Organic Chemistry Chemistry 251. Exam Points-

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1 rganic Chemistry Chemistry 251 Exam Points- This examination covers material contained in Pericyclic Reactions (chapter 22), and atural Products (chapter , ) in rganic Chemistry, by ornback. Show all work. 1) Reactions. Show products including stereochemistry. (21 points) h electrocyclic Me + Me 3 Si 1) eat 2) Cl hv (cycloaddition) plus two more 1) K 2 C 3 2) Br 3) heat ([5.5] sigmatropic) electrocyclic exact opposite of the first question except thermal

2 Reactions (continued) 2 4π + 2π endo addition sigmatropic [1.5] of course. Can continue to give a variety of isomers 2) epict the molecular orbitals of the pentadienyl cation. Label M and LUM and answer the following question. In the excited state is the C1-C2 bond length LGER or SRTER (circle one). int: verlap between symmetrical orbitals is bonding, dissymmetrical is anti-bonding and between an orbital and a nodes is non-bonding. (6 points) LUM M ere are the first three Ms with the four electrons of the cation shown If you excite an electron into the LUM then you move it from an orbital that is bonding to an orbital that is non-bonding This results in a weaker bond and longer bond distance 3) Shown below is a polyketide related to Emodin. In this product an oxygen function has been added and a methyl was lost. Suggest a means by which this octaketide is folded and reacts to form this compound, and note the added oxygen and where the methyl was lost. (6 points) add lost Me This cyclizes via aldol condesations and forms phenols by enolization.

3 4) ne of the biosynthetic steps in the production of Elemene, a sesquiterpene, is a [3.3] sigmatropic rearrangement. Show (arrow push) how Elemene arises from germacrene, shown below. The bolded bond is the bond that is broken in the migration. (6 points) Germacrene Elemene (depict) 5) esignate the quantitized molecular process that leads to the following absorptions. (9 pts) UV: electronic transitions MR: nuclei spin flips in a magnetic field IR: vibrational transitions 6) rder as rate of iels-alder reaction with ethylene, from fastest to slowest. (5 pts) Me 3 Si I II III IV I > III > IV > II

4 7) Give the two starting materials, shown below illustrate (arrow pushing): (16 points) Me Me methyl malonate methyl acetate a. Condensation to form a diketide C 3 C 3 beta keto-ester C 3 b. ow show how this product could be elaborated to make a fatty acid. (by this I mean the steps needed to make the 4 carbon unit which will go on to make fatty acids) ow using arrow pushing show how you could combine the diketide you made above (part a) with methyl acetate to form the isopentenol structure (I) shown below. You do not need to use a specific reagent, just show the transformation or use a term like oxidation or reduction. reduce one ester and hydrosyze the other C 2 Me C 3 I

5 8) As best you can, depict the bonding interaction and the unfavorable secondary overlap of this 6π + 4π electrocyclic reaction. Use the M of the triene and the LUM of the diene in your drawing. (6 points) bonding S 9) Consider the structures shown below. Each of these is a natural product and may be classified as a: Poison, Analgesic, Sesquiterpene, rnithine derived alkaloid, indolethylamine or Polyketide. There is one of each. Write it s classification below the given structure. Careful, some of your choices can fit more than one class so choose wisely. (12 points) sesquiterpene C3 Anagesic indolethylamine ornitine derived C 2 Ph poison 3 C Polyketide C 3 10) Shown below, is what appears to be [1.3] sigmatropic reaction which should be thermally forbidden suprafacially. And yet the product shown is formed in 95 % yield. Using depictions showing orbital movement, illustrate how this reaction could proceed in a thermally allowed

6 manner. Your answer should include the midpoint view of the moving group. int: the stereochemistry of the moving carbon (bolded bond) is key. (6 points) Ac Ac Ac rotating carbon The bond indicated by the arrows moves across the double bond to attach 1,3. This process is thermally disallowed because the bonding sigma bond would create an anti-bond on carbon 3. owever if the rotating carbon inverts, as indicated by the stereochemistry of the euterium then this concerted sigmatropic reaction would be allowed. This is illustrated in the box above. 10) Write a short paragraph on one of the natural products we discussed. More points will be given for unique answers. (7 points) These I will enjoy.