Reaction of 1,4-phthalazinedione with furfural: formation of the [5,6]benza-3a,7a-diazaindane system via an unusual skeletal rearrangement

reaction: see text] Oxidation of phthalahydrazide (1) with lead tetraacetate in the presence of furfural (3a) in methylene chloride gives 5,6]benza-3a,7a-diaza-3-carboxylindane-4,7-dione-1-ene (7a) in 64% yield. 5-Methylfurfural (3b) also reacted similarly to give the product 7b in 46% yield. Reaction of phathalazine-1,4-dione with thiophene-2-carboxaldehyde gives N-(2-formylthiophene)phthalahydrazide.