A series of novel podophyllotoxin derivatives containing structural modifications at C-4 (7– 14), C-4′(16–17), and the methylenedioxy A-ring (23–28) was synthesized and tested for inhibition of HIV replication. Four of these compounds (25–28) were previously reported to show EC50 values of< 0.001 μg/mL and therapeutic index (TI) values> 120. Three of the newly tested compounds (8, 12, and 20) showed good activity with EC50 values of 0.012, ...