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Cycloadditions involving (4n+2) pi electrons occur thermally, with the Diels-Alder being the prime example. How then could you make a five membered ring using six pi electrons?
The answer comes from using a 1,3-dipole (nitrone) which provides the four-electron component (compare diene). In the example shown, the nitrone oxygen provides 2 electrons from its lone pair and two come from the C=N double bond. Combine the two from the alkene component (dipolarophile) and you have got a five-membered transition state involving six pi electrons.
Richard Windsor – Undergraduate Final Year Project 2008