[4+3] Oxyallyl Cycloaddition

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This a [4+3] cycloaddition of an oxyallyl cation with 1,3-butadiene. There are 4 atoms in the butadiene (and the usual four electrons and the oxyallyl cation has 3 atoms but only two electrons, making it a [4+3] cycloaddition. This reaction is concerted with a total of six electrons like a Diels-Alder reaction and produces a seven-membered ring.

M. A. Battiste, P. M. Pelphrey and D. L. Wright, Chem. – A Eur. J., 2006, 12, 3438–3447.