The 1,4-dicarbonyl compound 22 was prepared by an
oxidative coupling of the benzyl methyl ketone
19. Dehydration of 22 gave the
3,4-diarylfuran 16 which upon functional group
transformations
and a subsequent intramolecular cyclization afforded only the
anti isomer of the furanothiacyclophane 27. Ring contraction following a Wittig
rearrangement−Hofmann elimination sequence led
to the isolation of anti furanocyclophanene 15b.
Valence isomerization of 15b to the
isoannulenofuran 14b could be achieved either photochemically or
thermally with 15b as the thermodynamically more stable isomer. Compound 14b was found to
exhibit only a very small ring current. The
diatropicity of 14b is clearly affected by a weak
participation of the oxygen in π-electron
delocalization and a steric effect of its external methyl groups
resulting in a deviation from planarity
of its molecular periphery. The thermal conversion of
15b to 14b was determined to have a
high
activation energy of 114 kJ mol-1. This
thermal process, in addition to involving the disruption
of
π-electron delocalization in two benzene and one furan rings, is
another example of an unsual
concerted, symmetry-forbidden reaction.