Stereoselective synthesis of new 8-oxabicyclo[3.2.1]oct-6-en-2-one and 8-oxabicyclo[3.2.1]octa-3,6-dien-2-one derivatives. The electron-releasing and electron-withdrawing effects of homoconjugated ketones

Two successive brominations of 8-oxabicyclo[3.2.1]oct-6-en-2-one 6 provided 3-exo,6-exo,7-endo-tribromo-7oxabicyclo[3.2.1]octan-2-endo-one 10 with high stereoselectivity. NaBH4 reduced 10 into 3-exo, 6-exo,7-endo-tribromo-8-oxabicyclo[3.2.1]octan-2-endo-ol exclusively. Selective elimination of one equivalent of HBr from 10 furnished 3-exo,7-dibromo-8-oxabicyclo[3.2.1]oct-6-en-2-endo-ol 13. Regio- and stereoselective addition of PhSeCl to enone 6, followed by alpha -ketone bromination and treatment with a base led selectively to 7-(phenylseleno)-8-oxabicyclo[3.2.1]octa-3,6-dien-2-one 20. The high selectivities observed are attributed to steric factors and to electronic factors such as electron-releasing or electron-withdrawing effects of homoconjugated carbonyl group.