A new gimmick for assigning absolute configuration

A New Gimmick for Assigning Absolute Configuration F. 0. Ayorinde Howard University, Washington, DC 20059 One of the most challenging aspects of stereochemistry for students is the assignment of configurational symbol (R or S). This is due, in part, to the difficulty in recognizing the three dimensional nature of molecules. Many published methods often rely on the student's ability to visualize molecules in three dimensions, thus compounding the problem.',2,3 On the other hand, the configurational symbol for a molecule represented as a Fischer projection is easily assigned when the atom or group of lowest sequence appears at the top (see (d) in example) or at the bottom of the projection formula. In that case, the two-dimensional array of the Fischer projection corresponds to the three-dimensional array of the molecule. Thus s one needs onlv to assirn ~riorities(a, b. c, d) to the g r o u ~ that are attached the chirai carbon basedon the ato& n&bers of the constituent atoms. after which a circular motion is made from (a) to (h) to (c) (ignoring (dl). The configuration is R if the direction of motion is clockwise

and S if the direction of motion is counterclockwise

i

B-C-C1

I

CHI

r]:

Step 1. Write a Fischer projection involving the chiral carbon:

Step 2. Assign priorities (a, b, c, d) to the groups attached to the chiral carhon. (d CH.

I

H-