Chirality generation

In most common chiral molecules, chirality arises from chiral tetravalent atoms. A conformation-independent chirality code (CICC) was developed that encodes the molecular chirality originating from a chiral tetravalent atom [42], For more generality, a conformation-dependent chirality code (CDCC) is used [43]. CDCC ti cats a molecule as a rigid set of points (atoms) linked by bonds, and it accounts for chirality generated by chirality centers, chirality axes, or chirality planes. 

The new centre of chirality generated by hemiacetal ring closure is called the anomeric centre. The two stereoisomers are referred to as anomers, designated a or (i according to the configurational relationship between the anomeric centre and a specified anomeric reference atom. 

The chiral crystallization of achiral materials and the asymmetric transformation in the chiral crystal environment are described. Many successful examples are presented however, it is still rare to find materials which show this behavior. Recently, new asymmetric reactions using chiral crystals in homogeneous conditions have been developed. [39],[40] These reactions used the frozen chirality generated by chiral... 

Sakamoto, M., Iwamoto, T., Nono, N., Ando, M., Arai, W., Mino, T., and Fujita, T. (2003) Memory of Chirality Generated by Spontaneous Crystallization and Asymmetric Synthesis Using the Frozen Chirality, J. Org. Chem., 68, 942-946. 

Scheme 3.12 Chirality generated by dimerization of an achiral monomer with two different spacers.

Recently, Sakamoto et al. reported an absolute asymmetric synthesis using the frozen chirality generated by chiral crystallization. Achiral asymmetricly substituted imide 68, which bonds between the nitrogen atom and the tetrahydronaph-tyl (TENAP) group, rotates freely at room temperature, crystallized in a chiral fashion, and the enantiomerization owing to the bond rotation was suppressed at low temperature (Scheme 33). Furthermore, the frozen molecular chirality could be transferred to optically active products in fluid solution [35]... 

Memory of chirality generated by spontaneous crystallization and asymmetric synthesis using the frozen chirality. J. Org. Chem., Vol. 68, pp. 942-946... 

In summary, oxypalladation of unsaturated carbon-carbon bonds followed by Pd—H elimination has been studied extensively with aUcenes using carboxylic acids and alcohols, but not much with alkynes. The chirality generated by this type of... 

The chiral 1-ce group was produced from the achiral 2-ce group in the photoisomerization as shown in Scheme 5.1. This gave an idea that the asynunetric induction would be observed if the isomerization occurs in the chiral crystal environment. A very high optical yield ( 82 % ee) was observed using / -2-amino-2-phenylethanol as an axial base ligand [14]. This section describes the chirality generation in the two types of 2-ce complex crystals with perpendicular and parallel conformations. 

The chirality generated by SADH was used in a subtler way by Hayashi et al. [34f] in their synthesis of the complex antitumoral agent (-l-)-fostriecin (Scheme 2.14). Propane-1,3-diol was converted in a sequence of seven steps into 1,3-enyne 71, SADH of 71 gave rise to chiral diol 72 in good yield and enantiomeric excess. A sequence of four steps transformed 72 into the alkyne-cobalt complex schematically... 

Chirality-transferring events between molecules are important in wide areas of supramolecular chemistry dealing with the design of asymmetric catalysts, memory devices,and sensors. Chirality generates diversity in molecular structures when it interacts with other chiralities to yield two structures with different energies (diastereomers). In a case where one of two chiral units is a racemic pair of enantiomers that exist in equilibrium (dynamic chirality), the other chiral unit perturbs the equilibrium to prefer a particular enantiomer through the formation of a complex. 

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