Manuela Michel

Ancistrobertsonines b, C, and D as well as 1,2-didehydroancistrobertsonine D from Ancistrocladus robertsoniorum

Abstract From the East African Liana Ancistrocladus robertsoniorum , four new naphthylisoquinoline alkaloids have been isolated using High Speed Countercurrent Chromatography (HSCCC) and High Performance Liquid Chromatography (HPLC). Their stereostructures were established by spectroscopic, chemical, and chiroptical methods. Two of the new compounds, ancistrobertsonines B and C, constitute O - and N -permethylated 5,1′-coupled and 5,8′-coupled regioisomeric naphthylisoquinolines, while ancistrobertsonine D and its 1,2-didehydro analogue are based on a 7,1′-coupling mode between the isoquinoline and naphthalene moieties. Biological tests revealed that some of the isolated alkaloids possess moderate antimalarial activities.

Cis- and trans-isoshinanolone from Dioncophyllum thollonii: absolute configuration of two `known', wide-spread natural products

Abstract From the rare West African liana, Dioncophyllum thollonii (Dioncophyllaceae), the known acetogenic tetralones, cis - and trans -isoshinanolone, were isolated. Exemplarily on this material, a new ruthenium-catalyzed oxidative degradation procedure, related to the well-established stereoanalysis of 1,3-dimethyltetrahydroisoquinolines, was elaborated. The method allows to unambiguously attribute the absolute configuration of these natural products, which likewise occur in several other plant species. For the rapid discrimination between the four possible stereoisomeric forms of isoshinanolone (i.e. the cis - and trans -diastereomers and their enantiomers), an HPLC-analytical procedure on a chiral stationary phase has been developed.

First total synthesis of the mastigophorenes C and D and of simplified unnatural analogs

Abstract The first total synthesis of the mastigophorenes C (2) and D (3), natural ‘dimeric’ sesquiterpenes isolated from the liverwort Mastigophora diclados with interesting biological activities, is described. As previously for mastigophorenes A (1) and B, the divergent synthetic approach was first optimized on a simplified model system with a tert-butyl group instead of the chiral cyclopentyl residue, also in order to find more easily available compounds with similar or even improved biological activity.

Gentrymine B, an N-Quaternary Ancistrocladus Alkaloid: Stereoanalysis, Synthesis, and Biomimetic Formation from Gentrymine A 1

Abstract The total synthesis of the N-quaternary isoquinoline alkaloid gentrymine B (1) and of its unnatural enantiomer as well as its oxidative degradation is described. A further proof of stereostructure and hints at the biosynthetic origin of the unusual S,S-configuration in gentrymine B (1) were obtained by an additional biomimetic synthesis of 1 from the related—but 1R-configured—natural product gentrymine A (2), by N-methylation and subsequent spontaneous epimerization at C-1.

Determination of the absolute configuration of chiral benzylic alcohols and their esters or ethers, by ruthenium-mediated oxidative degradation

Abstract A sensitive analytical method for the reliable determination of the absolute configuration of chiral benzylic alcohols and the corresponding methyl ethers is described. After protection of the hydroxy function by acylation, they are degraded by Ru(VIII)-mediated oxidation, yielding chiral α-oxygenated carboxylic acids, whose stereoanalysis is achieved by GC–MS on a chiral phase. The method easily works down to 1 mg of the analyte.