Application of 2H n.m.r. Spectroscopy to study the incorporation of 2H-labelled putrescines into the pyrrolizidine alkaloid retrorsine

Abstract

The biosynthesis of the retronecine (7) portion of the pyrrolizidine alkaloid retrorsine (5) has been studied in Senecio isatideus plants using ²H-labelled putrescines. The labelling pattern in retrorsine (9) derived biosynthetically from [2,2,3,3-²H₄]putrescine (8), established by ²H n.m.r. spectroscopy, showed ²H to be present at C-2, C-6α, C-6β, and C-7α. The retention of ²H at C-7α indicates that the introduction of the hydroxy group at the 7-position of retronecine does not involve keto or enol intermediates. The use of [1,1,4,4-²H₄]-putrescine (10) produced a sample of retrorsine (12) which gave ²H n.m.r. spectroscopic signals chiefly for C-3α, C-3β, and the C-9 pro-S position. The operation of deuterium isotope effects is used to explain this labelling pattern. The formation of (9S)-[9-²H₁]retrorsine (12) is consistent with stereospecific attack of a hydride donor on the reface of an aldehyde precursor. (R)[1-²H₁]Putrescine (19) afforded retrorsine (21) equally labelled with ²H at C-3β, C-5α, C-8α, and C-9 pro-S, while (S)-[1-²H₁]putrescine (22) yielded retrorsine (24) with only the C-3α and C-5β positions labelled with ²H. The stereochemistry of a number of the enzymic processes involved in retronecine biosynthesis has been established.