Studies on plant tissue cultures. Part 36. Biosynthesis of a retrochalcone, echinatin, and other flavonoids in the cultured cells of Glycyrrhiza echinata. A new route to a chalcone with transposed A- and B-rings

Abstract

Compounds labelled with radioisotopes were fed to the cultured cells of Glycyrrhiza echinata and the echinatin (1) obtained was degraded with alkali to p-hydroxyacetophenone (12) and 4-hydroxy-2-methoxybenzaldehyde (13) to determine the position of the incorporated radioactivity. The label from [3-¹⁴C]cinnamic acid and [3,5-³H₂]isoliquiritigenin was found only in compound (12) while the label from [1-¹⁴C]cinnamic acid and sodium [1-¹⁴C]acetate was found only in compound (13). These results establish the ‘retrochalcone’ nature of compound (1), in which, contrary to normal flavonoids, the A-ring and C₃-unit are of cinnamate origin and the B-ring is of acetatemalonate origin. ¹⁴C-Labelled precursors were synthesized and feeding experiments using a mutant strain of G. echinata cell culture revealed that licodione (9), a dibenzoylmethane, is an intermediate in the transposition of the αβ-unsaturated carbonyl group in the chalcone molecule during echinatin biosynthesis. The isotopic labelling of licodione (9) was also incorporated into 7,4′-dihydroxyflavone (6), although [carbonyl-¹⁴C]isoliquiritigenin labelled the flavone (6) more efficiently, while incorporation of compound (9) was very low compared with that of isoliquiritigenin (11) into the isoflavone formononetin (5). A feeding experiment with[1, 2-¹³C₂]iacetate further confirmed the polyketide moiety of the B-ring of chalcone (1) as well as the A-ring of isoflavone (5), both of which were labelled non-randomly with intact acetate units.