Sulphur-substituted pyrrolo[3,4-b]quinolines: Synthesis, chemistry and antimicrobial activity

2,3-Dihydro-2-propyl-3-propylimino-9-thioxo-pyrrolo[3,4-b]quinolin-1-one reacts with an aliphatic primary, secondary or tertiary amine to forma 1:1 substrate:amine (thiolate) complex. It also readily undergoes S-alkylation (with diazomethane or with an alkyl halide), S-acetylation (with acetic anhydride), hydrogenolysis (Raney nickel) with removal of sulphur, and acid-catalysed hydrolysis (with selective replacement of the 3-propylimino function by oxygen). Two novel di-(pyrrolo[3,4-b] quinolinyl) sulphide reaction products are described and their structures established. Treatment of these 'dimers' and related 9-alkylthio-substituted pyrroloquinolines with an aliphatic amine provides a convenient access to 'simple' or 'mixed' 2-alkyl-9-alkylamino-3- alkylimino-pyrroloquinoline derivatives. Preliminary antimicrobial (in vitro) tests indicate that: (a) the weak antimicrobial activity of the aforementioned 9-thioxo-pyrroloquinoline substrate against three selected Gram-positive pathogens is significantly enhanced in its amine complexes, and by the presence of a 6-fluoro atom in the quinoline moiety, and (b) that substitution of the 4-oxo-function in 4-oxo-3-quinolinecarboxylic acids by an ethylimino group leads to a marked reduction in antimicrobial properties.