A compound having a structure according to formula I: in which the dashed lines between positions 2-3 and positions 3. 7a in formula I represent locations for two double bonds present when X1R1 is present； the dashed lines between positions 3-3a and positions 7. 1 in formula I represent locations for two double bonds present when X2R2 is present； X1 is absent, or is lower alkylene； X2 is absent, or is lower alkylene； in which only one of X1R1 and X2R2 is present； X3 is absent, or is lower alkylene, lower alkylidene or -NH-； X5 is absent, or is lower alkylene； R1 is selected from the group consisting of aryl, C3-C12 cycloalkyl or heterocyclyl, any of which is optionally substituted in one or more positions with halogen, lower alkyl, hydroxy or lower alkoxy； R2 is selected from the group consisting of aryl, C3-C12 cycloalkyl or heterocyclyl, any of which is optionally substituted in one or more positions with halogen, lower alkyl, hydroxy or lower alkoxy； R3 is R4 is CH-aryl, in which aryl is optionally substituted in one or more positions with hydroxy, lower alkyl, lower alkoxy or halogen； or CH-heterocyclyl, wherein heterocyclyl is optionally substituted in one or more positions with hydroxy, lower alkyl, lower alkoxy or halogen； R5 is hydrogen； hydroxy； lower alkyl； lower alkoxy； hydroxy-lower alkylene-； carboxy； alkoxycarbonyl； aryloxycarbonyl； aryl-alkoxycarbonyl； NHR10； -C (O) NR11R11a； -O-C (O) -R12； oxo； or -C (O) R13； R6 is absent, or is -CH (R6a) -； R6a is hydrogen； lower alkyl； or aryl optionally substituted with one or more of halogen, hydroxy, lower alkoxy, carboxy or alkoxycarbonyl； R7 is aryl optionally substituted with one or more hydroxy, halogen, lower alkyl, carboxy, alkoxycarbonyl, lower alkoxy, hydroxy-alkylene-, -NH (R6a), aryloxy, arylalkoxy or aryl-lower alkylene； C3-C12 cycloalkyl optionally substituted with one or more hydroxy, halogen, lower alkyl, lower alkyl-aminocarbonyl, carboxy, alkoxycarbonyl, lower alkoxy, lower alkoxy-lower alkylene-
