Synthesis and bioactivity of the gibberellin, 18-hydroxy-GA₁ (GA₁₃₂)

As part of a study to confirm putative structural assignments to new gibberellins and to furnish sufficient quantities for biological investigations, a twenty step synthesis of 18-hydroxy GA₁ from gibberellic acid (GA₃) is described, allowing the confirmation of structure for a new gibberellin, GA₁₃₂, that occurs in developing grains of barley (Hordeum vulgare). The early part of the sequence involved cleavage of the C(3)-C(4) bond in the A-ring of a 3-oxo intermediate. The ring was then reformed as part of a "domino" process involving the conjugate addition of alkoxide to an α-methylene lactone moiety followed by an intramolecular aldol reaction. The bioactivities of the new GA, and its 18-hydroxy-GA ₄ relative, have been confirmed in dwarf barley growth and α-amylase induction assays.