Absolute configuration in a Fischer projection: a simple approach

Absolute Configuration in a Fischer Projection: A Simple Approach There appears to he no difficulty in assigning the absolute configuration from a Fischer projection when the lowest priority group is on either end of the vertical line. However, first year undergraduates often find it difficult to assign ahsolute configuration to a chiral center when the lowest priority group is either to the left or the right of the horizontal line. The methods available for assigning configuration involve imagining rotation of the molecule by 180° which has the effect of exchanging three substituents and thereby keeping the configuration the same. For example, in the fallawing molecule COOH

I H-&OH

I

COOH

I

=

OH-c-CH,

CH,

I

H

iR) To avoid such imagining, we suggest the following two simple procedures for assigning absolute configuration for an organic molecule having ehiral centers. Procedure I(Figure 1):

1) Interchange the lowest priority group an the horizontal line (in la), with the group on the lower end of the vertical line (giving ID). 2) Draw the mirror image of the above configuration (Ie). 3) Apply the sequence Rule which gives the absolute configuration (Fig. I).

-.

Ia

Ic

Ib

Figure 1. Conversion to R and S configuration from twodimensional representation.

Procedure II (Figure 2):

1) Neglect the lowest priority group present on the horizontal line :" TT ,A. &&.

2) Write the Fischer Projection as shown in IIa. 3) Convert IIa to IIb. I t does not involve any change of eonfiguration hut gives the absolute configuration directly on applying the sequence rule.

COOH

COOH

10.

H-t.. CIq

CH3

COOH H0-1(2 CHa

(R)

The methods are fairly applicable to simple molecules having more than one chiral centre.

U

IIa

IIh

Figure 2. Conversion to R and S configuration from lwodimensional representation.

Ravi Bhushan G. Bhattacharjee Department of Chemistry University of Roarkee ROORKEE-247 672. India

Volume 60

Number 3

March 1983

191