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The [2 + 4] cycloaddition products 4 are formed from 1 and 2 when the ammoniumyl salt 3 is added as initiator. The reaction is complete in only a few minutes at 0°C. In the absence of 3, compounds 1 and 2 only react at 80°C over two days, leading to [2 + 2] cycloadducts. An acid-catalyzed side reaction also produces cycloheptadienone. R = p-tolyl, p-anisyl.
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Schmittel et al. (1991) studied this question.
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