Absolute configuration of the creatonotines and callimorphines, two classes of arctiid-specific pyrrolizidine alkaloids

Abstract

Arctiids which as larvae sequester pyrrolizidine alkaloids (PAs) from their food plants are known to synthesize insect-specific PAs by esterifying necine bases derived from plant PAs with necic acids of insect origin. There are two classes of insect PAs, the creatonotines and the callimorphines. The creatonotines contain as necic acids either 2-hydroxy-3-methylbutyric acid (creatonotine A) or 2-hydroxy-3-methylpentanoic acid (creatonotine B). The three known callimorphines contain 2-hydroxy-2-methylbutanoic acid whose hydroxyl group can be either free (deacetylcallimorphine) or acetylated (callimorphine) or propionylated (homocallimorphine). Insect PAs are assumed to play an important role in the recycling of plant derived necine bases and the processing by trans-esterification of PA monoesters that cannot be directly transmitted to the insect's pupal and adult life-stages. The absolute configuration of the insect-specific necic acids was elucidated in the context of the suggested role of the insect PAs as insect-made mimics of plant monoester PAs of the lycopsamine type. For this purpose all needed stereoisomers were synthesized and a gas chromatography–mass spectrometry (GC–MS) method was established that allows the enantioselective separation and assignment of the stereochemistry of all insect specific necic acids as their methyl esters. The method could also be applied to the GC–MS analysis of the intact alkaloids which were hydrolyzed during injection and converted into their methyl esters. Analysis of the creatonotines and callimorphines isolated from the polyphagous arctiids Estigmene acrea and Grammia geneura that were fed with pure PAs and defined PA mixtures revealed the following absolute configuration: the callimorphines and creatonotine A were present in 2′ R configuration, whereas creatonotine B was found as mixture of (2′ R, 3′ S)- and (2′ S, 3′ S)-stereoisomers. The ratio of 2′ S to 2′ R was extremely variable ranging from 98% S to 94% R. The cause of the lack of stereospecificity is discussed particularly in respect of a possible epimerization of the hydroxyl group at C-2′ in analogy to the known epimerization at C-3′ of plant acquired PAs of the lycopsamine type.