A series of highly reactive cyclopentadienones were prepared in situ from the corresponding hydroxycyclopent-2-enones and trapped with a variety of quinones. Reaction of 1,4-naphthoquinone with 4-hydroxy-3,4-diphenyl-cyclopent-2-enone afforded 2,3-diphenylanthraquinone, whereas reaction of benzoquinone with this same cyclopentadienone precursor yielded a mixture of 6,7-diphenyl-1,4-naphthoquinone and 2,3,6,7-tetraphenyl anthraquinone. A number of other 2,3-diarylanthraquinones were likewise prepared in moderate yields from the reaction of 1,4-naphthoquinone with the appropriate 4-hydroxy-3,4-diarylcyclopent-2-enones. This method appears to be generally applicable to the synthesis of anthraquinone derivatives substituted at the 2- and 3-positions from inexpensive starting materials.Key words: anthraquinone, DielsAlder, cyclopentadienone, in situ.
The cycloaddition-ring transformation reaction sequence of pyrido[1,2-a]pyrazines with substituted naphthoquinones furnished a series of new highly substituted azaanthraquinones. Whereas monosubstituted naphthoquinones were normally leading to two regioisomeric products, in some cases a preference for only one regioisomer was observed. The amino derivative 3b which was isolated as the main product proved to be suitable for further modifications at the primary amino group. The derivatives obtained possess groups capable of connecting the molecule with other substructures for applications as functional dyes. The newly synthesized azaquinones show strong and very broad absorptions between 400 and 600 nm in their UV/Vis spectra