A compound having structural Formula I: or a pharmaceutically acceptable salt thereof wherein: C* indicates a potential chiral carbon atom; X is O, or NH; R 1 is phenyl unsubstituted or substituted by 1 to 3 halo, (C 1-6)alkyl unsubstituted or substituted by (C 1-3)alkyl, halo(C 1-6)alkyl, (C 3-7)cycloalkyl unsubstituted or substituted by 1 to 2 halo, 5- or 6-membered heteroaryl containing 1, 2, or 3 heteroatoms independently selected from N, O and S, wherein the heteroaryl is unsubstituted or substituted by 1 to 2 halo, (C 1-3)alkyl, or halo(C 1-3)alkyl, -(C 1-3)alkyl-(C 3-7)cycloalkyl, wherein the cycloalkyl is unsubstituted or substituted by 1 to 2 (C 1-3)alkyl, or halo, -(C 1-3)alkyl-phenyl, wherein phenyl is unsubstituted or substituted by 1 to 3 halo, or -(C 1-3)alkyl-heterocyclyl containing 1 or 2 heteroatoms independently selected from N and O, wherein the heterocyclyl is a 4-7 membered ring; R 2 is (C 1-3)alkyl, or (C 3-7)cycloalkyl; R 3 is phenyl unsubstituted or substituted by 1 to 2 halo, five- or six-membered heteroaryl containing 1, 2, or 3 heteroatoms independently selected from N, O and S, wherein heteroaryl is unsubstituted or substituted by 1 to 2 halo, or (C 1-6)alkyl; and R 4 is hydrogen, (C 1-3)alkyl, halo(C 1-3)alkyl, or (C 3-6)cycloalkyl. C* indicates a potential chiral carbon atom; X is O, or NH; O, or NH; R 1 is phenyl unsubstituted or substituted by 1 to 3 halo, (C 1-6)alkyl unsubstituted or substituted by (C 1-3)alkyl, halo(C 1-6)alkyl, (C 3-7)cycloalkyl unsubstituted or substituted by 1 to 2 halo, 5- or 6-membered heteroaryl containing 1, 2, or 3 heteroatoms independently selected from N, O and S, wherein the heteroaryl is unsubstituted or substituted by 1 to 2 halo, (C 1-3)alkyl, or halo(C 1-3)alkyl, -(C 1-3)alkyl-(C 3-7)cycloalkyl, wherein the cycloalkyl is unsubstituted or substituted by 1 to 2 (C 1-3)alkyl, or halo, -(C 1-3)alkyl-phenyl, wherein phenyl is unsubstituted or substituted by 1 to 3 halo, or -(C 1-3)alkyl-heterocyclyl containing 1 or 2 heteroatoms independently selected from N and O, wherein the heterocyclyl is a 4-7 membered ring; phenyl unsubstituted or substituted by 1 to 3 halo, (C 1-6)alkyl unsubstituted or substituted by (C 1-3)alkyl, halo(C 1-6)alkyl, (C 3-7)cycloalkyl unsubstituted or substituted by 1 to 2 halo, 5- or 6-membered heteroaryl containing 1, 2, or 3 heteroatoms independently selected from N, O and S, wherein the heteroaryl is unsubstituted or substituted by 1 to 2 halo, (C 1-3)alkyl, or halo(C 1-3)alkyl, -(C 1-3)alkyl-(C 3-7)cycloalkyl, wherein the cycloalkyl is unsubstituted or substituted by 1 to 2 (C 1-3)alkyl, or halo, -(C 1-3)alkyl-phenyl, wherein phenyl is unsubstituted or substituted by 1 to 3 halo, or -(C 1-3)alkyl-heterocyclyl containing 1 or 2 heteroatoms independently selected from N and O, wherein the heterocyclyl is a 4-7 membered ring; R 2 is (C 1-3)alkyl, or (C 3-7)cycloalkyl; (C 1-3)alkyl, or (C 3-7)cycloalkyl; R 3 is phenyl unsubstituted or substituted by 1 to 2 halo, five- or six-membered heteroaryl containing 1, 2, or 3 heteroatoms independently selected from N, O and S, wherein heteroaryl is unsubstituted or substituted by 1 to 2 halo, or (C 1-6)alkyl; and phenyl unsubstituted or substituted by 1 to 2 halo, five- or six-membered heteroaryl containing 1, 2, or 3 heteroatoms independently selected from N, O and S, wherein heteroaryl is unsubstituted or substituted by 1 to 2 halo, or (C 1-6)alkyl; and R 4 is hydrogen, (C 1-3)alkyl, halo(C 1-3)alkyl, or (C 3-6)cycloalkyl. hydrogen, (C 1-3)alkyl, halo(C 1-3)alkyl, or (C 3-6)cycloalkyl.