Macrocyclic conformation

The conversion of the six secondary into tertiary amine donor functions does not change the overall structure of the [M2(Lr)( -L )]+ complexes. That is the complexes retain their bioctahedral N3M( -SR)2( -L )MN3 core structures and the macrocycle conformation remains constant for a given coligand, see for instance the isostructural compounds 52-55 (211). This offers the opportunity to extend the rims of the binding pocket of the [M2(LR)(L )] complexes. 

Discrete dimers of the head-to-head type have been found in the structures of the Ag+ complex of (145)570 and the Na+ complex of (145)571 respectively. The complexes were recrystallized from carbon tetrachloride. In both complexes each metal is five-coordinated in the cavity provided by one anion, and there is an additional reaction with the second anion [through an Ag+-phenyl interaction or an Na+-carboxylate oxygen atom (Figure 32a)]. When the Na+ complex was crystallized from a solvent of medium polarity, acetone, the head-to-head dimer was recovered.571 In contrast, recrystallization from a polar medium, methanol, gave a monomeric complex in which one methanol of solvation was also present.572 In all of these complexes an intramolecular head-to-tail hydrogen bond was present to hold the ligand in its pseudo-macrocyclic conformation. 

An interesting and rare example of inverse electron demand transannular Diels-Alder reaction of the furanophane 65 was employed for the synthesis of the chatancin core as depicted in Equation (45) <2003JOC6847>. The diastereoselectivity of this reaction was controlled by the macrocyclic conformation of 65 in the protic reaction medium. 

Figure 7-65 shows the X-ray structure of the complex [7,16-bis(ferrocenylmethyl)-l,4,10,13-tetrathia-7,16-diaza-cyclooctadecane]copper(i), [CuL ] and that of the free ligand L [186]. The copper(i) centre assumes a distorted tetrahedral geometry. With respect to the macrocyclic conformation of the free ligand, which possesses two gauche and two anti C-S-C-C bonds and two anti N-C-C-S bonds, complexation with copper (i) maintains the two gauche and two anti C-S-C-C bonds, but the two N-C-C-S bonds become gauche. 

Harata, K. Macrocyclic conformation of methylated cyclodextrins, Minutes of the 5th International Symposium on... 

In Class I polyradicals,3 8 macrocyclic connectivity between the spin sites provides an alternative exchange pathway to bypass chemical defects or coupling defects. Furthermore, with adequate choice of the macrocycle, macrocyclic conformational restriction may prevent significant out-of-plane twisting associated with coupling defects. 

The subject of macrocyclic conformation is complex and will be illustrated here by reference to Kyba s systems. These 11- " and 14-membered phosphorus macrocycles exist in various conformations. The barrier to inversion for phosphines is approximately 146.4kJmol , but some conformations are remarkably stable. For example, 2,6,10-triphenyl-2,6,10-triphosphabi-cyclo[9.4.0]pentadeca-l l(l),12,14-triene (11) has three conformers (a-c in Figure 1). When a xylene solution of (11) is refluxed (135 °C, 1 h), only (a) is obtained when crystallized. The phenyl groups at the 2-P and 10-P remain cis, although the 6-P does invert, but the crystallization process favours conformer (a) over (b). The equilibrium (a) (b) is displaced to the LHS by removal of (a) as it... 

Figure 7.1 shows the structure of CyDs consisting of 6-9 glucose units. Some average parameters describing the macrocyclic conformation of native CyDs are summarized in Table 7.1. The pyranose ring of each glucose unit in native CyDs is relatively rigid and assumes the Ci chair conformation. Some structural characteristics of CyDs are illustrated in Fig. 7.2. Primary hydroxyl groups have rotational...

The complexing abilities of thiacrown ethers and related macrocycles depend on several factors that have not been fully investigated. These factors include ligand cavity size, spatial distribution of binding sites within the ring, types of donor atoms, hard and soft acid-base rule , cation diameter, solvent, macrocycle conformation, and other factors . 

Fusion of three aromatic residues to 9S3, as in (MeO)6-tribenzo-9S3, necessarily constrains the macrocycle conformationally. Klar and coworkers identified three likely conformations - termed crown , saddle , and pseudo-saddle - defined by the position of the phenylene moieties with respect to the plane of the three S atoms [83]. In the crown conformation, all three phenylene units lies on the same side of the S3 plane. In the saddle conformation, one phenylene ring lies in the S3... 

We report here the macrocyclic conformation of the complexed and the uncomplexed host molecules, hostrguest interactions, the molecular columns formed by stacking of the macrocycles and the guest dependent polymorphism (crystal structure). 

Figure 2. Oomparison of macrocyclic conformation between the complexed (left) and the uncomplexed 1 (right).

It is interesting to note that l-CHCl3 forms chiral crystals and ] -CH2Cl2 forms racemic crystals. 1 exhibits no chirality in solution because of the rapid axial-equatorial change of methylene hydrogens, but in solid the macrocyclic conformation is frozen. In the solid state, 1 exists as optically active isomers, "R"-conformer or "S"-conformer. 

Table II shows geometrical data describing the macrocyclic conformation of cyclodextrins and permethylated cyclodextrins. The radius and side length of the 0(4) heptagon of 3-cyclodextrin, 5,0 A and 4,38-4,39 A, respectively, are similar to those (5,00-5,01 A and 4.38-4,39 A, respectively) of permethylated 3-cyclodextrin. The 0(4) hexagon of permethylated a-cyclodextrin is somewhat larger than that of a-cyclodextrin the radius of the 0(4) hexagon is 4,23 A in a-cyclodextrin and 4.30 A in permethylated a-cyclodextrin.

The effect of the permethylation on the macrocyclic conformation appears most clearly in the change of 0(2) 0(3 ) distances and tilt-angles, The average 0(2) 0(3 ) distances of permethylated cyclodextrins are about 0.6 A larger than those of parent cyclodextrins. In a- and 3-cyclodextrin, secondary hydroxyl groups form intramolecular 0(2) 0(3 ) hydrogen bonds, as indicated by the 0(2) 0(3 ) distances of 2.79-2.89 A. These intramolecular hydrogen bonds may maintain the round macrocyclic... 

In most cases the signals of bridging methylene protons of cafix[4]arenes 1 together with the signals of the lower or upper rim substituents in H-NMR spectra clearly identify the macrocycle conformation as well as the spatial arrangement of the substituents [2]. Fnrthermore, Mendoza et al [3] introduced an exclusively useful rule that correlates C-NMR spectra of cafix[n]arenes ( = 4 - 6) with their conformations. Chemical shifts of bridging carbon atoms are equal to 8 31 if two adjacent aromatic groups are in the. yyn-orientation (u, u or d, d) and to 8 37, if they have anti-orientation (u, d). 

Replacing of methylene bridges in calix[4]arenes 1 with sulfur atoms in thiacalix[4]arene 2 and its derivatives greatly complicates the establishment of the macrocycle conformation. This is due to the absence of bridging methylene protons, which otherwise allow determination of calix[4]arene 1 conformation by their NMR spectra. Two-dimensional NMR techniques are employed in the establishment of the conformation of the thiacaUx[4]arene 2 molecules and its derivatives. Thiacalixarenes have larger cavity size, and their bridging sulfur atoms can be easily oxidized [4-6] to show new receptor properties [7-19],... 

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