Preparation of gibberellins A9 and A20 from gibberellic acid

Abstract

Methyl gibberellate has been converted via the methyl ester of 3-epigibberellin A₁ into the 3β-chloro- and 3β,13-dichloro-derivatives using triphenylphosphine and carbon tetrachloride. Hydrogenolysis of the chlorides with tributyltin hydride afforded gibberellins A₂₀ and A₉ as their methyl esters. Gibberellin A₉ methyl ester was also prepared from the gibberellin A₄/A₇ mixture. The stereochemistry of the conjugate reduction of the ring-A-unsaturated ketone is defined.