1H and 13C NMR studies on 5,11-dimethyl-5 H-indolo[2,3-b]quinoline and some of its derivatives

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The 1H and 13C NMR spectra of the parent 5-methyl-5 H-indolo[2,3 b-quinoline] and 5,11-dimethyl-5 H-indolo[2,3 b]quinoline and nine various methyl, methoxy and fluoro derivatives of the latter have been measured and analysed by the use of COSY, HETCOR, COLOC and selective decoupling experiments. This gave unambiguous assignment of all signals in the spectra studied; proton-proton coupling constants across three, four and five bonds have also been determined. The most interesting result found concerns a very large increase in the shielding of carbon 1 upon introduction of a methyl group at C11. This effect is dramatically enhanced by subsequent substitution with a methoxy group at C2.