A compound represented by formula or a pharmaceutically acceptable salt, wherein: R 1 and R 2 are independently selected from the group consisting of hydrogen, methyl and ethyl; R 1 and R 2 together can be =O; also, R 1 and R 2 together with the carbon atom connecting them can form a 3-membered carbon ring; A is selected from the group consisting of phenyl, R 3 -substituted phenyl, pyridinyl, R 4 -substituted pyridinyl, pyrimidinyl, R 5 -substituted pyrimidinyl, pyrrolyl, R 6 -substituted pyrrolyl, pyridazinyl, R 7 -substituted pyridazinyl, pyrazolyl, and R 8 -substituted pyrazolyl; R 3 is 1 to 5 substituents independently selected from the group consisting of halogen, (C 1 -C 3) alkyl, (C 2 -C 3) alkenyl, (C 3 -C 4) cycloalkyl, (C 1 -C 3) alkoxy, trifluoromethyl and cyano; R 4 is 1 to 4 substituents independently selected from the group consisting of halogen, (C 1 -C 3) alkyl, (C 2 -C 3) alkenyl, (C 3 -C 4) cycloalkyl, (C 1 -C 3) alkoxy, trifluoromethyl and cyano; R 5 is 1 to 3 substituents independently selected from the group consisting of halogen, (C 1 -C 3) alkyl, (C 2 -C 3) alkenyl, (C 3 -C 4) cycloalkyl, (C 1 -C 3) alkoxy, trifluoromethyl and cyano; R 6 is 1 to 4 substituents independently selected from the group consisting of (C 1 -C 3) alkyl, (C 2 -C 3) alkenyl and (C 3 -C 4) cycloalkyl; R 7 is 1 to 3 substituents independently selected from the group consisting of halogen, (C 1 -C 3) alkyl, (C 2 -C 3) alkenyl, (C 3 -C 4) cycloalkyl, (C 1 -C 3) alkoxy, trifluoromethyl and cyano; R 8 is 1 to 3 substituents independently selected from the group consisting of (C 1 -C 3) alkyl, (C 2 -C 3) alkenyl and (C 3 -C 4) cycloalkyl. R 1 and R 2 are independently selected from the group consisting of hydrogen, methyl and ethyl; R 1 and R 2 together can be =O; also, R 1 and R 2 together with the carbon atom connecting them can form a 3-membered carbon ring; A is selected from the group consisting of phenyl, R 3 -substituted phenyl, pyridinyl, R 4 -substituted pyridinyl, pyrimidinyl, R 5 -substituted pyrimidinyl, pyrrolyl, R 6 -substituted pyrrolyl, pyridazinyl, R 7 -substituted pyridazinyl, pyrazolyl, and R 8 -substituted pyrazolyl; R 3 is 1 to 5 substituents independently selected from the group consisting of halogen, (C 1 -C 3) alkyl, (C 2 -C 3) alkenyl, (C 3 -C 4) cycloalkyl, (C 1 -C 3) alkoxy, trifluoromethyl and cyano; R 4 is 1 to 4 substituents independently selected from the group consisting of halogen, (C 1 -C 3) alkyl, (C 2 -C 3) alkenyl, (C 3 -C 4) cycloalkyl, (C 1 -C 3) alkoxy, trifluoromethyl and cyano; R 5 is 1 to 3 substituents independently selected from the group consisting of halogen, (C 1 -C 3) alkyl, (C 2 -C 3) alkenyl, (C 3 -C 4) cycloalkyl, (C 1 -C 3) alkoxy, trifluoromethyl and cyano; R 6 is 1 to 4 substituents independently selected from the group consisting of (C 1 -C 3) alkyl, (C 2 -C 3) alkenyl and (C 3 -C 4) cycloalkyl; R 7 is 1 to 3 substituents independently selected from the group consisting of halogen, (C 1 -C 3) alkyl, (C 2 -C 3) alkenyl, (C 3 -C 4) cycloalkyl, (C 1 -C 3) alkoxy, trifluoromethyl and cyano; R 8 is 1 to 3 substituents independently selected from the group consisting of (C 1 -C 3) alkyl, (C 2 -C 3) alkenyl and (C 3 -C 4) cycloalkyl.