Cavitands chiral

The spherically shaped cryptophanes are of much interest in particular for their ability to bind derivatives of methane, achieving for instance chiral discrimination of CHFClBr they allow the study of recognition between neutral receptors and substrates, namely the effect of molecular shape and volume complementarity on selectivity [4.39]. The efficient protection of included molecules by the carcerands [4.40] makes possible the generation of highly reactive species such as cyclobutadiene [4.41a] or orthoquinones [4.41b] inside the cavity. Numerous container molecules [A.38] capable of including a variety of guests have been described. A few representative examples of these various types of compounds are shown in structures 59 (cyclophane) 60 (cubic azacyclophane [4.34]), 61a, 61b ([4]- and [6]-calixa-renes), 62 (cavitand), 63 (cryptophane), 64 (carcerand). 

The chiral cavitands 3.109 have been developed by combining the amino acid residue L-alanine with macrocyclic cavitands (calixarenes - Section 3.14).51 These ammonium ion receptors are able to complex a range of amino acids and their methylester hydrochloride salts, all of which contain an -NH3+ functionality capable of interaction with the carboxylate residues of the host. In general amino acids are bound only very weakly in aqueous solution, while association constants with the chiral methyl esters range from 620 M-1 for L-tryptophan methylester to 110 M-1 for L-alanine methylester. The methylester of glycine is not bound at all. Receptors related to 3.109 with variable four peptide loops arrayed around a central calixarene core have been used to bind to the surfaces of proteins. The... 

Cavitands III having chiral bridges, X, were studied in Langmuir monolayers and some pH-dependent enantiomeric recognition of amino acids in the subphase was reported.56... 

Enantiomerically pure hemicarcerands in which the two cavitands are connected by chiral bridges derived from l,T-binaphthalene 26a57 or from tartaric acid 26b,c58 should be mentioned here. They show chiral recognition in complexation as well as a chiral differentiation for guest release (Figure 3). 

In principle, two possibilities exist to create inherently chiral cavitands, namely the use of different linkers -X- or linkers having no symmetry plane. The first possibility was realized with cavitand 94.180 It possesses two adjacent methylene bridges and one bridging quinoxaline unit while two hydroxyl groups remain unreacted. This combination results in an asymmetric structure comparable to the AABC calix[4]arenes. 

As already mentioned, cavitand 126 forms a hydrogen-bonded capsule in the presence of a base. A single crystal X-ray structure could be obtained for the inclusion complex with pyrazine.285 Figure 38 shows its overall D4 symmetry (not regarding the guest) due to the twisting of the two cavitands. As mentioned before, this chiral arrangement is also found at low temperature in solution. 

Carbohydrates, Recognition of, p. 169 Carbonic Anhydrase Models, p. 178 Carcerands and Hernicarcerands. p. 189 Cation-n Interactions, p. 214 Cavitands, p. 219 Chiral Guest Recognition, p. 236 Classical Descriptions of Inclusion Compounds, p. 253 Classification and Nomenclature of Supramolecular Compounds, p. 261 Clathrate Hydrates, p. 274 Conzplexation of Fullerenes, p. 302 Concepts in Ciystal Engineering, p. 319 Crown Ethers, p. 326 Cryptands, p. 334 Cryptophanes, p. 340 Cyclodextrins, p. 398... 

Carcerands and Hemicarcerands. p. 189 Cavitands, p. 219 Chiral Guest Recognition, p. 236 Crown Ethers, p. 326... 

Organic cage compounds formed by covalent bonds alone have been synthesized with the help of dynamic covalent bond formation. Cage compounds through formation of imine bonds (hemicarcerands, cavitands, adamantoid nano-cages and chiral nano-cubes based on chiral... 

Collet et al. (138-140) have recently reported the elegant syntheses of cavitands composed of two cyclotriveratrylene units connected by three di-or trimethylene bridges (Figure 2.61). These hollow host molecules have been named cryptophanes, and are suitable for inclusion of lipophilic guests in the interior of their three-dimensional enforced cavities. The cryptophanes (139)-(142) are chiral, and have been obtained in enantiomerically pure forms and their absolute configurations established. 

Only one carbonyl group of each imide wall is left without a hydrogen bond donor, and this asymmetric arrangement causes a twist in the array of the imide walls that reduces the symmetry of the cavitands from 4 to C4 and creates a chiral space at two ends of the new assembly. As evidenced by 2D NMR spectroscopy, the two enantiomers of the new assembly interconvert at room temperature. The race-mization process occurs in a concerted fashion through rotation of all spacers in one direction, transitioned through an achiral structural intermediate. ... 

The use of a N-monosubstituted glycoluril was considered to short circuit the network of hydrogen bonds in the extended assembly. In fact, the addition of N-methylated glycoluril resulted in formation of a chiral deep cavitand in the presence of alkanes shorter than C,4and longer than C g (Figure 8.14). 

One of the early examples of resorcin[4]arenes with add-on features due to non-covalent chirality was deep cavitand amide 76 (Fig. 2.20) [98]. Cavitand 76 has doors at the upper rim that are craitroUed by a unidirectional cooperative belt of hydrogen bmids. The doors can close clockwise or counterclockwise, and, in the presence of additional chirality centers, one of the directions is preferred (de 50 %) [98]. The chiral vessels exhibit preferential binding of enantiomers of various small molecules, e. g. /rans-cyclohexanediol (60% de). Other resorcin[4]arenes with inherently chiral conformations were reported by Schmidt et al. [99] and the group of Szumna [100,101]. Amide substituted resorcin[4]arenes like 77,78 and 79 exist in inherently chiral kite conformatiOTis that are stabilized by an unidirectional... 

Fig. 2.20 Cavitands with a chiral system of hydrogen Ixmds (a) X-ray structure, (b) schematic view of the H-bond array in 76 (c) structures of cavitands 77-79, (d) crystal structure of 79...

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