A compound of formula: or pharmaceutically acceptable salt thereof, wherein L 1 is -C(R 6 R 7)-O- and L 2 is -C(R 13 R 14)-O-, L 1 is -C(R 6 R 7)-O- and L 2 is -O-C(R 13 R 14)-, L 1 is -O-C(R 6 R 7)- and L 2 is -C(R 13 R 14)-O-, L 1 is -C(R 6 R 7)-K 1 -C(R 6 R 7)- and L 2 is -O-C(R 13 R 14)-, or L 1 is -O-C(R 6 R 7)- and L 2 is -C(R 13 R 14)-K 1 -C(R 13 R 14)-; Y 1 and Y 2 are each -O-; X 1 and X 3 are each independently OH, SH, OR 15, or SR 15; X 2 and X 4 are each O; R 1, R 2, R 3, R 5, R 8, R 9, R 11 and R 12 are each H R 4, and R 10 are each independently H, OH, F, Cl, Br, I, CN, N 3, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, OR 15, SR 15, or N(R 15) 2; R 6, R 7, R 13 and R 14 are each independently H, CN, N 3, F, Cl, Br, I, COOR 15, CON(R 15) 2, OR 15, SR 15, N(R 15) 2, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 7 cycloalkyl, C 2 -C 10 heterocycloalkyl, C 6 -C 10 aryl, or C 2 -C 10 heteroaryl; each R 15 is independently H, -C(=Z)R 16, -C(=Z)OR 16, -C(=Z)SR 16, -C(=Z)N(R 16) 2, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 7 cycloalkyl, C 2 -C 10 heterocycloalkyl, C 6 -C 10 aryl, or C 2 -C 10 heteroaryl; each R 16 is independently H, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 7 cycloalkyl, C 2 -C 10 heterocycloalkyl, C 6 -C 10 aryl, or C 2 -C 10 heteroaryl; each Z is independently O, S, or NR 15; K 1 is a variable that represents -O-, -S-,; Base 1 and Base 2 are each independently: and wherein the C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 2 -C 10 heterocycloalkyl, C 6 -C 10 aryl, or C 2 -C 10 heteroaryl independently in each instance is optionally substituted with 1, 2, or 3 -OH; -SH; -NH 2; =O; =NH; =S; halogen; -N 3; C 6 -C 10 aryl optionally substituted with 1, 2, or 3 -OH, -CN, -O(C=O)OR B, -O(C=O)R B, or -COOR B; unsubstituted C 1 -C 6 alkyl; unsubstituted C 1 -C 6 alkoxy; unsubstituted C 1 -C 6 alkylthio; unsubstituted C 1 -C 6 alkylamino; unsubstituted C 1 -C 6 dialkylamino; -CN; -O(C=O)OR B; -O(C=O)R B; or -COOR B; wherein R B is H or unsubstituted C 1 -C 6 alkyl. or pharmaceutically acceptable salt thereof, wherein L 1 is -C(R 6 R 7)-O- and L 2 is -C(R 13 R 14)-O-, L 1 is -C(R 6 R 7)-O- and L 2 is -O-C(R 13 R 14)-, L 1 is -O-C(R 6 R 7)- and L 2 is -C(R 13 R 14)-O-, L 1 is -C(R 6 R 7)-K 1 -C(R 6 R 7)- and L 2 is -O-C(R 13 R 14)-, or L 1 is -O-C(R 6 R 7)- and L 2 is -C(R 13 R 14)-K 1 -C(R 13 R 14)-; Y 1 and Y 2 are each -O-; X 1 and X 3 are each independently OH, SH, OR 15, or SR 15; X 2 and X 4 are each O; R 1, R 2, R 3, R 5, R 8, R 9, R 11 and R 12 are each H R 4, and R 10 are each independently H, OH, F, Cl, Br, I, CN, N 3, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, OR 15, SR 15, or N(R 15) 2; R 6, R 7, R 13 and R 14 are each independently H, CN, N 3, F, Cl, Br, I, COOR 15, CON(R 15) 2, OR 15, SR 15, N(R 15) 2, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 7 cycloalkyl, C 2 -C 10 heterocycloalkyl, C 6 -C 10 aryl, or C 2 -C 10 heteroaryl; each R 15 is independently H, -C(=Z)R 16, -C(=Z)OR 16, -C(=Z)SR 16, -C(=Z)N(R 16) 2, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 7 cycloalkyl, C 2 -C 10 heterocycloalkyl, C 6 -C 10 aryl, or C 2 -C 10 heteroaryl; each R 16 is independently H, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 7 cycloalkyl, C 2 -C 10 heterocycloalkyl, C 6 -C 10 aryl, or C 2 -C 10 heteroaryl; each Z is independently O, S, or NR 15; K 1 is a variable that represents -O-, -S-,; Base 1 and Base 2 are each independently: and wherein the C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 2 -C 10 heterocycloalkyl, C 6 -C 10 aryl, or C 2 -C 10 heteroaryl independently in each instance is optionally substituted with 1, 2, or 3 -OH; -SH; -NH 2; =O; =NH; =S; halogen; -N 3; C 6 -C 10 aryl optionally substituted with 1, 2, or 3 -OH, -CN, -O(C=O)OR B, -O(C=O)R B, or -COOR B; unsubstituted C 1 -C 6 alkyl; unsubstituted C 1 -C 6 alkoxy; unsubstituted C 1 -C 6 alkylthio; unsubstituted C 1 -C 6 alkylamino; unsubstituted C 1 -C 6 dialkylamino; -CN; -O(C=O)OR B; -O(C=O)R B; or -COOR B; wherein R B is H or unsubstituted C 1 -C 6 alkyl.