Gibberellin analogues by reaction of 7-oxo-diterpenes with diacetoxyiodobenzene

In this work, the reaction of 7-oxo-diterpenes with diacetoxyiodobenzene (DIB) has been proved to be a good method for the preparation of gibberellin analogues, which can be applied to diterpenes with endocyclic or exocyclic double bonds. It has been studied with ent-kaur-16-ene, ent-trachylobane and ent-atis-16-ene diterpenes. Thus, the reaction of 7-oxo-ent-kaur-16-en-18-oic acid methyl ester and 7-oxo-ent-trachyloban-16-en-18-oic acid methyl ester with this reagent affords 4-epi-gibberellin A12 dimethyl ester and 4-epi-trachylobagibberellin A12 dimethyl ester, respectively. In some cases, especially with compounds functionalized at C-19, alternative reactions lead to the introduction in the substrate of a conjugated 5,6-double bond or to the formation of a ketal at the 6-position. Thus, the formation of gibberellin analogues, dehydrogenation products or 6-ketal derivatives depend on the neighbouring group participation of the C-18 (equatorial) and C-19 (axial) substituents at C-4.