Double helicates, chirality

Aniline, (V.iV-dimethylaniline and toluene form molecular complexes with (4a, b,13b)-13-methyl-16-oxo-17-norkaurane-18-carboxylic acid (isosteviol) (93) investigated by X-ray diffraction159. 93 forms individual 2 1 molecular complexes with aniline or N,N-dimethylaniline and toluene. The supramolecular crystal structure of these complexes is formed by chiral double helices involving isosteviol molecules. The helices are linked together by intermolecular hydrogen bonds between the carboxy and the carbonyl groups160. 

Nonetheless, much excellent use has been made of this system. Receptor 5, for example, extracts p-nitrobenzoate quantitatively from water into chloroform (43), and the chirality of the receptor allows the possibility of chiral anion recognition (44). Guanidinium has also been incorporated into devices, such as a hydrogen sulfite selective electrode (45). Recently, Mendoza and co-workers (46) reported a chiral double helical array of polyguanidinium strands assembled around sulfate templating anions, the first anion centered helical structure. 

Fig. 2. Schematic views of copper and silver helical arrangements (a) infinite single strand (b) 3-D single helical copper(I) complex with mixed ligands (c) dinuclear double helicate (d) trinuclear double helicate (e) infinite double helicate (f) infinite chiral double helicate of copper(II) with arginine and m-phthalate (g) triple helicate of silver(I).

Chiral bipyridine ligands have been explored for the asymmetric cyclopropanation of diazoacetate. For example, Lyle and Wilson reported that optically active C2-symmetric 2,2 -bipiridyl 248 served as a good ligand for asymmetric cyclopropanation in the presence of CuOTf and phenyl hydrazine (Scheme 1.116) [173]. Boyd et al. showed that a similar 2,2 -bipyridyl 249 also worked as an effective catalyst (Scheme 1.117) [174]. Mono-oxazoline-substituted 2,2 -bipyridyl derivatives 250-252 have been examined (Scheme 1.118) [175]. Chiral double helical oligopyridine... 

Self-assembled hexanuclear arene ruthenium metallo-prisms with 2,4,6-tripyridyl-l,3,5-triazine (ppt) subunits showed unexpected double helical chirality <06CC4691>. 

The double-helical rope formation in T-10-T was unexpected, since neither T-10-T nor its precursor thymine is chiral, as verified by CD spectroscopy. The authors speculate that photodimerization of two thymine derivatives leads... 

Helical intertwined stmctures should exhibit chirality. However, the helicates obtained from strands 451 are racemic mixtures of the right- and left-handed double helices. Jodry and Lacour resolved a dinuclear triple helicate 459... 

A particular point of interest included in these helical complexes concerns the chirality. The lielicates obtained from the achiral strands are a racemic mixture of left- and right-handed double helices. This special mode or recognition where homochiral supramolecular entities, as a consequence of homochiral. self-recognition, result from racemic components is known as optical self-resolution. It appears in certain cases from racemic solutions or melts (spontaneous resolution) and is often cited as one of the possible sources of optical resolution in the biological world. 

Fig. 7 Schematic view of the different approaches used to model the chiral interactions between DNA double helices, (a) DNA duplexes are viewed as clean cylinders with charged helical patterns of negative phosphates and positive adsorbed counterions. Reproduced with permission from [18], (b) Duplexes interact via steric interlocking of backbone and grooves and through electrostatic repulsion of the phosphate groups. Reproduced with permission from [14]...

We have already seen how a racemic mixture of chiral single helices forms a racemic mixture of two double-helicate enantiomers (Figure 10.74). Unlike the two half-apples in the Coup du Roi , however,... 

The achiral network of tetraphenylenes served as an inspiration for the design of a it-conjugated double helix. Double helical polymers, in which two polyphenylene helices are intertwined or tetraphenylenes are sequentially annelated, were recognized as the chiral building block of the network (Fig. 15.24) [107]. 

H.-Y. Peng, C.-K. Lam, T. C. W. Mak, Z. Cai, W.-T. Ma, Y.-X. Li, H. N. C. Wong, Chiral Rodlike Platinum Complexes, Double Helical Chains and Potential Asymmetric Hydrogenation Ligand Based on Linear Building Blocks ... 

Fig. 18 Supramolecular chiral aggregates based on bis-guanidinium functional groups A chiral tetra-guanidinium units self-assemble around sulfate anions into a left-handed double helical structure B chiral bis-guanidinium unit associates with a complementary bis-anionic counterpart leading to a left-handed double helix...

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