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Chem 110 Spring 2021 – Final – Prof. McKinnie
Tuesday, June 8, 2021
Please answer the following questions in the boxed spaces provided and upload your answer as a PDF document to Gradescope. The exam consists of 7 questions on 7 pages; there are additional resources on page 8 for your reference. (100 marks total) 1 a. Draw a detailed mechanism for the following synthetic transformation that includes more than one pericyclic reaction. Include all mechanistic arrows and intermediates. ( 8 marks) 1 b. Draw a detailed mechanism for the following synthetic transformation that includes more than one pericyclic reaction. Include all mechanistic arrows and intermediates. ( 6 marks) O OH heat O H heat H O F F
1 c. Draw a mechanism and the major regioisomer product of the following intermolecular Diels-Alder [4+2] cycloaddition. ( 6 marks) 2 a. For the 8p electrocyclization below, use Frontier Molecular Orbitals to define whether this reaction is: i) under thermal or photochemical (hu) conditions; and ii) is a conrotatory or disrotatory cyclization. More successful answers will include an image of the orbital responsible for the stereochemical outcome. ( 6 marks) 2 b. Draw the major product of the following electrocyclic ring-opening reaction on the left. Mechanisms and other potential products are not needed. ( 2 marks) 2 c. Describe why the following cascade of electrocyclization reactions on the right stops after forming the 6-4 bicyclic ring. ( 2 marks, 1 for each argument) S O O O heat mechanism and product thermal or h υ conrotatory or disrotatory (^)? ? major product heat heat H H H heat H H H H F thermal (heat) b) c)
4 a. The addition of hydrobromic acid across a cyclic symmetric diene is shown below alongside the 1,2 and 1,4 products. Fill in the boxes to define whether the products are the thermodynamic or kinetic product, and the temperature required to make this isomer the major product (either 0 or 40 °C). ( 4 marks) 4 b. On the energy diagram depicted below, please label the products in the right- hand boxes based on their overall stabilities and draw lines to each product from the carbocation intermediate through their respective transition state. (2 marks) 4 c. Draw all four (4) possible products from the addition of hydrochloric acid across this asymmetric diene. Mechanisms are not needed. ( 4 marks) HBr Br Br 1,2 addition product 1,4 addition product thermodynamic or kinetic product? temperature needed to make this isomer the major product? (0 or 40 °C) HBr Br Energy Reaction Progression H Br (^) transition state 2 energies product energies 1,2 or 1, product? carbocation intermediate starting materials transition state 1 HCl 4 possible products
- Below is a proposed synthetic scheme to make a cyclic tetrapeptide; however it unfortunately contains multiple errors that will prevent efficient synthesis of the final product. Analyze the scheme and identify four (4) problems with the synthetic proposal, list them in the first box below, and give a short description why this step will not work. For one (1) of the above problem steps, list a solution to fix that one step only (not the whole scheme). You can use the abbreviations listed along the scheme (SM, R1-6, I1-5, FP) for precise descriptions. ( 10 marks) Cbz N H S O OH tBu H 2 N O O O O Bn HATU, DIPEA Cbz N H S O H N tBu O O O O Bn NaOH (^) Cbz N H S O H N tBu OH O O O Bn H 2 N O O S tBu Cbz N H S O H N tBu N H O O O Bn O O S tBu H 2 , Pd H 2 N S O H N N H O O O Bn O O S tBu tBu BocHN NH 2 OH O DCC N H S O H N N H O O O Bn O O S tBu tBu O BocHN NH 2 N H S O H N N H O O OH OH O S O H 2 N NH 2 TFA, air **Starting Material (SM) Reagents 1 (R1) Intermediate 1 (I1) Reagent 2 (R2) Intermediate 2 (I2) Reagents 3 (R3) Intermediate 3 (I3) Reagents 4 (R4) Intermediate 4 (I4) Reagents 5 (R5) Intermediate 5 (I5) Reagents 6 (R6) Final Product (FP) Four (4) problems with the above synthetic proposal
Solution for one (1) of the above steps**
- Follow the synthetic roadmap and fill in the missing starting materials, reagents, or products as necessary (20 marks). CrO 3 , OH O O Si Ph Ph H^ Bu^4 N^ F 3 O+ H 3 O+ O Br 2 O O O C 12 H 13 NO CN
CN
- LDA
H H O Br Na 2 CO 3 O O (^) CN O heat
- LiAlH 4
- H 2 O workup O HN O NHBoc OH HN O NH 2
- NaCNBH 3 , 2) H 3 O+ C 14 H 20 N 2 O
- PCC
Bonus: Use clear mechanistic arrows and correctly labeled intermediates to show how this mono-deuterated bullvalene can have the D atom ‘move’ to 3 different positions at elevated temperature. ( 1 mark each, 3 marks maximum) Extra Resources: Double Bond Equivalents/Degrees of Unsaturation formula:
DBE = C + 1 – (H + X – N)
Conrotatory/Disrotatory table:
Thermal Photochemical
4 π electrons Conrotatory Disrotatory
4 π + 2 electrons Disrotatory Conrotatory
D
