Configurational chirality, designation

The R, S convention is a scheme which has largely superseded the D, i. system to denote configuration about a chiral centre in a molecule. The convention allows unequivocal designation of the absolute configuration in a description of the positions in space of ligands attached to a chiral centre, in relation to an agreed standard of chirality like a right-hand helix. 

FIGURE 1.19 Viewing angle as a means of designating the absolute configuration of compounds with a chiral axis, (a) (R )-2-Butanol (sequence clockwise) (b) (fi)-2-butanol (sequence counterclockwise). 

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 first (+)-discodermolide synthesis was completed by Stuart Schreiber s group at Harvard University and is outlined in Scheme 13.68. This synthesis was carried through for both enantiomers and established the absolute configuration of the natural material. The retrosynthetic plan outlined in Scheme 13.67 emphasizes the stereochemical triads found at C(2)-C(4), C(10)-C(12) and C(18)-C(20) and was designed to use a common chiral starting material. Each of the segments contains one of the stereochemical triads. 

R. S. Cahn (England), C. K. Ingold (England), and V. Prelog (Switzerland) devised the (R-S) system (Sequence rule) for designating the configuration of chiral carbon atoms. 

Helical Chirality. Helicity is a special case of chirality in which molecules are shaped as a right- or left-handed spiral like a screw or spiral stairs. The configurations are designed M and P, respectively, according to the helical direction. Viewed from the top of the axis, a clockwise helix is defined as P, whereas a counterclockwise orientation is defined as M. Thus, the configuration of example 9 is defined as M. 

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