Synthesis of 6-Membered rings

Question 1

What are the 5 ways you can synthesis 6-membered rings?

Answer 1

• Irreversible cyclisation
• Reversible cyclisation
• Robinson Annulation
• The Diels-Alder Reaction ([4+2] cycloaddition)
• Birch Reduction

Question 2

The following reaction is an example of a Robinson Annulation
What is the mechanism for the following reaction?

Answer 2

Question 3

The following reaction is an example of a Diels-Alder Reaction ([4+2] cycloaddition)
What is the mechanism for this reaction?
What happens to the spectrochemistry?

Answer 3

• The reaction is concerted where the dieneophile geometry is passed onto the product

Question 4

What conformation must the diene have for a Diels-Alder Reaction ([4+2] cycloaddition to occur)

Answer 4

the diene must be able to adopt the s-cis conformation
e.g. cyclopentadiene is very reactive in diels alder

Question 5

The following reaction is a Birch reduction
What is the mechanism for the following reaction?

Answer 5

Question 6

What is the following selectivity of the birch reaction with R as an electron withdrawing group

Answer 6

• as R stabilises a negative charge on the attached carbon within the transition state

Question 7

What is the following selectivity of the birch reaction with R as an electron donating group

Answer 7

Question 8

The following reaction below is an example of a way to make a 6-membered cyclic acetal
Whatt is the mechanism for the following reaction?
(Note: all step are reversible, so the more stable, thermodynamic, acetal is formed)

Answer 8

• The product has both group in the same plane (i.e. more energetically favourable to produce a product with both in axial) - left product