A compound of formula (I): ** Formula ** or a pharmaceutically acceptable salt thereof, wherein R1, R2, R4, R5 and R6 are independently selected from the group consisting of hydrogen, alkenyl, alkyl, haloalkyl, G1 and - (CR4aR5a) m-G1； R4a and R5a, each time they appear, are each independently hydrogen, halogen, alkyl or haloalkyl； G1, each time it appears, is independently aryl or heteroaryl, wherein each G1 is independently substituted or unsubstituted with 1, 2, 3, 4 or 5 substituents selected from the group consisting of alkyl, alkenyl, alkynyl, halogen, cyano, -G3, -NO2, -OR1b, -O- (CR4bR5b) m-G3, -OC (O) R1b, -OC (O) N (Rb) (R3b), -SR1b, - S (O) R2b , -S (O) 2R2b, -S (O) 2N (Rb) (R3b), -C (O) R1b, -C (O) OR1b, -C (O) N (Rb) (R3b), -C (OH) [(CR4bR5b) m-R4b] 2, -N (Rb) (R3b), - N (Ra) C (O) R1b, -N (Ra) C (O) O (R1b), -N ( Ra) C (O) N (Rb) (R3b), - (CR4bR5b) m-G3, - (CR4bR5b) m-NO2, - (CR4bR5b) m-OR1b, - (CR4bR5b) m-OC (O) R1b, - (CR4bR5b) m-OC (O) N (Rb) (R3b), - (CR4bR5b) m-SR1b, - (CR4bR5b) mS (O) 2R2b, - (CR4bR5b) m- S (O) 2N (Rb) (R3b), - (CR4bR5b) mC (O) R1b, - (CR4bR5b) mC (O) OR1b, - (CR4bR5b) mC (O) N (Rb) (R3b), - (CR4bR5b) mN (Rb) (R3b ), - (CR4bR5b) mN (Ra) C (O) R1b, - (CR4bR5b) mN (Ra) C (O) O (R1b), - (CR4bR5b) mN (Ra) C (O) N (Rb) ( R3b), cyanoalkyl, and haloalkyl； Ra and Rb, each time they appear, are each independently hydrogen, alkyl or haloalkyl； R1b and R3b, each time they appear, are each independently hydrogen, alkyl or haloalkyl； R2b, each time it appears, is independently alkyl or haloalkyl； R4b and R5b, each time they appear, are each independently hydrogen, halogen, alkyl or haloalkyl； m, each time it appears, is independently 1, 2, 3, 4 or 5； G3, each time it appears, is independently aryl or heteroaryl, wherein each G3 is independently substituted or unsubstituted with 1, 2, 3, 4 or 5 substituents selected from the group consisting of alkyl, alkenyl, alkynyl, halogen, cyano, -NO2, -OR1b, -OC (O) R1b, -OC (O) N (Rb) (R3b), -SR1b, -S (
