A compound having the structure or a pharmaceutically acceptable salt thereof, wherein R6 is hydrogen, &mdash；OH, &mdash；C1-6alkyl, &mdash；OC1-6alkyl, aryl, or halogen, or together with R7, forms an oxo group, or together with R7 and the atom to which they are attached, form a 5-7-membered carbocycle ring or a 5-7-membered heterocyclic ring having 1, 2, or 3 heteroatoms, wherein alkyl and aryl are unsubstituted or independently mono-, di-, or tri-substituted with R14； R7 is hydrogen, C1-6alkyl, &mdash；CF3, aryl, &mdash；O-aryl, &mdash；O&mdash；C1-6alkyl, &mdash；C(O)OC1-6alkyl, &mdash；C(R15R16)OH, a 5-7-membered heteroaryl having 1, 2, 3 or 4 nitrogen atoms, or halogen, or together with R6, forms an oxo group, or together with R6 and the atom to which they are attached, form a 5-7-membered carbocycle ring or a 5-7-membered heterocyclic ring having 1, 2, or 3 heteroatoms wherein alkyl, aryl and heteroaryl are unsubstituted or independently mono-, di- or tri-substituted with R14； R12 is hydrogen, C1-6alkyl, or &mdash；(CH2)1-2OH, or together with R13 and the nitrogen atom to which they are attached, form a 4- to 7-membered heterocyclic ring containing one nitrogen atom and 0 or 1 oxygen atoms, wherein said ring is unsubstituted or independently mono-, di- or tri-substituted with halogen or &mdash；C1-6alkyl； R13 is &mdash；C1-6alkyl or &mdash；(CH2)1-2OH, or together with R12 and the nitrogen atom to which they are attached, form a 4- to 7-membered heterocyclic ring containing one nitrogen atom and 0 or 1 oxygen atoms, wherein said ring is unsubstituted or independently mono-, di- or tri-substituted with halogen or &mdash；C1-6alkyl.
