Organic Chemistry Qualifying and Comprehensive Exam

Transcription

1 rganic Chemistry Qualifying and Comprehensive Exam Chemistry Division Illinois Institute of Technology April 16 th, 2010 Page 1 of 9

2 Instructions: This is a closed book exam and you have three hours to complete it. You may mark on the exam, but please write all your answers in the blue book that is given to you. Part three may ask for you to answer directly on the exam paper, then please do so. Use a separate blue book for each of the three parts and put your name and Part number on the front of the book. Budget your time wisely. PART 1: rganic Synthesis rganic Literature Synthesis Name Reactions and Mechanisms 1. Give a mechanistic explanation (including reaction type, key intermediates or transition states, stereochemistry, and so on) for each step shown below leading to the synthesis of Roseophilin (I). Identify any Named Reactions involved in the synthesis. Explain these terms: regiospecific, regioselective, stereospecific, and stereoselective reactions If applicable in the synthesis, explain why a reaction would be regiospecific, regioselective, stereospecific, or stereoselective. 2. Indicate the chiral centers found in intermediate 11 shown below in scheme 3 and label them as R or S. Explain your answers. What is the total number of stereoisomers possible for this molecule? 3. Indicate how you might synthesize pyrrole 15, shown below in scheme 4, from pyrrole? Page 2 of 9

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4 Retrosynthetic analysis: The flavor ingredient, Nootkatone, (see below) has been a synthetic target for different research labs engaged in natural products syntheses. Using a retro-synthetic approach, out line a possible strategy for its synthesis from materials likely found in the Aldrich catalog. Think about the big picture and less on detail. Page 4 of 9

5 PART 2: Physical rganic Chemistry 1. The reaction of sulfur ylide A with an aldehyde generated an epoxide (X = ) as shown below. (rg Lett, 2007, 9, 5481) S A + X k 1 k 2 intermediate A Ph Ph The following observations were made: i) The reaction of ylide A and benzaldehyde in CD 3 CN provided trans-epoxide as the exclusive product. ii) k /k D (aldehyde proton) = 0.93 iii) k 12C /k 13C = iii) ρ = 2.50 a) Identify intermediate compound A. b) Show how the experimental results can be applied to deduce the details of the mechanism: rate determining step, substitution effects, stereochemistry, etc. 2. For the following reactions (rg Lett 2002, 4, 533; JCS Perkin Trans 1, 1985, 2307), identify the intermediate compounds and products and give the mechanisms of the reactions. i) Br 2, hv ii) S Et 3 N N 2 N S iii) PhN=C= Et P Ph Ph i) n-buli -78oC ii) iii) Na, DMF Compound B (C ) Page 5 of 9

6 Cl 3 C i)p-ts, Et, reflux ii) TESCl, DMAP, Et 3 N, C 2 Cl 2 iii) DIBAL, C 2 Cl 2 iv)c 3 C 2 PPh 3 + Cl - LiMDS, TF v) 3 C Compound C N 2 C 3 Ac, 100 o C, 24 h 3. Explain the following experimental results. a) Compound D is ten times faster than compound E in solvolysis in aqueous ethanol. Both of the compounds provide compound F as the solvolysis product. X X D E F b) Provide a mechanism. i) NCl, pyr ii) hv C=N Page 6 of 9

7 PART 3: Structural Analysis by Spectroscopy 1. Use the following expanded spectra to answer the following questions or provide an answer. DRAW N TIS EXAM for (a), but (b) and (c) answer in blue book. a. Draw the stick diagram analysis below the spectra of this single hydrogen. Try to use straight lines because you will be asked for the coupling constants in part b. b. What are the different coupling constants for this hydrogen and it s neighbors? (Arbitrarily label them J ab, J ac, etc. for all the coupling constants that you determine using a as the one shown.) c. Using the splitting pattern that you determined from part (a), draw a logical spin system for this a and use the notation you choose in part (b) for labeling your hydrogens ( b, c, etc.). Draw the number of hydrogens on each of the attached carbons that would make sense for the spin system. If you need to add carbons or other groups that would make a complete answer, add them. (Redraw in blue book) a Page 7 of 9

8 2. Using the letters below the spectra, place them correctly on the spectra near the hydrogens that they represent. A ne experiment showed that when the peak at 1.75ppm was irradiated there was an enhancement at 4.86ppm (C) but not at 4.93ppm (D). (Enhancement indicates a through space correlation). nly the peaks with integrations are part of the structure. Write the letter that is below the spectra in the boxes of the next to the hydrogens on the given structure. (Draw on this paper) I GF E D C B A Expansion of ppm region I G F Page 8 of 9

9 3. Using the following spectroscopic data of compound A, determine the structure of compound A. (Note the MS used chemical ionization with methane) Page 9 of 9