Terpenoids. Part XXXVIII. Ring B contraction of kauranolides and related compounds into gibberellane-type compounds

Abstract

A detailed investigation of ring B contraction has been carried out with several derivatives containing the ent-3β,20-epoxy-3,16α-dimethoxy-17-norkaurane skeleton (1). The 19,6β-lactone 7α-methanesulphonate (3) on treatment with base and subsequent methylation gave the gibberellane aldehyde (8) and the epimeric norkaurane derivative (9). The 6β-hydroxy-7α-methanesulphonate (4) on similar treatment gave a similar result. The 19,6α-lactone 7α-methanesulphonate (6) on treatment with potassium hydroxide in t-butyl alcohol–water and subsequent methylation afforded only the desired product (8), in almost quantitative yield. The ring B contraction reaction did not proceed with the 6β-methanesulphonate 7α-acetate (7). The results are rationalised in terms of stereochemical considerations.