Revisit of the Dessy−White Intramolecular Acetylene−Acetylene [2 + 2] Cycloadditions

In this experiment, a series of thermal reactions of 4,4‘-disubstituted 2,2‘-bis(phenylethynyl)biphenyls
with 2,3,4,5-tetraphenylcyclopenta-2,4-dienone were carried out under neat conditions and in diphenyl
ether at temperatures between 260 and 270 °C to give rise to 9,10,11,12,13,14-hexaphenylcycloocta[l]phenanthrenes as the adducts in 12−23% yields. We traced these results to the intramolecular [2 + 2]
thermal cyclization of 2,2‘-bis(phenylethynyl)biphenyls to form 1,2-diphenylcyclobuta[l]phenanthrenes,
which were further trapped as bridged-ketone Diels−Alder adducts, followed by thermal decarbonylative
ring opening, which gave rise to the products.