A compound of formula I wherein R 1 is hydrogen, halogen, C 1-6 alkyl, C 1-6 alkylamino or C 1-6 alkoxy; R 2 is hydrogen; halogen; C 1-6 alkyl, which is unsubstituted or once, twice or three times substituted by fluoro; C 1-6 alkoxy, which is unsubstituted or once, twice or three times substituted by fluoro; cyano; C 3-7 cycloalkyl; hydroxy or phenyl-C x H 2x -O-; R 3 is hydrogen; halogen; C 1-6 alkyl, which is unsubstituted or once, twice or three times substituted by fluoro; cyano; pyrrolidinyl; phenyl-C x H 2x - N (C 1-6 alkyl)-; C 1-6 alkoxycarbonylpiperazinyl; or R 7 -O-, wherein R 7 is hydrogen; C 1-6 alkyl, which is unsubstituted or substituted with one to three substituents independently selected from fluoro, hydroxy and C 2-6 alkenyl; C 1-6 alkoxyC 1-6 alkyl; C 1-6 alkoxyC 1-6 alkoxyC 1-6 alkyl; aminoC 1-8 alkyl; C 1-6 alkylcarbonylaminoC 1-8 alkyl; C 1-6 alkylsulfonylaminoC 1-8 alkyl; C 1-6 alkylsulfanylC 1-6 alkyl; C 1-6 alkylsulfonylC 1-6 alkyl; cyanoC 1-6 alkyl; C 3-7 cycloalkylC 1-6 alkyl; cyanoC 3-7 cycloalkylC 1-6 alkyl; phenylC 1-6 alkyl; pyrrolidinylcarbonylC 1-6 alkyl; C 2-6 alkynyl; hydroxyC 1-6 alkylC 2-6 alkynyl; aminoC 1-6 alkoxyC 1-6 alkyl; C 1-6 alkylaminoC 1-6 alkoxyC 1-6 alkyl; diC 1-6 alkylaminoC 1-6 alkoxyC 1-6 alkyl; carboxyC 1-6 alkyl; or C 1-6 alkoxycarbonylaminoC 1-8 alkyl; heteroarylC 1-6 alkyl, wherein heteroaryl is N -containing monocyclic heteroaryl; or heterocycloalkylC 1-6 alkyl, wherein heterocycloalkyl is monocyclic heterocycloalkyl; R 4 is hydrogen, halogen, C 1-6 alkyl, cyano or C 1-6 alkoxy; provided that R 1, R 2, R 3 and R 4 are not hydrogen simultaneously; R 5 is hydrogen or C 1-6 alkyl; R 6 is hydrogen; C 1-6 alkyl, which is unsubstituted or once, twice or three times substituted by fluoro; C 3-7 cycloalkyl, which is unsubstituted or once, twice or three times substituted by fluoro or C 1-6 alkyl; or phenyl-C x H 2x -; x is 1-6; with the proviso that 6-methyl-2-oxo-9-pyrrolidin-1-yl-6,7-dihydrobenzo[a]quinolizine-3-carboxylic acid, 9-fluoro-6-methyl-2-oxo-6,7-dihydrobenzo[a]quinolizine-3-carboxylic acid, and 9,10-difluoro-6-methyl-2-oxo-6,7-dihydrobenzo[a]quinolizine-3-carboxylic acid are excluded; or pharmaceutically acceptable salts, or enantiomers thereof. R 1 is hydrogen, halogen, C 1-6 alkyl, C 1-6 alkylamino or C 1-6 alkoxy; R 2 is hydrogen; halogen; C 1-6 alkyl, which is unsubstituted or once, twice or three times substituted by fluoro; C 1-6 alkoxy, which is unsubstituted or once, twice or three times substituted by fluoro; cyano; C 3-7 cycloalkyl; hydroxy or phenyl-C x H 2x -O-; R 3 is hydrogen; halogen; C 1-6 alkyl, which is unsubstituted or once, twice or three times substituted by fluoro; cyano; pyrrolidinyl; phenyl-C x H 2x - N (C 1-6 alkyl)-; C 1-6 alkoxycarbonylpiperazinyl; or R 7 -O-, wherein R 7 is hydrogen; C 1-6 alkyl, which is unsubstituted or substituted with one to three substituents independently selected from fluoro, hydroxy and C 2-6 alkenyl; C 1-6 alkoxyC 1-6 alkyl; C 1-6 alkoxyC 1-6 alkoxyC 1-6 alkyl; aminoC 1-8 alkyl; C 1-6 alkylcarbonylaminoC 1-8 alkyl; C 1-6 alkylsulfonylaminoC 1-8 alkyl; C 1-6 alkylsulfanylC 1-6 alkyl; C 1-6 alkylsulfonylC 1-6 alkyl; cyanoC 1-6 alkyl; C 3-7 cycloalkylC 1-6 alkyl; cyanoC 3-7 cycloalkylC 1-6 alkyl; phenylC 1-6 alkyl; pyrrolidinylcarbonylC 1-6 alkyl; C 2-6 alkynyl; hydroxyC 1-6 alkylC 2-6 alkynyl; aminoC 1-6 alkoxyC 1-6 alkyl; C 1-6 alkylaminoC 1-6 alkoxyC 1-6 alkyl; diC 1-6 alkylaminoC 1-6 alkoxyC 1-6 alkyl; carboxyC 1-6 alkyl; or C 1-6 alkoxycarbonylaminoC 1-8 alkyl; heteroarylC 1-6 alkyl, wherein heteroaryl is N -containing monocyclic heteroaryl; or heterocycloalkylC 1-6 alkyl, wherein heterocycloalkyl is monocyclic heterocycloalkyl; R 4 is hydrogen, halogen, C 1-6 alkyl, cyano or C 1-6 alkoxy; provided that R 1, R 2, R 3 and R 4 are not hydrogen simultaneously; R 5 is hydrogen or C 1-6 alkyl; R 6 is hydrogen; C 1-6 alkyl, which is unsubstituted or once, twice or three times substituted by fluoro; C 3-7 cycloalkyl, which is unsubstituted or once, twice or three times substituted by fluoro or C 1-6 alkyl; or phenyl-C x H 2x -; x is 1-6; with the proviso that 6-methyl-2-oxo-9-pyrrolidin-1-yl-6,7-dihydrobenzo[a]quinolizine-3-carboxylic acid, 9-fluoro-6-methyl-2-oxo-6,7-dihydrobenzo[a]quinolizine-3-carboxylic acid, and 9,10-difluoro-6-methyl-2-oxo-6,7-dihydrobenzo[a]quinolizine-3-carboxylic acid are excluded; or pharmaceutically acceptable salts, or enantiomers thereof.