Tropone is a mere ketone for cycloadditions to ketenes

Tropone (1) reacts with ketenes 2 to yield [8 + 2] cycloadducts, the gamma-lactones 3. The concerted [8 + 2] cycloaddition path is formally symmetry-allowed, but we established that it is unfavorable. Careful low-temperature NMR (H-1, C-13, and F-19) spectroscopies of the reaction of diphenyl ketene (2b) or bis(trifluoromethyl) ketene (2c) with tropone (1) allowed the direct detection of a beta-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 beta-lactones 5b and 5c were converted to the gamma-lactones A 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 beta-lactone 5c via the first zwitterionic intermediate. The P-lactone 5c is further isomerized to the product gamma-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).