Phloroglucinol Derivatives as Potent Photosystem II Inhibitors

Studies on structure/activity relationships o f phloroglucinol derivatives tha t had been de­ signed based on the structures o f grandinol and hom ograndinol. potent photosystem II (PS II) inhibitors in Eucalyptus grandis, revealed th a t two electron-withdraw ing groups which differ by their electron-w ithdraw ing pow er on a phloroglucinol nucleus were essential for activity. A larger difference in the electron-w ithdraw ing pow er between the two groups enhanced the ac­ tivity, and 3-nitro-phloroglucinecarboxam ides and the corresponding thioam ides were the m ost active ones when they had p roper lipophilic side chains. Their binding dom ain seems to overlap with those o f D C M U and atrazine, w hereas they m ay approach to the site in a similar m anner to tha t o f phenol type inhibitors. A ccordingly, the phloroglucinol derivatives repre­ sent a new type o f PS II inhibitors.