Linear catenation

The multibranched dodeca-arabinosaccharide 174 (Scheme 5.38) is a relevant part of the of LAM glycolipid of M. tuberculosis, illustrated in Figure 5.2. The structure features a repeated 3,5-branched motif, represented by the encircled units, along with linear catenated arabino-1 —> 5 units. Lu and Fraser-Reid proposed two retro-synthetic plans (based on dissections A and B, Scheme 5.38), which employ as the sole source NPOE 175 [102,103]. In option A, a nona-arabinan donor would be delivered to the primary hydroxy] of a tri-arabinan acceptor, whereas in option B two identical tetra-arabinan donors would be delivered to the tetra-arabino acceptor. 

The propensity for iodine to catenate is well illustrated by the numerous polyiodides which crystallize from solutions containing iodide ions and iodine. The symmetrical and unsymmetrical 13 ions (Table 17.15) have already been mentioned as have the I5- and anions and the extended networks of stoichiometry (Fig. 17.12). The stoichiometry of the crystals and the detailed geometry of the polyhalide depend sensitively on the relative concentrations of the components and the nature of the cation. For example, the linear ion may have the following dimensions ... 

Figure 2. Structural properties of alginate are shown, with the linear array of Haworth structures given at the top, the conformational structure given next, and the effect of calcium on the formation of complexes between two polymeric strands of alginate given at the bottom. The epimerase catalyzed conversion of / ( —4) linked D-mannuronate to a(l—4) linked L-guluronate residues of poly(ManA) to the catenated structure of poly(GulA) and the formation of the eggbox structure upon the complexing of two polymer strands with Ca. (Reproduced with permission from reference 7. Copyright 1988 Elsevier.)...

Aromatic cyclic chains are more stable than aliphatic catenated carbon chains at elevated temperatures. Thus linear phenolic and melamine polymers are more stable at elevated temperatures than polyethylene, and the corresponding cross-linked polymers are even more stable. In spite of the presence of an oxygen or a sulfur atom in the backbones of polyphenylene oxide (PPO), polyphenylene sulfide (PPS), and polyphenylene sulfone, these polymers are... 

In recent years, dehydrocoupling reactions catalysed by early transition metal complexes have become an increasingly important method for generating catenated species of the p-block elements. In addition to producing cyclic oligomers, this approach is used to prepare linear oligomers and polymers such as polysilanes and polystannanes of the type H(MR2) H (M = Si, Sn) (see Section 10.1.4). ... 

Figure 4. (a) Two interpenetrating adamantane units in the structure of Zn(CN)2. Circles represent Zn and connections between Zn atoms are provided by linear bridging cyanides, (b) A single cyclohexane-like Zn6(CN)6 ring catenated by six distinguishable rings. 

Shimada and coworkers recently reported the synthesis of the poly[2]catenand 56, which is structurally related to the poly[2]catenand 51b (Scheme 20) [60]. Similarly to what has been observed for the co-polycondensation of the dihy-droxy[2]catenand 50b with the terephthalic acid derivative 47, the reaction of the diamino[2]catenand 53 with adipoyl dichloride affords mainly the Pretzel -shaped compound 54 [3, 41, 60]. The poly[2]catenand 56 has been obtained via the polycondensation of the diamino[2]catenate 54 with adipoyl dichloride, yielding the poly[2]catenate 55. Subsequently, the demetalation of the latter affords the linear poly[2]catenand 56 with M = 8.1xl05, using polystyrene standards, which corre-... 

It is clear from comparison of the reactivity towards polycondensation of the difunctionalized [2]catenand 50b and 53 with their corresponding difunctionalized [2]catenates 50a and 54, respectively, that the mobility of the interlocked macrocycles of catenanes plays a fundamental role for the nature of the resulting polycondensates - either cyclic oligo[2]catenanes 52,54 or linear high molecular-weight poly[2]catenanes 51b,56 (Schemes 18-20). 

Chain helices (catenal helices) are composed of beaded lines in which particular points, separated by line segments, are distinguished in function from others. Here we may distinguish iterative chains, in which the motifs of points and fines are repeated regularly, and non-iterative chains in which no such repetition occurs. The backbones of linear homopolymers are iterative chains which may be imagined to assume, on the one hand, two kinds of regular conformation — the zig-zag chain (all dihedral... 

Ribbed helices (costal helices) are important in organic chemistry because linear polymers contain side chains as well as backbones. We may, then, discern not only the catenal helix of the backbone, but the intercostal helix formed by all of the ribs and the infracostal helicesof the individual side chains. The intercostal helix may be iterative (as in an isotactic head-to-tail vinyl polymer or homogeneous poly-a-amino acid) or non-iterative (as in a random copolymer, an atactic polymer or typical protein). The intracostal helices can best be analyzed as short-chain crooked lines, as in Section III. Important as costal helicity is, it is secondary to catenal helicity and we therefore limit our attention to the primary helicity, that of long chains. Indeed, we limit our attention to catenal helices having chain motifs of two atoms and two bonds as found in head-to-tail vinyl homopolymers ... 

There are two distinct approaches to catenane synthesis the statistical approach, and approaches relying on self-assembly, so-called directed synthesis . The statistical approach relies on the small chance that macrocyclisation may occur while a linear precursor is threaded through a macrocyclic component. Because this is a rather unlikely eventuality, it naturally results in low yields of interlinked product and is chiefly of historical interest. It was this kind of statistical approach that resulted in the first synthesis of a [2] catenane by Wasserman in 1960 (10.64), from cyclisation of the long-chain diester 10.65 while threaded through the annulus of a deuterated C34 cycloalkane 10.66 (Scheme 10.11), 57 Although the overall yield of the catenation reaction was less than 1 %, the existence of the catenane was firmly established. The relatively polar [2] catenane product, along with other polar macrocyclisation reaction products and... 

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