[4+3] Oxyallyl Cycloaddition

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This a [4+3] cycloaddition of and 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 Diles-ALder reaction and produces a 7 membered ring.