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

摘要

Reaction of (Z,Z)-1,5-cyclooctadiene (1) with pivaloyl tetrafluoroborate at -78 degrees 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-endo-8-hydroxybicyclo[3.3.0]octane 4, the reaction mixture was refluxed at 42 degrees 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 degrees C, 7 and 8 give rise to a mixture of carboxonium salts 9-11, which rearrange into pure 9 after prolonged reflux.