ANSWER KEY PAGE 1 of 11 UNIVERSITY OF MANITOBA DEPARTMENT OF CHEMISTRY CHEM 3390 STRUCTURAL TRANSFORMATIONS IN ORGANIC CHEMISTRY FINAL EXAMINATION Dr. Phil Hultin Tuesday December 13, 2011 9:00 am. NAME: STUDENT NUMBER: Section 1: Concepts and terminology (20 Marks) Section 2: Stereochemistry and conformation (20 Marks) Section 3: Reaction products and reagents (25 Marks) Section 4: Mechanisms and experimental explanations (25 Marks) Section 5: Applications of spectroscopy (10 Marks) TOTAL (100 Marks): Put all answers in the space provided for each question. You may use the backs of the sheets if necessary, but none of the answers requires a long written answer. Clearly drawn chemical structures will greatly assist the marking process and will reduce the likelihood that your answer will be misinterpreted. You may use molecular models during the exam. Note that a spectroscopic data sheet is not provided.
ANSWER KEY PAGE 2 of 11 1. (20 Marks Total) CONCEPTS AND TERMINOLOGY. In the table below, FIFTEEN of the terms, reagents or concepts listed on the left have a corresponding definition or description in the right hand column; FIVE do not. Match the term with the appropriate definition or description by writing the corresponding letter in the box at the left; If the term does not have a matching definition or description write X. P Structural isomers A: Another term for anchimeric assistance. X Transition state B: Energy difference between staggered and eclipsed conformations. H Haptophilic effect C: The species formed on protonation of a Brønsted base. X Stereospecific reaction D: Relationship between pairs of structural elements in an achiral molecule whose alteration leads to mirror-image products. M Gauche conformation E: Destabilizing steric interactions in allylic fragments. B Barrier to rotation F: A highly polarizable electron donor. R Ylides G: Local minimum in potential energy for a reaction process. D Enantiotopic H: Control of hydrogenation stereochemistry by coordination of alcohol groups to catalyst. A Neighbouring group participation I: Any reaction that forms only one of several possible stereoisomeric products. Q HSQC J: Compounds able to act as electron acceptors. X Lindlar catalyst K: A geometric parameter that changes during the conversion a reactant into a product and whose value can be taken for a measure of the progress of an elementary reaction. N Conformational isomers L: Forms trans diols from alkenes. T Anomeric effect M: K Reaction coordinate N: S Hydrogenolysis O: X Lewis bases P: C Conjugate acid Q: L Prévost reaction R: A contiguous 4-atom fragment having a dihedral angle of approximately 60º. Stereoisomers that can be interconverted by rotations around single bonds. Promotes conversion of alkynes to trans alkenes in the presence of H 2 (g). Compounds of the same formula that differ in their connectivity. Two-dimensional NMR technique to correlate chemical shifts of different nuclei. Compounds in which an anionic site Y is attached directly to a heteroatom X + carrying a formal positive charge. F Soft base S: Reductive cleavage of a bond. X A-value T: Unusual stabilization present in gauche conformations of C-O-C-X fragments.
ANSWER KEY PAGE 3 of 11 2. (20 Marks Total) STEREOCHEMISTRY AND CONFORMATION a. (5 Marks) Draw a clear perspective view showing the preferred conformation of tri-o-acetyl-β-d-arabinopyranosyl bromide (A). b. (5 Marks) The conformations of α,βunsaturated carbonyl compounds are somewhat like those of 1,3-dienes. Gas-phase microwave spectroscopy data for propenal, propenoic acid and methyl propenoate are shown. For propenal (X = H), the planar s-trans geometry is significantly favoured, but for the acid (X = OH) and the ester (X = OCH 3 ) the difference between s-cis and s-trans is quite small. Briefly explain these observations. For propenal, there is minimal A 1,3 strain in the s- trans conformation, while the carbonyl oxygen encounters more A 1,3 strain in the s-cis geometry. In the acid and ester compounds the difference in size between the carbonyl O and the OH/OR groups is much less, so the A 1,3 strain in either s-cis or s-trans is similar. Thus, the two rotamers are not greatly different in energy.
ANSWER KEY PAGE 4 of 11 c. (5 Marks) The name inositol refers to the 9 possible stereoisomers of 1,2,3,4,5,6-hexahydroxycyclohexane. Of these 9 isomers, only two are chiral and form an enantiomeric pair. Draw the structures of the two chiral inositols using any representation that clearly indicates their stereochemical configurations. While you could approach this by drawing all nine isomers and then identifying the chiral ones, it is quicker to remember that to be chiral the structure must not have any planes of symmetry or points of internal symmetry. Draw the basic structure with all the OH groups on the same side, and then flip OH groups one at a time to destroy all planes of symmetry. d. (5 Marks) The [2.2]-metacyclophane carboxylic acid B is chiral, although a pure enantiomer of B can be racemized by heating. Draw the two enantiomers of B, and briefly explain why how racemization occurs.
ANSWER KEY PAGE 5 of 11 3. (25 Marks Total) REACTION PRODUCTS AND REAGENTS For each of the following, provide the missing product or reagent(s)/solvent(s)/conditions. Show stereochemistry whenever it is appropriate. a. (2) b. (2) c. (2) d. (2) e. (2)
ANSWER KEY PAGE 6 of 11 f. (2) Some people might have drawn the isomer of this with the carbonyl and allyl groups exchanged. This is less likely since initial ionization of the diol would favour the tertiary cation, leading to the product shown. However, substantial part marks will be given for the other isomer if correctly drawn. g. (3) h. (10 2 for each product, 2 for final reagent, 1 for each box in first step)
ANSWER KEY PAGE 7 of 11 4. (25 Marks Total) MECHANISMS AND EXPERIMENTAL EXPLANATIONS a. (8 Marks) Provide a detailed stepwise mechanism for the following reaction, which occurs in buffered aqueous formic acid solution (ph ~ 3.7). This was one of the practice mechanism problems in the textbook, Chapter 4.
ANSWER KEY PAGE 8 of 11 b. (8 Marks) Provide a mechanism to explain the following reaction which occurs in the presence of 2 equivalents of pyridinium chlorochromate.
ANSWER KEY PAGE 9 of 11 c. (9 Marks) Give mechanisms for the formation of both the major product A and the minor byproduct B in the following reaction.
ANSWER KEY PAGE 10 of 11 5. (10 MARKS TOTAL) APPLICATIONS OF SPECTROSCOPY When 3,4-dihydro-6-methoxynaphthalene was subjected to the reaction sequence shown, product B was obtained. The structure of A should be obvious, but the NMR spectra for B are on the next page. a. What was the structure of product B? b. Assign the 1 H NMR signals to appropriate protons in your proposed structure. c. Use mechanistic arguments to explain how B was formed from A. The first step obviously forms the epoxide from the alkene. In the second step, the epoxide is protonated and opens up to form the benzylic cation. A pinacol-like 1,2-shift then forms the ketone, similar to box question g on page 6 of this exam.
ANSWER KEY PAGE 11 of 11 13 C NMR of Compound B Note: solvent signals have been removed 1 H NMR of Compound B Spectrum reconstructed from data using Spinworks solvent signal not shown. Organic Radicals? (lol)