Unusual reactions of methylsulfonyl esters: syntheses of 3α-methyl and 3β-methyl gibberellin A20

Abstract

Gibberellin A₃ has been converted to 3α-methylGA₂₀ 8 in 33% yield via catalytic hydrogenation of the 3-exo-methylene derivative 14. In an attempt to prepare 3β-methylGA₂₀ 9, the 3α-methanesulfonate 25 has been treated with lithium dimethylcuprate; only the β-keto sultones 26 and 27 have been isolated (84% yield). In contrast, under similar conditions the 3α-methanesulfonate 35 gave the 2-oxopropylsulfonyloxy derivatives 36 and 37. Synthesis of 3β-methylGA₂₀ has been achieved via reaction of the 3α-trifluoromethanesulfonate 38 with lithium dimethylcuprate. 3α-MethylGA₂₀ and 3β-methylGA₂₀ show similar activity to GA₂₀ (and significantly less activity than GA₃) in the stimulation of stem elongation of dwarf rice seedlings.