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 using BuLi, Acid Chloride, LDA and a bromoalkene?

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 (other reagents: LDA + bromoalkene)

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