The synthesis and biological evaluation of corosolic acid derivatives and related compounds as inhibitors of rabbit muscle glycogen phosphorylase a is described. Within this series of compounds, 8 (IC(50)=7.31 microM), 12d (IC(50)=3.26 microM), and 12e (IC(50)=5.1 microM) exhibited more potent activities than the parent compound 1 (IC(50)=20 microM). SAR of these compounds is also discussed.