Synthesis and biological activity of substituted 2,4-diaminopyrimidines that inhibit Bacillus anthracis

► An improved synthesis of 2,4-diaminopyrimidine-based antibiotics is reported. ► Antibiotic synthesis includes a novel sealed-tube Heck reaction. ► The agents synthesized show activity in the μg/mL range. ► The size and planarity of the phthalazine C-1 substituent influences the activity. ► Steric and electronic factors affect the binding of the agents to DHFR.