Chirality 3+4

Question 1

What does the vastatin group do?

Answer 1

They are lipid lowering

Question 2

What is the importance of more asymmetric carbons present in a molecule?

Answer 2

• The more stereoisomers are possible
• For a compound with n asymmetric carbon atoms there 2^n possible stereoisomers

Question 3

What are meso isomers?

Answer 3

Miso compounds with 2 or more asymmetrical carbons and a plane of symmetry - they are achiral

Question 4

State the importance of meso-DMSA.

Answer 4

• Used medically to chelate toxic metals in cases of metal poisoning
• Particularly in children as more sensitive

Question 5

How can we identify stereoisomers with 2 chiral carbons?

Answer 5

• Look for non-superimposable mirror images - enantiomers
• Non-superimposable, not mirror images of the same structural formula - diastereoisomers
• Superimposable, has mirror plane - meso compound

Question 6

State the properties of diasterisomers.

Answer 6

• Have the chemical reactivity as they have the same functional groups
• Different physical properties e.g. melting point, solubility

Question 7

What Is meant by resolution?

Answer 7

The separation of enantiomers from wither racemic mixture or enantiomerically enriched mixture

Question 8

What is chiral chromatography?

Answer 8

Normally HPLC or GC where racemic solution is passed over a chiral stationary phase giving rapid and reversible diastereotopic interaction

Question 9

What enables the separation of diastereoisomers?

Answer 9

The different physical characteristics.

Question 10

What can be used to separate diastereoisomers ?

Answer 10

– Use a solvent in which one diastereoisomer is very soluble,
the other is less soluble
– Both diastereoisomers dissolve
– The less soluble one crystallises out and is filtered
– Evaporate the solvent to get the other pure
diastereoisomer

Question 11

What happens in a stereoselective reaction?

Answer 11

Preferential formation of one stereoisomer over another

Question 12

What happens in a stereospecific reaction?

Answer 12

Each stereoisomeric reactant produces a different stereoisomeric product or a different set of products.

Question 13

What happens if chiral molecule shows no stereoselectivity?

Answer 13

Results in racemisation

Question 14

What happens if chiral molecule can show stereoselectivity?

Answer 14

• Invert stereochemistry or
• Conserve stereochemistry
• Preferentially react or form particular stereoisomers

Question 15

What are stereochemical outcomes important in?

Answer 15

• Metabolism of drugs
• Drug action at receptors
• Toxicity through undesirable activity in stereoisomers

Question 16

Describe a non-concerted reaction.

Answer 16

• The bond breaking and making
  occurs in different steps
• Intermediates are formed: C+, C* or C- (depending on reactant)
• Intermediates allow changes to stereochemistry, e.g. SN1, E1,
  some electrophilic/radical additions to alkenes
• Racemisation results if intermediate formed

Question 17

Describe a concerted reaction.

Answer 17

• All bond making and breaking happens in the same step
• Stereochemistry is usually defined, e.g. SN2, E2 reaction
• Stereochemistry either inverted or conserved in concerted reactions
• Biological systems, such as enzymes, generally prefer
  reactions to produce defined products

Question 18

Where does non-concerted reaction of SN1 occur?

Answer 18

At sp3 hybridised carbons

Question 19

Describe the steps in a non concerted Sn1 reaction.

Answer 19

• Intermediate carbocation formed: planar
  – Stereochemical definition lost
• Reactant forms new bond to C
  – Can attack planar intermediate from above or below plane equally easily
  – 3D tetrahedral carbon re-formed with 1:1 mix of enantiomers
• No stereoselectivity

Question 20

Where does a concerted SN2 reaction occur?

Answer 20

sp3 hybridised carbons

Question 21

Describe the steps in a concerted Sn2 reaction.

Answer 21

• Polarised bond between C and heteroatom (Br) causes slight
  positive charge (d+) on C making it electrophilic
• Nucleophile attacks electrophilic Cd+, breaks existing polarised C-Br
  bond as new bond is made – one step reaction
• No intermediate formed
• Only one product, with stereochemistry defined by attack of
  nucleophile horizontally opposed to leaving group

Question 22

What is the advantage of active stereoisomers?

Answer 22

• They can bind better to target receptor to produce stronger effect, however, toxicity can also arise in this way