Question

a-Santonin, isolated from the flower heads of certain species of Artemisia, is an anthelmintic (meaning against intestinal worms). This terpene is used in oral doses of \(60 \mathrm{mg}\) to rid the body of roundworms such as Ascaris lumbricoides. It has been estimated that over one-third of the world's population is infested with these slender, threadlike parasites. \(\alpha\)-Santonin (a) Locate the three isoprene units in santonin and show how the carbon skeleton of farnesol might be coiled and then cross-linked to give santonin. Two different coiling patterns of the carbon skeleton of farnesol can lead to santonin. Try to find them both. (b) Label all chiral centers in santonin. How many stereoisomers are possible for this molecule? (c) Calculate the index of hydrogen deficiency for santonin.

Short answer

Question: Identify the three isoprene units in the α-Santonin molecule and determine two different ways the carbon skeleton of farnesol can be coiled and cross-linked to generate α-Santonin. Label all the chiral centers in α-Santonin and determine how many stereoisomers are possible for this molecule. Calculate the index of hydrogen deficiency for α-Santonin.

Step-by-step solution

a) Isoprene units and coiled carbon skeleton of farnesol

Firstly, analyze the structure of α-Santonin and you can see the three isoprene units within the molecule (each isoprene unit contains a 5-carbon chain). And for the farnesol molecule, you have to consider 3 isoprene units with a 15-carbon total skeleton. As there are two possible ways of coiling the carbon skeleton of farnesol to generate α-Santonin, this step requires drawing the two possible coiling patterns and cross-linking them to produce the α-Santonin molecule. While working on this step, pay attention to the connectivity of each carbon and proper positioning of the functional groups in the resultant α-Santonin structures.

b) Chiral centers and stereoisomers

To identify chiral centers in the α-Santonin molecule, look for any carbon atom that has four different substituents bonded to it. Mark these chiral centers and count the total number of such centers. For a molecule with n chiral centers, the maximum number of possible stereoisomers is given by the formula: max possible stereoisomers = \(2^n\). Use this formula to determine the total number of possible stereoisomers for α-Santonin.

c) Index of hydrogen deficiency

The index of hydrogen deficiency (IHD) indicates how many degrees of unsaturation (rings or double bonds) are present in the molecule. The IHD formula is given as: IHD = \(((2 * C - H + N - X) / 2)\) Where C is the number of carbon atoms, H is the number of hydrogen atoms, N is the number of nitrogen atoms, and X is the number of halogen atoms in the molecule. Using the molecular formula of α-Santonin, input the values of C, H, N, and X in the IHD formula to calculate the index of hydrogen deficiency for α-Santonin.