Biosynthesis of phenanthroindolizidine alkaloids: incorporation of 2-pyrrolidin-2-ylacetophenone and benzoylacetic acid and derivatives
2-Pyrrolidin-2-ylacetophenone (12) and its oxygenated derivatives, (13) and (14), bearing 14C and 3H labels are synthesized and are shown to be intact precursors for the Phenanthroindolizidine alkaloid, tylophorinine (16) in Tylophora asthmatica; the three amines, singly labelled with tritium, are shown to be precursors for tylophorine (3) and tylophorinidine (17). Benzoylacetic acid (9) and p-hydroxybenzoylacetic acid (10)but not 4-hydroxy-3-methoxybenzoylacetic acid, are also precursors for tylophorinine (16). The results allow partial description of the biosynthetic pathways to Phenanthroindolizidine alkaloids. A degradation is described which allowed the location of label in 2-pyrrolidin-2-ylacetophenone, derived from[5-14C]ornithine, to be established.