Alkylideneallyl cations ring opening

The regioselectivity of ring opening of 4a-c is successfully controlled by the choice of reaction conditions. Lewis acid in dichloromethane and hydrogen chloride in alcohol are suitable conditions for generation of an alkylideneallyl cation, which was employed for studying its reactivity with nucleophiles such as... 

Product distribution in the reaction of 4 with furan depends on the reaction conditions as well as on the oxy group of the acetal substrates 4a-c. The diverse products formed in the reaction of 4a-c with furan are rationalized by the reaction pathways illustrated in Scheme 13. All products arise from nucleophilic addition of furan to alkylideneallyl cation intermediate 5M (5S), which is generated by acid-mediated ring opening of cyclopropanone acetals 4a-c (Scheme 5). The [4 + 3] cycloadduct 23 is simply formed via 27, and the furanyl... 

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