Synthesis of 12-hydroxy C-20-gibberellins

Gibberellic acid (GA(3)) (6) has been converted into 12-hydroxylated derivatives of gibberellins GA(19) and GA(24) with a view to confirming tentative assignments of structure to 12-hydroxy C-20-gibberellins isolated from several plant species. The conversions were initiated by reducing 12-substituted analogues of 6 with Li-NH3 to afford either diene carboxylic acid 8 or its 12 alpha-epimer, then introducing C20 substituents by means of the copper catalysed intramolecular cyclopropanations of the derived diazoketones 9 and 19. Li-NH3 reductions of the resulting cyclopropyl ketones 11 and 22 furnished ketones 12, 22 and 23, which could be oxidatively cleaved by treatment of their enolate anions with molecular oxygen, thereby providing the key intermediate aldehydes 13, 24 and 25 from which the target GAs could be prepared. (C) 1997, Elsevier Science Ltd.