MC test: Taxol - a chiral auxiliary case study

Multiple choice test on D.7 Taxol - a chiral auxiliary case study

Use the following 'quiz' to test your knowledge and understanding of this sub-topic. As this relates to a sub-topic on the options you may need access to the IB data booklet.

If you get an answer wrong, read through the explanation carefully to learn from your mistakes.

Which method can be used to separate a racemic mixture of two enantiomers into its individual components?



 

In chiral column chromatography, silica gel is bonded to chiral molecules (usually chiral sugars) to form the chiral stationary phase. The enantiomers will then separate as they run down the column because one of the enantiomers will interact more strongly with the column and be eluted more slowly.

 

Where does the drug paclitaxel (taxol) occur naturally?

Paclitaxel is found in the bark of the Pacific yew tree (Taxus brevifolia).

 

Which type of disease is paclitaxel (taxol) used to treat?

Paclitaxel is used to treat several different types of cancer including ovarian cancer, breast cancer, lung cancer, cervical cancer, and pancreatic cancer.

 

Beccatin III is a natural product that can be used as a precursor in the semi-synthesis of paclitaxel (taxol).

How many chiral centres does beccatin III posses?

The full wedges and/or the dotted-line wedges show the positions of the 9 chiral carbon atoms. Each chiral carbon atom contains four different groups attached to it.

 

Which property can be used to distinguish between different enantiomers of paclitaxel (taxol)?

Enantiomers have identical physical properties except for the direction and amount that they rotate the plane of plane-polarised light.

 

Which properties must be possessed by a chiral auxiliary?

I. It must be optically active.

II. It should be easy to remove after the desired intermediate has been formed.

III. It should be able to bond covalently to the non-chiral substrate.

Chiral auxiliaries must be able to bond to the starting material (substrate) and be easy to remove in order to complete the synthesis of the desired enantiomer. In order for the desired enantiomer to be formed the chiral auxiliary must be chiral.

 

Which instrument can be used to differentiate between different enantiomers of Beccatin III, a natural product that can be used as a precursor in the semi-synthesis of paclitaxel (taxol)?

Because they contain the same molar mass, the same functional groups and the same structural formulas the different enantiomers will have identical spectra (or virtually identical spectra even at high resolution) but a polarimeter will detect how much and in what direction the different enantiomers will rotate the plane of plane-polarised light.

 

Compound X exists as two enantiomers, A and B. A pure sample of enantiomer A rotates the plane of plane-polarised light clockwise by 13o and has a melting point if 131 oC. After a sample of compound X had been prepared in the laboratory it was found to melt at 131 oC and rotate the plane of plane-polarised light anticlockwise by 11o. What can be deduced from this observation?

Because there is no lowering of melting point the sample must be pure X. Pure B will rotate the plane of plane-polarised light anticlockwise by 13o so the sample must be a mixture consisting mainly of enantiomer B with some enantiomer A present.

 

Specific stereoisomers of 2-phenylcyclohexanol are used as chiral auxiliaries in the synthesis of paclitaxel (taxol). The structures of the four possible stereoisomers of 2-phenylcyclohexanol are shown and labelled, A, B, C and D.

Which is a true statement?

A and D are both cis-2-phenylcyclohexanol whereas B and C are both trans-2-phenylcylcohexanol.

 

In the presence of a chiral auxiliary (CA), non-chiral starting material S reacts with a reagent R to form the product S−R. The process goes through two intermediates and the chiral auxiliary is recycled once it has been removed.

Which row describes the the two intermediates and the product correctly?

  1st. intermediate 2nd. intermediate Product
1 single enantiomer single enantiomer diastereomer
2 diastereomer single enantiomer single enantiomer
3 diastereomer diastereomer single enantiomer
4 single enantiomer diastereomer single enantiomer

The starting material reacts with the chiral auxiliary to give an optically active intermediate. When this intermediate reacts with the chiral reactant the configuration of the second intermediate is affected by the configuration of the existing chiral centre and a diastereomer is formed. The chiral auxiliary is then removed leaving the product with the desired stereochemistry.

 

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