Cyclization reactions of long-lived 10,10-dimethyl-9-phenylethynyl-9,10-dihydrophenanthren-9-yl cation

According to the NMR data, long-lived 10,10-dimethyl-9-phenylethynyl-9,10-dihydrophenanthren-9-ylium in acid medium undergoes cyclization whose direction is determined by the acidity of the medium. The cyclization in HSO₃F-SbF₅ superacid involves the aromatic ring in the phenanthrene core as nucleophilic component and yields 5a,6-dimethyl-4-phenyl-4,5,5a,6-tetrahydroacephenantrylene-4,6-bis(ylium). In trifluoroacetic acid the nucleophilic component is the side-chain phenyl group, and the cyclization product is neutral 8b,14b-dimethyl-8b,14b-dihydrobenzog]chrysen-10-yl trifluoroacetate. Both cyclization directions are observed in moderate-strength trifluoromethanesulfonic acid.