Synthesis and Diatropicity of trans-2‘,5‘,10b,10c-Tetramethylfurano[3,4-e]-10b,10c-dihydropyrene. A Valence Isomerization To Form a Novel Isoannulenofuran at the Expense of Two Benzene and One Furan Rings

Abstract

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.

Synthesis and Diatropicity of trans-N-Cyclohexyl-2‘,5‘,10b,10c-tetramethyl- pyrrolo[3,4-e]-10b,10c-dihydropyrene. The First Example of an Iso[17]annulenopyrrole

“…(a) Synthesis. 2,5-Diarylfurans are known to undergo acid-catalyzed hydrolysis to afford the corresponding 1,4-diketones, which could be cyclized again to form either 2,5-diarylthiophenes or 2,5-diarylpyrroles . A potential route to the pyrrole-annelated thiacyclophane 15 , and perhaps the thiophene-annelated thiacyclophane 14 , was using the furan-annelated thiacyclophane 13 16 as a precursor.…”