Chirality conformationally mobile systems

Biological Discrimination of Enantiomers 189 5-6 Racemic Mixtures 191 5-7 Enantiomeric Excess and Optical Purity 192 5-8 Chirality of Conformationally Mobile Systems 193 5-9 Chiral Compounds without Asymmetric Atoms 195 5-10 Fischer Projections 197... 

However, in a cyclohexane system we also need to consider the conformational mobility that generates two different chair forms of the ring (see Section 3.3.2). Let us consider 3-methylcyclohexanecarboxylic acid. This has two chiral centres, and thus there are four configurational stereoisomers. These are the enantiomeric forms of the trans and cis isomers. 

The conformational mobility for compounds 52 was found to be higher than that for calix[4]arenes. A barrier of AG = 11.7 kcal mol-1 determined for R = H is equal to the value found for trihydroxy-p-f-butyl-calix[4]arene (11.6 kcal mol-1).108 The pairs of broad doublets observed for the Ar-CH2-Ar protons (A8 = 0.74 ppm) at -60 °C agree with a cone-like conformation of the macrocycle where the nitrogen is involved in the system of intramolecular hydrogen bonds. Due to the directionality of the bridge, this cone conformation is chiral. Dias-tereomeric cone conformations should exist therefore for the cysteine bridged... 

Mislow and Bickart (258) have recently discussed the properties, and specified the limitations and essential features, of models that can be used for the prediction of chirality of a molecular system. In the simplified and idealized representation of molecular stracture, nonessential features are deliberately left out the model summarizes some selected aspects of the system and completely disregards or even falsifies, others. The model must be adequate to the time scale in which the phenomenon is observed. In particular, in mobile conformational systems it should refer to a time-averaged structure. In other words, the model can have a higher symmetry than that observed under static conditions (e.g., by X-ray diffraction in the crystalline state or by NMR under slow exchange conditions) (259). 

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