Mechanism and stereochemistry of conjugate reduction of enones from gibberellins A3 and A7

Abstract

Conjugate reduction of the methyl esters of 3-didehydrogibberellin A₃ 13-acetate and of 3-didehydrogibberellin A₇, in aprotic solvents by borohydride (or borodeuteride), is shown to introduce hydrogen (or deuterium) at the 1β- and 3β-positions in the products, 3-epi-gibberellin A₁ 13-acetate and 3-epi-gibberellin A₄ methyl esters. The third hydrogen (or deuterium) comes from the proton (or deuteron) source used in the work-up. A mechanism for conjugate reduction of enones is proposed. The products from the borodeuteride reduction of 3-didehydrogibberellin A₇ methyl ester with proton and deuteron work-up were chemically converted into [1β-²H₁]-, [1β,2²H₂]-, and [1β3α-²H₂]-gibberellin A₄ and the stereochemistries of the deuterium atoms were determined from the deuterium content of the metabolites, formed from these labelled gibberellins in cultures of Gibberella fujikuroi, mutant B1-41a.