Pyrrolizidine alkaloid biosynthesis. Synthesis of 3H-labelled trachelanthamidine and isoretronecanol and their incorporation into three pyrrolizidine bases (necines)

Abstract

(±)-[5-³H]Isoretronecanol (22) and (±)-[5-³H]trachelanthamidine (24) were prepared by 1,3-dipolar cycloaddition of N-formyl[5-³H]proline with ethyl propiolate followed by reduction steps. These ³H-labelled 1-hydroxymethylpyrrolizidines together with [1,4-¹⁴C]putrescine were fed to Senecio isatideus which produces retrorsine (1); S. pleistocephalus which yields rosmarinine (8); and Cynoglossum officinale which affords echinatine (5). The double labelling experiments demonstrated that isoretronecanol is incorporated much more efficiently into rosmarinine than into retrorsine or echinatine, whereas trachelanthamidine is a much more efficient precursor for retrorsine and echinatine. Base hydrolysis of retrorsine and echinatine labelled with [5-³H]trachelanthamidine and of rosmarinine labelled with [5-³H]isoretronecanol established that most of the ³H-label was in the base portions, retronecine (2), heliotridine (6), and rosmarinecine (9), respectively. Further degradation of retronecine and rosmarinecine showed that most of the radioactivity was confined to the β-alanine (4) portion. The biosynthetic pathways to isoretronecanol and trachelanthamidine apparently diverge prior to the formation of these 1-hydroxymethylpyrrolizidines, probably during the cyclisation of an immonium ion (14) to form the 1-formylpyrrolizidines (15) and (17).