Ipobscurines C and D: macrolactam-type indole alkaloids from the seeds of Ipomoea obscura

• Published in: Phytochemistry (Oxford) 2003-04, Vol.62 (8), p.1257-1263
• Main Authors: Jenett-Siems, Kristina, Weigl, Robert, Kaloga, Macki, Schulz, Jutta, Eich, Eckart
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Chemical constitution; Chromatography, High Pressure Liquid; Convolvulaceae; Fundamental and applied biological sciences. Psychology; Hydroxycinnamic acid amide-type conjugates; Indole alkaloids; Indole Alkaloids - chemistry; Indole Alkaloids - isolation & purification; Ipobscurines; Ipomoea - chemistry; Ipomoea obscura; Lactams - chemistry; Lactams - isolation & purification; Neolignans; Nuclear Magnetic Resonance, Biomolecular; Plant physiology and development; Seeds - chemistry; Serotonin; Spectrometry, Mass, Electrospray Ionization; Spectrometry, Mass, Fast Atom Bombardment; Stereoisomerism
• ISSN: 0031-9422; 1873-3700
• DOI: 10.1016/S0031-9422(02)00756-2

Separation of the methanolic seed extract of Ipomoea obscura afforded five indole alkaloids, three of them (ipobscurines B-D) being new natural products of a unique structural type characterized as serotonin hydroxycinnamic acid amide-type conjugates with a second phenylpropanoid moiety forming an ether with the 5-OH position of the indole nucleus. Due to an oxidative phenolic coupling between the two phenylpropanoid moieties of the supposed precursor ipobscurine B two 21-membered macrolactams with a phenol ether partial structure are formed: the trans–cis isomers ipobscurines C and D. Their structures were established on the basis of spectral data. Moreover, total synthesis of the racemic erythro- and threo-ipobscurine B 4′,4″-dimethyl ethers and the comparison with the corresponding derivative of natural (−)-ipobscurine B proved an erythro configuration of the latter. Ipobscurines C and D are macrocyclic serotonin-neolignanoid-type lactams; structure elucidation was achieved by spectral data. The acyclic lactam B is a putative precursor; total synthesis of its 4,4′-dimethyl ether and corresponding derivatisation of natural B supported the structure and proved an erythro-configuration.