Diels–Alder Versus Michael Adducts of 2‐Methylfuran and Maleate: Consecutive Synthetic Access to Both Product Types

Originally, the Diels–Alder reaction was not carried out, as one might assume, by Diels and Alder, but by Albrecht who reported this reaction already in 1906 and, thus, more than two decades earlier than Diels and Alder in their pioneer publication. However, Albrecht concluded a wrong structure for the product as he proposed a Michael adduct. In 1928, Diels and Alder then successfully identified the real structure and discovered the reaction class of [4 + 2] cycloaddition reactions, for which they were awarded Nobel Prize. Unexpectedly, we found in our experiments, for the conversion of 2‐methylfuran and a maleate, that there is a synthetic relationship of the wrongly proposed and real types of Diels–Alder products. In detail, the Michael adduct is formed under catalytic conditions from the real Diels–Alder product through rearrangement. Subsequent theoretical studies revealed that the wrongly proposed structure is even more stable than the real Diels–Alder product and accordingly such Albrecht product should be formed in a thermodynamically controlled reaction. Thus, our results also suggest that the original proposal of Albrecht from 1906—which was corrected by Diels and Alder in 1928—was not entirely unrealistic. Although this synthesized Michael adduct is not obtained directly from the diene and dienophile, it, ironically, is formed from the real Diels–Alder product via this type of rearrangement reaction.