Lecture 11 - Stereochemistry 4/4
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What is racemisation?
The process of making a racemic mixture.
Draw the two stereoisomers of Thalidomide - controversial drug for morning sickness used in the 60s. Which enantiomer caused birth defects and which was effective?
• S = birth defects • R = anti- sickness
Scientists tried to produce a drug containing only the effective R enantiomer of thalidomide. But, our body’s naturally racemise Thalidomide via keto - enol tautomerisation to produce a mixture of the two. Draw the keto enol tautomer of thalidomide
The keto enol tautomer of thalidomide does not contain a ……….. ………… . It is therefore not chiral, it is……. and contains a ……. …………
stereogenic centre achiral mirror plane
When the proton attacks the ex stereogenic centre that we see in the keto enol tautomer of thalidomide, it can be added from the front or the back of the molecule to produce…
the R/S enantiomer.
Give two compounds that can racemise. How do they do so?
thalidomide and ibuprofen. Keto-enol tautomerisation
Some enantiomers cannot racemise. Give an example.
citalopram.
What is citalopram? It is sold as a racemic mixture, which is the active and inactive enantiomer? What is the daily dose?
SSRI S = active R = inactive 20mg
Why is escitalopram more spenny than citalopram? What is daily dose?
S enantiomer has been isolated. 10mg
How can an enantiomer be active and one inactive? Did we not say enantiomers have identical physical properties?
Enantiomers in isolation have no physical differences. But, when there is interaction between different chiral; molecules we generate diastereomeric complexes/ diastereoisomers. Diastereoisomers- different physical properties. For example, one enantiomer would fit into an enzyme (also a chiral compound) and one would not. Different diastereoisomers.
Stereoisomers with one stereogenic centre…
produces an S and an R. These are enantiomers.
What labels are given to molecules with two stereogenic centres?
SS , RR , SR, RS
Out of SS, RR, SR and RS, which are the enantiomers ?
SS AND RR SR AND RS
Which are diastereoisomer out of RR, RS, SS, SR?
RR, RS RR, SR SR, SS RS, SS
What do we call it when diastereoisomers differ at only one stereocentre? eg. RRRS
epimers.
Can a molecule with 2 stereogenic centres be achiral?
Yes. There may be a mirror plane within the molecule- achiral