A compound having the structure (I) or a pharmaceutically acceptable salt thereof, wherein R3 is hydrogen, deuterium, —OH, —OC1-6alkyl, or halogen; R8 is hydrogen, deuterium, or C1-6alkyl; R11 and R12 are independently hydrogen, —C1-6alkyl, —OH, —OC1-6alkyl, or halogen; R13 and R14 are independently —C1-6alkyl, —(CH2)1-2OH, or —OC1-6alkyl, or, together with 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 mono-, di- or tri-substituted with halogen or —C1-6alkyl; R15 is (CR1R2)nC(O)OR16, wherein n is 0, 1 or 2, —C(O)NHCH(R17)OR16, or —C(O)NHCH(R17)C(O)NHCH(R18)C(O)OR16; R16 is hydrogen, C1-6alkyl, or (CH2)1-2N+R19R20R21; R1, R2, R4, R5, R6, R7, R9, R10, R17, R18, R19, R20, and R21 are independently hydrogen or —C1-6alkyl; and stereoisomers thereof, and pharmaceutically acceptable salts thereof, and pharmaceutically acceptable salts of stereoisomers thereof.