Pyrrolizidine alkaloid biosynthesis. Incorporation of 13C-labelled precursors into rosmarinine

Abstract

Biosynthesis of the rosmarinecine (8) portion of the pyrrolizidine alkaloid rosmarinine (7) has been investigated in Senecio pleistocephalus plants using ¹³C-labelled precursors. These plants were fed with [1-¹³C]putrescine dihydrochloride (12) and [2,3-¹³C₂] putrescine (13) dihydrochloride, and the labelling patterns in the biosynthetically derived rosmarinine were established by ¹³C n.m.r. spectroscopy. Two molecules of putrescine are incorporated to about the same extent into rosmarinecine (8). Incorporation of [1-amino-¹⁵N,1-¹³C]putrescine (14) dihydrochloride into rosmarinine produced a labelling pattern which was consistent with conversion of the two putrescine molecules into a C₄–N–C₄ symmetrical intermediate. This intermediate was identified as homospermidine (4) by feeding [1,9-¹³C₂] homospermidine (15) trihydrochloride to S. pleistocephalus plants. Intact incorporation of this precursor was demonstrated by observation of two enriched ¹³C n.m.r. signals for C-8 and C-9 of rosmarinine.