Application of 2H n.m.r. spectroscopy to study the incorporation of enantiomeric [2-2H]-labelled putrescines into the pyrrolizidine alkaloid retrorsine

Abstract

A sample of (2R)-[2-²H]putrescine (13) dihydrochloride was prepared from (2S)-aspartic acid (8), and (2S)-[2-²H]putrescine (15) dihydrochloride was synthesized from (2R)-aspartic acid. Feeding experiments carried out with these precursors on Senecio isatideus plants gave retrorsine (5) containing ²H, and the distribution of ²H from each experiment in retrorsine was determined by ²H n.m.r. spectroscopy. All of the ²H was confined to the base component of the alkaloid, retronecine (4). Retrorsine (14), derived biosynthetically from (2R)-[2-²H]putrescine (13) dihydrochloride was labelled with ²H at C-2 and C-6α, while retrorsine (16), produced from (2S)-[2-²H]putrescine (15) dihydrochloride contained ²H labels at C-6β and C-7α. These labelling patterns demonstrate that hydroxylation at C-7 of retronecine (4) proceeds with retention of configuration. In addition, the formation of the 1,2-double bond of retronecine involves removal of the pro-S hydrogen and retention of the pro-R hydrogen at the carbon atom which becomes C-2 of retronecine.