SYNTHESIS OF 12-HYDROXY C-20-GIBBERELLINS

Gibberellic acid (GA(3)) (6) has been converted into 12-hydroxylated d erivatives of gibberellins GA(19) and GA(24) with a view to confirming tentative assignments of structure to 12-hydroxy C-20-gibberellins is olated from several plant species. The conversions were initiated by r educing 12-substituted analogues of 6 with Li-NH3 to afford either die ne carboxylic acid 8 or its 12 alpha-epimer, then introducing C20 subs tituents by means of the copper catalysed intramolecular cyclopropanat ions of the derived diazoketones 9 and 19. Li-NH3 reductions of the re sulting cyclopropyl ketones 11 and 22 furnished ketones 12, 22 and 23, which could be oxidatively cleaved by treatment of their enolate anio ns with molecular oxygen, thereby providing the key intermediate aldeh ydes 13, 24 and 25 from which the target GAs could be prepared. (C) 19 97, Elsevier Science Ltd.