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 ( 1 H, 13 C, and 19 F) 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 ).