Chirality 3+4 Flashcards
What does the vastatin group do?
They are lipid lowering
What is the importance of more asymmetric carbons present in a molecule?
- The more stereoisomers are possible
- For a compound with n asymmetric carbon atoms there 2^n possible stereoisomers
What are meso isomers?
Miso compounds with 2 or more asymmetrical carbons and a plane of symmetry - they are achiral
State the importance of meso-DMSA.
- Used medically to chelate toxic metals in cases of metal poisoning
- Particularly in children as more sensitive
How can we identify stereoisomers with 2 chiral carbons?
- Look for non-superimposable mirror images - enantiomers
- Non-superimposable, not mirror images of the same structural formula - diastereoisomers
- Superimposable, has mirror plane - meso compound
State the properties of diasterisomers.
- Have the chemical reactivity as they have the same functional groups
- Different physical properties e.g. melting point, solubility
What Is meant by resolution?
The separation of enantiomers from wither racemic mixture or enantiomerically enriched mixture
What is chiral chromatography?
Normally HPLC or GC where racemic solution is passed over a chiral stationary phase giving rapid and reversible diastereotopic interaction
What enables the separation of diastereoisomers?
The different physical characteristics.
What can be used to separate diastereoisomers ?
– 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
What happens in a stereoselective reaction?
Preferential formation of one stereoisomer over another
What happens in a stereospecific reaction?
Each stereoisomeric reactant produces a different stereoisomeric product or a different set of products.
What happens if chiral molecule shows no stereoselectivity?
Results in racemisation
What happens if chiral molecule can show stereoselectivity?
- Invert stereochemistry or
- Conserve stereochemistry
- Preferentially react or form particular stereoisomers
What are stereochemical outcomes important in?
- Metabolism of drugs
- Drug action at receptors
- Toxicity through undesirable activity in stereoisomers
Describe a non-concerted reaction.
- 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
Describe a concerted reaction.
- 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
Where does non-concerted reaction of SN1 occur?
At sp3 hybridised carbons
Describe the steps in a non concerted Sn1 reaction.
- 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
Where does a concerted SN2 reaction occur?
sp3 hybridised carbons
Describe the steps in a concerted Sn2 reaction.
- 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
What is the advantage of active stereoisomers?
- They can bind better to target receptor to produce stronger effect, however, toxicity can also arise in this way
