Otmar Hofer

Beiträge zur Chemie der Pyrrolpigmente, 18. Mitt.: Pyrromethenone—Partialstrukturen von Gallenpigmenten: Struktur und Eigenschaften in Lösung

Depending on solvent polarity and concentration a monomer—dimer equilibrium for pyrromethenones was deduced by means of absorption and fluorescence spectroscopy and measurement of a colligative property. The monomer could be characterized as asyn-(Z) form with a dihedral angle at the methine single bond of about fourty degrees using the lanthanide shift technique and nuclearOverhauser effect studies. Two types of dimer geometry with differing bond strengths were found by comparingPPP calculation results with the measured absorption spectra and the liquid crystal induced circular dichroism.The (Z) isomers can be transformed to the corresponding (E) isomers by means of a photoreaction. The latter is thermally stable but rather sensitive against oxygen, light and protons, an effect which varies strongly with the substitution pattern of the pyrrole fragment thus enabling isolation in certain cases only.

New unsymmetrically substituted tetrahydrofurofuran lignans from artemisia absinthium: Assignment of the relative stereochemistry by lanthanide induced chemical shifts

Abstract The isolation of thirteen tetrahydrofurofuran lignans from the roots of A. absinthium and six closely related species is reported. A new group of four lignans, all stereoisomers of 1- [(3,4-methylenedioxy-5-methoxy) phenyl]-4-(3,4,5-trimethoxyphenyl)-tetrahydro-1H,3H-furo[3,4-c]furan, was found. The compounds, named sesartemin, episesartemin A, episesartemin B, and diasesartemin, were characterized by 1 H NMR (including lanthanide induced shifts), UV, IR and MS. A fifth new lignan of the sesamin type (the eq/eq isomer of fargesin) could be identified as a minor constituent. The relative configurations of two further products (fargesin and epiashantin) could be confirmed by the lanthanide induced shift technique.

Tryptamine derived amides from Clausena indica

Abstract Leaf extracts from different Sri Lankan provenances of Clausena indica were compared by HPLC linked with diode array detection. Remarkable chemical differences towards different types of amides were observed between the collections from the northern dry semi-evergreen forests and the central montane rainforests. Apart from the already known phenethyl cinnamide, four new tryptamine derived amides were detected with different acid moieties: the cinnamic acid amides named balasubramide and prebalamide and the isovaleric acid amides named madugin and methylmadugin. Balasubramide is characterised by an eightmembered lactam ring. All structures were established by spectral analyses.

Sulphur-containing amides from Glycosmis species with different antifungal activity

Leaf extracts from different Sri Lankan provenances of Glycosmis mauritiana and G. pentaphylla have been compared by HPLC analyses and tested for antifungal activity. All collections of G. pentaphylla are uniformly characterized by the known alkaloids arborine, skimmianine and arborinine, whereas the different provenances of G. mauritiana differ by displaying chemical polymorphism of different sulphur-containing amides. Three new amides, named illukumbin B, methylillukumbin B and methylillukumbin A, have been isolated and their structures are described on the basis of spectroscopic evidence. They show different antifungal activities in bioautographic tests and appear to be accumulated mainly in individuals collected from more humid and elevated habitats.

Thapsakins: possible biogenetic intermediates towards insecticidal cyclopenta[b]benzofurans from Aglaia edulis

Abstract Nine new flavaglines, cyclopenta[bc]benzopyrans (thapsakins) and benzo[b]oxepines (thapoxepines), were isolated from the lipophilic root extract of Aglaia edulis together with two known cyclopenta[b]benzofurans, aglaroxin A and pannellin. The structures were established on the basis of extensive use of NMR spectroscopic methods (C,H-COSY, NOESY, HMBC, lanthanide induced shifts). Aglaroxin A and pannellin exhibited the strongest insect toxicity toward neonate larvae of Spodoptera littoralis. Possible biogenetic connections between the three skeletal types are outlined and chemosystematic implications of flavagline formation are discussed.

Pyranoquinolones and acridones from Vepris bilocularis

Besides already known furoquinolones and acridones, three new pyrano-2-quinolones and one new acridone were isolated from the leaves of south Indian Vepris bilocularis. The novel alkaloids were identified by spectroscopic means and named 7-methoxyflindersine, 7-prenyloxyflindersine, N-methyl-7-prenyloxy-flindersine and vebilocine.

Stress induced carbazole phytoalexins in Glycosmis species

Induced formation of a series of carbazole alkaloids was observed in leaves of Glycosmis parviflora and G. pentaphylla after wounding, UV-irradiation, and particularly after inoculation with the fungus Botrytis cinerea. Chemical variation between different provenances and even individuals of G. parviflora led to an accumulation of different derivatives from which three proved to be undescribed natural products. Their structures were identified by spectroscopic methods and named carbalexins A, B, and C. Bioautographic tests on TLC plates with Cladosporium herbarum exhibited strong antifungal activity for the new carbalexins as well as for the already known 2-hydroxy-3-methylcarbazole, but only weak effects for the pyranocarbazole glycoborinine. Detailed experiments with marked infection areas confirmed the restricted accumulation of carbazole derivatives which could not be detected in non-infected areas of the same leaf. Apart from carbazoles, in some individuals of G. parviflora an additional accumulation of the pyranoquinolones flindersine and methylflindersine was induced, which supports the already previously discussed biogenetic connections between carbazoles and prenylated quinolones.

Sulfur containing cinnamides with antifungal activity from glycosmis cyanocarpa

Abstract Bioassay guided analysis of the methanolic leaf extract of Glycosmis cyanocarpa (Rutaceae) led to the isolation of a new type of sulfur containing cinnamides with antifungal activity: sinharine (cinnamic acid methylsulfidoethylamide) (2) and the corresponding N-methyl derivative methylsinharine (4). In addition, the dominating furoquinoline kokusaginine (3) together with small amounts of skimmianine (5) and the carbazole glycozolidol (1) were also isolated. A series of quinolone and quinazolone alkaloids were detected as minor components by reversed phase HPLC. The structures of 2 and 4 were elucidated by means of spectroscopic methods (IR, UV, 1H NMR, 13C NMR, MS). Temperature dependent 1H NMR and lanthanide induced shifts (LIS) established the stereochemistry of the two conformers of 4.

Sulphones derived from methylthiopropenoic acid amides from Glycosmis angustifolia

Abstract Seven new sulphones derived from methylthiopropenoic acid amides have been isolated as dominant components from the methanolic leaf extract of Glycosmis angustifolia collected in Sri Lanka. All derivatives contained substituted phenethylamine moieties with prenyloxy (dambullins) or geranyloxy groups (gerambullins) in the para -position. Additional oxidation of the geranyl side chain leads to a diol (gerambullindiol) and to ketones (methylgerambullone and methylisogerambullone). The major component, methyldambullin, exhibited moderate antifungal activity.

Relative and absolute configurations of two naturally occurring acetylenic spiroketal enol ether epoxides

Abstract The underground parts of Artemisia selengensis afforded a new C 14 acetylenic six-ring spiroketal enol ether epoxide. A closely related diastereomeric compound has been isolated previously, however, the relative configuration of the epoxy ring ( anti or syn to a reference oxygen) could not be established. The relative stereochemistries of the anti-syn pair were elucidated by 1 H-lanthanide induced shifts and comparison of 13 C NMR data. The absolute configurations were deduced via the corresponding carbinols using the method of Horeau . The results were confirmed by CD measurements and equilibration experiments of anti ⇌ syn pairs.