Tropone Is a Mere Ketone for Cycloadditions to Ketenes

Tropone (1) reacts with ketenes 2 to yield [8+2] cycloadducts, the γ‐lactones 3. The concerted [8+2] cycloaddition path is formally symmetry‐allowed, but we established that it is unfavorable. Careful low‐temperature NMR (1H, 13C, and 19F) spectroscopies of the reaction of diphenyl ketene (2b) or bis(trifluoromethyl) ketene (2c) with tropone (1) allowed the direct detection of a β‐lactone intermediates 5b,c and novel norcaradiene species 6b,c in head‐to‐head configurations. The [2+2] cycloadducts 5b,c equilibrated with the norcaradienes 6b,c. The β‐lactones 5b and 5c were converted to the γ‐lactones 3b and 3c, respectively, in quantitative yields. The DFT calculations showed that the concerted [8+2] cycloaddition is unfavorable. The first step of the calculated reaction 1+2c is a cycloaddition which leads to a dioxetane intermediate. This initial [2+2] cycloadduct is isomerized to the β‐lactone 5c via the first zwitterionic intermediate. The β‐lactone 5c is further isomerized to the product γ‐lactone 3c via the second zwitterion intermediate. Thus, 3c is not formed via the well‐established two‐step mechanism including zwitterionic intermediates but via a five‐step mechanism composed of a [2+2] cycloaddition and subsequent isomerization (Scheme 12).