Acylation of 1,5-Cyclooctadiene with Pivaloyl Tetrafluoroborate. Stereoselective Synthesis of endo-2-Pivaloyl-endo-8-hydroxybicyclo[3.3.0]octane. Study of the Reaction Mechanism

Reaction of (Z,Z)-l,5-cyclooctadiene (1) with pivaloyl tetrafluoroborate at -78 °C gives endo-2-pivaloylbicyclo[3.3.0]oct-7-ene (2) and endo-2-pivaloylbicyclo[3.3.0]oct-7-ene (3) in a 5:1 ratio. If the reaction mixture was stored at room temperature before hydrolysis, a mixture of hydroxy ketones 4-6 is obtained. The ratio for compounds 4-6 depends on the particular temperature conditions; to give pure endo-2-pivaloyl-erado-8-hydroxybicyclo[3.3.0]octane 4, the reaction mixture was refluxed at 42 °C for 36 h before hydrolysis. The NMR study of the reaction mixture showed that the primary product is protonated ketone 8 equilibrating with its isomer 7 via protonation-deprotonation. At temperatures higher than -20 °C, 7 and 8 give rise to a mixture of carboxonium salts 9-11, which rearrange into pure 9 after prolonged reflux.