Preparation of cyclopentadienes and diazocyclopentadienesvia cyclopentenolones and cyclopentenones

Click Chemistry with Cyclopentadiene

Cyclopentadiene is one of the most reactive dienes in normal electron-demand Diels-Alder reactions. The high reactivities and yields of cyclopentadiene cycloadditions make them ideal as click reactions. In this review, we discuss the history of the cyclopentadiene cycloaddition as well as applications of cyclopentadiene click reactions. Our emphasis is on experimental and theoretical studies on the reactivity and stability of cyclopentadiene and cyclopentadiene derivatives.

From enolates to anthraquinones

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, Diels–Alder, cyclopentadienone, in situ.