Glycoluril capsules

A molecular capsule can be composed of more than two units also a sphere can be obtained assembling together a generic number n of slices. Rebek and collaborators, in their extensive exploration of the properties and features of self-assembled capsules [36], developed a L-shaped molecule characterized by the presence at one end of the glycoluril hydrogen bond system, and at the other end, separated by an aromatic ring, a sulfamide moiety (Fig. 14). 

Fig. 20 Chiral homodimeric tennis ball capsule from an achiral component bearing two different glycoluril residues. The R1 and R2 groups have been removed for clarity...

Differently from the previous chiral softball, in this case the dissymmetric inner surface of the cavity easily sensed chiral guests, providing different diastereoisomeric complexes in different amounts. Each diastereoisomer is characterized by four different NH resonances because of the complete loss of symmetry of the complex. The system described was extensively investigated pursuing the synthesis of a total of six different glycoluril monomers (21-26) with various spacers in terms of size and electronic properties on the two sides of the bicyclic six-membered unit, and the relative homodimeric capsules were studied with a total of 20 chiral enantiopure guests (Fig. 23 and Table 2) [53]. 

Chirality can be introduced in capsules by substitution. Examples of chiral recognition inside the cavities are scarce as introduction of asymmetry in the periphery does not translate inside the cavity. Thus, introduction of different substituents at both side glycolurils in tennis balls and soft balls, such as in 35 (R1=p-C6H4OC6H15, R2 = C02-i-pentyl, R3 = H), removes one of the two symmetry planes... 

Figure 3.27 An expanded, six-component capsule is formed by addition of glycoluril 54 to cavitand 37, which would otherwise form a dimeric (37)2 capsule.

Fig. 2 Expansion of the capsule with glycoluril spacers creates extended capsule 1.24.1 in racemic form (the peripheral alkyl and aryl groups have been removed for viewing clarity). The cartoon representation used elsewhere in this work is also shown...

As previously mentioned, the belt of glycolurils that inserts into the capsule increases the length by some 7 A and the volume by nearly 50% [83]. In the expanded capsule tetradecane relaxes into an extended conformation and this is evident from changes in the H NMR spectrum. The NMR signals move downfield as the methylenes of the alkane move away from the walls and toward the center of the structure. The different NMR spectra of tetradecane in the original vs the expanded capsule is shown in Fig. 16. 

Fig. 16 The reversible compression and expansion of encapsulated n-tetradecane. The conformation of the n-tetradecane was deduced from the upheld regions of the H NMR spectra which are shown together with the respective host guest complexes. In the expanded capsule 1.24.1 the signals move downfield as the methylenes move away from the capsule s walls and toward its center. The chiral space of the expanded assembly is reflected by the diastereotopic signals for the methylenes. The glycolurils insert under basic conditions acidic conditions precipitate the glycouril and regenerate the capsule 1.1 with coiled tetradecane inside...

Fig. 17 Energy minimized structures and approximate dimensions of capsules extended by glycolurils. The length refers to the accessibility of methyl groups in the inner space. Peripheral groups have been removed for viewing clarity... 

Hybrid capsules composed of two glycoluril derivatives incorporating different backbone spacer groups have also been demonstrated to form in solutions containing a mixture of two homo-dimers chosen from the above series. In this... 

Figure 3.6 A pseudo-spherical capsule composed of bridged glycoluril subunits linked by hydrogen bonds ...

Rebek has conducted extensive studies on curve-shaped molecules that self-assemble to form homodimers, in the form of molecular capsules, in both solution and the solid state.34 In particular, Valdes et al. have demonstrated the ability of two glycoluril units separated by a benzene spacer to self-assemble in the solid state via eight N-H O hydrogen bonds to form a capsule with a structure that conforms to a tennis ball (Fig. 16).34a A disordered guest, identified as probably being methanol, occupied the interior. A related dimer based on an ethylene spacer was also prepared and shown to accommodate a guest, also identified as probably being methanol, in the crystalline state.34b... 

A tetrameric capsule of ideal D2 symmetry has been reported by Rebek.65 In particular, a molecule with glycoluril and sulfamide functionality self-assembled in the solid state via 24 hydrogen bonds to form a finite assembly that accommodated 2,6-adamantanedione as a guest. The volume of the cavity of the capsule was determined to be approximately 184 A3 (Fig. 35). Each carbonyl oxygen atom of the... 

Fig. 5.12. Rebek s softballs — dimeric self-assembling capsules. The cavity volume can be tuned by the length of the spacer introduced between the center piece and glycoluril binding sites of the monomers. The methylated monomer (bottom) is not able to form dimeric capsules. Several hydrogen bonding donors and acceptors (arrows) that are responsible for dimerization, are implemented in each building block. A computer-generated model of an example of a dimeric capsule is shown in the box. Right Typical guest molecules which can be encapsulated in the cavity.

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