (C6 nonsubstituted sub-unit and C6 substituted subunit), 53–50 (C6 substituted sub-unit) ppm. HRMS: calcd. for [C106H160N6O72 + Na]+ 2692.90;found 2692.21.

Supporting Information (see footnote on the first page of this arti-cle): NMR spectra.

Acknowledgments

J. P. is grateful to the Région Nord-Pas-Calais and the CentreNational de la Recherche Scientifique (CNRS) for financial sup-port (2010–2013). The authors are grateful to Roquette Frères(Lestrem, France) for a gift of β-cyclodextrin.

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Published Online: December 19, 2013