Rectus configuration

If the order of decreasing precedence of the three highest ranked substituents appears in a clockwise sense the absolute configuration is R (Latin rectus... 

If the observed order of priority of the remaining three functions (a > 6 > c) is in a clockwise direction, the absolute configuration is designated R (rectus or right) if counterclockwise, the configuration is S (sinister or left). The concepts of stereochemistry and chiraUty have been extensively discussed and reviewed (27—29). 

If the order of decreasing precedence of the three highest ranked substituents appears in a clockwise sense, the absolute configuration is R (Latin rectus, "right," "correct"). If the order of decreasing precedence is counterclockwise, the absolute configuration is S (Latin sinister, "left"). 

Secondly, we have labeled one of the substituted methanes 5-fluorochloro bromomethane and the other f -fluorochlorobromomethane. S (sinister, left) and R (rectus, right) are labels that are useful in designating the absolute configuration of chiral molecules in space. The rules for assigning S stereochemistry in one case and R stereochemistry in the other are somewhat complex but it doesn t matter, so never mind. The point is that we have a way of talking about the two possibilities. 

The Cahn-Ingold-Prelog rules (1956) are used to designate the configuration of each chiral C in a molecule in terms of the symbols R and S. These symbols come from the Latin, R from rectus (right) and S from sinister (left). Once told that the configuration of a chiral C is R or S, a chemist can write the correct projection or Fischer structural formulas. In our statement of the three rules the numerals 1, 2, 3, and 4 are used some chemists use letters a, b, c. and d in their place. 

The view down the axis and toward the group of lowest priority (d), which lies behind the page. The right-handed turn indicates the configuration R (rectus = right) the opposite configuration is S (sinister = left). 

A chiral carbon is designated as being R or S as follows. The carbon is viewed from the direction opposite to the ligand of lowest priority. If the priority order of the remaining three ligands decreases in a clockwise direction, the absolute configuration of the molecule is said to be R (Latin, rectus = right). If it decreases in the counterclockwise direction it is said to be S (Latin, sinister = left. For example ... 

There are several conventions to indicate the configuration of a stereocenter. The most precise is the IUPAC system that names the configuration around a stereocenter as R (for right handed rectus) or S (for left handed sinister). You have probably seen the R/S descriptors on drug information sheets. The details of the R/S system are beyond our purposes here, but any standard organic chemistry text will discuss how to assign these descriptors. 

Figure 3.23 Configuration of tetrahedral enantiomers R (rectus) - right-handed S (sinister) - left-handed carbon R substituents are arranged in decreasing order of priority according to the rules of precedence R1 > R2 > R > R4...

The central point of chirality in a molecule is known as the stereo-centre and diastereoisomers occur when there is more than one stereo-centre in a molecule. Stereo-centres are given the absolute configurations of Rectus (R) and Sinister (S). Diastereoisomers that differ in absolute configuration at the stereo-centres are called epimers. Diastereoisomers differ in conformation so it is actually possible to purify these mixtures by normal phase and reversed phase HPLC. However, better separation factors are often obtained using chiral stationary phases. 

View the molecule so that the bond from the chirality center to group number 4 is pointed directly away from you. Now determine whether the direction of the cycle proceeding from group I to 2 to 3 and back to I is clockwise or counterclockwise. If this rotation is clockwise, the configuration is R (from the Latin word for right, rectus). If the rotation is counterclockwise, the configuration is S (from the Latin word for left, sinister). 

By convention, the chiral center is viewed with a, b and c pointing toward the observer, and d pointing away. The path a to b to c to a can be either clockwise (IX) in which case the configuration is designated (R) (rectus), or counterclockwise (X) which means an (S) configuration (sinister). 

QSAR quantitative structure-activity relationships R(S) Rectus (Sinister) configurational symbols of the Cahn-Ingold-Prelog protocol RTC rat tail clip assay RTF rat tail-flick assay RTA/A rat tail-flick antagonist assay RTP rat tail pressure assay SAR structure-activity relationships sc subcutaneous SSB stereospecific binding TFA trifluoroacetic acid Ts tosyl(p-tolylsulfonyl)... 

When a molecule contains only one chiral center, the two stereoisomers are known as enantiomers. These may be referred to or labeled using the configurational descriptors as either R rectus meaning righthanded) or S (sinister meaning left handed), or alternatively, D (dextrorotatory) or l (levorotatory). The D and l configurational descriptors are... 

---