STEREOSELECTIVE REACTIONS OF "2-OXABICYCLO[3.3.0]HEPT-6-EN-3-ONES LEADING TO FUNCTIONALIZED ALPHA-METHYLENE-GAMMA-LACTONES AND DELTA-ALPHA-BETA-BUTENOLIDES

4-Bromo-4-methyl-2-oxabicyclo[3.3.0]oct-6-en-3-ones have been oxidized with peracid. The resulting epoxides suffer regioselective attack by nucleophiles at C-7. In this way, stereospecific pathways to 6-acetoxy-4-methylene-2- oxabicyclo[3.3.0]oct-7-en-3-ones (3) and (4) have been found in which the necessary dehydrobrominations are effected by triethylamine or potassium acetate and 18-crown-6. The latter reagents have been used also to form the Δαβ-butenolides (26) and (27) from the bromolactones (28) and (2), respectively.