The First Example of the Friedel-Crafts Cyclization Leading to (10-Hydroxy-9,10-dihydroanthr-9-yl)phosphonium Salts without the Expected Bradsher Dehydration.

The reaction of ( ortho- acetalaryl)arylmethanols with various phosphines PR 1 R 2 R 3 (R 1 = R 2 = R 3 = Ph; R 1 = R 2 = Ph, R 3 = Me and R 1 = R 2 = Me, R 3 = Ph) under acidic conditions (e.g., HCl, HBF 4 , TsOH) unexpectedly led to the formation of (10-hydroxy-9,10-dihydroanthr-9-yl)phosphonium salts instead of the corresponding anthryl phosphonium salts. The cyclization occurred according to the Friedel-Crafts mechanism but without the usually observed Bradsher dehydration, giving cyclic products in the form of cis/trans isomers and their conformers. In case of electron-rich and less-hindered dimethylphenylphosphine, all four stereoisomers were recorded in 31 P{ 1 H} NMR spectra, while for the other phosphines, only the two most stable cis / trans stereoisomers were detected. This study was supported by DFT and NCI calculations in combination with FT-IR analysis.