Synthesis of 3,3',5-trihydroxybiphenyl-2-carboxylic acid, a component of the bitterest natural product amarogentin and its coenzyme A and N-acetyl cysteamine thiol esters.

3,3',5-Trihydroxybiphenyl-2-carboxylic acid (6), an ester component of the bitter-tasting natural products amarogentin and amaroswerin, was synthesized in six steps in 13.6% overall yield. Its N-acetyl cysteamine thiol ester (9) and its coenzyme A thiol ester (8), a likely biosynthetic precursor of the amarums, were also prepared.

Biosynthesis of Erythrina alkaloids in Erythrina crista-galli.

A precursor application system was developed to allow the study of Erythrina alkaloid formation in Erythrina crista-galli. Fruit wall tissue of this species was recognized as the major site of alkaloid biosynthesis. The application of radioactively and 13C-labelled potential precursors showed that the hitherto assumed precursor (S)-norprotosinomenine was not incorporated into the Erythrina alkaloids. In contrast, (S)-coclaurine as well as (S)-norreticuline were metabolized to erythraline and erythrinine, respectively, suggesting that a coclaurine-norreticuline pathway is operative in Erythrina alkaloid formation. Feeding of [1-13C]-labelled (S)-norreticuline with subsequent NMR spectroscopy demonstrated that the resulting erythraline was exclusively labelled at position C-10. Therefore, the participation of a symmetrical intermediate of the diphenoquinone type in Erythrina alkaloid biosynthesis can be excluded.

(-)-2C-methyl-D-erythrono-1,4-lactone is formed after application of the terpenoid precursor 1-deoxy-D-xylulose.

Application of [1,2-14C]1-deoxy-D-xylulose, the committed precursor of terpenoids, thiamine and pyridoxol, to a variety of plant species resulted in the labelling of an unknown metabolite. The isolation and purification of this metabolite from Ipomoea purpurea plants fed with 1-deoxy-D-xylulose (DX), followed by NMR analysis, resulted in the identification of its structure as (-)-2C-methyl-D-erythrono-1,4-lactone (MDEL). MDEL has been previously isolated as a stress metabolite of certain plants. A hypothetical biosynthetic scheme is given.