Submicromole structure elucidation: Cryptolepicarboline-a novel dimeric alkaloid from Cryptolepis sanguinolenta

Cryptolepis sanguinolenta (Lindl.) Schlechter (Asclepiadaceae), a shrub indigenous to West Africa, has been employed by traditional healers in the treatment of various fevers, including malaria. Column chromatography of the alkaloid fraction obtained from a defatted ethanolic extract of the roots of the plant afforded the previously characterized alkaloids cryptolepine, quindoline and cryptospirolepine. Continued extensive column chromatography, followed by preparative high‐performance liquid chromatography, resulted in the isolation of nine new alkaloids, including approximately 100 μg of a novel indoloquinoline‐β‐carboline dimeric alkaloid, cryptolepicarboline. The structure was elucidated through the use of a combination of spectroscopic techniques which included UV, Fourier transform, electron impact mass spectrometry, and 500 MHz NMR spectroscopy. NMR data included 1D 1H and 13C reference spectra, the latter recorded in 40 μl of DMSO‐d6 using a new Varian heteronuclear Nano‐probe and in 140 μl of DMSO‐d6 using a Nalorac carbon‐optimized microprobe. These 13C NMR spectra allow the first direct comparison of these two probe formats. Homonuclear 2D NMR data acquired included both COSY and ROESY spectra. The homonuclear 2D experiments were variously performed in both a homonuclear Nano‐probe and micro‐inverse detection probes. Heteronuclear shift correlation experiments included HMQC, HMBC and IDR (inverted direct response)‐HMQC‐TOCSY spectra, which were uniformly performed using a 500 MHz Nalorac micro inverse‐detection probe.