10-Hydroxy-10,9-boroxarophenanthrenes: Versatile synthetic intermediates to 3,4-benzocoumarins and triaryls

10-Hydroxy-10,9-boroxarophenanthrenes were obtained as unexpected major products upon BBr₃-mediated O-demethylation of 2-methoxybiaryls. The formation likely proceeds via intramolecular electrophilic aromatic cyclization of a reactive dibromoaryloxyborane intermediate. Essentially quantitative yields of 10-hydroxy-10,9-boroxarophenanthrenes were also obtained from 2-hydroxybiaryl and BCl₃/AlCl₃ with use of a modified literature procedure. As synthetic intermediates, 10-hydroxy-10,9- boroxarophenanthrenes were efficiently converted to 3,4-benzocoumarins and triaryls through Pd-catalyzed CO insertion and Suzuki reaction.