Intramolecular Anionic Diels−Alder Reactions of 1-Aryl-4-oxahepta-1,6-diyne Systems in DMSO

Base-promoted cycloaddition reactions of 1-aryl- or 1-aryl-7-substituted-4-oxahepta-1,6-diyne
systems in DMSO have proven to involve an anionic intramolecular Diels−Alder process taking place even
at room temperature in spite of the reaction suffering from temporary disruption of aromaticity. Although
initially formed α-arylallenide anion can be protonated by DMSO, it can be back to the allenide anion probably
because of a small acidity difference between α-arylallene and DMSO. The α-arylallenide anion in
combination with the α-aryl substituent can constitute an anionic diene structure that undergoes the
intramolecular Diels−Alder reaction involving the C(6)-yne part, a very fast process probably because of
the increased HOMO−1 level of the anionic diene, as shown by DFT calculations. Diversified substituted
naphthalenes, benzofurans, phenanthrenes, and quinolines, including biaryl architectures, are available
from 4-oxahepta-1,6-diynes in a highly expeditious way.