Dearomatizing anionic cyclization and novel skeletal rearrangement: High yield formation of multiply substituted bicyclic or polycyclic spirocyclopentadienes and phenanthrene derivatives from 4-aryl 1-lithio-1,3-butadienes

Both 4-phenyl 1-lithio-1,3-butadienes and 4-naphthyl 1-lithio-1,3-butadienes underwent highly efficient and selective intramolecular nucleophilic addition of the butadienyllithium to the aromatic rings, resulting in full dearomatization of the phenyl rings and partial dearomatization of the naphthyl rings. When the reactions were carried out at lower temperatures, ipso intramolecular nucleophilic attack took place exclusively to afford the spirocyclopentadiene derivatives upon hydrolysis or further treatment with a variety of electrophiles. 4-Naphthyl 1-lithio-1,3-butadienes and 4-phenyl 1-lithio-1,3-butadienes were found to proceed in this reaction under similar conditions, with the former being faster even at -78 degrees C. However, when the reaction of 4-naphthyl 1-lithio-1,3-butadienes was carried out at higher temperatures, such as 75 degrees C, an interesting skeletal rearrangement took place to afford the vicinal attack products, tetrasubstituted phenanthrenes, via a dearomatization/rearomatization process. Mechanistic investigation revealed that the kinetically favored ipso attack intermediates might undergo thermal skeletal rearrangement via 1,2-alkyl shift.