The biosynthesis of the gibberellins. Part III
Seven 14C-labelled compounds, including gibberellins A12(Ib), A13(IIc), and A14(IIIc), have been prepared and tested as precursors in the biosynthesis of gibberellic acid by Gibberella fujikuroi(see Table). It is suggested that gibberellic acid is biosynthesised from (–)-kaurene through 10β-formyl-1β,4aα-dimethyl-8-methylenegibbane-1 α-carboxylic acid (VIIIc) and gibberellin A14 or the corresponding 10-aldehyde (XV). Gibberellin A13 is not thought to be a precursor of gibberellic acid.
Although several kaurene derivatives, (VIIa), (XIVc), and (XIVe), having the 16-methylene group, undergo ring contraction to gibbanes, e.g., (VIIIa), with base, those having a 16-keto-group, (VIIb) and (XIVd), do not. This appears to be due to a long-range effect associated with the 16-keto-group.