1. Option D HL

2.  When a molecule contains a carbon atom bonded to four different groups, it is said to be chiral and two mirror images (known as enantiomers) exist.  These enantiomers can behave very differently in the body as a result of their different shapes. E.g. one enantiomer can combine with a specific protein because its functional groups are in correct position.

3.  Developed in Germany in the 1953.  Used as a tranquilizer (1957- 1962).  Prescribed to combat morning sickness in the early months of pregnancy.  Marketed widely in Europe but never approved by the FDA in the USA.  Caused major birth defects in ABOUT 10,000 children whose mothers had taken thalidomide. 3

4.  Thalidomide has two optical isomers, one of which is a powerful teratogen. 4 Chiral Carbon

5. Thalidomide is both a tranquilizer and a teratogen 5 It is sometimes used to treat some symptoms of Hansen’s disease. (Leprosy)

6.  Many drug with chiral center are synthesized from materials that have no chiral center. The product is racemic mixture. To obtain single enantiomer chiral auxiliary center is used.

7.  A chiral auxiliary is one enantiomer of an optically active substance that is temporarily incorporated into a non-chiral molecule to produce a single enantiomer of a product in an organic synthesis reaction.  Chiral auxiliary allows reagent to approach from the one side of the molecule, so forcing the reaction to produce only one type of enantiomer.  Once reaction is complete chiral auxiliary is removed

9.  Chiral auxiliaries are successfully used in semi-synthesis of anticancer drug Paclitaxel (Taxol).

10.  Paclitaxel is used to treat several forms of cancer – mainly breast, ovarian and lung cancer. It is usually given intravenously as part of a course of chemotherapy to treat cancer. Paclitaxel acts by preventing cell division – it does this by binding to microtubules in the cytoplasm, preventing them from breaking down during cell division.

11.  Paclitaxel was originally obtained from Pacific yew tree bark – however, it took the bark from more than one tree to provide enough paclitaxel to treat just one patient and so semi-synthetic processes were developed that involved making paclitaxel from another natural product derived from the needles of yew trees.  Semi-synthesis of the drug allowed it to be made on a larger scale and reduced the environmental impact – extracting the drug from its natural source results in killing of the trees.  Nowadays paclitaxel is also made by fermentation using plant cell cultures.

12.  Optically active enantiomers can be distinguished using polarimeter, because they rotate plane polarized light in opposite directions  A simple polarimeter consists of a source of light (usually a sodium lamp producing one specific wavelength), two polarising filters, a sample tube and a scale to measure the degree of rotation of the plane-polarised light

14.  An enantiomer that rotates plane-polarised light clockwise (to the right – dextrorotatory) is called the (+)-enantiomer and one that rotates the plane anticlockwise (to the left – levorotatory) is called the (−)-enantiomer.  An enantiomer can be identified from its specific rotation. The specific rotation, [α], is worked out from the angle though which the plane-polarised light is rotated in degrees (α), the path length of solution that the light passes through (l) in dm and the concentration of the solution (c) in g/cm −3 :.

15.  Science is both systematic and creative. Systematic screening of a large number of plant extracts identified paclitaxel as a possible anticancer drug. When the demand for paclitaxel exceeded the supply from natural sources scientists had to develop ways of making this important drug. A great deal of creativity was involved in designing ways of making paclitaxel.