Quinones and related compounds in higher plants. Part 11. Role of 2-carboxy-2,3-dihydro-1,4-naphthoquinone and 2-carboxy-2-(3-methyl-but-2-enyl)-2,3-dihydro-1,4-naphthoquinone in the biosynthesis of naphthoquinone congeners of Catalpa ovata callus tissues

Abstract

Dilution analyses after administration of ¹⁴C-labelled 4-(2-carboxyphenyl)-4-oxobutanoic acid (3) to the callus tissues of Catalpa ovata demonstrated the following results: (i) 2-Carboxy-4-oxo-1-tetralone (COT), 2-carboxy-4-hydroxy-1-tetralone (CHT), prenyl-COT (4), and prenyl-CHT (5) are on the biosynthetic pathway of naphthoquinones; (ii) the route passing through COT → prenyl-COT (4)→ catalponone (6) is the main pathway of the biosynthesis of naphthoquinones, whereas there exists a subsidiary route passing through CHT → prenyl-COT (5)→ catalponol (2); (iii) regarding the key intermediates prenyl-COT (4) and catalponone (6), (2S)-prenyl-COT [(2S)-(4)] and (2R)-catalponone [(2R)-(6)] participate in the biosynthesis and both prenylation and decarboxylation proceed stereoselectively; and (iv) the acid (3) was incorporated into menaquinone-1 (7), 1-hydroxy-2-methylanthraquinone (8), 3-hydroxydehydro-iso-α-lapachone (9), etc., in the callus tissues. Furthermore, administration of 4-([carboxy-¹⁴C]-2-carboxyphenyl)-4-hydroxy[4-³H]butanoic acid (32) to the original Catalpa plant and the callus tissues revealed that (3) is cyclised to COT and then reduced to CHT.