Tricyclic compounds and their use as phosphodiesterase inhibitors

A compound of Formula I:

• or a pharmaceutically acceptable salt thereof, wherein:
• Ring A is a fused (6-membered)nitrogen-containing heteroaryl ring selected from the group consisting of pyridinyl, pyrazinyl, pyrimidinyl, and pyridazinyl, each of which is independently optionally substituted with one to three R8;
• R1 is selected from the group consisting of (C3-C6)cycloalkyl, (4- to 10-membered)heterocycloalkyl, (C6-C10)aryl, and (5- to 14-membered)heteroaryl, each of which is independently optionally substituted with one to three R9;
• R2 is selected from the group consisting of hydrogen, (C1-C6)alkyl, (C3-C8)cycloalkyl, (4- to 6-membered)heterocycloalkyl, and (5- to 6-membered)heteroaryl, wherein said (C1-C6)alkyl, (C3-C8)cycloalkyl, (4- to 6-membered)heterocycloalkyl, and (5- to 6-membered)heteroaryl are independently optionally substituted with one to three R8;
• R3a, where chemically permissible, is selected from the group consisting of hydrogen, halogen, oxo, cyano, hydroxy, —SF5, nitro, N(R5)(R6), (C1-C6)alkylthio, (C1-C6)alkyl, (C1-C6)alkoxy, and (C3-C8)cycloalkyl, wherein said (C1-C6)alkylthio, (C1-C6)alkyl, (C1-C6)alkoxy, and (C3-C8)cycloalkyl are independently optionally substituted with one to three substituents selected from the group consisting of halogen, oxo, cyano, hydroxy, —SF5, (C1-C6)alkylthio, nitro, —C(═O)—R5, and —N(R5)(R6);
• when present, R3b, where chemically permissible, is selected from the group consisting of hydrogen, halogen, oxo, cyano, hydroxy, —SF5, nitro, N(R5)(R6), (C1-C6)alkylthio, (C1-C6)alkyl, (C1-C6)alkoxy, and (C3-C8)cycloalkyl, wherein said (C1-C6)alkylthio, (C1-C6)alkyl, (C1-C6)alkoxy, and (C3-C8)cycloalkyl are independently optionally substituted with one to three substituents selected from the group consisting of halogen, oxo, cyano, hydroxy, —SF5, (C1-C6)alkylthio, nitro, —C(═O)—R5, and —N(R5)(R6);
• R4a is hydrogen;
• when present, R4b is hydrogen;
• R5 and R6 at each occurrence are each independently selected from the group consisting of hydrogen and (C1-C6)alkyl;
• R7 is (C1-C6)alkyl;
• when present, R8 at each occurrence is independently selected from the group consisting of halogen, oxo, cyano, hydroxy, —SF5, nitro, N(R5)(R6), (C1-C6)alkylthio, (C1-C6)alkyl, and (C1-C6)alkoxy, wherein said (C1-C6)alkylthio, (C1-C6)alkyl, and (C1-C6)alkoxy are independently optionally substituted with one to three substituents selected from the group consisting of halogen, oxo, cyano, hydroxy, —SF5, (C1-C6)alkylthio, nitro, —C(═O)—R5, and —N(R5)(R6).
• when present, R9 at each occurrence is independently selected from the group consisting of halogen, oxo, cyano, hydroxy, —SF5, nitro, (C1-C6)alkylthio, (C1-C6)alkyl, (C1-C6)alkoxy, N(R5)(R6), —S(O)2R7, and —S(O)2N(R5)(R6), wherein said (C1-C6)alkylthio, (C1-C6)alkyl, and (C1-C6)alkoxy are independently optionally substituted with one to three substituents selected from the group consisting of halogen, oxo, cyano, hydroxy, —SF5, (C1-C6)alkylthio, nitro, —C(═O)—R5, and —N(R5)(R6);
• ---- is absent (forming a single bond) and n is 1.