Nudicaulins – a group of indole-phenylpropanoid hybrid alkaloids from Island poppy

Iceland poppy (Papaver nudicaule) is a flowering plant growing in nordic landscapes. The wild type has white or pale yellow petals, while garden varieties have also red or orange flowers. NMR spectroscopic and mass spectrometric analyses of the differently coloured petals revealed that anthocyanidins, flavonoids, and nudicaulins, a novel type of indole-phenylpropanoid hybrid alkaloids, contribute to the pigmentation. The nudicaulins are responsible for the coloration of yellow flowers, which could enhance their attraction for pollinating insects. A hypothesis suggesting a pentacyclic skeleton of nudicaulins was reported (Schliemann et al., 2006, Phytochemistry 67, 191-201). The constitution of the nudicaulin aglycon was revised, the relative configuration of nudicaulins I and II was assigned by extensive NMR and semisynthetic experiments, and the absolute stereochemistry was assigned by experimental CD measurements and quantum chemical calculations (Tatsis et al., Organic Letters 2012, http://dx.doi.org/10.1021/ol303211w; cooperation with A. Schaumlöffel and Prof. G. Bringmann, University of Würzburg). The unique nudicaulin aglycon skeleton, which is composed of an indole unit and a glycosylated phenylpropanoid moiety, attracts the interest for biosynthetic studies. Retrobiosynthetic studies were performed in coopration with W. Eisenreich, Technical University of Munich, using 13CO2 as a general precursor. This approach has the advantage that the isotopologue profile analyzed by 13C NMR spectroscopy and mass spectrometry reflects the biosynthetic relationship of the measured metabolites and allows conclusions about the biosynthetic pathway. Further 13C isotope labelling experiments and gene expression analyses are in progress.