Allyl cations, 2-methoxy cycloaddition reactions

Modes of cycloaddition of alkylideneallyl cation are also controlled by the reaction conditions. [4 + 3] Cycloaddition occurs in the reaction with furan. The [4 + 3] cycloaddition with furan was observed for the siloxy-substituted allyl cation 5S, but not for the methoxy-substituted allyl cation 5M. The lower electrophilicity of 5S may prefer the concerted pathway of [4 + 3] cycloaddition in competition with the stepwise pathway to yield a [3 + 2] cycloadduct and an electrophilic substitution product. 

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