The R/S System: A New and Simple Approach to Determining Ligand Priority and a Unified Method for the Assignment and Correlation of Stereogenic Center Configuration in Diverse Stereoformulas

Abstract

A new approach providing an "at-a-glance" priority order of ligands attached to a stereogenic center in organic molecules and a unified method for assigning and correlating stereogenic center absolute configuration in diverse stereochemical representations is presented. The approach to assigning ligand priority uses ligands in their proper bonding connectivity and proposes the following "application rules": (i) carbon-oxygen/carbon-nitrogen bond: higher number of bonds gives higher priority; (ii) hydrogen atoms: the fewer the number of H atoms linked to the first or outward atom, the higher is the precedence; (iii) chain propagation vs chain termination: in case of a tie in the number of H atoms, chain propagation (presence of next outward atom) precedes chain termination (absence of next outward atom). For assigning absolute configuration, the procedure is as follows. For a stereogenic center, any two ligands in a plane are chosen and then linked by a clockwise arc taking the stereogenic atom as the geometric center. The stereoformula is then transformed into a Fischer projection in which the ligands at the initial and final positions of the clockwise arc occupy the vertical top and vertical bottom, and those in front and in the rear take up the horizontal right and horizontal left positions, respectively. The Fischer projection drawn is then used to determine the R/S configuration. The correlation of absolute configuration between varied representations can be made in terms of a clockwise arc and a front (or rear) ligand defined for a stereogenic center. These procedures are illustrated with suitable examples.