Design, synthesis and biological evaluation of betulinic acid derivatives as potential inhibitors of 3CL-protease of SARS-CoV-2

•Betulinic acid derivatives show significant inhibitory activity against SARS-CoV-2 3CLpro.•The oxidation of the hydroxyl group at C-3 and esterification of the carboxylate group at C-28 can improve the inhibitory activity.•The inhibition rates of BA02 and BA05 are 78.1% and 82.5%, respectively.