Synthesis of gibberellins A8 and A56 from gibberellin A3

Abstract

A mixture of gibberellin A3 derivatives with 1(10)-ene-2β,3β-diol and 1(10)-ene-2α,3β-diol (2:5) groups, readily obtained from gibberellin A3, has been used for a new and simple synthesis of gibberellin A8 and its esters. The hydrolysis of GA3 and the iodolactonization of a mixture of the 2-epimers was carried out in aqueous solution in a single flask, as also was a synthesis of GA56 from GA3 by a method that we have modified. The mixture of 1β-iodides of GA8 and GA56 was separated by chromatography on SiO2 in the form of methyl or p-bromophenacyl esters which were then deiodinated and the methyl or p-bromphenacyl ester of GA8 was isolated. Free GA8 was obtained by the dephenylation of the latter ester. By two-dimensional NMR spectroscopy we succeeded in assigning all the signals in the13C and1H NMR spectra of the methyl esters of GA8 and GA56. In an attempt to obtain GA5 methyl ester by the action of trimethylchlorosilane/sodium iodide on the 2α,3β-diol system in GA56 methyl ester, the 8,13-epimer of the latter was formed, the structure of its molecule being established from the results of X-ray structural analysis.