Asymmetric synthesis, chiral auxiliaries and methods of asymmetric induction - Question Bank

1. Which of the following is a method of asymmetric induction that does NOT typically involve metal catalysts?
A) Noyori Asymmetric Hydrogenation
B) Sharpless Asymmetric Epoxidation
C) Organocatalysis (e.g., using proline)
D) Jacobsen Epoxidation
2. In the context of asymmetric synthesis, 'chiral memory' refers to:
A) The ability of a chiral catalyst to retain its configuration.
B) The ability of a chiral auxiliary to transfer its chirality to the product and be recovered.
C) The inherent chirality of the starting material being preserved.
D) The process of remembering the R/S configuration.
3. What does 'stereospecificity' refer to in a chemical reaction?
A) A reaction that produces a racemic mixture.
B) A reaction where a specific stereoisomer of the reactant yields a specific stereoisomer of the product.
C) A reaction that creates a new achiral center.
D) A reaction that converts an R enantiomer to an S enantiomer.
4. Diastereoselective reactions create products with a higher proportion of one diastereomer over others. This is often achieved by:
A) Using a chiral catalyst
B) Utilizing existing stereocenters in the substrate to influence the formation of new stereocenters
C) Forming a racemic mixture
D) Employing achiral reagents
5. Which statement best describes the 'chiral pool' approach to asymmetric synthesis?
A) Synthesizing chiral molecules from achiral starting materials using chiral catalysts.
B) Using readily available, inexpensive chiral natural products as starting materials.
C) Resolving racemic mixtures using chiral chromatography.
D) Attaching chiral auxiliaries to achiral substrates.
6. Organocatalysis, using small organic molecules as catalysts, can also be used for asymmetric synthesis. A prominent example is:
A) The Sharpless epoxidation
B) The Noyori hydrogenation
C) Organocatalytic Michael additions using proline derivatives
D) Asymmetric Diels-Alder reactions with Lewis acids
7. What is a key difference between chiral auxiliaries and chiral catalysts?
A) Auxiliaries are used in stoichiometric amounts; catalysts are used in sub-stoichiometric amounts.
B) Auxiliaries are always chiral; catalysts are always achiral.
C) Auxiliaries are recovered unchanged; catalysts are consumed.
D) Auxiliaries are temporarily attached; catalysts are never attached.
8. The use of chiral phosphine ligands like BINAP in transition metal catalysis is an example of:
A) Chiral auxiliary approach
B) Stoichiometric chiral reagent approach
C) Chiral catalyst approach
D) Chiral resolving agent approach
9. What is a common chiral auxiliary derived from an amino acid?
A) Camphor
B) Menthol
C) Proline
D) Lactic acid
10. Asymmetric synthesis is particularly important in the pharmaceutical industry because:
A) Chiral drugs often have different pharmacological activities or toxicities for different enantiomers
B) Achiral drugs are more stable
C) Synthesis of enantiomerically pure compounds is always cheaper
D) Most biological systems are achiral
11. Which type of molecule has internal symmetry elements such that it is superimposable on its mirror image, despite having chiral centers?
A) Enantiomer
B) Diastereomer
C) Meso compound
D) Racemate
12. When assigning R/S configuration, if the lowest priority group is pointing away from the viewer, a clockwise arrangement of the other three groups corresponds to:
A) S configuration
B) R configuration
C) M configuration
D) P configuration
13. What is the purpose of the Cahn-Ingold-Prelog priority rules?
A) To determine the boiling point of isomers
B) To assign the absolute configuration (R or S) to a stereocenter
C) To predict the reactivity of enantiomers
D) To calculate the enantiomeric excess
14. Reagent-controlled asymmetric synthesis relies on:
A) The inherent chirality of the substrate
B) A chiral reagent or catalyst that directs the stereochemical outcome
C) The solvent used in the reaction
D) The temperature of the reaction
15. Which of the following is an example of a substrate-controlled asymmetric synthesis?
A) Using a chiral catalyst
B) Using a chiral auxiliary attached to the substrate
C) Using chiral chromatography
D) Using a chiral resolving agent
16. A key challenge in using chiral auxiliaries is:
A) Their high cost and availability
B) The difficulty in their removal and recycling
C) Their tendency to racemize the product
D) Their inability to induce chirality
17. What is kinetic resolution?
A) Separating enantiomers based on different physical properties
B) Using a chiral reagent to react preferentially with one enantiomer in a racemic mixture
C) Forming diastereomers and separating them
D) Achieving high enantioselectivity in a synthesis
18. In the asymmetric reduction of a prochiral ketone, a chiral reducing agent or catalyst influences the approach of the hydride to:
A) The carbonyl carbon, leading to preferential formation of one enantiomer
B) The oxygen atom, leading to dehydration
C) The alpha-carbon, leading to enolate formation
D) The entire molecule, causing racemization
19. Which type of reaction is most likely to convert a prochiral ketone into a chiral alcohol?
A) Nucleophilic addition
B) Electrophilic substitution
C) Radical halogenation
D) Friedel-Crafts alkylation
20. What is a 'prochiral' molecule?
A) A molecule that is already chiral
B) A molecule that can be converted into a chiral molecule by a reaction that forms a new stereocenter
C) A molecule with no plane of symmetry
D) A molecule that exists as a racemic mixture
21. The (R) and (S) nomenclature for stereocenters is based on:
A) The direction of light rotation
B) The sequence rules developed by Cahn-Ingold-Prelog
C) The physical properties of the molecule
D) The alphabetical order of substituents
22. Polarimetry measures the rotation of plane-polarized light by a chiral compound. A positive rotation is denoted by (+), and a negative rotation by (-). These are:
A) Related to absolute configuration (R/S)
B) Independent of absolute configuration
C) Always R configurations
D) Always S configurations
23. Which of the following is NOT a method for determining enantiomeric purity?
A) Chiral chromatography
B) NMR spectroscopy with chiral shift reagents
C) Polarimetry
D) Mass spectrometry
24. Classical resolution of enantiomers often involves forming salts with a chiral acid or base. These salts are:
A) Enantiomers
B) Diastereomers
C) Racemates
D) Meso compounds
25. What is the role of a chiral resolving agent?
A) To catalyze an asymmetric reaction
B) To temporarily attach to a racemic mixture to form diastereomers, which can then be separated
C) To induce chirality in a substrate
D) To cleave a chiral auxiliary
26. The Jacobsen epoxidation is known for its high enantioselectivity in the epoxidation of:
A) Alkenes bearing electron-withdrawing groups
B) Terminal alkenes
C) Internal alkenes
D) Allylic alcohols
27. What type of substrates are commonly reduced in Noyori asymmetric hydrogenation?
A) Alkenes
B) Alkynes
C) Ketones and imines
D) Alcohols
28. The Noyori asymmetric hydrogenation typically employs a chiral ruthenium-phosphine complex as a:
A) Chiral auxiliary
B) Chiral resolving agent
C) Chiral catalyst
D) Chiral solvent
29. What are the key components of the Sharpless asymmetric epoxidation catalyst system?
A) Titanium tetraisopropoxide, tert-butyl hydroperoxide, and a chiral tartrate ester
B) Osmium tetroxide, NMO, and a chiral diamine
C) Diborane, THF, and a chiral phosphine
D) Lithium aluminum hydride and a chiral alcohol
30. The Sharpless asymmetric epoxidation is a famous example of:
A) Chiral auxiliary mediated epoxidation
B) Asymmetric induction using a chiral catalyst system
C) Diastereoselective epoxidation
D) Racemic epoxidation
31. Chiral ligands are often used in conjunction with:
A) Stoichiometric chiral auxiliaries
B) Achiral metal centers to form chiral catalysts
C) Achiral organic bases
D) Chiral resolving agents
32. If a reaction yields 90% of the (R)-enantiomer and 10% of the (S)-enantiomer, what is the enantiomeric excess (ee)?
A) 10%
B) 80%
C) 90%
D) 100%
33. What does 'enantiomeric excess' (ee) measure?
A) The percentage of chiral product formed
B) The difference in energy between transition states
C) The degree to which one enantiomer is favored over the other
D) The amount of unreacted starting material
34. Which concept is central to achieving asymmetric induction?
A) Thermodynamic control
B) Kinetic control
C) Chiral discrimination
D) Steric hindrance
35. A chiral catalyst is a catalyst that is:
A) Achiral and speeds up the reaction
B) Chiral and speeds up the reaction
C) Achiral and directs stereochemistry
D) Chiral and does not affect stereochemistry
36. Methods of asymmetric induction can involve:
A) Achiral catalysts
B) Chiral catalysts, chiral reagents, or chiral auxiliaries
C) Simple heating or cooling
D) Non-chiral solvents only
37. What is 'asymmetric induction'?
A) The process of creating a racemic mixture
B) The process by which a chiral influence directs the stereochemical outcome of a reaction
C) The separation of enantiomers from a racemic mixture
D) The formation of a new achiral center
38. In the context of Evans' auxiliaries, N-acylation followed by deprotonation creates an enolate that reacts with electrophiles with high:
A) Regioselectivity
B) Chemoselectivity
C) Diastereoselectivity
D) Enantioselectivity
39. Evans' oxazolidinones are a well-known class of:
A) Chiral catalysts
B) Chiral resolving agents
C) Chiral auxiliaries
D) Chiral ligands
40. After the desired chiral center is formed using a chiral auxiliary, it is typically:
A) Left intact in the final product
B) Oxidized to a ketone
C) Hydrolyzed or cleaved to be removed
D) Recycled without modification
41. What is the primary goal when using a chiral auxiliary?
A) To increase reaction rate
B) To achieve high diastereomeric excess (de)
C) To simplify product purification
D) To reduce the number of byproducts
42. Which of the following is a key characteristic of a chiral auxiliary?
A) It is easily removed after the chiral center is formed
B) It remains permanently attached to the product
C) It must be achiral
D) It must not influence the stereochemical outcome
43. What is a 'chiral auxiliary' in asymmetric synthesis?
A) A reagent that causes racemization
B) A chiral molecule temporarily incorporated into a substrate to induce diastereoselectivity
C) A catalyst that is achiral
D) A solvent that promotes chiral recognition
44. Asymmetric synthesis aims to produce:
A) A racemic mixture
B) A single enantiomer or an enantiomerically enriched product
C) A mixture of diastereomers
D) A compound with no stereocenters
45. A mixture containing equal amounts of two enantiomers is known as a:
A) Diastereomer
B) Meso compound
C) Racemic mixture
D) Epimer
46. What does the term 'enantiomers' refer to?
A) Stereoisomers that are mirror images of each other
B) Stereoisomers that are not mirror images of each other
C) Isomers with different functional groups
D) Isomers with the same molecular formula but different connectivity
47. Which of the following is a common example of a chiral molecule?
A) Water (H₂O)
B) Methane (CH₄)
C) Lactic acid
D) Carbon dioxide (CO₂)
48. A molecule that is non-superimposable on its mirror image is called:
A) Achiral
B) Chiral
C) Meso compound
D) Racemate
49. What is the fundamental requirement for a molecule to exhibit optical isomerism?
A) Presence of a double bond
B) Presence of a chiral center
C) Presence of a functional group
D) Presence of an aromatic ring
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