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 BBr(3)-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 BCl(3)/AlCl(3) 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.