Confirmation of structure and synthesis of three new 11β-OH C₂₀ gibberellins from loquat fruit

Three new 11β-hydroxy C₂₀ gibberellins have been isolated from immature loquat fruit and their structures were established as 11β-hydroxy-GA₁₂, 11β-hydroxy-GA₁₅ and 11β-hydroxy-GA₅₃, respectively, by direct GC-MS comparisons with authentic samples obtained from gibberellic acid by multistep syntheses. An advanced intermediate (30) was prepared in 20 steps from which 6 11β-hydroxy C₂₀ gibberellins were prepared by parallel routes involving up to a further 5 steps for each sequence. The key steps involved a much improved synthesis of gibberellenic acid derivatives, a Lewis acid catalysed cyclisation of a diazoketone, a domino-hydroboration of a diene and oxidative cleavage of a ketone derived enolate.