A compound of formula I: or a pharmaceutically acceptable salt thereof, wherein: one of R 2' and R 3' is -L 1 -R x, and the other is hydrogen; L 1 is a covalent bond or a straight or branched C 1-6 aliphatic group, wherein one or more methylene groups are independently and optionally replaced by -NR a - or -O-; R x is selected from the group consisting of NH 2, guanidino, 4-7 membered optionally substituted saturated heterocyclic ring having 1-2 heteroatoms independently being nitrogen, and 5-6 membered heteroaromatic ring having 1-2 heteroatoms independently being nitrogen; each of R 3, R 4, R 5, R 6, R 7, R 8, R 4', R 5', and R 6' is independently selected from the group consisting of hydrogen, halogen, -CN, -CF 3, -OR, -NR 2, -NO 2, -COOR, -CONR 2, and -R; each R a is independently hydrogen or C 1-3 aliphatic; and each R is independently hydrogen or optionally substituted C 1-6 aliphatic; wherein suitable monovalent substituents on a substitutable carbon atom of an "optionally substituted" group are independently halogen; -(CH 2) 0-4 R ∘; -(CH 2) 0-4 OR ∘; -O(CH 2) 0-4 R ∘, -O-(CH 2) 0-4 C OR ∘; -(CH 2) 0-4 CH(OR ∘) 2; -(CH 2) 0-4 SR ∘; -(CH 2) 0-4 Ph, which may be substituted with R ∘; -(CH 2) 0-4 O(CH 2) 0-1 Ph which may be substituted with R ∘; -CH=CHPh, which may be substituted with R ∘; -(CH 2) 0-4 O(CH 2) 0-1 -pyridyl which may be substituted with R ∘; -NO 2; -CN; -N 3; -(CH 2) 0-4 N(R ∘) 2; -(CH 2) 0-4 N(R ∘)C R ∘; -N(R ∘)C R ∘; -(CH 2) 0-4 N(R ∘)C NR ∘ 2; -N(R ∘)C NR ∘ 2; -(CH 2) 0-4 N(R ∘)C OR ∘; -N(R ∘)N(R ∘)C R ∘; -N(R ∘)N(R ∘)C NR ∘ 2; -N(R ∘)N(R ∘)C OR ∘; -(CH 2) 0-4 C R ∘; -C R ∘; -(CH 2) 0-4 C OR ∘; -(CH 2) 0-4 C SR ∘; -(CH 2) 0-4 C OSiR ∘ 3; -(CH 2) 0-4 OC R ∘; -OC (CH 2) 0-4 SR- SC SR ∘; -(CH 2) 0-4 SC R ∘; -(CH 2) 0-4 C NR ∘ 2; -C NR ∘ 2; -C SR ∘; -SC SR ∘, -(CH 2) 0-4 OC NR ∘ 2; -C N(OR ∘)R ∘; -C C R ∘; -C CH 2 C R ∘; -C(NOR ∘)R ∘; -(CH 2) 0-4 SSR ∘; -(CH 2) 0-4 S 2 R ∘; -(CH 2) 0-4 S 2 OR ∘; -(CH 2) 0-4 OS 2 R ∘; -S 2 NR ∘ 2; -(CH 2) 0-4 S R ∘; -N(R ∘)S 2 NR ∘ 2; -N(R ∘)S 2 R ∘; -N(OR ∘)R ∘; -C NR ∘ 2; -P 2 R ∘; -P R ∘ 2; -OP R ∘ 2; -OP (OR ∘) 2; SiR ∘ 3; -(C 1-4 straight or branched alkylene)O-N(R ∘) 2; or -(C 1-4 straight or branched alkylene)C O-N(R ∘) 2, wherein each R ∘ is independently hydrogen, C 1-6 aliphatic, -CH 2 Ph, -O(CH 2) 0-1 Ph; wherein suitable substituents on a substitutable nitrogen of an "optionally substituted" group include -R †, -NR † 2, -C R †, -C OR †, -C C R †, -C CH 2 C R †, -S 2 R †, -S 2 NR † 2, -C NR † 2, -C NR † 2, or -N(R †)S 2 R †; wherein each R † is independently hydrogen, C 1-6 aliphatic, unsubstituted -OPh, or an unsubstituted 5-6-membered saturated, partially unsaturated, or aryl ring having 0-4 heteroatoms independently selected from nitrogen, oxygen, or sulfur. one of R 2' and R 3' is -L 1 -R x, and the other is hydrogen; L 1 is a covalent bond or a straight or branched C 1-6 aliphatic group, wherein one or more methylene groups are independently and optionally replaced by -NR a - or -O-; R x is selected from the group consisting of NH 2, guanidino, 4-7 membered optionally substituted saturated heterocyclic ring having 1-2 heteroatoms independently being nitrogen, and 5-6 membered heteroaromatic ring having 1-2 heteroatoms independently being nitrogen; each of R 3, R 4, R 5, R 6, R 7, R 8, R 4', R 5', and R 6' is independently selected from the group consisting of hydrogen, halogen, -CN, -CF 3, -OR, -NR 2, -NO 2, -COOR, -CONR 2, and -R; each R a is independently hydrogen or C 1-3 aliphatic; and each R is independently hydrogen or optionally substituted C 1-6 aliphatic; wherein suitable monovalent substituents on a substitutable carbon atom of an "optionally substituted" group are independently halogen; -(CH 2) 0-4 R ∘; -(CH 2) 0-4 OR ∘; -O(CH 2) 0-4 R ∘, -O-(CH 2) 0-4 C OR ∘; -(CH 2) 0-4 CH(OR ∘) 2; -(CH 2) 0-4 SR ∘; -(CH 2) 0-4 Ph, which may be substituted with R ∘; -(CH 2) 0-4 O(CH 2) 0-1 Ph which may be substituted with R ∘; -CH=CHPh, which may be substituted with R ∘; -(CH 2) 0-4 O(CH 2) 0-1 -pyridyl which may be substituted with R ∘; -NO 2; -CN; -N 3; -(CH 2) 0-4 N(R ∘) 2; -(CH 2) 0-4 N(R ∘)C R ∘; -N(R ∘)C R ∘; -(CH 2) 0-4 N(R ∘)C NR ∘ 2; -N(R ∘)C NR ∘ 2; -(CH 2) 0-4 N(R ∘)C OR ∘; -N(R ∘)N(R ∘)C R ∘; -N(R ∘)N(R ∘)C NR ∘ 2; -N(R ∘)N(R ∘)C OR ∘; -(CH 2) 0-4 C R ∘; -C R ∘; -(CH 2) 0-4 C OR ∘; -(CH 2) 0-4 C SR ∘; -(CH 2) 0-4 C OSiR ∘ 3; -(CH 2) 0-4 OC R ∘; -OC (CH 2) 0-4 SR- SC SR ∘; -(CH 2) 0-4 SC R ∘; -(CH 2) 0-4 C NR ∘ 2; -C NR ∘ 2; -C SR ∘; -SC SR ∘, -(CH 2) 0-4 OC NR ∘ 2; -C N(OR ∘)R ∘; -C C R ∘; -C CH 2 C R ∘; -C(NOR ∘)R ∘; -(CH 2) 0-4 SSR ∘; -(CH 2) 0-4 S 2 R ∘; -(CH 2) 0-4 S 2 OR ∘; -(CH 2) 0-4 OS 2 R ∘; -S 2 NR ∘ 2; -(CH 2) 0-4 S R ∘; -N(R ∘)S 2 NR ∘ 2; -N(R ∘)S 2 R ∘; -N(OR ∘)R ∘; -C NR ∘ 2; -P 2 R ∘; -P R ∘ 2; -OP R ∘ 2; -OP (OR ∘) 2; SiR ∘ 3; -(C 1-4 straight or branched alkylene)O-N(R ∘) 2; or -(C 1-4 straight or branched alkylene)C O-N(R ∘) 2, wherein each R ∘ is independently hydrogen, C 1-6 aliphatic, -CH 2 Ph, -O(CH 2) 0-1 Ph; wherein suitable substituents on a substitutable nitrogen of an "optionally substituted" group include -R †, -NR † 2, -C R †, -C OR †, -C C R †, -C CH 2 C R †, -S 2 R †, -S 2 NR † 2, -C NR † 2, -C NR † 2, or -N(R †)S 2 R †; wherein each R † is independently hydrogen, C 1-6 aliphatic, unsubstituted -OPh, or an unsubstituted 5-6-membered saturated, partially unsaturated, or aryl ring having 0-4 heteroatoms independently selected from nitrogen, oxygen, or sulfur.