Synthesis of a tritium labelled pyrrolizidine alkaloidisenecioyl retronecine

Retronecine (7β-hydroxy-1-hydoxymethyl-1,2-dehydro-8α-pyrrolizidine) (II) which is prepared from the hydrolysis of monocrotaline (I) was oxidized with manganese dioxide in the presence of potassium cyanide to methyl 1,2-dehydro-7β-hydroxy-8α-pyrrolizidine-1-carboxlate (III). Retronecine (II') was 3H labelled at the 1 methyl proton by the reduction of III with LiAl3H4. The semi-synthetic pyrrolizidine alkaloid disenecioyl retronecine (Di-3,3-dimethylacrylic acid ester of retronecine). having a high specific activity. was prepared by the acylation of 3H retronecine (II') with senecioyl chloride (3,3-dimethylacryloyl chloride).