Asymmetric synthesis 1

Question 1

The following molecule has no defined chemistry
i.e. it produces a racmic mixture
What are the two enantiomers?

Answer 1

Question 2

What is the formula for enantiomeric excess?

Answer 2

ee = % major enantiomer - % minor enatiomer
(i.e. 80%-20% = 60%ee)

Question 3

This following reaction uses Mosher’s acid chloride to produce a resolution of racemic alcohols or amines
(X = NH₂ or OH)

Answer 3

The diastereoisomers can be separated by chromotography/fractional distilliation etc

Question 4

Once we have seperated out the diasteroisomers, how do we reform our spectrospecific product
X= NH, O

Answer 4

Question 5

What is the core advantage and disadvantage of obtaining a single enantiomer through Mosher’s acid chloride

Answer 5

• Advantage: Simple, cheap (100% enantiomerically pure)
• Disadvantage: max 50% yield (more often than not do you get this much tho)

Question 6

We can use CBS reagent to produce enantiomerically pure ketone reductions
(L=large R group, S=small R group)
What is the product

Answer 6

• hydride is delivered to top face
• up to 95% ee if small group is v small and large group is v large
• CBS is a chiral reagent

Question 7

What is the intermediate formed within this reaction using CBS to form an enantiomerically pure product?

Answer 7

Question 8

With a chiral auxiliary, we use the addition of temporary stereochemistry to enable diasteroselective reactions, before the temporary stereochemical element to give enantioenriched products
How do we forms Evans Oxazolidinone?

Answer 8

• up to 99% de (diasteromeric excess) - which can be increased

Question 9

Evans chiral enolate alkylation is another example of Chiral auxiliaries
How do you form Evans chiral enolate reagent?

Answer 9

Question 10

Once we have formed Evans chiral enolate, how does the reaction proceed

Answer 10

• This allows for the production of the opposite sterogenic centre to be made
• Up to 99% de

Question 11

An issue with using Evans Chiral auxiliaries is it needs removing once the reaction has occured
Using the compound on the right, describe how LiAlH₄ can be used to remove the auxiliary group?

Answer 11

• Benefit of this mean Auxiliary is simple to install and gives a diasteroselective reaction
• And is easy to remove to give enatiomerically enriched product

Question 12

What is the other way we can remove an auxiliary after it has reacted

Answer 12