A compound of Formula or Formula: or a pharmaceutically acceptable salt thereof, wherein: Z is oxygen, sulfur, or N-H; L 1 is or L 2 is or R 1 is hydrogen or -C R x; R 2 is hydrogen or -C R y; R x and R y are each independently selected from cycloalkyl, heteroaryl, heterocyclyl, alkoxy, and alkyl, said alkyl being substituted by one or more substituents independently selected from aryl, alkenyl, cycloalkyl, heterocyclyl, heteroaryl, -OR 3, -C OR 4, -NR a R b, and -C NR c R d, wherein any said aryl and heteroaryl may optionally be substituted with one or more substituents independently selected from alkyl, haloalkyl, arylalkyl, heterocyclyl, heterocyclylalkyl, halogen, cyano, nitro, -C OR 4, OR 5, -NR a R b, (NR a R b) alkyl, and (MeO) P O-, and wherein any said cycloalkyl and heterocyclyl may optionally be fused onto an aromatic ring and may optionally be substituted with one or more substituents independently selected from alkyl, hydroxyl, halogen, aryl, -NR a R b, oxo, and -C OR 4; R 3 is hydrogen, alkyl, or arylalkyl; R 4 is alkyl or arylalkyl; R 5 is hydrogen, alkyl, or arylalkyl; R a and R b are independently selected from hydrogen, alkyl, cycloalkyl, arylalkyl, heteroaryl, -C R 6, -C OR 7, -C NR c R d, and (NR c R d) alkyl, or alternatively, R a and R b, together with the nitrogen atom to which they are attached, form a five- or six-membered ring or bridged bicyclic ring structure, wherein said five- or six-membered ring or bridged bicyclic ring structure optionally may contain one or two additional heteroatoms independently selected from nitrogen, oxygen, and sulfur and may contain one, two, or three substituents independently selected from C 1 to C 6 alkyl, C 1 to C 4 haloalkyl, aryl, hydroxyl, C 1 to C 6 alkoxy, C 1 to C 4 haloalkoxy, and halogen; R 6 is alkyl; R 7 is alkyl, arylalkyl, cycloalkyl, or haloalkyl; and R c and R d are independently selected from hydrogen, alkyl, arylalkyl, and cycloalkyl. Z is oxygen, sulfur, or N-H; L 1 is or L 2 is or R 1 is hydrogen or -C R x; R 2 is hydrogen or -C R y; R x and R y are each independently selected from cycloalkyl, heteroaryl, heterocyclyl, alkoxy, and alkyl, said alkyl being substituted by one or more substituents independently selected from aryl, alkenyl, cycloalkyl, heterocyclyl, heteroaryl, -OR 3, -C OR 4, -NR a R b, and -C NR c R d, wherein any said aryl and heteroaryl may optionally be substituted with one or more substituents independently selected from alkyl, haloalkyl, arylalkyl, heterocyclyl, heterocyclylalkyl, halogen, cyano, nitro, -C OR 4, OR 5, -NR a R b, (NR a R b) alkyl, and (MeO) P O-, and wherein any said cycloalkyl and heterocyclyl may optionally be fused onto an aromatic ring and may optionally be substituted with one or more substituents independently selected from alkyl, hydroxyl, halogen, aryl, -NR a R b, oxo, and -C OR 4; R 3 is hydrogen, alkyl, or arylalkyl; R 4 is alkyl or arylalkyl; R 5 is hydrogen, alkyl, or arylalkyl; R a and R b are independently selected from hydrogen, alkyl, cycloalkyl, arylalkyl, heteroaryl, -C R 6, -C OR 7, -C NR c R d, and (NR c R d) alkyl, or alternatively, R a and R b, together with the nitrogen atom to which they are attached, form a five- or six-membered ring or bridged bicyclic ring structure, wherein said five- or six-membered ring or bridged bicyclic ring structure optionally may contain one or two additional heteroatoms independently selected from nitrogen, oxygen, and sulfur and may contain one, two, or three substituents independently selected from C 1 to C 6 alkyl, C 1 to C 4 haloalkyl, aryl, hydroxyl, C 1 to C 6 alkoxy, C 1 to C 4 haloalkoxy, and halogen; R 6 is alkyl; R 7 is alkyl, arylalkyl, cycloalkyl, or haloalkyl; and R c and R d are independently selected from hydrogen, alkyl, arylalkyl, and cycloalkyl.