Prelog-Klyne

Fig. 1.4 Proposed labels for favored dihedral angles in the Cahn-Ingold-Prelog (A), Prelog-Klyne (B), and Michl-West (C) nomenclatures.

A helical unit is inherently chiral. Its chirality sense or helicity can assume two values that correspond to the sign of the torsional angle. Although the definition of the sign is arbitrary (sec Section 1.1.1.). a universal scientific convention2 is followed (Klyne Prelog convention) ... 

Conformation Hard (Klyne-Prelog Convention) and Soft Descriptions... 

An important application of the Klyne-Prelog convention is the characterization of conformations of rings2. The convention not only allows classical concepts, such as the chair and boat conformations of cyclohexane, to be defined but also allows complicated situations, as is characteristic for medium rings, to be treated in a very convenient and general way. 

Linkage bonds are defined with reference to the atoms having the highest Klyne-Prelog priority,21 i.e., 0-C-0-C-C(CH0H)2C02. ... 

Klyne and Prelog [72] have proposed a very useful nomenclature for the description of steric relationships across single bonds. In the system A-X-Y-B, the torsion angle about X-Y is defined as exemplified in LXVII, A and B being selected according to criteria derived from the sequence rule (for details, see [72]). The nomenclature of Klyne and Prelog is given in Table 6. 

The absolute chemistry of reserpine has been derived directly (135), by making use of Klyne s extension of the Hudson lactone rule (147) as applied to reserpic acid and its lactone, as well as by the application of Prelog s asymmetric synthesis (148) to methyl reserpate (149). These results are in agreement with the conclusions obtained by more indirect but no less accurate means (91). 

The barrier to internal rotation must be included in the force field. Usually, the internal rotational energy is a frmction of the torsion angle co. (Fig. 1). Klyne and Prelog have defined the torsional angle co for the connected atoms A—C—C—D, as depicted previously. Rotation of A towards D along the shortest arc, viewing the molecule along the C—C axis, is defined as a positive value when clockwise. 

A better term would be all-antiperiplanar (terminology of W. Klyne and V. Prelog, Experientia 16 (1960) 521) to avoid confusion with trans as being used for one of the geometric isomers of double bonds and ring systems Icis-trans system). 

Klyne, W., and Prelog, V. Description of steric relationships across single bonds. Experientia 16, 521-568 (1960). 

Figure 2-2 Description of conformations about a single bond in the terminology of Klyne and Prelog - using the Newman projection. Group A is on the front atom at the top the conformation is given for each possible position of group B on the other atom.

Before discussion of stereoelectronic effects on conformations, we will need a way to describe conformations. We will follow the accepted convention of Klyne and Prelog described below. This convention specifies one proximal and one distal group, designated here as A and B, and separates the conformational space defined by the A-X-Y-B dihedral into eight well-defined regions as shown in Figure 6.2. 

Figure 6.2 Convention for description of conformations suggested by Klyne and Prelog. (Source Testa 2013 (1). Reproduced with permission of John Wiley and Sons.)...

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