A compound of formula: or a pharmaceutically acceptable salt thereof, wherein: A 1, A 2 and A 3 are each independently CH or N; L 1 is a linker selected from -O- and -CH 2 -; L 2 is absent or a -(C 1 -C 6)alkylene- linker; wherein said -(C 1 -C 6)alkylene- linker is optionally substituted with one to three groups selected from -OH, halo, -(C 1 -C 6)alkyl; D is a ring selected from (a) -(C 3 -C 7)cycloalkyl, (C 6 -C 10)aryl, and -(5- to 11-membered)heteroaryl; (b) -(4- to 11-membered)heterocycloalkyl, comprising an O or S ring atom and optionally 1 to 3 additional ring heteroatoms selected from N, O, and S; (c) a 6 to 11-membered fused bicyclic, bridged bicyclic or spirocyclic heterocyclic radical comprising a N ring atom and optionally 1 to 3 additional ring heteroatoms selected from N, O, and S; and (d) a group selected from 2-oxo-pyrrolidin-1-yl, 2-oxo-pyrrolidin-3-yl, 2-oxo-pyrrolidin-5-yl, 1-methyl-2-oxo-pyrrolidin-4-yl, and 2-oxo-piperidin-5-yl wherein each of said D rings is optionally substituted with one to three R 1 groups; and wherein each of said D rings is further optionally substituted, where possible, by one or two groups independently selected from (=O) and (=S); each R 1 is independently selected from halo, -OH, -CF 3, -CN, -(C 1 -C 6)alkyl, -O(C 1 -C 6)alkyl, -C R 2, - C OR 2, -C N(R 2) 2, -N(R 2) 2, -N(R 2)C R 2, -S 2 R 2, -N(R 2)-S 2 -R 2, -(C 3 -C 6)cycloalkyl, -(5- to 11-membered)heterocycloalkyl, -(C 6 -C 10)aryl, and 0-(5- to 11-membered)heteroaryl; wherein each of said, -(C 1 -C 6)alkyl, -O(C 1 -C 6)alkyl, -(C 3 -C 6)cycloalkyl, -(5- to 11-membered)heterocycloalkyl, -(C 6 -C 10)aryl, and -(5- to 11-membered)heteroaryl of said R 1 group is optionally substituted with one to three groups selected from halo, -OH, -CF 3, -(C 1 -C 6)alkyl, -C OH, -C OC 1 -C 6)alkyl, -C (C 1 -C 6)alkyl), -NH 2, -NH(C 1 -C 6)alkyl, N((C 1 -C 6)alkyl) 2 and -CN; each R 2 is independently selected from the group consisting of -H, -(C 1 -C 6)alkyl, -(C 3 -C 6)cycloalkyl, -(5- to 11-membered)heterocycloalkyl, -(C 6 -C 10)aryl, and -(5- to 11-membered)heteroaryl; wherein each of said, -(C 1 -C 6)alkyl, -O(C 1 -C 6)alkyl, -(C 3 -C 6)cycloalkyl, -(5- to 11-membered)heterocycloalkyl, -(C 6 -C 10)aryl, and -(5- to 11-membered)heteroaryl of said R 2 group is optionally independently substituted by one to three groups selected from halo, -OH, -CF 3, -(C 1 -C 6)alkyl, -NH 2, -NH(C 1 -C 6)alkyl, -N((C 1 -C 6)alkyl) 2 and -CN. A 1, A 2 and A 3 are each independently CH or N; L 1 is a linker selected from -O- and -CH 2 -; L 2 is absent or a -(C 1 -C 6)alkylene- linker; wherein said -(C 1 -C 6)alkylene- linker is optionally substituted with one to three groups selected from -OH, halo, -(C 1 -C 6)alkyl; D is a ring selected from (a) -(C 3 -C 7)cycloalkyl, (C 6 -C 10)aryl, and -(5- to 11-membered)heteroaryl; (b) -(4- to 11-membered)heterocycloalkyl, comprising an O or S ring atom and optionally 1 to 3 additional ring heteroatoms selected from N, O, and S; (c) a 6 to 11-membered fused bicyclic, bridged bicyclic or spirocyclic heterocyclic radical comprising a N ring atom and optionally 1 to 3 additional ring heteroatoms selected from N, O, and S; and (d) a group selected from 2-oxo-pyrrolidin-1-yl, 2-oxo-pyrrolidin-3-yl, 2-oxo-pyrrolidin-5-yl, 1-methyl-2-oxo-pyrrolidin-4-yl, and 2-oxo-piperidin-5-yl wherein each of said D rings is optionally substituted with one to three R 1 groups; and wherein each of said D rings is further optionally substituted, where possible, by one or two groups independently selected from (=O) and (=S); (a) -(C 3 -C 7)cycloalkyl, (C 6 -C 10)aryl, and -(5- to 11-membered)heteroaryl; (b) -(4- to 11-membered)heterocycloalkyl, comprising an O or S ring atom and optionally 1 to 3 additional ring heteroatoms selected from N, O, and S; (c) a 6 to 11-membered fused bicyclic, bridged bicyclic or spirocyclic heterocyclic radical comprising a N ring atom and optionally 1 to 3 additional ring heteroatoms selected from N, O, and S; and (d) a group selected from 2-oxo-pyrrolidin-1-yl, 2-oxo-pyrrolidin-3-yl, 2-oxo-pyrrolidin-5-yl, 1-methyl-2-oxo-pyrrolidin-4-yl, and 2-oxo-piperidin-5-yl wherein each of said D rings is optionally substituted with one to three R 1 groups; and wherein each of said D rings is further optionally substituted, where possible, by one or two groups independently selected from (=O) and (=S); each R 1 is independently selected from halo, -OH, -CF 3, -CN, -(C 1 -C 6)alkyl, -O(C 1 -C 6)alkyl, -C R 2, - C OR 2, -C N(R 2) 2, -N(R 2) 2, -N(R 2)C R 2, -S 2 R 2, -N(R 2)-S 2 -R 2, -(C 3 -C 6)cycloalkyl, -(5- to 11-membered)heterocycloalkyl, -(C 6 -C 10)aryl, and 0-(5- to 11-membered)heteroaryl; wherein each of said, -(C 1 -C 6)alkyl, -O(C 1 -C 6)alkyl, -(C 3 -C 6)cycloalkyl, -(5- to 11-membered)heterocycloalkyl, -(C 6 -C 10)aryl, and -(5- to 11-membered)heteroaryl of said R 1 group is optionally substituted with one to three groups selected from halo, -OH, -CF 3, -(C 1 -C 6)alkyl, -C OH, -C OC 1 -C 6)alkyl, -C (C 1 -C 6)alkyl), -NH 2, -NH(C 1 -C 6)alkyl, N((C 1 -C 6)alkyl) 2 and -CN; each R 2 is independently selected from the group consisting of -H, -(C 1 -C 6)alkyl, -(C 3 -C 6)cycloalkyl, -(5- to 11-membered)heterocycloalkyl, -(C 6 -C 10)aryl, and -(5- to 11-membered)heteroaryl; wherein each of said, -(C 1 -C 6)alkyl, -O(C 1 -C 6)alkyl, -(C 3 -C 6)cycloalkyl, -(5- to 11-membered)heterocycloalkyl, -(C 6 -C 10)aryl, and -(5- to 11-membered)heteroaryl of said R 2 group is optionally independently substituted by one to three groups selected from halo, -OH, -CF 3, -(C 1 -C 6)alkyl, -NH 2, -NH(C 1 -C 6)alkyl, -N((C 1 -C 6)alkyl) 2 and -CN.