A compound of the formula: or a pharmaceutically acceptable salt thereof, wherein R 1 is selected from the group consisting of hydrogen, acetyl, alkyl, fluoroalkyl and cycloalkyl； each of R2 and R3 is independently selected from the group consisting of hydrogen and alkyl, or R2 and R3 taken together form a 3- to 6-membered ring； Ring A of the formula: is selected from the following: and in which: the dotted line represents an optional link； R4 and R5 are each independently selected from the group consisting of hydrogen and fluorine, with the proviso that R5 is present only when the bond represented by the dotted line is absent； R6 is selected from the group consisting of hydrogen, alkyl and fluorine； one of R7 and R8 is hydrogen； and the other of R7 and R8 is selected from the group consisting of: a) NR18R19； b) SR20 or O (C = O) N (R20) (R21), and c) NR22 (C = O) NR26R27, NR22 (C = O) CH (NR28R29) R30 or NR22 (C = O) C (OR23) R30R30b； and d) NR22SO2R31 or NR22SO2N (R22) (R23)； or R7 and R8 taken together with the carbon atom to which each is attached form a group of the formula - C = C (Ra) (Rb), in which each of Ra and Rb is independently selected from the group that consists of hydrogen, alkyl and cycloalkyl； each of R9 and R10 is independently selected from the group consisting of hydrogen, halogen, alkyl, and cyano； each of R11 and R12 is independently selected from the group consisting of hydrogen, heteroaryl, heterocycle, aryl, arylalkyl, aryloxy, arylcarbonyl, arylcarbonyloxy, arylalkoxy, alkylsulfonyl, arylsulfonyl and trifluoromethylsulfonyl； or one of R10 and R11 or R11 and R12 taken together with the atoms to which each is attached form an aromatic or heteroaromatic ring of 5 to 6 members； R13 is selected from the group consisting of alkyl, cycloalkyl, aryl and heteroaryl； R18 is hydrogen or C1-C6 alkyl and R19 is selected from the group consisting of aryl and heteroaryl, or R18 and R19 at each occurrence are taken together to form a 3- to 8-membered het
