The partial synthesis of beyergibberellin A4 and A9 methyl esters

Abstract

Beyergibberellins A4 and A9 methyl esters were synthesized from methyl gibberellate via gibberellin A1 methyl ester and its 3-epimer. The C/D ring junction of the 3-epimer was isomerized to afford the 16-oxo-8,13-isogibberellin and 3-hydroxy group removed by tributyltin hydride reduction of the corresponding chloride. Alternatively the C/D ring junction of the gibberellin A1 methyl ester was isomerized and the 3-hydroxy group was converted into its trimethylsilylethoxymethyl derivative. Reduction of the 16-ketones and elimination of the corresponding 16-sulphonate esters over alumina or with collidine afforded the title compounds.