title: Semi-automated separation of the epimeric dehydropyrrolizidine alkaloids lycopsamine and intermedine: preparation of their N-oxides and NMR comparison with diastereoisomeric rinderine and echinatine.
abstract: BACKGROUND
  The diversity of structure and, particularly, stereochemical variation of the dehydropyrrolizidine alkaloids can present challenges for analysis and the isolation of pure compounds for the preparation of analytical standards and for toxicology studies.
  
  
  OBJECTIVE
  To investigate methods for the separation of gram-scale quantities of the epimeric dehydropyrrolizidine alkaloids lycopsamine and intermedine and to compare their NMR spectroscopic data with those of their heliotridine-based analogues echinatine and rinderine.
  
  
  METHODS
  Lycopsamine and intermedine were extracted, predominantly as their N-oxides and along with their acetylated derivatives, from commercial samples of comfrey (Symphytum officinale) root. Alkaloid enrichment involved liquid-liquid partitioning of the crude methanol extract between dilute aqueous acid and n-butanol, reduction of N-oxides and subsequent continuous liquid-liquid extraction of free base alkaloids into CHCl3 . The alkaloid-rich fraction was further subjected to semi-automated flash chromatography using boronated soda glass beads or boronated quartz sand.
  
  
  RESULTS
  Boronated soda glass beads (or quartz sand) chromatography adapted to a Biotage Isolera Flash Chromatography System enabled large-scale separation (at least up to 1-2 g quantities) of lycopsamine and intermedine. The structures were confirmed using one- and two-dimensional (1) H- and (13) C-NMR spectroscopy. Examination of the NMR data for lycopsamine, intermedine and their heliotridine-based analogues echinatine and rinderine allowed for some amendments of literature data and provided useful comparisons for determining relative configurations in monoester dehydropyrrolizidine alkaloids. A similar NMR comparison of lycopsamine and intermedine with their N-oxides showed the effects of N-oxidation on some key chemical shifts. A levorotatory shift in specific rotation from +3.29° to -1.5° was observed for lycopsamine when dissolved in ethanol or methanol respectively.
  
  
  CONCLUSIONS
  A semi-automated flash chromatographic process using boronated soda glass beads was standardised and confirmed as a useful, larger scale preparative approach for separating the epimers lycopsamine and intermedine. The useful NMR correlations to stereochemical arrangements within this specific class of dehydropyrrolizidine alkaloid cannot be confidently extrapolated to other similar dehydropyrrolizidine alkaloids. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.
authors.affiliation: Poisonous Plant Research Laboratory, Agriculture Research Service, US Department of Agriculture, 1150 East 1400 North, Logan, Utah, 84341, USA.
authors.forename: Steven M
authors.initials: SM
authors.lastname: Colegate
authors.forename: Dale R
authors.initials: DR
authors.lastname: Gardner
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Betz
authors.forename: Kip E
authors.initials: KE
authors.lastname: Panter
mesh_terms: D001331:Automation; D005591:Chemical Fractionation / Q000379:methods*; D020844:Comfrey / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D018517:Plant Roots; D011763:Pyrrolizidine Alkaloids / Q000737:chemistry*
chemical_list: D011763:Pyrrolizidine Alkaloids; C000597572:intermedine; C018336:indicine
keywords: Boronated soda glass chromatography; HPLC-ESI/MS; NMR; Symphytum officinale; comfrey; dehydropyrrolizidine alkaloids; echinatine; intermedine; lycopsamine; optical rotation; rinderine