Oxocarbenium ions cycloaddition

Cycloaddition reactions of vinyl oxocarbenium ions with the formation of (9-heterocycles 03T2371. 

Formal inverse-demand [4 + 2] cycloaddition reaction of the in situ-generated cationic aryl 2-oxadiene oxocarbenium ions with electron-rich alkenes provides polysubstituted chromans in high yields. The diastereose-lectivity is dependent on the substitution pattern of the alkene generally... 

Reaction of cyclic acetals with alkynyl ethers promoted by BF3 gives unsaturated lactones in a process proposed to involve initial ketene intermediate 157 which reacts by an intramolecular [2 + 2] cycloaddition giving an in situ generated oxocarbenium ion and then ring expansion (Eqn (4.96)). Both larger and smaller unsaturated lactones can be prepared by the same procedure (Eqn (4.97)). ... 

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