Matthias Wenzel

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.

Octadehydromichellamine, a structural analog of the anti-HIV michellamines without centrochirality☆

Abstract The synthesis of octadehydromichellamine ( 4 ), as the fully dehydrogenated structural analog of the naturally occurring michellamines ( 1 ), is described. This derivative is the first michellamine-type quateraryl without centrochirality and thus constitutes a distinctly simplified structural michellamine analog. Key step of the total synthesis is the twofold coupling of a bis- O -triflate activated central binaphthalene building block 9 with 2 eq. of the isoquinoline boronic acid 8 , to give the quateraryl 11 , whose deprotection delivers the target molecule 4 , in an apparently stereochemically pure form. Octadehydromichellamine ( 4 ) shows a good order of anti-HIV activity and, compared with natural michellamines, enhanced antimalarial activity against Plasmodium falciparum .

Dioncophylline D and 8-O-Methyldioncophylline D, 7,8′-Coupled naphthylisoquinoline alkaloids from triphyophyllum PELTATUM

Abstract Theisolation and structural elucidation of two novel naphthylisoquinoline alkaloids, dioncophylline D and its 8- O -methyl analogue, from the West African liana Triphyophyllum peltatum are described. Their structures are based on the rare 7,8′-coupling between the two molecular moieties. The absolute configuration at the stereocentres of the isoquinoline half was established by oxidative degradation to give R -3-aminobutyric acid and d -alanine. Both alkaloids are characterized by a rapid rotation around the biaryl axis. The two new alkaloids are apparently found in young (carnivorous) plants, exclusively.

Labelled precursors for biosynthetic studies on naphthylisoquinoline alkaloids

The isotope labelled monocyclic ketones 5 and 8, postulated precursors to the presumably acetogenic naphthylisoquinoline alkaloids, have been synthesized for biogenetic experiments to Ancistrocladaceae and Dioncophyllaceae plants. Key step of the preparation of 1-(2′-[carbonyl-14C]acetyl-3′,5′-dibenzyloxyphenyl)-2-propanone ([14C]-13) is the C-acetylation of the arylpropanone 10 with the mixed pivalic acetic anhydride ([14C]-11). The resulting pyrylium salt [14C]-12, which is stable and can be stored, is cleaved directly before the feeding experiment to give the diketone [14C]-13 and deprotected to give the free phenolic target molecule [14C]-5. This synthetic route is applicable also to the prepation of 1-(2′-[13C2]acetyl-3′-hydroxyphenyl)-2-propanone ([13C2]-5) for biosynthetic experiments with NMR analysis. For the preparation of the oxygen-poorer 13C-labelled diketone 1-(2′-[methyl-13C]acetyl-3′-hydroxyphenyl)-2-propanone ([13C]-8), an ‘indanone-route’ has been elaborated.