An Easy Way To Convert a Fischer Projection into a Zigzag Representation Sandra Signorella and Luis F. Sala Facultad d e Ciencias Bioqulmicas y Farmaceuticas, UNR, Suipacha 531, 2000, Rosario, Argentina A frequent difficulty for undergraduate students taking a basic organic chemistry course arises when they have to understand the relationship between the different conformational representation of acyclic sugar derivatives, especiallv between Fischer and ziezae conformations. We have u prepared a simple way of transforming a Fischer projection into a zigzag form. T o perform this, the student has to know the characteristics of each type of representations and the principal differences between them;' for example: (1) The Fischer projection, though an important representation in the classification of saccharides, represents the molecule in a conformation having the largest groups eclipsed. On the other hand, the zigzag conformation is the more stable form since ell the carbon-carbon bonds are in a staggered relationship. (2) In the Fischer projection the substituents are distributed to the right and to the left of the carbon chain but always above the pime of the paper, whereas the zigzag represent&ion has the advantage of showing the main carbon chain lying approximateIv in the olene of the oaoer end the substituent " erouos . located above and below this pike.
-
Relationship
Fisctw projection
Zigzag conformation
1.2-threo 1.2-erymro
an UIW oppaaiteride on the same side
on me same side on Me opposite slde
An application of this rule is shown in the figure, where the principal group (R1) is placed a t the top of the molecule, observing the Fischer projection from the left. However, this method might be also applied if R1 were the principal group. This method for correlating representations has been recently and successfully applied in a stereochemistry course on carbohydrate chemistry, where the students had only a minimum knowledge of organic topics.
The instructions followed to go from one representation to the other are the following: (1) Observe the Fischer projection from one side of the vertical chain (left or right). (2) Place the ~rincioalarour, downward. (31 Draw the rarhm chain in a zigrag form, always placing the principal group in a diagonal line that is sloping down infothe "lane.. to the left or to therieht.de~cndine. on the sidechosen for . - . . observing the Fischer projection. ( 4 ) Place the substituent bonded to the carbon adjacent to the principal group above the plane if it is placed toward the observer in item 2, or below the plane if it is located on the opposite side. (5) Locate the remaining substituents employing the following rules:
'
Plgman. W.; Horion, D. In The Carbohydrates, 2nd ed.: Plgman, W.: HoriOn, D., Eds.: Academic: New York, 1980; Voi. IA, Chapter 1.
Appiication of the conversion mamod.
Volume 68
Number 2
February 1991
105
