Synthesis of Cyclobutanes

Question 1

What are the two ways you can synthesise cyclobutanes?

Answer 1

1) Irreversible cyclisation (cannot use reversible due to ring strain)
2) Via [2+2] cycloaddition

Question 2

[2+2] cycloadditions are thermally disallowed, meaning?

Answer 2

It only occuring under irradiation (i.e. under UV light)

Question 3

What is the product of this reaction?

Answer 3