CHAPTER 4: Question 5

3D explanations

This compound contains two stereocentres, which are indicated by the red carbon atoms in the diagrams below. Hence, there are a maximum of four stereoisomers. In this case however, the substituents attached to the two stereocentres are identical, as a result of which there are only three stereoisomers (the compound drawn in the text and two others). Wedge / hash diagrams of the two stereoisomers (A and B) not given in the text are shown below, as are 3D structures of all three isomers.

 
To construct the table, the stereochemistry at each stereocentre is assigned using the CIP rule (cf. Chapter 3). In each case, CIP rule 1 will determine that the OH group hash priority 1 and the hydrogen atom priority 4. CIP rules 2 and 3 then allow the acid group to be given priority 2 and the remaining substituent to be assigned priority 3. Viewing the structure with the group of priority four at the rear then allows the absolute configuration to be defined as R (other three substituents decrease in priority in a clockwise direction) or S (other three substituents decrease in priority in an anti-clockwise direction), giving the table shown below.

Structure drawn in text     Isomer A                                     Isomer B

R                                       S                                                 R
R                                       S                                                 S

From the table, it is apparent that isomer A is the enantiomer of the structure given in the text since it has the opposite absolute configuration at both stereocentres. Isomer B is a diastereomer of both isomer A and the isomer in the text, since it has the same configuration at one stereocentre but the opposite configuration at the other stereocentre. Isomer B is a meso compound since it contains two stereocentres with the same substituents attached to each, but with opposite absolute configurations.

Enantiomers have identical physical properties except for the sign of their specific rotations. Thus, isomer A will have the same melting point (170-172oC) as the isomer in the text, and will have a specific rotation of -12.4. Isomer B is a meso compound and so is achiral and will have a specific rotation of 0. The melting point of this isomer cannot be predicted from the information given, since it is a diastereomer of the isomer in the text and diastereomers have different physical properties.

The isomer in the text is the l-isomer, since the two stereocentres both have the same (R) absolute configuration. Similarly, isomer A is the l-isomer, since the two stereocentres both have the same (S) absolute configuration. Isomer B however is the u-diastereomer since one stereocentre has the (R)-configuration whilst the other has the (S)-configuration. To assign the stereochemistry using the syn / anti nomenclature, the compounds must be drawn in flying wedge projections. The structure in the text is already drawn in this way, as are the two diagrams of isomers A and B shown above. For the isomer in the text, the two non-hydrogen substituents attached to the main carbon chain at the two stereocentres are on the same side of the molecule (these are the two OH groups and are both drawn with hashed bonds). Hence, this is the syn-isomer. Similarly, for isomer A, the two non-hydrogen substituents attached to the main carbon chain at the two stereocentres are on the same side of the molecule. Hence, this is also the syn-isomer. For isomer B however, the two substituents are on opposite sides of the molecule, so this is the anti-isomer. To assign the stereochemistry using the erythro / threo nomenclature, the compounds must first be converted into Fischer projections as shown below. (The 3D structures above may be helpful in carrying out these rotations) Thus the structures are rotated until all vertical bonds are pointing away from the viewer and all horizontal bonds towards the viewer. All wedges and hashes are then replaced by normal bonds and bonds to hydrogen atoms are deleted. For the isomer in the text, and isomer A, the two OH groups are on opposite sides of the Fischer projection, so this is the threo-isomer whilst for isomer B, the two OH groups are on the same side of the projection so this is the erythro-isomer.

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