pubs.acs.org/crystal Published on Web 02/05/2010 r 2010 American Chemical Society

DOI: 10.1021/cg9009064

2010, Vol. 101124–1129

Enantioresolution and Chameleonic Mimicry of 2-Butanol with an

Adamantylacetyl Derivative of Cholic Acid

JavierMiragaya, Aida Jover, Francisco Fraga, FranciscoMeijide, and Jos�e V�azquez Tato*

Departamentos de Quımica Fısica y Fısica Aplicada, Facultad de Ciencias, Universidad de Santiago deCompostela, Avda. Alfonso X El Sabio s/n, 27002 Lugo, Spain

Received August 3, 2009; Revised Manuscript Received December 14, 2009

ABSTRACT: [3β,5β,7R,12R]-3[(Adamantyl-1-acetyl)-amino]-7-12-dihydroxycholan-24-oic acid (AdCH2CA) was synthesizedby the reaction between 1-adamantyl acetyl chloride and the methyl ester of 3β-amino-cholic acid and hydrolysis of the ester.The acid was recrystallized from racemic 2-butanol (0.1% water). Crystals are orthorhombic (P212121) and form inclusioncomplexes with water and 2-butanol with a 1:1:1 stoichiometry. Only the S-enantiomer is included into the structure ofthe crystal, exhibiting a chameleonic mimicry with the steroid bilayers. The isolation of crystals allows the enantioresolution ofthe racemate with a high purity (≈99%) of S-2-butanol. The steroid molecules are disposed in an antiparallel orientation in thehydrophobic layer and a parallel orientation in the hydrophilic one.

Introduction

Practical methods of enantiomer separation are importantboth in the research laboratory and in several industries. Themain methods, consisting of the use of inclusion complexa-tion, biological methods, and HPLC, have been reviewed in arecent book edited by Toda.1 When a chiral host compoundincludes selectively one enantiomer of a racemic guest com-pound, optical resolution of the guest can be accomplished. Inthese cases, guest molecules are accommodated in a cavityformed by the host compound or belonging to it and, fre-quently, the process is accomplished in the solid state duringrecrystallization. Although for a full knowledge of the processthe crystal must be resolved, the absence of crystals suitablefor X-ray analysis does not mean that resolution (at leastpartial) of a racemic mixture is not occurring.

Bile acids and their derivatives form inclusion crystals withmanyorganic compounds.2-5Amongother factors that affectthe steroidal assembly in the crystalline state, the effect of thelength of the side chain of the main natural bile acids has beensystematically studied byMiyata et al.6 bymaking one-by-oneinsertions and suppressions ofmethylene spacers into the side-chain of steroidal bile acids. For example, bishomocholicacid (with two additional methylene units) includes variousorganic substances,7 while bisnorcholic acid (with two de-creased units) does not.4 This causes a diversity of hostframeworks with characteristic hydrogen-bonding networks,8

a fact also observed for cholanamide crystals in which manyalcohols are included.9 It is interesting to notice that theinclusion behavior of aliphatic alcohols in cholamide and incholic acid is completely different since 54 alcohols can beincluded in the bilayer-type structures of cholamide, but onlytwo in those of cholic acid.10,11

Since bile acids are highly asymmetric, it can be expectedthat enantioresolution could take place in the steroidal crys-tals. From the first paper published by Miyata et al.12 relatedto the optical resolution of lactones by the inclusion methodusing cholic acid as the host, the number of chiral guests that

have been efficiently resolved by their selective complexa-tion with bile acids has increased steadily. Among them wecan mention lactones (cholic acid),12-14 alcohols (cholan-amide, 3-epideoxycholic acid, lithocholanamide),10,15-17 sulf-oxides (dehydrocholic acid),18 cyclic ketones (cholic anddeoxycholic acids),19 epoxides (cholic acid),20 amines (cholicacid),21N-nitrosopiperidines (cholic and deoxycholic acids),22

and cyclic amides (dehydrocholic acid).23 Hosts are given inparentheses. Generally, it is not so easy to perform enantiore-solutions with a high enantiomeric excess. The subject hasbeen reviewed by Bortolini et al.24

We have recently synthesized several bile acid derivatives inwhich the hydrophobicity of natural bile acids has beenextended by attaching a bulk hydrophobic group at C3. Asa consequence, different packing structures in crystals25,26 andnew supramolecular structures (as lamellae25 and tubes27,28)were obtained in solution. In particular, the packing structurein crystals ofmodifiedbile acids is highly dependent on the sizeof the group and length of the binding bridge between theattached group and the steroid nucleus. For instance, when anorbornyl-2-acetyl derivative of cholic acid26 (NbCH2CA) isrecrystallized from DMSO, acetone, and 2-propanol, allguests are located in the region of the bridge forming ahydrogen bond with the amide of the bridge. However, whenthe hydrophobic adamantyl group is attached to the 3-posi-tion of cholic acid (compound named Ad-HC),25 the ada-mantyl moieties are mutually interlocked in the crystalwithout leaving free space for solvent guests.

The remarkable effects of the length of the side chain on theassembly of bile acids, largely studied by Miyata et al.,8

together with the last mentioned observations, suggest thatthe length of the bridge between the two residues and thesize of the hydrophobic bulky residue should be investigatedmore systematically. For this reason, we have synthesized anewderivative (AdCH2CA,Figure 1) obtained by reacting the1-adamantyl acetic acidwith the 3β-aminoderivative of cholicacid.Compared to its predecessorAd-HC, the bridge betweenthe two residues has been extended by a methylene group.Because of the importanceof the enantioresolutionmentionedabove, we have recrystallizedAdCH2CA in racemic 2-butanolwith the confidence of getting the enantioresolution of the

*To whom correspondence should be addressed. E-mail: jose.vazquez@usc.es.

Article Crystal Growth & Design, Vol. 10, No. 3, 2010 1125

mixtures because of the high asymmetry of the host molecule.As Aburaya et al.29 pointed out, enantioresolution of second-ary aliphatic alcohols still remains a challenging problembecause of the fourth substituent at the stereogenic carbon isthe small hydrogen atom. Several methods have been pro-posed for the enantioresolutionof 2-butanol. Among themwecan mention, apart from cholanamide,15 enzymatic meth-ods,30 HPLC,31 metal complexes of O,O0-dibenzoyltartaricacid,32 and reaction with specific compounds as 2-methoxy-2-(1-naphthyl)propionic acid.33,34

Experimental Section

Synthesis of [3β, 5β, 7r, 12r]-3[(Adamantyl-1-acetyl)-amino]-7-12-dihydroxycholan-24-oic acid (AdCH2CA). The compound isobtained by the reaction between 1-adamantyl acetyl chloride andthemethyl ester of 3β-amino-cholic acid. To synthesize 1-adamantylacetyl chloride, a mixture of 0.59 g of 1-adamantyl acetic acid and2 mL of thionile chloride are refluxed during 2 h under a CaCl2 trap

and concentrated under a vacuum. The methyl ester of 3β-amino-cholic acid is obtained from 3β-amino-cholic acid (the synthesisof this amine derivative from commercial cholic acid has beendescribed elsewhere).35,36 1.5 g of this compound is dissolved in20 mL of chloroform and 2 mL of triethylamine under nitrogenand cooled in an ice-salt bath (-10 to -15 �C). The previouslyobtained 1-adamantyl acetyl chloride dissolved in chloroform isadded dropwise under stirring and the reaction mixture is main-tained during 18 h at r.t. before concentration in a rotavapor. Themixture is purified in a silica gel column using 20:1 ethyl acetate/methanol as eluent and dried in a vacuum oven. Yield: 40%. Thelast step is the hydrolysis of the ester derivative. For this purpose,0.8 g of the methyl ester in 15 mL of 1 M KOH in methanol arerefluxed during 1 h.After evaporation ofmethanol, 200mLofwaterand concentrated HCl are added to neutralize the potassium salt.The suspension formed is filtered, washed with water, and dried at70 �C with P2O5. Yield: 90%.

1H NMR of the methyl ester of AdCH2CA (DMSO, 300 MHz,δ/ppm): 7.38 (d NH-CdO); 3.55 (s OCH3); 2.43-1 (steroid nucleusand adamantine protons); 0.89 (dH21); 0.84 (sH19); 0.57 (sH18).

13CNMR of the methyl ester of AdCH2CA (DMSO; 75 MHz, δ/ppm):174.51 (O-C24dO), 170.06 (NH-C26dO), 71.72 (C12), 66.98 (C7),51.86 (OCH3), 23.53 (C19), 17.59 (C21), 12.99 (C18). MALDI-TOF,SDHB matrix, m/z: M: 598.55, [M þ Na]þ: 622.5, [M þ K]þ: 636.55;theoreticalM: 597.44, [MþNa]þ: 620.43, [MþK]þ: 636.40.MALDI-TOF of the methyl ester of AdCH2CA, SDHB matrix, m/z:M: 584.40, [M þ Na]þ: 606.40, [M þ K]þ: 622.38; theorethical M:583.42, [M þ Na]þ: 606.41, [M þ K]þ: 622.39.

For crystallization, racemic 2-butanol (p.a 99% and 0.1% ofwater; Panreac, Barcelona, Spain) was used. Fifteen milligrams ofAdCH2CA was dissolved into 1 g of racemate and the mixture washeated and left to reach room temperature. Colorless crystals wereformed after 1 month.

X-ray Diffraction.A colorless prismatic crystal of the compoundwas mounted on a glass fiber and used for data collection. Datawere collected on a Bruker AXS APEXII-CCD area dectectordiffractometer. Molecular graphics were from Mercury (http://www.ccdc.cam.ac.uk/prods/mercury) and Accelrys DS Visualizerv2.0 (http://accelrys.com/products/discovery-studio/visualization/discovery-studio-visualizer.html). A summary of the crystal data,

Figure 1. Structure and conventional numbering of AdCH2CA.

Table 1. Crystal Data, Data Collection and Refinement

solvent racemic 2-butanolempirical formula C36H57NO5, C4H10O, H2Oformula weight 675.96temperature (K) 100 (2)wavelength (A) 0.71073crystal system, space group orthorhombic, P212121a (A) 9.4616(16)b (A) 17.690(2)c (A) 22.405(4)R (�) 90.00β (�) 90.00γ (�) 90.00cell volume (A3) 3750.1(10)Z, calculated density (g/cm3) 4, 1.197absorption coefficient (mm-1) 0.08F(000) 1488crystal size (mm3) 0.19 � 0.08 � 0.07theta range (data collection) (o) 1.47-26.46index ranges -11 e h e 11, 0 e k e 22,

0 e l e 27data/restraints/parameters 4311/3/457goodness-of fit on F2 0.978final R indices [I > 2σ(I)] R1 = 0.0555, wR2 = 0.1207R indices (all data) R1 = 0.1460, wR2 = 0.1552ΔFmax and ΔFmin (e A

-3) 0.309 and -0.278

Figure 2. Crystal packing of AdCH2CA recrystallized from 2-butanol/water viewed along the a axis.

Figure 3. Mimicry of 2-butanol with the AdCH2CA molecule.

1126 Crystal Growth & Design, Vol. 10, No. 3, 2010 Miragaya et al.

and experimental details are listed in Table 1. CCDC-742154 con-tains the supplementary crystallographic data for the crystal. Thesedata can be obtained free of charge from The Cambridge Crystallo-graphic Data Center via www.ccdc.cam.ac.uk/data_request/cif.

Optical rotations were determined on a Dr. Kernchen PropolDigital Automatic polarimeter (resolution 0.0001�) at the sodiumDline at 20 �C, with a path length of 0.2 dm.

Results and Discussion

A summary of the crystal data is listed in Table 1. Crystalsare orthorhombic (P212121) and form inclusion complexeswith water and 2-butanol with a 1:1:1 stoichiometry.

The crystal packing along the a-axis is shown in Figure 2.Abilayer structure with hydrophobic and hydrophilic regions isclearly distinguished with the β sides of the steroid moleculesdisposed in an antiparallel orientation andmethyl groupsC18and C19 having an R-interdigitation. The distance betweensteroid h planes26,37 with a back-to-back hydrophobic inter-action is 5.3 A�, while the distance between steroid planes withhydrophilic interactions is 6.0 A�. Thus, the width of thebilayer in the crystal is 11.3 A, half of the c length of the unitcell. However, the orientation of the steroidal molecules atthe hydrophilic interface of the bilayers is parallel. Thisarrangement (parallel and antiparallel for hydrophilic andhydrophobic layers, respectively) is just the opposite of thatobserved byAburaya et al.29 for the inclusion of 2,2-dimethyl-3-hexanol by cholamidewhere the hydrophilic interface of thebilayers is antiparallel while the lipophilic one is parallel.This bilayer structure is different from those observed byYoswathananont et al.11 of any of the 58 inclusion crystals ofcholanamide with aliphatic alcohols since such a reversion inthe lipophilic sides is very rare in the case of bile acids and theirderivatives. It is noteworthy the parallelism between thesteroid nucleus plane and the direction of the lamellae.

In Figure 2, the 2-butanol molecule exhibits a remarkablechameleonic mimicry with the steroid molecules. A “third”methyl group (of the ethylene group of 2-butanol), directedtoward the hydrophobic layer, as well as a “third” hydroxylgroup directed toward the hydrophilic layer, are observed. Thetwo carbon atoms linked to the stereogenic carbon atom of2-butanol are in the same plane of the steroid nucleus sincetheir distances to this plane (in yellow color in Figure 2, top)are 0.116 (methyl) and 0.049 (methylene) A�. Furthermore, thedistance of the hydroxyl group of 2-butanol to the hydrophilicplane (defined by the hydroxyl groups of AdCH2CA; in redcolor inFigure 2, bottom) is only 0.063 A�, while the distance ofthe methyl group of the ethylene residue of 2-butanol to the

hydrophobic plane (definedby themethyl groupsof the steroidnucleus of AdCH2CA; in blue color in Figure 2, bottom) isnegligible (equal to 0.009 A�). That is, 2-butanol is fullymimeticwith the facial amphiphilicity of the steroid (Figure 3).

It is time to recognize that only S-2-butanol is included inthe crystal although the recrystallizationprocesswas performin racemic 2-butanol; that is, AdCH2CA crystal exhibits afull enantioselectivity. To confirm this enantioselectivity,optical rotation measurements were carried out. It is neces-sary to consider that both components of the crystal(2-butanol and AdCH2CA) are optically active. When crys-tals are redissolved in racemic 2-butanol, the observedopticalrotation is the sum of the contribution of each component,that is,R=RS-2-butanolþRAdCH2CA

, where each contributionis given by Ri = [R]D20 � l � ci/100 (l is the path lengthin decimeters, ci is the concentration of the i-th component ing/100 mL, and [R]D20 is the specific rotationmeasured at 20 �Cat the wavelength of the sodium D line). Because of thestoichiometry of the crystal, themolar concentration of bothcomponents is the same, and the following equation results

R ¼ ½R�20D ðS-2-butÞþMAdCH2CA

MS-2-but½R�20D ðAdCH2CAÞ

� �

� l � cS-2-but=100

(MAdCH2CAand MS-2-but being the molar mass of each

component). That is, the R vs cS-2-but plot should be linear,

Figure 4. Carbon-carbon distances of interdigitated methyl goups of steroid molecules and distances of the C4 of 2-butanol to methylsteroidal groups and adamantyl methylene groups.

Figure 5. Parallel and transversal orientation of the hydrogen atomand the methyl group of 2-butanol with respect to the direction ofthe steroid molecules.

Article Crystal Growth & Design, Vol. 10, No. 3, 2010 1127

with a slope value of 35.0� (calculated by using the valuesofþ20.64� andþ13.52� for [R]D20AdCH2CAand [R]D20(S-2-but),respectively. Measurements of the optical rotation were madein the concentration range 0.8-6.0 mg/mL of cS-2-but, and theobtained experimental slope is 34.8 ( 0.2�. Therefore, withinan experimental error of ≈1% the isolated S-2-butanolfrom the crystal is enantiomerically pure. If the guest were theR enantiomer the expected value for the slope would be 29.6�.

This powerful enantioselectivity means that the cavitywhere the guest is accommodated is chiral. We will analyzethis chirality in a similar way as the four location modeldoes.38 For this purpose, some distances from 2-butanolatoms to their neighbors will be considered.39

The orientation of the hydroxy group of 2-butanol can beconsidered fixed toward the hydrophilic layer of the crystal,forming a hydrogen bond with a water molecule. Similarly,the ethylene group of butanol has the right length for itsinterdigitation with the methyl groups of the steroid and amethylene group of the adamantyl residue. Figure 4 shows

that the distances between carbon atoms for these interactionsare very similar to those of the interdigitated steroid methylgroups. Thus, the hydroxy and ethylene groups are perpendi-cular to the laminar structure of the crystal. Consequently, theother two groups have to be in parallel and transversalorientations with respect to the longitudinal direction of thesteroid molecules, defining the configuration R or S of thechiral carbon atom (see Figure 5 for the actual orientation ofthese groups).

Figure 6 shows that there are six hydrogen atoms in theneighborhood of hydrogenwhich are located at a distance lessthan 4 A�. Itmust be noticed that the closest ones (at 2.34, 2.40,and 3.2 A�) are almost transversal with respect to the long-itudinal direction. This means that under the geometricalrestrictions imposed by the host molecules the methyl groupdoes not have enough space to be oriented toward thetransversal direction, except if the gap between parallel steroidmolecules is broadening. The methyl group finds a betteraccommodation along the longitudinal axis, parallel or anti-parallel to the direction of the steroid molecules. A watermolecule (which is forming a hydrogen bond with the alcoholhydroxy group, see below) is almost at the opposite directionof the actual methyl group location, at a distance of 3.76 A� ofthe stereogenic carbon of the alcohol. Thus, there is not freespace for the inversion of the chiral atom without the dis-placement of the water molecule, disturbing the whole lamel-lar structure of the crystal. Furthermore, there are twohydrogen atoms of the adamantyl residue located at less than4.5 A� from the stereogenic carbonatomwhichwould alsobeara steric hindrance for this alternative location of the methylgroup after chiral inversion. Therefore, the inversion of thestereogenic carbon atom of 2-butanol would oblige a fullrearrangement of the bilayer structure of the crystal. Ob-viously, the parallel orientation of the methyl group corre-sponds to theminimum energy for the actual bilayer structureof the crystal.

Further clarification is given by Figure 7, in which S-2-butanol is included in the bilayer structure of the crystal,squeezed by the steroid molecules. For comparison, R-2-butanol is also drawn to illustrate that this enantiomer cannot

Figure 7. Front and back views of the S-2-butanol after sequential rotations of 45� of the crystal AdCH2-CA. Mirror images of theR enantiomer are also shown.

Figure 6. Hydrogen (linked to the stereogenic carbon)-hydrogen(closest neighbor molecules) distances.

1128 Crystal Growth & Design, Vol. 10, No. 3, 2010 Miragaya et al.

be accommodated in this place. In the figure, the sequentialimages correspond to rotations of the crystal around anaxis located on the guest molecule. For comparison, theR enantiomer is drawn as shadow mirror images.

Finally, Figure 8 shows a visualization of butanol cush-ioned by the surface of the surrounding molecules whichinteract with it, forming the chiral cavity where the guest isincluded.

Four steroid molecules are required to fully describe thehydrogen network of the crystal. The water molecule playsa central role as it interacts with three of these steroidmolecules and with the S-2-butanol molecule which acts onlyas a donor toward water; that is, it does not exhibit doublehooks (Figure 9).9,26 Each bile acidmolecule is linked to threeother steroid molecules. This hydrogen bond network inter-connects the hydrophilic groups of the lamellar structure. Thenetwork scheme and lengths are shown inFigure 9, all of thembeing within typical values for hydrogen bond interactions.TheN-Hgroup of the amide bond does not participate in thehydrogen bond network.

Conclusion

When AdCH2CA is recrystallized in racemic 2-butanol,only the S-enantiomer is included in the bilayer structure ofthe crystal, allowing the enantioresolution of the racemate byisolation of crystals. The steroid molecules are disposed inantiparallel orientation in the hydrophobic layer and parallelin the hydrophilic one. The guest S-2-butanol exhibits achameleonic mimicry with the steroid bilayers suggesting thatthis behavior can be further exploited in designing new hostmolecules for the enantioresolution of other alkyl alcohols.

Acknowledgment. The authors thank the Ministerio deCiencia y Tecnologıa, Spain, (Project MAT2006-61721) forfinancial support. J.M. also thanks theMinisterio deCiencia yTecnologıa, Spain, for a scholarship.

References

(1) Toda, F., Ed.; Enantiomer Separation: Fundamentals and PracticalMethods; Kluwer Academic Publishers: Norwell, MA, 2004.

(2) Miyata,M.; Sada,K. InComprehensive Supramolecular Chemistry;D. D.MacNicol, T., F., Bishop, R., Eds.; Elsevier: Oxford, 1996; Vol. 6,p 147.

(3) Nakano, K.; Sada, K.; Kurozumi, Y.; Miyata, M. Chem.—Eur. J.2001, 7, 209.

(4) Miyata, M.; Tohnai, N.; Hisaki, I. Molecules 2007, 12, 1973.(5) Miyata, M.; Tohnai, N.; Hisaki, I. Acc. Chem. Res. 2007, 40,

694.(6) Sugahara, M.; Sada, K.; Miyata, M. Chem. Commun. 1999, 293.(7) Sada, K.; Sugahara, M.; Kato, K.; Miyata, M. J. Am. Chem. Soc.

2001, 123, 4386.(8) Kato, K.; Sugahara, M.; Tohnai, N.; Sada, K.; Miyata, M. Cryst.

Growth Des. 2004, 4, 263.(9) Sada, K.; Kondo, T.; Miyata, M.; Miki, K. Chem. Mater. 1994, 6,

1103.(10) Sada, K.; Kondo, T.;Miyata,M.Tetrahedron: Asymmetry 1995, 6,

2655.(11) Yoswathananont,N.; Sada,K.;Nakano,K.;Aburaya,K.; Shigesato,

M.; Hishikawa, Y.; Tani, K.; Tohnai, N.; Miyata, M. Eur. J. Org.Chem. 2005, 5330.

(12) Miyata, M.; Shibakami, M.; Takemoto, K. J. Chem. Soc., Chem.Commun. 1988, 655.

(13) Sada, K.; Nakano, K.; Hirayama, K.; Miyata, M.; Sasaki, S.;Takemoto, K.; Kasai, N.; Kato, K.; Shigesato, M.; Miki, K.Supramol. Chem. 2001, 13, 35.

(14) Nakamura, S.; Imashiro, F.; Takegoshi, K.; Terao, T. J. Am.Chem. Soc. 2004, 126, 8769.

(15) Briozzo, P.; Kondo, T.; Sada, K.; Miyata, M.; Miki, K. ActaCrystallogr. 1996, B52, 728.

(16) Kato, K.; Aburaya, K.;Miyake, Y.; Sada, K.; Tohnai, N.;Miyata,M. Chem. Commun. 2003, 2872.

(17) Aoki, Y.; Hishikawa, Y.; Sada, K.; Miyata, M. Enantiomer 2000,5, 95.

(18) Bortolini, O.; Fantin, G.; Fogagnolo, M.; Medici, A.; Pedrini, P.Chem. Commun. 2000, 365.

(19) Bertolasi, V.; Bortolini, O.; Fogagnolo, M.; Fantin, G.; Pedrini, P.Tetrahedron: Asymmetry 2001, 12, 1479.

(20) Bortolini, O.; Fantin, G.; Fogagnolo, M.; Medici, A.; Pedrini, P.Chem. Lett. 2000, 1246.

(21) Sada, K.; Maeda, T.; Miyata, M. Chem. Lett. 1996, 837.(22) Gdaniec,M.;Milewska,M. J.; Polonski, T.Angew. Chem., Int. Ed.

1999, 38, 392.(23) Bortolini, O.; Fogagnolo, M.; Fantin, G.; Medici, A. Chem. Lett.

2003, 32, 206.

Figure 9. Hydrogen bond network for AdCH2CA crystal in water/butanol.

Figure 8. Solvent surface of the neighbors next to S-2-butanol. For clarity, some atoms have not been considered and the surface is drawn intwo left and right halves.

Article Crystal Growth & Design, Vol. 10, No. 3, 2010 1129

(24) Bortolini, O.; Fantin, G.; Fogagnolo, M. Chirality 2005, 17, 121.(25) Soto, V. H.; Jover, A.; Galantini, L.; Pavel, N. V.; Meijide, F.;

V�azquez Tato, J. J. Phys. Chem. B 2006, 110, 13679.(26) Miragaya, J.; Jover, A.; Fraga, F.; Meijide, F.; V�azquez Tato, J.

Steroids 2009, 74, 735.(27) Soto, V. H.; Jover, A.;Meijide, F.; V�azquez Tato, J.; Galantini, L.;

Pavel, N. V. Adv. Mater. 2007, 19, 1752.(28) Galantini, L.; Leggio, C.; Jover, A.;Meijide, F.; Pavel, N. V.; Soto,

V. H.; V�azquez Tato, J.; Di Leonardo, R.; Ruocco, G. Soft Matter2009, 5, 3018.

(29) Aburaya, K.; Hisaki, I.; Tohnai, N.; Miyata, M. Chem. Commun.2007, 4257.

(30) Pfaller, R.; Schneider, C.Eur. Pat. Appl., EP 1829974A1 20070905,2007.

(31) Okamoto, Y.; Cao, Z.; Aburatani, R.; Hatada, K.Bull. Chem. Soc.Jpn. 1987, 60, 3999.

(32) Mravik, A.; Bocskei, Z.; Simon,K.; Elekes, F.; Izasaki, Z.Chem.—Eur. J. 1998, 4, 1621.

(33) Harada, N.; Watanabe, M.; Kuwahara, S.; Sugio, A.; Kasai, Y.;Ichikawa, A. Tetrahedron: Asymmetry 2000, 11, 1249.

(34) Taji, H.; Kasai, Y.; Sugio, A.; Kuwahara, S.; Watanabe, M.;Harada, N.; Ichikawa, A. Chirality 2002, 14, 81.

(35) Anelli, P. L.; Lattuada, L.;Uggeri, F.Synth. Commun. 1998, 28, 109.(36) Ryu, E.-H.; Ellern, A.; Zhao, Y. Tetrahedron 2006, 62, 6808.(37) Alvarez, M.; Jover, A.; Carrazana, J.; Meijide, F.; Soto, V. H.;

V�azquez Tato, J. Steroids 2007, 72, 535.(38) Mesecar, A. D.; Koshland, D. E., Jr. Nature 2000, 403, 614.(39) Koshland, D. E., Jr. Biochem. Mol. Biol. Educ. 2002, 30, 27.