Article pubs.acs.org/JAFC
Structure of Echivulgarine, a Pyrrolizidine Alkaloid Isolated from the Pollen of Echium vulgare Eric Cairns,† Muhammad Ali Hashmi,‡ A. Jonathan Singh,‡,§ Galen Eakins,‡,∥ Matthias Lein,‡,⊥ and Robert Keyzers*,‡,§ †
AsureQuality Ltd., P.O. Box 31242, Lower Hutt 5040, New Zealand School of Chemical & Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand § Centre for Biodiscovery, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand ∥ MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand ⊥ Centre for Theoretical Chemistry and Physics (CTCP), New Zealand Institute for Advanced Study, Massey University Auckland, Auckland, New Zealand
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S Supporting Information *
ABSTRACT: 1,2-Dehydropyrrolizidine alkaloids are common toxic metabolites isolated from plants within the Boraginaceae, in particular from the genera Heliotropium and Echium. Previous studies have deduced the structures of these often potent bioactives based upon mass spectrometric evidence, but these identifications have not established conclusive connectivity and configurational data. Herein, we describe the isolation and full structural characterization of echivulgarine, occurring in the pollen of Echium vulgare and correct the structure previously ascribed to the compound, using a comprehensive combination of both experimental and calculated nuclear magnetic resonance and electronic circular dichroism spectroscopic data. KEYWORDS: structural reassignment, DFT, absolute configuration, qNMR
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INTRODUCTION Plant pollens and the resulting floral honeys of the Boraginaceae genera Helitropium and Echium are common reservoirs of the potent hepatotoxic 1,2-dehydropyrrolizidine alkaloids (DHPAs). A large variety of DHPAs based upon the necine base skeleton have been isolated from these plants and plant-based materials, with more than 215 congeners (not counting N-oxides) reported to date1,2 since the seminal studies,3−8 beginning in the 1960s, of the identification and biological profiling of these highly toxic metabolites. In some cases, the free amine alkaloids and their Noxides have shown similar levels of toxicity in vitro, even though the oxides are generally considered as less toxic excretable liver metabolites.9 For example, calves fed with typical doses of riddelliine N-oxide present in Riddell groundsel (Senecio riddellii) were more affected than those fed with doses of the parent alkaloid, with 100% morbidity noted for the N-oxide treatment, while those given the parent alkaloid did not exhibit any symptoms of S. riddellii toxicity; therefore, the free base content alone of S. riddellii is not an effective measurement of the overall oral toxicity of the plant.10 The various DHPA-producing plants are often found as weeds growing in the local vicinity of other agricultural crops or rangeland pastures. DHPAs can therefore enter the human food chain either by inadvertent incorporation of the toxic weed with harvested plants or by bees transferring toxic pollen and the subsequent accumulation of the DHPAs in honey,1,11 a route to exposure first suggested in 1981.12 The potential risks associated with the incorporation of DHPA in food have been reviewed.11,13 Some DHPA structures have been only tentatively identified on the basis of comparison of mass spectrometric fragmentation © XXXX American Chemical Society
patterns, obtained using a large variety of mass spectrometer and ion source types. Boppré et al.14 conducted an extensive investigation of the content of known honey DHPAs in pollen collected from the anthers of Echium vulgare at high concentrations (8000−14000 ppm). The majority of the known DHPAs were found as their N-oxides, with several new congeners detected and tentatively identified from ion-trap tandem mass spectrometry (MS/MS) studies. One of the compounds, named echivulgarine, 1a or 1b,14 had been detected previously as a minor component (estimated
