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Expect the unexpected: Classic organic chemistry continues to hold surprises. Cyclopentadiene and simple α-alkylated acroleins, in the presence of AlCl3 and Et2AlCl, furnish unconventional [4+3] cycloadducts (see scheme). The “expected” Diels–Alder adducts are intermediates in the reaction.
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Nieß et al. (2004) studied this question.
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