Pyrrolizidines spectra

Segall and coworkers described the in vitro mouse hepatic microsomal metabolism of the alkaloid senecionine (159) (Scheme 34). Several pyrrolizidine alkaloid metabolites were isolated from mouse liver microsomal incubation mixtures and identified (222, 223). Preparative-scale incubations with mouse liver microsomes enabled the isolation of metabolites for mass spectral and H-NMR analysis. Senecic acid (161) was identified by GC-MS comparison with authentic 161. A new metabolite, 19-hydroxysenecionine (160), gave a molecular ion consistent with the addition of one oxygen atom to the senecionine structure. The position to which the new oxygen atom had been added was made evident by the H-NMR spectrum. The three-proton doublet for the methyl group at position 19 of senecionine was absent in the NMR spectrum of the metabolite and was replaced by two signals (one proton each) at 3.99 and 3.61 ppm for a new carbinol methylene functional group. All other H-NMR spectral data were consistent for the structure of 160 as the new metabolite (222). 

The venomous constituent of the cryptic thief ant, Solenopsis xenovenenum, has been identified as the 3-heptyl-5-methylpyrrolizidine (50) from its mass spectrum and the fact that a related pyrrolidine (51) has been isolated from another species of ant.47 This is the first reported occurrence of a 3,5-dialkyl-pyrrolizidine, and its structure was confirmed by synthesis. Reductive amination of the known triketone (52) with sodium cyanoborohydride and ammonium acetate gave a mixture of four isomers of 3-heptyl-5-methylpyrrolizidine, which were separated by preparative g.l.c. The stereochemistry of the ring-junction of each isomer was established from its i.r. and n.m.r. spectra. 

Clivorine, the alkaloid of Ligularia clivorum Maxim., was earlier shown to be a pyrrolizidine alkaloid whose basic constituent is otonecine (64). The acid component is a hydroxydicarboxylic acid which readily lactonises, on attempted isolation, to clivonecic acid (65). The structure of this acid has now been firmly established on the basis of chemical and spectroscopic evidence, and its stereochemistry has also been discussed. The present proposal" is that clivonecic acid has the S-configuration at C-2 since the o.r.d. spectrum of tetrahydroclivonecic acid exhibits a positive Cotton effect, in agreement with the modified octant rule. ... 

The mass spectra of pyrrolizidine alkaloids have been reviewed by Russian authors. The n.m.r. spectrum of retrorsine (42), obtained from Senecio selloi... 

Edgar JA, Colegate SM, Boppre M, Molyneux RJ. Pyrrolizidine alkaloids in food A spectrum of potential health consequences. Food Addit Conlam Part A - Chem Anal Control Exposure Risk Assessm 2011 28(3) 308-24. 

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