Assignment of the proton and carbon NMR spectra of the indoloquinoline alkaloid cryptolepine

The relatively rare alkaloid cryptolepine, 5‐methyl‐5 H ‐indolo[3,2‐ b ]quinoline, was isolated from Cryptolepis sanguinolenta (Lindl.) Schlecter ( Asclepiadaceae ). Unequivocal proton and carbon nmr assignments in dimethyl sulfoxide are reported based on two‐dimensional nmr methods, including COSY, inverse‐detected direct (HMQC), and long‐range (HMBC) correlations. Several of the assignments were confirmed using one‐dimensional SIMBA spectra ‐ a new, selective, one‐dimensional analogue of the proton‐detected long‐range heteronuclear shift correlation experiment (HMBC). The SIMBA experiment was also used in an attempt to observe a two‐bond coupling from the H11 proton to C10a at several optimizations.