Plant analysis by butterflies: occurrence of cyclopentenylglycines in Passifloraceae, Flacourtiaceae, and Turneraceae and discovery of the novel nonproteinogenic amino acid 2-(3'-cyclopentenyl)glycine in Rinorea.

Following records about feeding habits of nymphalid butterflies, a novel nonproteinogenic L-amino acid, (S)-2-(3'-cyclopentenyl)glycine (11), was discovered in Rinorea ilicifolia, a species where the presence of a cyclopentanoid natural product of this kind was neither known nor anticipated from the taxonomic point of view. Another novel amino acid, (2S,1'S,2'S)-2-(2'-hydroxy-3'-cyclopentenyl)glycine (12), the stereochemistry of which was determined by single-crystal X-ray diffraction, was shown to occur in species belonging to Flacourtiaceae, Passifloraceae, and Turneraceae. These species, many of which serve as hosts for nymphalid butterflies (Acraeinae, Heliconiinae, Argynninae), also produce 2-(2'-cyclopentenyl)glycine. Cyclopentenylglycines are proposed to be novel chemical recognition templates for plant-insect interactions. Ratios between the epimers of (2S)-2-(2'-cyclopentenyl)glycine, which co-occur in plants, were determined by (1)H NMR spectroscopy. Contrary to a previous report, the (2S,1'R) epimer always appears to predominate over the (2S,1'S) epimer. Stereochemical aspects of biosynthesis of natural cyclopentanoid cyanogenic glycosides are discussed in relation to these findings.