Pyrrolizidine alkaloid biosynthesis: stereochemistry of the formation of isoleucine in Senecio species and of its conversion into necic acids

Incorporation experiments with isoleucine (1) and (2)-aminobutanoic acid [as (20)], stereospecifically labelled with tritium at C-4 and C-3 respectively, have shown that the ethyl migration step during biosynthesis of isoleucine (1) in Senecio species takes place with retention of configuration. It was also shown that coupling of two molecules of isoleucine (1) to give the ten-carbon necic acids of the senecic (2) type takes place with loss of the C-4 pro-S and retention of the C-4 pro-R hydrogen from both components. A new synthesis of βγ-unsaturated amino-acids has been developed which provides for control of the configuration of the olefinic system.