Substituted 9‑Anthraldehydes from Dibenzocycloheptanol Epoxides via Acid-Catalyzed Epoxide Opening/Semipinacol Rearrangement

Starting
from benzaldehyde derivatives, the corresponding dibenzocycloheptenol
could be prepared in five steps. Under both substrate (secondary vs
tertiary alcohol and the substituents on the aromatic ring(s)) and
condition control, the subsequent epoxidation and acid-catalyzed epoxide
opening/semipinacol rearrangement/aromatization afforded the corresponding
9-anthraldehydes in good yields, up to 88% over two steps. The presence
of the electron-withdrawing group(s) on the aromatic ring(s) suppressed
the rate of the epoxidation while the subsequent semipinacol rearrangement
step required heating; the presence of the electron-donating group(s),
on the other hand, frequently led to the decomposition during the
epoxidation. From the mechanistic studies, the semipinacol rearrangement
of the epoxide could precede the ionization at the bisbenzylic position,
yielding the aldehyde intermediate. The ensuing dehydrative aromatization
led to the formation of 9-anthraldehyde. Conversely, nucleophilic
addition to the aldehyde and dehydrative aromatization with concomitant
loss of formic acid led to anthracene.