Biosynthesis of Oleoside-, 10-Hydroxyoleoside- and Ligustaloside-Type Glucosides from Secologanin

(1)-type [e.g. oleuropein (2)], 10-hydroxyoleoside (3)-type [e.g. 10-acetoxyoleuropein (4)] and ligustaloside (5)-type secoiridoid glucosides [e.g. ligustalosides A, B (5a, 5b)] were found in oleaceous plants and their hypothetical biosynthetic pathway was proposed as depicted in Fig. 1. Later, secologanoside (6)-type [e.g. oleuroside (7)] and 8epi-kingiside (8)-type glucosides [e.g. 8-epi-kingiside (8) and jasmolactones A, B (9a, 9b)] were isolated together with various glucosides of the three above types from oleaceous plants. The biosynthesis of the glucosides of the two types 6 and 8 was also presumed to proceed along the above-mentioned pathway (Fig. 1). In this pathway, in addition to the mechanism for the formation of structures around C-8, -9 and -10 in these glucosides, another problem is whether epoxidation of C-8—C-10 occurs before or after the oxidation of the aldehyde group at C-7 to the carboxy group. Regarding the biosynthesis of 1-type glucosides, earlier experiments with radio-active precursors have shown that 1-type glucosides were biosynthesized from loganin (11) via secologanin (10) and not via 7-ketologanin (12). Recent experiments with deuterium-labeled precursors, on the other hand, have shown that these glucosides were formed from 7-epi-loganin (13) via 12. Further experiments were required to