Neighbouring group participation on the bromination of methyl gibberellate: X-ray molecular structure of methyl 3,13-di-O-acetylgibberellate 16β,17-dibromide and ent-3α,13-diacetoxy-17-bromo-10β,16β-dihydroxy-20-norgibberell-1-ene-7,19-dioic acid 19,10-lactone 7-methyl ester

Abstract

Bromination of methyl gibberellate at C-16 with trimethyl(phenyl)ammonium perbromide has been shown to be non-stereospecific. However only the (16R)-epimer readily rearranged to the 8,13-epigibberellin. The stereochemistry of the (16S)-epimer was established by X-ray analysis. Bromination of methyl diacetylgibberellate afforded a (16R)-hydroxy-17-bromide. Bromination under strictly anhydrous conditions led to the isomerization of ring A to the Δ¹-19,2-lactones. Reaction with bromine in the presence of sodium hydrogen carbonate also gave a 1β,2α-dibromo adduct.