Some stereocontrolled reactions of bicyclo [3.2.0]heptan-6-ones and 2-oxabicyclo [3.3.0]octan-3-ones

Abstract

2-Oxabicyclo[3.3.0]oct-6-en-3-one reacts with bromine through the formation of exo- and endo-bromonium ions. The regioselectivity of the reactions of bromonium ions derived from 4-bromo-4-methyl-2-oxabicyclo[3.3.0]oct-6-en-3-ones hinges on the configurations of the substituents at C-4. These bromonium ions are attacked by nucleophiles at C-7, the carbon centre more distant from the lactone ring, with high selectivity. Similarly, acid catalysed ring cleavages of epoxides derived from bicyclo[3.2.0]hept-2-en-6-one and 2-oxabicyclo[3.3.0]oct-6-en-3-ones take place with marked selectivity. Conformational preferences of the adjacent ketone or lactone ring in the intermediate bromonium ions and protonated epoxides are invoked to explain the various selectivities observed.