Asymmetric synthesis is a cornerstone of pharmaceutical chemistry, enabling selective preparation of chiral molecules with high *enantioselectivity* and controlled *absolute configuration*. For B.Pharm students, understanding concepts like *chirality*, *enantioselective catalysis*, *optical purity*, and methods such as chiral auxiliaries, asymmetric hydrogenation, and enzymatic resolution is essential for designing safer and more effective chiral drugs. This introduction covers practical principles, common catalysts (BINAP, oxazaborolidine, Sharpless reagents), analytical techniques (polarimetry, chiral HPLC, X‑ray), and strategies to improve *stereoselective synthesis*. Clear mastery of these topics helps predict biological activity and regulatory requirements for chiral pharmaceuticals. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the primary goal of asymmetric synthesis in pharmaceutical chemistry?
- To produce racemic mixtures
- To selectively synthesize one enantiomer over the other
- To increase reaction temperature
- To avoid use of catalysts
Correct Answer: To selectively synthesize one enantiomer over the other
Q2. Which term describes two stereoisomers that are non-superimposable mirror images?
- Conformers
- Enantiomers
- Geometric isomers
- Isotopes
Correct Answer: Enantiomers
Q3. A carbon atom bonded to four different substituents is called:
- Aromatic
- Prochiral
- Achiral
- Chiral centre (stereogenic centre)
Correct Answer: Chiral centre (stereogenic centre)
Q4. Which method is commonly used to assign R or S absolute configuration?
- Cahn–Ingold–Prelog priority rules
- Fischer projection inversion
- Optical rotation sign alone
- Infrared spectroscopy
Correct Answer: Cahn–Ingold–Prelog priority rules
Q5. Enantiomeric excess (ee) is defined as:
- The difference in concentration between two diastereomers
- The sum of both enantiomer concentrations divided by two
- The excess percentage of one enantiomer over the racemate
- The absolute optical rotation value
Correct Answer: The excess percentage of one enantiomer over the racemate
Q6. Which technique directly determines absolute configuration of a crystalline compound?
- Chiral HPLC
- Polarimetry
- X-ray crystallography with anomalous dispersion
- IR spectroscopy
Correct Answer: X-ray crystallography with anomalous dispersion
Q7. Sharpless asymmetric epoxidation is most effective on which substrate type?
- Internal alkynes
- Allylic alcohols
- Terminal alkanes
- Aromatic rings
Correct Answer: Allylic alcohols
Q8. The CBS reduction uses which catalytic type to achieve enantioselective ketone reduction?
- Chiral oxazaborolidine catalyst
- Chiral phosphine ligand only
- Simple sodium borohydride
- Lewis acid without chirality
Correct Answer: Chiral oxazaborolidine catalyst
Q9. Which chiral auxiliary is commonly associated with Evans’ methodology?
- Oxazolidinone
- MTPA (Mosher) acid
- Tetrachlorocatechol
- Triphenylphosphine
Correct Answer: Oxazolidinone
Q10. In asymmetric hydrogenation, BINAP ligands are most often complexed with which metal?
- Iron
- Rhodium or Ruthenium
- Sodium
- Magnesium
Correct Answer: Rhodium or Ruthenium
Q11. Which term describes a molecule with stereocentres but is overall achiral due to an internal plane of symmetry?
- Meso compound
- Enantiomer
- Constitutional isomer
- Optical isomer
Correct Answer: Meso compound
Q12. Which analytical method measures optical rotation to estimate optical purity?
- Mass spectrometry
- Polarimetry
- NMR spectroscopy
- UV-Vis without chiral reagents
Correct Answer: Polarimetry
Q13. Diastereomers differ from enantiomers in that they:
- Are mirror images of each other
- Have identical physical properties in achiral media
- Are not mirror images and have different physical properties
- Always rotate plane-polarized light equally and oppositely
Correct Answer: Are not mirror images and have different physical properties
Q14. Which approach involves converting a racemate into a mixture that can be separated due to different diastereomeric interactions?
- Chiral pool synthesis
- Diastereomeric salt formation (resolution)
- Direct racemization
- Pericyclic reaction
Correct Answer: Diastereomeric salt formation (resolution)
Q15. The Felkin–Anh model helps predict:
- The rate of radical reactions
- The preferred face of nucleophilic attack on a carbonyl adjacent to a stereocentre
- UV absorption maxima
- Boiling points of stereoisomers
Correct Answer: The preferred face of nucleophilic attack on a carbonyl adjacent to a stereocentre
Q16. Mosher’s method typically uses which reagent to assign absolute configuration of alcohols or amines?
- (+)-Camphorsulfonic acid
- α-Methoxy-α-(trifluoromethyl)phenylacetic acid (MTPA) derivatives
- Sodium borohydride
- EDTA
Correct Answer: α-Methoxy-α-(trifluoromethyl)phenylacetic acid (MTPA) derivatives
Q17. Dynamic kinetic resolution differs from simple kinetic resolution because it:
- Uses only achiral catalysts
- Converts the undesired enantiomer back to the racemate continuously while resolving
- Occurs only at absolute zero temperature
- Requires chromatographic separation only
Correct Answer: Converts the undesired enantiomer back to the racemate continuously while resolving
Q18. Which statement about enantiomers in an achiral environment is true?
- They have different melting points
- They have different NMR spectra in achiral solvent
- They have identical physical properties except for direction of optical rotation
- They react at very different rates with achiral reagents
Correct Answer: They have identical physical properties except for direction of optical rotation
Q19. Which catalytic system is most associated with asymmetric hydrogenation developed by Noyori?
- Ru-BINAP with diamine ligands
- Pd/C without ligand
- NaBH4
- TiCl4 only
Correct Answer: Ru-BINAP with diamine ligands
Q20. In Cahn–Ingold–Prelog rule application, priorities are assigned based on:
- Boiling point of substituents
- Atomic number of atoms directly attached to stereocentre
- Optical rotation magnitude
- Electronegativity only
Correct Answer: Atomic number of atoms directly attached to stereocentre
Q21. Which is a common chiral ligand used in asymmetric catalysis for hydride transfer?
- BINAP
- EDTA
- Triethylamine
- Water
Correct Answer: BINAP
Q22. Which process separates enantiomers using a chiral stationary phase?
- Achiral gas chromatography
- Chiral HPLC (chiral chromatography)
- Distillation under reduced pressure
- Simple filtration
Correct Answer: Chiral HPLC (chiral chromatography)
Q23. What is the formula for calculating enantiomeric excess (ee)?
- ee = (major − minor) / (major + minor) × 100%
- ee = major + minor
- ee = molecular weight difference
- ee = optical rotation × 2
Correct Answer: ee = (major − minor) / (major + minor) × 100%
Q24. Which reaction is typically stereospecific rather than stereoselective?
- Nucleophilic substitution SN2
- Free radical halogenation
- Racemization
- Photochemical cis–trans isomerization
Correct Answer: Nucleophilic substitution SN2
Q25. Chiral pool synthesis relies on:
- Starting from readily available chiral natural products
- Using only achiral reagents
- High temperature racemization
- Photochemical chiral induction
Correct Answer: Starting from readily available chiral natural products
Q26. Which catalyst class is commonly used in organocatalysis for asymmetric aldol reactions?
- Proline or secondary amines
- Transition metal complexes only
- Peroxides
- Aliphatic hydrocarbons
Correct Answer: Proline or secondary amines
Q27. For a racemic pharmaceutical mixture, which statement is generally true?
- Both enantiomers always have identical biological activity
- One enantiomer may be active while the other is less active or harmful
- Regulatory agencies do not require enantiomeric characterization
- Racemic mixtures are always preferred for synthesis
Correct Answer: One enantiomer may be active while the other is less active or harmful
Q28. Which reagent set is typically used in Sharpless asymmetric dihydroxylation?
- OsO4 with chiral ligands (AD-mix)
- LiAlH4 only
- PCC and base
- HCl and heat
Correct Answer: OsO4 with chiral ligands (AD-mix)
Q29. Optical purity is another term for:
- Isotopic enrichment
- Enantiomeric excess
- Molecular weight distribution
- Melting point depression
Correct Answer: Enantiomeric excess
Q30. Which mechanism typically leads to inversion of configuration at a stereocentre?
- SN1
- SN2
- Radical substitution with retention
- Pericyclic sigmatropic shift without inversion
Correct Answer: SN2
Q31. When using a chiral catalyst, the phenomenon where the chiral catalyst dictates product stereochemistry regardless of substrate chirality is called:
- Substrate-controlled selectivity
- Catalyst-controlled selectivity (reagent-controlled)
- Thermal selectivity
- Photochemical control
Correct Answer: Catalyst-controlled selectivity (reagent-controlled)
Q32. Which of the following is an example of enzymatic resolution?
- Using lipase to selectively hydrolyze one ester enantiomer
- Heating a racemate to racemize it
- Using NaBH4 to reduce a ketone nonspecifically
- Using silica gel to remove impurities
Correct Answer: Using lipase to selectively hydrolyze one ester enantiomer
Q33. Which descriptor indicates the measured direction of plane-polarized light rotation?
- R/S
- α (alpha) or +/− (specific rotation sign)
- CIP
- Bp (boiling point)
Correct Answer: α (alpha) or +/− (specific rotation sign)
Q34. In asymmetric induction, the existing stereocentre that influences formation of a new stereocentre is called:
- Prochiral centre
- Auxiliary centre
- Control or directing stereocentre
- Meso centre
Correct Answer: Control or directing stereocentre
Q35. Which technique provides information about enantiomer ratio by using chiral shift reagents in NMR?
- 2D COSY without reagents
- Chiral NMR with shift reagents
- Mass spectrometry
- Infrared spectroscopy
Correct Answer: Chiral NMR with shift reagents
Q36. Which is true about stereoselectivity and stereospecificity?
- Stereoselective implies one stereoisomer favored; stereospecific implies mechanism gives stereochemical outcome dependent on starting stereochemistry
- They mean the same thing
- Stereospecific means no stereochemistry involved
- Stereoselective requires complete inversion always
Correct Answer: Stereoselective implies one stereoisomer favored; stereospecific implies mechanism gives stereochemical outcome dependent on starting stereochemistry
Q37. Which chiral reagent pair is commonly used for enantioselective dihydroxylation at low temperature?
- KMnO4 and NaOH
- AD-mix-α or AD-mix-β (Sharpless AD)
- HBr and peroxide
- PCC and silica
Correct Answer: AD-mix-α or AD-mix-β (Sharpless AD)
Q38. What does the term prochiral mean?
- Already chiral and cannot become achiral
- A molecule that can become chiral in a single modification
- A racemate that cannot be resolved
- A chiral compound with two stereocentres
Correct Answer: A molecule that can become chiral in a single modification
Q39. Which factor does NOT affect enantioselectivity of a catalytic asymmetric reaction?
- Ligand structure
- Reaction temperature
- Solvent and additives
- Color of the reaction flask
Correct Answer: Color of the reaction flask
Q40. Which reagent is commonly used for enantioselective epoxidation of allylic alcohols developed by Sharpless?
- m-CPBA alone
- Ti(OiPr)4 with diethyl tartrate (DET) and t-BuOOH
- LiAlH4 and water
- PCl5 and base
Correct Answer: Ti(OiPr)4 with diethyl tartrate (DET) and t-BuOOH
Q41. What is the matching/mismatching effect in asymmetric synthesis?
- Interaction where substrate and chiral reagent either reinforce or oppose each other’s stereochemical preference
- Effect describing color changes in chiral solutions
- Term for temperature dependence only
- Only applies to achiral catalysts
Correct Answer: Interaction where substrate and chiral reagent either reinforce or oppose each other’s stereochemical preference
Q42. Which of the following best describes kinetic resolution?
- Both enantiomers react at the same rate
- One enantiomer reacts faster than the other, enabling separation
- It converts enantiomers into identical products
- It always gives 100% yield of pure enantiomer
Correct Answer: One enantiomer reacts faster than the other, enabling separation
Q43. Which measurement is directly proportional to the concentration and path length, used in polarimetry?
- Specific rotation [α]
- Refractive index only
- Optical rotation (observed) αobs
- pH
Correct Answer: Optical rotation (observed) αobs
Q44. Which statement is correct about asymmetric organocatalysis?
- It uses only metal-based chiral catalysts
- It employs small organic chiral molecules as catalysts without metals
- It cannot achieve high enantioselectivity
- It is limited to gas-phase reactions
Correct Answer: It employs small organic chiral molecules as catalysts without metals
Q45. What role does a chiral auxiliary play in asymmetric synthesis?
- Acts as a temporary chiral controller that is attached and removed later
- Permanently converts substrate to racemate
- Is used only as a solvent
- Provides thermal energy to the reaction
Correct Answer: Acts as a temporary chiral controller that is attached and removed later
Q46. Which is a common way to improve enantioselectivity in a catalytic asymmetric reaction?
- Optimize ligand structure and reaction conditions
- Ignore solvent effects
- Use excess achiral base only
- Run reaction at arbitrarily higher temperature
Correct Answer: Optimize ligand structure and reaction conditions
Q47. Which analytical method can separate enantiomers based on diastereomer formation with a chiral reagent followed by achiral analysis?
- Forming diastereomeric derivatives (e.g., Mosher esters) and analyzing by NMR or HPLC
- Direct IR analysis without derivatization
- Simple distillation
- Thin-layer chromatography without chiral reagents
Correct Answer: Forming diastereomeric derivatives (e.g., Mosher esters) and analyzing by NMR or HPLC
Q48. Which term describes the scenario when a single reagent converts different stereoisomeric substrates into different stereoisomeric products?
- Stereospecific reaction
- Stereoselective but nonspecific
- Racemization
- Isomerization only
Correct Answer: Stereospecific reaction
Q49. Which modern technique can provide chiroptical information used to assign absolute configuration besides X‑ray?
- Circular dichroism (CD) spectroscopy
- GC-FID without chiral column
- Simple UV-Vis without chiral analysis
- Melting point determination only
Correct Answer: Circular dichroism (CD) spectroscopy
Q50. Which strategy combines racemization of the undesired enantiomer with selective transformation to give high yield of one enantiomer?
- Kinetic resolution without racemization
- Dynamic kinetic resolution (DKR)
- Simple crystallization of the racemate
- Photochemical racemization only
Correct Answer: Dynamic kinetic resolution (DKR)
