Four-Membered Rings Photochemistry in Synthesis

Background Needed for this Chapter Reference to Clayden, Organic Chemistry Chapter 35 Pericyclic reactions I Cycloadditions. 

So special reactions are often used to make cyclobutanes. In the next chapter we shall see that thermal cycloadditions of alkenes with ketenes give four-membered rings, but the commonest method is photochemical cycloaddition. You are already aware that Diels-Alder reactions (chapter 17) occur easily when a diene 6 and a dienophile 7 are heated together and six-membered rings 8 are formed. Have you ever wondered why four-membered rings 9 are not formed instead Orbital symmetry allows cycloadditions involving six Ti-electrons but not those involving four 7r-electrons.2 

The 2 + 2 cycloadditions do occur in the excited state so these are photochemical reactions.3 They work best if one component (usually an enone) absorbs the light to form the excited state 

Organic Synthesis The Disconnection Approach. Second Edition Stuart Warren and Paul Wyatt 2008 John Wiley Sons, Ltd 

Most cyclobutanes offer a choice between two 2 + 2 disconnections and the choice can often be made by considering the availability of the starting materials. We already know from 11 that compounds like this can be made by irradiation of ethylene with enones, here 15, so we shall focus on the alternative 16b. The starting material for an intramolecular photo-cycloaddition would be dienone 17. It was decided to make this by oxidation of 18 because this alcohol was easy to make.7 

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