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