Cycloadditions of allyl cations, 22. Allyl alcohols as precursors of allyl cations

Allyl alcohols such as 3‐ethoxy‐2‐methyl‐3‐buten‐2‐ol (4), the silylated alcohol 8 and also 2‐methyl‐3‐pentyn‐2‐ol (12) have been converted into the corresponding trifluoroacetates (5, 9, 13), which in the presence of zinc halide/ethyldiisopropylamine react with conjugated dienes to form bridged seven‐membered rings (14, 16, 18, 20–24) in good to excellent yields. Thus, unlike all other oxyallyl precursors studied so far, 5 gives the bridged 1‐ethoxy‐1,4‐cycloheptadiene (14, 16, 18, 20) under the present conditions rather than the thermodynamically more stable 4‐cyclohepten‐1‐one (15, 17, 19, 21) which is formed from the initial enol ether by hydrolysis. It is concluded that structurally complex allyl cations can now be generated in very mild conditions, and owing to the presence of ethyldiisopropylamine, even in weakly alkaline solutions.