Isolation and elucidation of the structure of homocryptolepinone

The structure of a new indolobenzazepine alkaloid, homocryptolepinone, isolated from extracts of the roots of the indigenous Ghanaian medicinal plant Cryptolepis sanguinolenta, is reported. The structure was determined using mass spectrometric, one‐dimensional nOe difference nmr, and inverse‐detected two‐dimensional nmr experiments which included HMQC, IDR‐(Inverted Direct Response)‐HMQC‐TOCSY, and HMBC experiments. The structure of homocryptolepinone is significant in that it may provide insight into the biogenesis of the benzpyrrolizinobenzazepine portion of the structure of the complex spiro nona‐cyclic alkaloid cryptospirolepine previously isolated in these laboratories from C. sanguinolenta, and which has no precedent in alkaloid chemistry.