# Synthesis of a Regioisomer of syn -Cryptophane-B: Crystal Structure and Study of Its Chiroptical Properties

Thierry Brotin, Nicolas Daugey, Sandrine Denis-Quanquin, Erwann Jeanneau, Nicolas Vanthuyne, Thierry Buffeteau

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Thierry Brotin, Nicolas Daugey, Sandrine Denis-Quanquin, Erwann Jeanneau, Nicolas Vanthuyne, et al.. Synthesis of a Regioisomer of syn -Cryptophane-B: Crystal Structure and Study of Its Chiroptical Properties. Journal of Organic Chemistry, In press, 10.1021/acs.joc.5c00755 . hal-05122352

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# Synthesis of a Regioisomer of *syn*-Cryptophane-B: Crystal Structure and Study of its Chiroptical Properties

Thierry Brotin,*,† Nicolas Daugey,§ Sandrine Denis-Quanquin,† Erwann Jeanneau,‡ Nicolas Vanthuyne,& Thierry Buffeteau.*,§

## TOC/Abstract graphic:

<figure><img src="image_2.png" /></figure>

**ABSTRACT:** Cryptophane *syn*-1, a compound decorated with three acetate functions and three benzyl groups, has been isolated in very small quantities from the reaction aimed at preparing functionnalized *syn*- and *anti*-cryptophanes (Brotin et al., *J. Org. Chem.* **2018**). This new compound has been fully characterized and its X-ray structure is reported. Compound *syn*-1 shows an unusual arrangement of the substituents never encountered for cryptophane derivatives. The preparation of this product in larger quantities and subsequent reactions enabled the synthesis of the chiral *syn*-2 compound, a regioisomer of the *syn*-cryptophane-B

<footnote>† Université Lyon 1, Ecole Normale Supérieure de Lyon, CNRS UMR 5182, Laboratoire de Chimie, 46 allée d'Italie, F-69364, Lyon, France</footnote>

<footnote>§ Université Bordeaux, CNRS, Bordeaux INP, ISM, UMR 5255, F-33405 Talence, France</footnote>

<footnote>‡ Centre de Diffractométrie Henri Longchambon, Université Lyon 1, 5 rue de la Doua, F-69100 Villeurbanne, France</footnote>

<footnote>& Aix Marseille Univ, Centrale Med, CNRS, FSCM, Chiropole, F-13397 Marseille, France</footnote>
derivative. The two enantiomers of *syn*-2 have been separated by HPLC on chiral stationary phase and isolated in small quantities. The X-ray structure of *syn*-2 was reported and its chiroptical properties were determined from polarimetry, electronic (ECD) and vibrational (VCD) circular dichroism experiments. Finally, VCD combined with theoretical calculations allowed the determination of the (−)₅₈₉-*MP* absolute configuration for *syn*-2.

INTRODUCTION

*Syn*- and *anti*-cryptophane derivatives with small cavity sizes ($V_{vdw} \sim 100 \text{ Å}^3$) are a class of hollow poly-aromatic molecules exhibiting good affinity for guest molecules that interact only weakly with their surrounding environment.¹ For example, the ability of these compounds to encapsulate rare gases like xenon or small inert gases molecules like methane paved the way for the development of new materials such as molecular tracers for Nuclear Magnetic Resonance Imaging or methane detection devices.²⁻⁴

Future applications for cryptophane derivatives will depend on chemist's ability to functionalize these compounds with chemical functions with well-defined properties. However, the high molecular symmetry generally encountered with these systems and the limited options to introduce hetero-atoms onto the cryptophane skeleton make difficult the synthesis of new cryptophanes, especially for those with the *syn* relative configuration. Thus, only cryptophanes decorated with strongly electron-donating substituents (OMe, NH) can be easily prepared, and the synthesis of new cryptophanes generally requires subsequent reactions from these molecules or requires new synthetic strategies.⁵

In 2018, in order to extend the chemistry of these poly-aromatic compounds we reported the simultaneous synthesis in moderate yield of *syn*-3- and *anti*-4-cryptophanes with C₃-symmetry, compounds decorated with three acetate functions and three benzyl groups (Chart 1).⁶ Compounds *syn*-3 and *anti*-4 are the protected version of the cryptophane derivatives
decorated with three phenol and three benzyl groups. These latter compounds can be used to
easily prepare a large range of cryptophane with different symmetries (D<sub>3</sub>, C<sub>3h</sub>, C<sub>3</sub>). For
instance, we were able to prepare the *syn*-cryptophane-B (chart 1a) (symmetry C<sub>3h</sub>),<sup>7</sup> the
diastereomer of *anti*-cryptophane-A (chart 1b) prepared by Collet and co-workers in 1981.<sup>8</sup>

<figure><img src="image_3.png" /><figcaption>X = OBn, Y = OAc; syn-3
X = Y = OMe; syn-cryptophane-B</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>X = OBn, Y = OAc; anti-4
X = Y = OMe; anti-cryptophane-A</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>compound-1</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>syn-1</figcaption></figure>

Chart 1: structures of compounds a) syn-3 and syn-cryptophaneB, b) anti-4 and anti-cryptophane-A, c) compound 1, syn-1.

Interestingly, a thorough examination of the different fractions collected during the synthesis of compounds syn-3 and anti-4 also suggested the presence of an unknown cryptophane-type molecule in trace amounts. Furthermore, this species appears to possess C<sub>1</sub>-symmetry as evidenced by the multiple equally intense <sup>1</sup>H NMR signals in the aromatic region. This intriguing result encouraged us to investigate in greater detail to elucidate the structure of this new cryptophane (chart 1c).

Here, we report the complete characterization of this new cryptophane by NMR spectroscopy and X-ray crystallography. Subsequent reactions allowed the preparation of compound **2**, which is a chiral regioisomer of *syn*-cryptophane-B. Crystal structure of compound **2** has been obtained by slow evaporation of a THF/EtOH mixture and 2D-NMR experiments were used to fully attribute protons and carbons of compound **2**. HPLC on chiral stationary phase allow us to isolate small quantities of the two enantiomers of **2** in order to
study their chiroptical properties. Electronic (ECD) and vibrational circular dichroism (VCD) combined with density functional theory (DFT) calculations were used to determine the absolute configuration of these two enantiomers.

## RESULTS AND DISCUSSION

**Characterization and molecular structure of compound 1.** As compound 1 (Chart 1) is only produced in low yield (~ 4-5 %), the reaction used to prepare simultaneously compound 1, *syn*-3 and *anti*-4 compounds was repeated several times to isolate a sufficient quantity of compound 1 for its characterization. Thus, thirty identical reactions were necessary to isolate about 500 mg of the desired material compound 1. Purification proved tedious, requiring three chromatography columns to remove the two other isomers (*syn*-3 and *anti*-4), which are produced in larger quantities. ¹H and ¹³C NMR spectroscopy of compound 1 revealed complex NMR spectra (Figures S1-S2), suggesting that this compound has *C*<sub>1</sub>-symmetry contrary to the *C*<sub>3</sub>-symmetry of compounds *syn*-3 and *anti*-4. The ¹H NMR spectrum of 1 also revealed that of the twelve aromatic protons in the cryptophane skeleton, two are coupled with a constant *J* = 8.5 Hz, giving some clues to its structure. However, determining the relative *syn* or *anti* configuration for this compound seems to be a difficult task to solve exclusively by NMR spectroscopy alone. Thankfully, the determination of its structure was facilitated by obtaining single crystals suitable for X-ray crystallography (Figure 1, Figure S3 and Table S1). Similarly, single crystals of *syn*-3 were also obtained by slow evaporation of a CH<sub>2</sub>Cl<sub>2</sub>/EtOH mixture (Figure 2, Figure S4, Table S2). The structure of the unknown compound 1 reveals a *syn*-relative configuration and differs only of compound *syn*-3 by the unusual arrangement of one acetate group on the cryptophane backbone. Compound *syn*-1 is also a chiral compound and possesses a *C*<sub>1</sub>-symmetry. This new compound was fully characterized by ¹H and ¹³C NMR spectroscopy as well as HRMS spectrometry (Figures S1, S2 and S5). The assignment of all
protons and carbons has been achieved for this derivative thanks to 2D NMR spectroscopy (COSY, HSQC, HMBC) (Figures S7-S13).

Compound syn-1 crystallizes in the P2₁/n space group (monoclinic crystal system) with four molecules per unit cell. The molecule accommodates one chloroform molecule and the cavity size of syn-1 has been estimated to be Vvdw ~ 113 Å³ from the ORTEP structure. The three acetate groups are perpendicular to the plane formed by the benzene ring with angles θ = 73.7°, 103.3° and 87.4°. The dihedral angles of the three ethylenedioxy linkers are reported for this compound in Supporting Information (Table S3). The unusual position of one acetate group in syn-1 leads to a distortion of the cyclotribenzylene (CTB) cap to which it is connected (Figure 1). Consequently, the two diastereomeric axial (Hₐ) and equatorial (Hₑ) protons of the CTB cap located close to this acetate moiety appear significantly shifted with respect to the other CH₂(a,e) groups as it can be seen on the COSY 2D-NMR spectra of syn-1 (Figure S7). For instance, the equatorial Hₑ proton (28), which can be easily identified on the ¹H NMR spectrum of syn-1 appears downfield shifted relative to the other Hₑ protons of syn-1 (24-27 and 29). It is separated by a chemical shift (Δδ = 0.7 ppm) from the Hₐ proton (28) with a coupling constant ²J(a,e) = 13.5 Hz. Unlike the syn-1 compound, the CTB cap of syn-3 compound has no such deformed structure (Figure 2). This compound crystallizes in the P1̄ space group (triclinic crystal system) with 2 molecules per unit cell. The inner cavity of syn-3 accommodates a CH₂Cl₂ molecule and the size of its cavity has been calculated to be Vvdw ~ 117 Å³ from the ORTEP structure. The dihedral angles of the three ethylenedioxy bridges are reported in Supporting Information (Table S3).
<figure><img src="image_7.png" /><figcaption>Figure 1: View of the molecular structure of cryptophane *syn*-1. Hydrogen atoms and solvent molecules were omitted for clarity.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Figure 2: View of the molecular structure of cryptophane *syn*-3. Hydrogen atoms and solvent molecules were omitted for clarity.</figcaption></figure>

Cryptophane *syn*-1 has an unusual structure, never encountered for cryptophane derivatives. Of course, as this compound is produced in very small quantities, we can't rule out the formation of this unusual regioselectivity takes place for other cryptophanes. It is
noteworthy that Martinez and co-workers reported recently the synthesis of a *C*<sub>1</sub>-symmetric
hemi-cryptophane where the CTB unit displayed an unusual arrangement of the three methoxy
substituents.⁹ As observed with the *syn*-1 compound, this unexpected arrangement also took
place during the last Friedel-Crafts ring-closing reaction but, unlike compound 1, it was
produced in significantly higher yield (42%), probably due to the greater flexibility of the three
linkers of this hemi-cryptophane.

The isolation of this derivative makes *syn*-1 a unique compound in the cryptophane
series, worthy of further study. Firstly, this compound has a *syn* relative configuration. *Syn*-
cryptophanes are relatively scarce compared to their *anti*-congeners and their properties have
not been yet studied in details.¹⁰ Secondly, the unusual position of the acetate substituent gives
rise to a dissymmetrical cavity that has never been studied. *C*<sub>1</sub>-dissymmetrical derivatives have
been for instance prepared by our group and other, but they require multi-step synthesis and
have always an *anti*-configuration.¹¹,¹² These compounds offer interesting features, such as
enantioselective complexation properties.¹³ Finally, the presence of protecting groups that can
be easily removed at different stage of the synthesis allow us to prepare new *syn* derivatives
with *C*<sub>1</sub>-symmetry.

**Synthesis of syn-2.** Compound *syn-1* is a chiral cryptophane decorated with six protected phenol groups. Three of them, located on the same CTB cap, are protected by three benzyl groups, while the other three are protected by acetate groups. Thus, compound *syn-1* seems to be an interesting chemical platform to prepare new *syn*-cryptophanes. As an example, compound *syn-1* was used to prepare compound *syn-2*, which is a regioisomer of the *syn*-cryptophane-B (achiral). Contrary to *syn*-cryptophane-B, compound *syn-2* is a chiral compound and only few of them have been studied by chiroptical techniques.

Two different routes can be used to obtain the *syn-2* compound, since two different
protecting groups are used in *syn-1* to protect the phenolic groups. As we anticipate that the
benzyl groups will offer better solubility to the intermediate compounds, facilitating their purification on silica gel, we decided to remove the three acetate groups first. In addition, benzyl groups are less sensitive to hydrolysis thus providing to these intermediates better stability.

Scheme 1 shows the synthetic pathway to prepare *syn*-2 from compound *syn*-1. The three acetate groups of *syn*-1 were removed under basic conditions using a mixture of KOH/H₂O (0.25 M) and THF at 60°C to give rise to compound *syn*-5 in 76% (isolated yield). Alkylation of the three hydroxyl groups with iodomethane was then carried out in DMF in the presence of cesium carbonate. This operation afforded compound *syn*-6 within a 84% yield. However, two passages were necessary to convert all the phenolic groups. It should be noted that this problem has already been encountered in the synthesis of the enantiopure *syn*-cryptophane-B isotopologues.⁷ The ability of cryptophanes containing phenol functions to accommodate alkali cations in their cavities may be mentioned in our difficulties to achieve the alkylation reaction in a single step.¹⁴ The removal of the three benzyl groups of *syn*-6 was then carried out in a mixture of CH₂Cl₂ and MeOH in presence of a catalytic amount of Pd/C (10%) to give compound *syn*-7 in 88% yield. Finally, a second nucleophilic substitution reaction using iodomethane in DMF in presence of cesium carbonate gives rise to *rac*-*syn*-2 in 74% yield. Here again, two runs were needed to convert all the phenolic functions into their corresponding ether derivatives.
<figure><img src="image_9.png" /><figcaption>syn-1</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>syn-7</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>rac-syn-2</figcaption></figure>

<caption>(+)-CD<sub>254</sub>-syn-2</caption>

<caption>(-)-CD<sub>254</sub>-syn-2</caption>

Scheme 1: Synthetic route to prepare enantiopure compounds (+)-CD<sub>254</sub>-syn-2 and (-)-CD<sub>254</sub>-syn-2 from compound syn-1.

All the intermediates 5-7 were characterized by ¹H and ¹³C NMR, COSY 2D-NMR spectroscopy and HRMS spectrometry (Figures S5-S6 and S14-S22). 2D-COSY-NMR experiments were used to identify more easily the two diastereomeric axial (H<sub>a</sub>) and equatorial (H<sub>e</sub>) protons, which are shifted from the others due to the unusual arrangement of the substituent. The ¹H NMR spectrum of syn-5 shows three exchangeable protons located respectively at δ = 6.34, 5.95 and 5.69 ppm and correspond to protons of the three phenol groups (Figure S14). Similarly, the ¹H NMR spectrum of syn-7 also displays three exchangeable protons located at δ = 6.09, 6.07 and 5.54 ppm that correspond to the protons of the phenol
groups (Figure S20). Compound *rac-syn-2* was characterized by ¹H and ¹³C NMR and HRMS spectroscopy (Figure S6 and Figures S23-S24). Thanks to 2D-NMR spectroscopy (HSQC and HMBC), the assignment of all protons and carbons has also been achieved for compound *syn-2* (Figures S25-S30). X-ray quality crystals were also obtained for this compound by slow evaporation of a THF/EtOH mixture and an ORTEP structure is shown in Figure 3 (see also Figure S31 and Table S4).

<figure><img src="image_12.png" /><figcaption>Figure 3: View of the molecular structure of compound *rac-syn-2*. Hydrogen atoms and solvent molecules were omitted for clarity.</figcaption></figure>

Compound *rac-syn-2* crystallises in the P2₁/C space group (monoclinic crystal system) with 4 molecules per unit cell. The cavity size of *syn-2* has been estimated to be V<sub>vdw</sub> = 132 Å³ from the ORTEP structure. The dihedral angles for the three O-CH₂CH₂-O linkers are reported in Table S3. In the solid state, this compound encapsulates preferentially an ethanol molecule. Four methoxy groups are localized in the plane of the aromatic, whereas the two others methoxy groups, facing each other, are perpendicular to the plane formed by the benzene rings. The methoxy group showing the unusual arrangement is one of them. As observed for compound
*syn*-1, one CTB cap appears significantly distorted compared to the ORTEP structures of the *syn*-cryptophane-B and its isotopologue.⁶,⁷

**Chiroptical properties of syn-2.** The two enantiomers of *syn-2* have been separated by HPLC using a chiral stationary phase Chiralpak ID (an immobilized polysaccharide phase) and the two enantiomers were obtained in small quantities with high enantiomeric purity ee > 99.5 %. The first eluted compound [(+)-CD<sub>254</sub>-syn-2] was eluted at *t* = 5.4 min and shows a positive circular dichroism (CD) signal at 254 nm. The second eluted compound [(-)-CD<sub>254</sub>-syn-2] was eluted at *t* = 6.9 min and it displays a negative CD signal at the same wavelength (Figure S32). The chiroptical properties of these two enantiomers have been studied by polarimetry, ECD and VCD spectroscopy. Polarimetry (Figure S33, Table S5) performed in CH<sub>2</sub>Cl<sub>2</sub> at several wavelengths shows for the two enantiomers opposite specific optical rotation values in the range of the experimental error, which was estimated to be 5 %. For instance, the first eluted enantiomer, [(+)-CD<sub>254</sub>-syn-2], shows at 589 nm a positive specific optical rotation value [α]<sup>25</sup><sub>589</sub> = + 51.3 ± 2.6 10<sup>-1</sup> deg cm<sup>2</sup> mol<sup>-1</sup>. The second eluted enantiomer, [(-)-CD<sub>254</sub>-syn-2], shows an opposite value [α]<sup>25</sup><sub>589</sub> = -52.8 ± 2.6 10<sup>-1</sup> deg cm<sup>2</sup> mol<sup>-1</sup>.

ECD spectroscopy was another chiroptical tool used to study the two enantiomers of *syn-2*. Figure 4 shows the UV-visible and ECD spectra of [(+)-CD<sub>254</sub>-syn-2] and [(-)-CD<sub>254</sub>-syn-2] recorded in CH<sub>2</sub>Cl<sub>2</sub>. As expected, the UV-visible spectra of these two enantiomers are identical and show two transitions in the 230-350 nm spectral range. One broad band is observed around λ = 290 nm, corresponding to the symmetry forbidden <sup>1</sup>L<sub>b</sub> (Platt's notation) transitions of the six aromatic rings. A second more intense band is also observed at shorted wavelengths (λ = 235 nm) and corresponds to the other symmetry forbidden <sup>1</sup>L<sub>a</sub> transitions of benzene rings.<sup>15</sup> It should be noted that the shape and intensity of these two spectra are almost superimposable to the UV-visible spectrum of *syn*-cryptophane-B (Figure S34), thus suggesting
that UV-visible spectroscopy is not very sensitive to the particular arrangement of the methoxy substituents in *syn*-2.

<figure><img src="image_13.png" /><figcaption>Figure 4: a) UV-visible and b) ECD spectra of [(+)-CD<sub>254</sub>-syn-2] (black spectra) and [(-)-CD<sub>254</sub>-syn-2] (red spectra).</figcaption></figure>

The ECD spectra of the two enantiomers of *syn*-2 show in the absorption region (230 – 310 nm) several Cotton bands with opposite signs. For instance, the ECD spectrum of first eluted enantiomer [(+)-CD<sub>254</sub>-syn-2] displays in the 230-310 nm region six well-resolved positive or negative Cotton bands (reported in L mol<sup>-1</sup> cm<sup>-1</sup>) of different magnitude (Δε = + 1.0, 304 nm; Δε = -5.1, 292 nm, Δε = +15.8, 276 nm; Δε = + 3.4, 260 nm; Δε = -13.9, 250 nm and Δε = +10.0, 231 nm). The band at 292 nm also displays a shoulder. The enantiomer [(-)-CD<sub>254</sub>-syn-2] shows a perfect mirror image with Cotton ECD bands of similar magnitude and of opposite signs.
Unlike UV-visible spectroscopy, ECD spectroscopy is a chiroptical tool which is more sensitive to small conformational or structural changes because the rotational strength associated with each excited state is highly dependent on geometric factors. Thus, small changes in the structure of the cryptophanes can modify significantly their ECD spectra.¹⁶,¹⁷ The nature of the solvent or the nature of the guest molecule also have a significant impact on the conformation of the cryptophane backbone, thus resulting in the change of their ECD spectra.¹⁸ In the case of cryptophane derivatives, these spectral changes are more easily detected in the ¹L<sub>b</sub> region of the spectrum whereas the ¹L<sub>a</sub> region is usually less affected. The nature of the substituents, their position on the aromatic rings and the relative syn/anti configuration of the cryptophane are other parameters that can affect the shape of the ECD spectra of cryptophanes.

The ECD spectra of [(+)-CD<sub>254</sub>-syn-2] and [(-)-CD<sub>254</sub>-syn-2] show more complex structures than the ECD spectra of syn/anti cryptophane congeners with higher molecular symmetry (D<sub>3</sub>- or C<sub>3</sub>-symmetry).⁷ Here, the particular position of a methoxy group results in a break of molecular symmetry. Thus, for symmetry reasons, the six excited states that make up the Cotton bands in the ¹L<sub>b</sub> and ¹L<sub>a</sub> regions become non-degenerate. As a consequence, a more complex ECD spectrum is expected for the two enantiomers of syn-2 than that generally observed with the higher-symmetry cryptophanes. For instance, only two Cotton bands are observed in the ¹L<sub>b</sub> region of anti-cryptophane-A (D<sub>3</sub>-symmetry)¹⁸ and an isotopologue of syn-cryptophane-B (C<sub>3</sub>-symmetry).⁷

VCD spectroscopy combined with DFT calculations has rapidly become an indispensable tool to determine the absolute configuration of organic molecules.¹⁹ For example, this chiroptical technique has been used with confidence to determine the absolute configuration of cryptophane and hemicryptophane derivatives, as well as cage-shaped
molecules.²⁰ The infrared (IR) and VCD spectra of the two enantiomers [(+)-CD₂₅₄-syn-2] and [(-)-CD₂₅₄-syn-2] are reported in Figure 5.

<figure><img src="image_14.png" /><figcaption>Figure 5: IR and raw VCD spectra of [(+)-CD₂₅₄-syn-2] (black spectra) and [(-)-CD₂₅₄-syn-2] (red spectra) enantiomers recorded in CD₂Cl₂ (path length 250 µm, 15 mM) at 298 K.</figcaption></figure>

The VCD spectra of [(+)-CD₂₅₄-syn-2] and [(-)-CD₂₅₄-syn-2] show relatively weak (|Δε| < 0.1) bands in the 1700 – 1000 cm⁻¹ region and a quasi-perfect mirror image is observed for the two enantiomers. The intensity of the VCD bands measured for syn-2 derivative is lower than those obtained for *anti*-cryptophane-A (|Δε| < 0.5), *anti*-nonamethoxy cryptophane (|Δε| < 0.3) and *anti*-dodecamethoxy cryptophane (|Δε| < 0.4).²⁰ᵃ This intensity is also lower than those measured for syn-nonamethoxy cryptophane (|Δε| < 0.2) and syn-Tris-aza-cryptophane (|Δε| < 0.2).²⁰ᵃ,²¹ Conversely, the VCD bands
measured for *syn-2* are about twice bigger than those observed for the two enantiomers of the isotopologue of *syn*-cryptophane-B, where H/D substitution was used to convert an achiral molecule (i.e cryptophane-B) into a chiral one.⁷

Theoretical calculations of *MP-syn-2* have been performed at the DFT level using B3LYP functional and 6-31G** basis set. The definition of the two stereo-descriptors has been reported in the literature.²² The first letter corresponds to the stereo-descriptor of the CTB cap bearing the methoxy substituents with usual position. The predicted VCD spectrum of *MP-syn-2* is compared to the experimental VCD spectrum of the [(-)-*CD*₂₅₄-*syn-2*] enantiomer in Figure 6. A fairly good agreement is observed between the two spectra allowing us to assign with confidence the absolute configuration of the two enantiomers of *syn-2*. Thus, the *MP* stereo-descriptor can be attributed to the [(-)-*CD*₂₅₄-*syn-2*] enantiomer, giving the absolute configuration (−)₅₈₉-*MP-syn-2*. By analogy, absolute configuration (+)₅₈₉-*PM-syn-2* can be attributed to the [(+)-*CD*₂₅₄-*syn-2*] enantiomer.

<figure><img src="image_15.png" /><figcaption>Figure 6: Comparison between the experimental VCD spectrum of the [(-)-*CD*₂₅₄-*syn-2*] enantiomer and the predicted VCD spectrum at the B3LYP/6-31G** level (IEPCM=DCM) for the *MP-syn-2* configuration.</figcaption></figure>

## CONCLUSION
*Syn-1* has been isolated as a trace compound from the synthesis of the protected *syn-3* and *anti-4* derivatives, compounds decorated with three acetate groups and three benzyl groups. Thanks to an X-ray crystallography study, the structure of *syn-1* has been established and this compound has been fully characterized by mass spectrometry and NMR spectroscopy allowing the assignment of all protons and carbons. Interestingly, *syn-1* compound displays an unusual regioselectivity, never observed in cryptophane derivatives. Indeed, the unusual position of an acetate group breaks the molecular symmetry, making the *syn-1* compound a chiral derivative with *C*<sub>1</sub>-symmetry. This compound represents an interesting molecular platform since two different chemical functions (i.e acetate and benzyl) are protecting the phenol groups. It is thus possible from this original platform to prepare new *C*<sub>1</sub>-symmetrical cryptophane just as its congeners *syn-3* and *anti-4*.

Herein, the production of larger quantities of *syn-1* was carried out to prepare compound *syn-2*, a chiral regioisomer of the *syn*-cryptophane-B (achiral). The complete assignment of protons and carbons has been carried out by NMR and an X-ray structure of this compound has also been obtained and discussed. Then, the two enantiomers of *syn-2* have been separated by HPLC technique using a chiral stationary phase. Chiroptical techniques such as ECD and VCD were used to characterize these enantiomers and to determine their absolute configuration. Based on these experiments the stereo-descriptors (+)₅₈₉-*PM*-syn-2 and (-)₅₈₉-*MP*-syn-2 have been attributed to the [(+)-CD₂₅₄-syn-2] and [(-)-CD₂₅₄-syn-2] enantiomers, respectively.

The original structure of *syn-2* makes this compound interesting because the unusual position of the substituents creates a non-symmetrical lipophilic cavity that would be suitable for the encapsulation of chiral substrates and enantioselective molecular recognition. A better understanding of the parameters such as temperature, nature of the solvent and nature of the catalyst, that govern the formation of cryptophane *syn-1*, seems also suitable to have access more easily to this compound and to produce this derivative in larger quantities.
## EXPERIMENTAL SECTION

**Cryptophane syn-1.** Syn-1 was isolated as a trace compound from the reaction aimed at purifying compounds syn-3 and anti-4 by column chromatography on silica gel. Thirty experiments allowed us to isolate 500 mg of cryptophane syn-1 as a colourless glassy product.

¹H NMR (300 MHz, CD₂Cl₂, 25°C): δ = 7.70 – 7.30 (m, 15 H), 7.31 (d, *J* = 8.5 Hz, 1H), 7.27 (s, 1H), 6.96 (s, 1H), 6.85 (s, 1H), 6.83 (s, 1H), 6.82 (s, 1H), 6.77 (s, 1H), 6.74 (s, 1H), 6.62 (d, *J* = 8.5 Hz, 1H), 6.60 (s, 1H), 6.49 (s, 1H), 6.16 (s, 1H), 5.25 (d, *J* = 13.5 Hz, 1H), 5.23 (d, *J* = 13.5 Hz, 1H), 5.17 (m, 2H), 5.01 (d, *J* = 13.5 Hz, 1H), 4.98 (d, *J* = 13.5 Hz, 1H), 4.69 (d, *J* = 13.5 Hz, 1H), 4.65 (m, 4H), 4.67 (d, *J* = 13.5 Hz, 1H), 4.61 (d, *J* = 13.5 Hz, 1H), 4.53 (d, *J* = 13.5 Hz, 1H), 4.47 (d, *J* = 13.5 Hz, 1H), 4.51 (d, *J* = 13.5 Hz, 1H), 4.29 (m, 1H), 4.20 – 4.05 (m, 3H), 3.98 (m, 1H), 3.81 (d, *J* = 13.5 Hz, 1H), 3.82 – 3.68 (m, 2H), 3.50 (d, *J* = 13.5 Hz, 1H), 3.45 (d, *J* = 13.5 Hz, 1H), 3.39 (d, *J* = 13.5 Hz, 1H), 3.38 (m, 1H), 3.31 (d, *J* = 13.5 Hz, 1H), 3.29 (d, *J* = 13.5 Hz, 1H), 2.52 (s, 3H), 2.15 (s, 3H), 2.04 (s, 3H). ¹³C{¹H} NMR (75.5 MHz, CD₂Cl₂, 25°C) δ = 170.0, 169.4, 169.1, 150.4, 149.9, 149.6, 149.1, 148.3, 147.5, 147.4, 146.8, 146.0, 141.7, 141.3, 138.6 (3C), 138.5, 137.9, 137.7, 135.6, 135.5, 135.0, 134.5, 133.0, 132.9, 132.7, 132.3, 132.1, 131.6, 129.4 (2C), 129.3 (2C), 129.2 (2C), 128.8, 128.4, 128.3, 127.8, 127.6 (2C), 127.5 (2C), 127.4 (2C). 124.5, 124.4, 123.2, 122.3, 120.6, 117.7, 116.7, 116.6, 114.7, 114.4, 114.2, 72.3, 72.2, 71.7, 71.6, 70.4, 69.6, 69.4, 67.6 (2C), 37.1, 36.7, 36.6, 36.4, 36.3, 29.6, 21.5, 20.9, 20.8. HRMS(ESI-TOF) *m/z* [M+Na]⁺ calculated for C₇₅H₆₆NaO₁₅, 1229.4294 found 1229.4274.

**Synthesis of cryptophane syn-5.** A solution of KOH/H₂O (0.25 M; 25 mL) was added to a THF (25 mL) solution that contains compound syn-1 (0.5 g, 0.41 mmol) resulting from the purification of compounds syn-3 and syn-4. The mixture was heated (oil bath) overnight to 40°C under an argon atmosphere. The THF solvent was removed under reduced pressure and 25 mL of H₂O was added to the remaining solution. After cooling the solution to 0°C, it was acidified
with conc. HCl. The product was extracted three times with CH₂Cl₂. The organic layer was then washed once with H₂O and dried over Na₂SO₄. After filtration, the solvent was removed by rotary evaporation and the residue was purified by column chromatography on silica gel (CH₂Cl₂/Acetone: 95/5) to give compound syn-5 as a colourless solid. It was washed on a frit with a Et₂O/AcOEt (50/50) solution to give syn-5 (0.34 g, 76%) as a glassy product.

¹H NMR (300 MHz, CD₂Cl₂, 25°C): δ= 7.65-7.33 (m, 15H), 7.30 (s, 1H), 6.80 (d, *J* = 8.5 Hz, 1H), 6.79 (s, 1H), 6.77 (s, 1H), 6.76 (s, 1H), 6.71 (s, 1H), 6.70 (2s, 2H), 6.68 (s, 1H), 6.63 (s, 1H), 6.42 (s, 1H), 6.40 (d, *J* = 8.5 Hz, 1H), 6.34 (s, 1H), 5.94 (s, 1H), 5.69 (s, 1H), 5.12 (d, *J* = 11.5 Hz, 1H), 5.09 (d, *J* = 11.5 Hz, 1H), 5.08 (d, *J* = 11.5 Hz, 1H), 5.0 (d, *J* = 11.5 Hz, 1H), 4.98 (d, *J* = 11.5 Hz, 1H), 4.86 (d, *J* = 11.5 Hz, 1H), 4.70-4.16 (m, 7H), 4.61 (d, *J* = 13.5 Hz, 1H), 4.57 (d, *J* = 13.5 Hz, 1H), 4.48 (2xd, *J* = 13.5 Hz, 2H), 4.43 (d, *J* = 13.5 Hz, 1H), 4.23 (d, *J* = 13.5 Hz, 1H), 4.16-3.95(m, 2H), 4.04 (d, *J* = 13.5 Hz, 1H), 3.85 (m, 1H), 3.70 (m, 1H), 3.59 (m, 1H), 3.39 (d, *J* = 13.5 Hz, 1H), 3.33 (2d, *J* = 13.5 Hz, 2H), 3.32 (2xd, *J* = 13.5 Hz, 2H).

¹³C {¹H} NMR (75.5 MHz, CD₂Cl₂, 25°C) δ= 149.1, 148.4, 148.1, 147.3, 146.9, 146.7, 146.4, 145.5, 145.4, 144.9, 144.7, 143.4, 138.4, 138.0, 137.9, 136.1, 134.2, 133.9, 133.8, 133.75, 133.4, 133.2, 132.8, 132.6, 131.1, 130.9, 129.4 (2C), 129.1 (5C), 128.8, 128.6, 128.5 (2C), 128.4 (2C), 127.7 (2C), 126.8, 120.0, 118.9, 118.7, 118.6, 118.0, 117.9, 117.3, 116.9, 116.8, 116.2, 115.1, 112.6. 73.0, 72.8, 71.8, 70.2, 68.7, 68.3, 68.2, 67.7, 67.6, 36.8, 36.7 (2C), 36.3, 36.1, 28.7. HRMS(ESI-TOF) *m/z* [M]⁻ calcd for C₆₉H₅₉O₁₂, 1079,4012 found 1079,3997.

**Synthesis of cryptophane syn-6.** Excess of methyl iodide (0.3 mL, 4.88 mmol) was added via a syringe to a solution of compound syn-5 (0.38 g, 0.35 mmol) and cesium carbonate (0.48 g, 1.48 mmol) in DMF (6 mL). The mixture was heated (oil bath) at 65°C under an argon atmosphere. CH₂Cl₂ (20 mL) and H₂O were added to the solution and the product was extracted twice with CH₂Cl₂. The combined organic layers were then washed 5 times with water to remove DMF. The organic layer was dried over Na₂SO₄ and then filtered. Next, evaporation of
the solvent under reduced pressure gave rise to a residue that was studied by ¹H NMR spectroscopy. This technique revealed that the alkylation reaction was incomplete. Thus, the residue was passed through a short silica gel column (CH₂Cl₂/Acetone: 95/5) and the product mixture was collected and reused for a second pass applying the same experimental conditions.

The same purification procedure was applied to give compound syn-6 as a colourless glassy product (0.33 g, 84%). ¹H NMR (300 MHz, CDCl₃, 25°C): δ = 7.54-7.35 (m, 15H), 7.20 (s, 1H), 6.81 (s, 1H), 6.75 (s, 1H), 6.70 (s, 5H), 6.69 (s, 1H), 6.60 (s, 1H), 6.57 (s, 1H), 6.38 (s, 1H), 5.19 (d, *J* = 12.5 Hz, 1H), 5.14 (d, *J* = 12.5 Hz, 1H), 5.12 (d, *J* = 12.5 Hz, 1H), 4.94 (d, *J* = 12.5 Hz, 1H), 4.93 (d, *J* = 12.5 Hz, 1H), 4.91 (d, *J* = 12.5 Hz, 1H), 4.55 (d, *J* = 13.5 Hz, 1H), 4.53 (d, *J* = 13.5 Hz, 1H), 4.51 (d, *J* = 13.5 Hz, 1H), 4.45 (2xd, *J* = 13.5 Hz, 2H), 4.42 - 4.02 (m, 6H), 4.35 (d, *J* = 13.5 Hz, 1H), 4.01 (d, *J* = 13.5 Hz, 1H), 3.97 - 3.85 (m, 5H), 3.86 (s, 3H), 3.53 - 3.40 (m, 1H), 3.47 (s, 3H), 3.42 (s, 3H), 3.39 (d, *J* = 13.5 Hz, 1H), 3.36 (d, *J* = 13.5 Hz, 1H), 3.33 (d, *J* = 13.5 Hz, 1H), 3.28 (2xd, *J* = 13.5 Hz, 2H). ¹³C{¹H} NMR (75.5 MHz, CDCl₃, 25°C) δ=150.4, 150.3, 149.7, 149.3, 149.1, 148.3, 148.1, 147.8, 147.3, 147.1, 146.9, 146.0, 137.7 (2C), 137.5, 135.9, 135.5, 135.1, 134.8, 134.2, 133.1, 132.9, 132.6, 132.2, 132.1, 131.5, 131.1, 128.9 (4C), 128.8 (2C), 128.6, 127.8 (2C), 127.1 (2C), 126.8 (2C), 126.6 (2C), 125.6, 123.5, 123.4, 123.1, 122.9, 117.7, 117.4 (2C), 115.5, 115.2, 114.8, 114.5, 72.2, 71.6, 71.3, 71.1 (3C), 70.6, 69.5, 66.5, 60.2, 55.8, 55.6, 36.3 (2C), 36.2 (2C), 36.1, 29.40. HRMS(ESI-TOF) m/z [M+Na]⁺ calcd for C₇₂H₆₆NaO₁₂, 1145,4446 found 1145,4437.

**Synthesis of cryptophane syn-7.** Catalytic amount of Pd/C (10%) was added to a stirred solution of cryptophane syn-6 (0.3 g, 0.27 mmol) in CH₂Cl₂ (15 mL) and MeOH (15 mL). Then, Hydrogen gas was bubbled for half an hour. Then, the mixture was left overnight at room temperature under a hydrogen atmosphere and under stirring. The solution was filtrated over celite and the solvents were then evaporated under reduced pressure. An ¹H NMR spectrum does not show any residual benzyl groups left. The solid residue was purified on silica gel
(CH₂Cl₂/Acetone: 90/10) to give compound syn-7 (0.22 g, 88 %) as a colourless glassy product.

¹H NMR (300 MHz, CDCl₃, 25°C): δ = 7.28 (s, 1H), 6.81 (d, *J* = 8.5 Hz, 1H), 6.72 (s, 1H), 6.71 (s, 1H), 6.70 (s, 1H), 6.68 (s, 1H), 6.65 (3xs, 3H), 6.64 (s, 1H), 6.62 (s, 1H), 6.59 (d, *J* = 8.5 Hz, 1H), 6.09 (s, 1H), 6.07 (s, 1H), 5.54 (s, 1H), 4.62 (d, *J* = 13.5 Hz, 1H), 4.57 (d, *J* = 13.5 Hz, 1H), 4.53 (d, *J* = 13.5 Hz, 1H), 4.52 (d, *J* = 13.5 Hz, 1H), 4.55 – 4.44 (m, 4H), 4.47 (d, *J* = 13.5 Hz, 1H), 4.40 – 4.20 (m, 4H), 4.35 (d, *J* = 13.5 Hz, 1H), 4.13 (m, 1H), 3.95 – 3.80 (m, 1H), 3.96 (d, *J* = 13.5 Hz, 1H), 3.80 – 3.65 (m, 2H), 3.83 (s, 3H), 3.77 (s, 3H), 3.76 (s, 3H), 3.44 (d, *J* = 13.5 Hz, 1H), 3.37 (d, *J* = 13.5 Hz, 1H), 3.35 (d, *J* = 13.5 Hz, 1H), 3.34 (d, *J* = 13.5 Hz, 1H), 3.32 (d, *J* = 13.5 Hz, 1H). ¹³C {¹H} NMR (75.5 MHz, CDCl₃, 25°C) δ = 149.9 (2C), 148.8 (2C), 147.1, 146.5, 146.1, 145.6, 144.6, 144.2, 143.9, 143.5, 134.8 (2C), 134.7, 133.5, 133.4 (2C), 133.2, 131.8 (2C), 131.7, 131.0, 129.5, 125.2, 119.0, 118.5, 117.5, 117.0, 116.9 (2C), 116.7, 116.3, 116.1, 115.9, 113.7, 69.4, 69.0, 67.7, 67.5, 67.4, 66.7, 59.7, 56.2, 55.9, 36.7, 36.6, 36.1, 36.0, 35.8, 29.8. HRMS(ESI-TOF) *m/z* [M+Na]⁺ calcd for C₅₁H₄₈NaO₁₂, 875,3038 found 875.3033.

**Synthesis of cryptophane syn-2.** Excess of methyl iodide (0.146 mL, 24 mmol) was added to a solution of syn-7 (0.168 g, 0.2 mmol) in DMF (5 mL) in presence of cesium carbonate (0.26g, 0.79 mmol). The mixture was heated (oil bath) for 18 hours at 60°C under an argon atmosphere. CH₂Cl₂ (10 mL) and water (10 mL) were added. The aqueous layer was extracted once with CH₂Cl₂. The organic layer was washed three times with water and then dried over Na₂SO₄. Evaporation of the solvent leaves a residue. ¹H NMR spectroscopy shows that the alkylation reaction is incomplete. Thus, this crude product was reused for a second pass applying the same experimental conditions. The product obtained after evaporation of the solvent was purified on silica gel to give compound syn-2 (0.13 g, 74 %) as a colourless glassy solid. ¹H NMR (300 MHz, CD₂Cl₂, 25°C): δ = 7.28 (s, 1H), 7.10 (d, *J* = 8.5 Hz, 1H), 6.84 (s, 1H), 6.78 (d, *J* = 8.5 Hz, 1H), 6.76 (2xs, 2H), 6.72 (s, 1H), 6.71 (2xs, 2H), 6.70 (s, 1H), 6.63
(s, 1H), 6.60 (s, 1H), 4.65 (d, *J* = 13.5 Hz, 1H), 4.56 (2xd, *J* = 13.5 Hz, 2H), 4.52 (d, *J* = 13.5 Hz, 1H), 4.47 (d, *J* = 13.5 Hz, 1H), 4.55 - 4.44 (m, 2H), 4.40 - 4.21 (m, 4H), 4.35 (d, *J* = 13.5 Hz, 1H), 4.12 (m, 1H), 4.04 (d, *J* = 13.5 Hz, 1H), 3.95 - 3.80 (m, 3H), 3.88 (s, 3H), 3.84 (s, 3H), 3.77 (s, 3H), 3.76 (2xs, 6H), 3.80-3.65 (m, 2H), 3.66 (s, 3H), 3.41 (d, *J* = 13.5 Hz, 1H), 3.39 (d, *J* = 13.5 Hz, 1H), 3.385 (d, *J* = 13.5 Hz, 1H), 3.38 (d, *J* = 13.5 Hz, 1H), 3.34 (d, *J* = 13.5 Hz, 1H). ¹³C{¹H} NMR (75.5 MHz, CDCl₃, 25°C) δ= 150.5, 150.4, 150.3, 150.1, 150.0, 149.4, 148.9, 147.6, 147.2, 146.2, 146.0, 145.6, 135.6, 135.4, 135.2, 135.1, 134.3, 134.1, 133.1, 132.4, 132.0, 131.7, 131.6, 131.5, 125.2, 123.2, 121.9, 119.2, 118.7, 117.4, 115.8, 115.5, 114.5, 114.4, 114.0, 113.5. 71.4, 70.1, 69.5, 68.6, 68.2, 67.9, 60.5, 56.4, 56.3 (2C), 56.0, 55.9, 36.8 (2C), 36.6, 36.5, 36.4, 29.5. HRMS(ESI-TOF) *m/z* [M+Na]⁺ calcd for C₅₄H₅₄NaO₁₂, 917.3507 found 917.3500.

**Single-Crystal X-ray diffraction.** Suitable crystals of *syn*-1, *syn*-2, *syn*-3 were selected and mounted on a Rigaku-OD Synergy-S single-crystal diffractometer equipped with an Hypix-100 detector. Intensities were collected at 100K with Copper radiation (l=1.54184 Å) by means of the CrysalisPro software.²³ Reflection indexing, unit-cell parameters refinement, Lorentz-polarization correction, peak integration and background determination were carried out with the CrysalisPro software.²⁴ An analytical absorption correction was applied using the modeled faces of the crystal.²⁴ The resulting set of *hkl* was used for structure solution and refinement. The structures were solved with the ShelXT structure solution program using the intrinsic phasing solution method and by using Olex2 as the graphical interface.²⁵,²⁶ The model was refined with version 2018/3 of ShelXL using least-squares minimization.²⁷ CCDC 2427157-2427159 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
**NMR spectrometry.** ¹H and ¹³C NMR spectra of *syn*-5, *syn*-6 and *syn*-7 were recorded at 300 and 75.5 MHz, respectively on a Bruker Avance III 300 MHz spectrometer equipped with a BBFO broadband probe. COSY-2D NMR spectra of *syn*-2, *syn*-5, *syn*-6 and *syn*-7 were recorded at 300 MHz. Full ¹H and ¹³C assignment for *syn*-1 and *syn*-2 compounds was performed at 400 MHz and 100.6 MHz (¹H, ¹³C, HMQC, HSQC) on a Bruker Avance III 400 MHz spectrometer equipped with a Prodigy cryoprobe. High resolution edited-HSQC and HMBC were acquired with 1200 to 2000 points in the indirect dimension, using non-uniform sampling to keep the duration of the 2D experiments under 3 hours. The reference was set at δ = 7.24 ppm for CHCl₃ (¹H), δ = 5.32 ppm for CHDCl₂ (¹H), δ = 72.0 ppm for CDCl₃ (¹³C) and δ = 54.0 ppm for CD₂Cl₂ (¹³C). Coupling constants are given with an uncertainty of ± 0.5 Hz.

**High Resolution Mass spectrometry.** Mass spectra (HRMS) were performed by the Centre de Spectrométrie de Masse, University of Lyon. Analyses were performed with a hybrid quadrupole-time of flight mass spectrometer, microToF QII equipped with an Electrospray ion source. Data Analysis® 4.0 was used for instrument control, data collection, and data treatment. HRMS analyses were performed in full scan MS with a mass range from 50 to 2000 Da at an acquisition rate of 1 Hz. Transfer parameters were: RF Funnel 1, 200 V; RF Funnel 2, 200 V; hexapole, 50 V; transfer time, 70 µs and PrePulse storage time, 1 µs Before each acquisition batch, external calibration of the instrument was performed with a sodium formate clusters solution.

Column chromatographic separations were carried out over Merck silica gel 60 (0.040-0.063 mm). Analytical thin layer chromatography (TLC) was performed on MERCK silica gel TLC plates F-254. The solvents were distilled prior to use: DMF and CH₂Cl₂ from CaH₂, THF from Na/benzophenone.

**HPLC chromatography.** Separation of the two enantiomers of *syn*-2 was conducted by using analytical HPLC column (Chiralpak ID, 250 × 4.6 mm, EtOH/CH₂Cl₂ (30/70), T =
298 K). Detection was performed by UV-visible (λ = 254 nm) and Circular Dichroism spectroscopy (λ = 254 nm). Sample preparation: About 133 mg of *rac*-syn-2 were dissolved in 5 mL of the mobile phase. Chromatographic conditions: Chiralpak ID (250 × 10 mm), EtOH/CH₂Cl₂ (30/70) as mobile phase, flow-rate = 5 ml/min, UV detection at 254 nm. Injections (stacked): 20 times 250 µL, every 10 minutes.

First fraction: 65 mg of the first eluted enantiomer with an ee > 99.5%. The solid was then washed on a small frit with Et₂O (20 mL) and transferred to an Eppendorf flask (m ~ 40 mg).
Second fraction: 65 mg of the second eluted enantiomer with an ee > 99.5%. The solid was then washed on a small frit with Et₂O (20 mL) and transferred to an Eppendorf flask (m ~ 40 mg).
The enantiomeric purity for each enantiomer was checked by analytical HPLC.

**Polarimetric, UV-visible, and ECD measurements.** Optical rotations of the two enantiomers of *syn*-2 were measured in CH₂Cl₂ at several wavelengths on a polarimeter with a 10 cm cell thermostated at 293 K. UV-visible spectra were recorded (1 scan) in CH₂Cl₂ at 293 K using 0.4375 cm path length quartz cell. The concentration of (+)₅₈₉-*PM*-syn-2 was c = 1.17 10⁻⁴ M. The concentration of (−)₅₈₉-*MP*-syn-2 was c = 1.13 10⁻⁴ M. The concentration of *syn*-cryptophane-B was c = 0.71 10⁻⁴ M. ECD spectra were recorded in the 230-350 nm spectral range at 293 K using 0.2 cm path length quartz cell. The concentration of the two enantiomers was the same as those used for recording UV-Visible spectra. The spectra reported in Figure 4 are the average of 2 scans. The baseline (solvent alone) was subtracted and smoothing was then applied to the resulting ECD spectra.

**VCD Measurements.** IR and VCD spectra of *syn*-2 enantiomers were recorded with a ThermoNicolet Nexus 670 FTIR spectrometer equipped with a VCD optical bench.²⁸ In this optical bench, the light beam was focused by a BaF₂ lens (191 mm focal length) to the sample, passing an optical filter, a BaF₂ wire grid polarizer (Specac), and a ZnSe photoelastic modulator (Hinds Instruments, Type II/ZS50). The light was focused by a ZnSe lens (38.1 mm focal
length) onto a 1x1 mm² HgCdTe (ThermoNicolet, MCTA* E6032) detector. IR and VCD spectra were recorded at a resolution of 4 cm⁻¹ by co-adding 50 scans and 36000 scans (12h acquisition time), respectively. The sample was held in a fixed path length (250 µm) cell with BaF₂ windows. IR and VCD spectra of syn-2 enantiomers were measured in CD₂Cl₂ at a concentration of 15 mM. For comparison with predicted VCD spectrum, baseline corrections of the experimental VCD spectra were performed by subtracting the two opposite-enantiomer VCD spectra of syn-2 with division by two. In all experiments, the photoelastic modulator was adjusted for a maximum efficiency at 1400 cm⁻¹. Calculations were done with the standard ThermoNicolet software, using Happ and Genzel apodization, de-Haseth phase-correction, and a zero-filling factor of one.

**DFT Calculations.** All semi-empirical and DFT calculations were carried out with Gaussian 16.<sup>29</sup> Preliminary conformer distribution search of *MP-syn-2* was performed at the molecular mechanics level of theory, employing MMFF94 force fields incorporated in ComputeVOA and GaussView 6.1 software packages.<sup>30</sup> 2048 conformers within roughly 8 kcal/mol of the lowest energy conformer were kept. All the geometries were optimized at the semi-empirical level of theory, using PM7 Hamiltonian. These optimized geometries were then calculated at the DFT level using B3LYP functional and 6-31G** basis set with the use of IEFPCM model of solvent (DCM). Finally, only the 11 lowest energetic geometries were kept within 3.5 kcal/mol. Vibrational frequencies, IR and VCD intensities were calculated at the same level of theory. For comparison to experiment, the calculated frequencies were scaled by 0.97 and the calculated intensities were converted to Lorentzian bands with a half-width at half-maximum (HWHM) of 7 cm⁻¹.

**ASSOCIATED CONTENT**
**Data Availability Statement**

The data underlying this study are available in the published article and its online Supporting Information.

**Supporting Information.**

¹H and ¹³C NMR spectra of *syn*-1, *syn*-2, *syn*-5, *syn*-6 and *syn*-7. COSY 2D-NMR (300 MHz) spectra of *syn*-5, *syn*-6 and *syn*-7. HSQC, HMBC 2D-NMR (400 MHz) spectra of *syn*-1 and *syn*-2. HRMS of *syn*-1, *syn*-2, *syn*-5, *syn*-6 and *syn*-7. ORTEP structures and crystallographic data of *syn*-1, *syn*-2 and *syn*-3. HPLC and Polarimetry of *syn*-2. UV-visible spectra of *syn*-2 and *syn*-cryptophane-B. Dihedral angles for *syn*-1, *syn*-2, *syn*-3.

**AUTHOR INFORMATION**

Corresponding author

*E-mail: thierry.brotin@ens-lyon.fr

ORCID

Thierry Brotin: 0000-0001-9746-4706

Erwann Jeanneau: 0000-0002-8861-4140

Nicolas Vanthuyne: 0000-0003-2598-7940

Thierry Buffeteau: 0000-0001-7848-0794

Nicolas Daugey: 0000-0002-6455-9458

Sandrine Denis-quanquin: 0000-0002-6746-7831

**Author Contributions**

The manuscript was written through contributions of all authors. All authors have given
approval to the final version of the manuscript.
## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGEMENTS

The French National Research Agency (ANR) is acknowledged for financial support of Project ANR CAORSS (ANR-21-CE29-0006-01). Computational Facilities were provided by the Mésocentre de calcul intensif Aquitain (MCIA) of the University of Bordeaux and financed by the conseil Régional de la Nouvelle Aquitaine.

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