09 Synthesis of Cholesterol Epoxide

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CHM25601 Epoxidation of Cholesterol 1 © Trustees of Purdue University. Permission for reproduction or further distribution must be obtained in writing from the Department of Chemistry. The Synthesis of Cholesterol Epoxide P RELAB R EADING /V IDEO : Review the Experimental Techniques listed below in Zubrick. Relevant videos for these techniques are posted on Brightspace. Review the relevant reactions in your lecture textbook. This is a 1+ week experiment. I NTRODUCTION Cholesterol is a member of a class of molecules called steroids . It is the principal component in gallstones; in fact, a common laboratory experiment for organic chemistry courses is the isolation of cholesterol from gallstones. You’ve undoubtedly heard about the molecule since it’s a primary cause of arteriosclerosis (hardening of the arteries). However, we also require a certain amount of cholesterol for good health (the average person has about 200 g, primarily in brain and nerve tissue), and the compound is synthesized naturally in the body. Some other examples of steroids are shown below. In this experiment, we will use 3-chloroperoxybenzoic acid (MCPBA) to epoxidize the alkene in cholesterol. This is called a Prilezhaev Reaction, after Nikolai Prilezhaev who first published the reaction in 1910.

CHM25601 Epoxidation of Cholesterol 2 © Trustees of Purdue University. Permission for reproduction or further distribution must be obtained in writing from the Department of Chemistry. The bond making and bond breaking steps are thought to be concerted: (from Brown, et.al. Organic Chemistry , 5 th ed. 2009 Chapter 11. Find this mechanism in your text and read about it!) Note that the reaction in this experiment is stereospecific; that is, the only product formed is the one where the epoxide group is trans to the OH group. This is because the methyl groups on the tetracyclic backbone sterically impede the approach of the 3-chloroperoxybenzoic acid from the “top” side of the molecule (if you’re having trouble visualizing this, make a model) . Note also that two different melting points are given for the product because this particular compound can crystallize in either of two different forms (i.e., either of two different crystal structures) depending on the conditions used to isolate the compound. P RE -L AB Q UESTIONS 1. What is the purpose of the lab? What specific techniques will you be using? 2. Provide the scheme and mechanism of the reaction you will be performing during the lab. 3. Provided below are the list of chemicals you will be using in this lab. Please fill in the table below.

CHM25601 Epoxidation of Cholesterol 3 © Trustees of Purdue University. Permission for reproduction or further distribution must be obtained in writing from the Department of Chemistry. Chemical Quant ity Used Molecular Weight Moles Density MP/BP Appearance Hazards Cholesterol m -chloroper- benzoic acid (MCPBA) Ethyl acetate sodium bicarbonate Product Calcium Chloride 4. What is the Limiting Reagent (LR) for the reaction (show calculation)? Calculate the theoretical yield based on the LR. 5. Draw the apparatus setup you will be using in this lab. 6. Does the epoxidation of trans -2-hexene produce a single enantiomer, a meso compound or a racemic mixture? Explain your reasoning and show the mechanism for the reaction of trans -2-hexene with MCPBA. 7. In steroids, the “face” (or “side”) of the molecule in which the methyl groups at the ring junctions are pointing is referred to as the β “ face ” ; the other “ face ” (or “ side ” ) of the molecule is referred to as the α “ face ” . Is the orientation of the epoxide group in cholesterol epoxide α , or β ? 8. What structural feature do all steroids have in common? List two different steroids (not including those listed in the Introduction to this experiment), draw their structures, and state their biological relevance. 9. What is the structural difference between a carboxylic acid and a percarboxylic acid? Why is commercial MCPBA often less than 75% pure, and what are the impurities? (Hint: Google is a very nice search engine.) 10. When the reaction mixture is worked-up, it is first washed three times with 5% sodium bicarbonate, and then with a saturated NaCl solution. Explain why. 11. What are the types of waste you are generating in this experiment and how are you disposing of them? 12. Please provide a flowchart of the procedure you will be following in this lab, highlighting the key steps.

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