Synthesis of 8-azaprotosappanin A derivatives via intramolecular palladium-catalyzed ortho C-H activation/C-C cyclization and their antibacterial activity.

A novel synthetic protocol for the construction of eight-membered heterocycles by intramolecular palladium-catalyzed ortho C-H activation/C-C cyclization was proposed. With protosappanin A as the lead compound, 25 derivatives of 8-azaprotosappanin A were prepared in good yields by this protocol. Besides, a plausible reaction mechanism of the intramolecular cyclization was proposed. This strategy could be widely used in the synthesis of some natural products and drugs with large heterocycles due to the fast reaction rate and the mild conditions. In vitro antimicrobial activities of the synthesized compounds were assessed against the strains of Gram-positive bacteria and linezolid and ciprofloxacin were selected as the standard drugs. Some of the synthesized compounds were found to have excellent antibacterial activities.