A compound of Structural Formula I: ** Formula ** or a pharmaceutically acceptable salt thereof, in which: X is selected from halo, -R, -OR, -SR, -S (O) mR, -N (R ) 2, -N (R) C (O) R, N (R) C (O) OR 'and N (R) S (O) mR', where: each R is independently selected from H, (C1) alkyl -C6), carbocyclyl or heterocyclyl； or two R groups taken together with the atom or atoms to which they are attached form a 4-7 membered non-aromatic heterocyclyl； and R 'is (C1-C6) alkyl, carbocyclyl or heterocyclyl； Ring A is selected from: ** Formula ** R1 is selected from hydrogen, -alkyl (C1-C8), -alkylene (C0-C6) -carbocyclyl, -alkylene (C0-C6) -heterocyclyl, -alkylene (C1 -C6) -O-(C1-C6) alkyl, -alkylene (C2-C6) -O-carbocyclyl, -alkylene (C2-C6) -O-heterocyclyl, -S (O) m-C1-C6 alkyl , -S (O) m-carbocyclyl, -S (O) m-heterocyclyl, -alkylene (C2-C4) -S (O) m-carbocyclyl, -alkylene (C2-C4) -S (O) m-heterocyclyl , -C (O) - [C (R4) (R4)] 0-4-N (R2) (R3), -C (O) -alkyl (C1-C6), -C (O) -heterocyclyl, - C (O) -carbocyclyl, -S (O) m- [C (R4) (R4)] 0-4-N (R2) (R3) and -S (O) m-alkylene (C1-C4) -carbocyclyl , -S (O) m-C 1 -C 4 alkylene-heterocyclyl； or R1 taken together with a ring atom adjacent to the nitrogen atom to which R1 is attached forms a saturated heterocyclic ring fused to ring A； R6a is selected from hydrogen and methyl； and R6 is selected from hydrogen, (C1-C4) alkyl optionally substituted with hydroxy or phenyl； or R6 taken together with R1 and the nitrogen atom and the carbon atom to which they are respectively bonded form a pyrrolidinyl or piperidinyl ring fused to ring A, wherein the pyrrolidinyl or piperidinyl ring is optionally substituted with -OH or -F； or R6 and R6a are taken together with the carbon atom to which both are linked to form a cyclopropyl ring； and R7a and R7b are each hydrogen or are taken together to form>； = O each of R2 and R3 is independently selected from hydrogen, (C1-C8) alkyl, -alkylene (C0-C6) -carbocyclyl, -alkylene (C0 -C6) -heterocyclyl, -alkylene
