The structure of the vat dye pyranthrone, a regioselective double Scholl Reaction product

The structure of the vat dye pyranthrone (2), first synthesized by Scholl in 1912, formed by the double Scholl reaction (AlCl 3 /NaCl melt) of 1,6-dibenzoylpyrene (1), was verified for the first time by 1 H- and 13 C-two-dimentional correlation NMR spectroscopy in nitrobenzene-d 5 at 150 °C. The analogous reaction of 1,8-dibenzoylpyrene (3) did not give the direct double Scholl reaction product, forming pyranthrone (2) instead. The reaction indicated the 1,8-dibenzoylpyrene (3) →1-benzoylpyrene →1,6-dibenzoylpyrene (1) Friedel-Crafts acyl rearrangement via monodeacylation. These Scholl reactions were not only regioselective but also stereoselective: pyranthrone (2) was formed from the less-stable diastereomer/conformer 1 E ,6 E -dibenzoylpyrene, not from the most stable diastereomer/conformer 1 Z ,6 Z -dibenzoylpyrene. Other products of these reactions were 8 H -dibenzo[ def,qr ]chrysene-8-one (9) and 11 H -indeno[ 2 , 1-a ]pyrene-11-one (10), formed from 1-benzoylpyrene by a direct mono-Scholl reaction.