CATIONIC RING-OPENING OLIGOMERIZATION OF THE 2 STEREOISOMERS OF 4-BROMO-6,8-DIOXABICYCLO[3.2.1]OCTAN-7-ONE

Ring-opening oligomerization of the two stereoisomers of a bicyclic lactone 4-bromo-6,8-dioxabicyclo left bracket 3. 2. 1. right bracket octan-7-one was investigated in dichloromethane and chloroform at minus 40 and 0 degree C using antimony pentachloride, phosphorus pentafluoride, and trifluoromethanesulfonic acid as initiators. The axial isomer showed a tendency to cyclodimerize, particularly at higher temperature, whereas the equatorial isomer was much less reactive and it afforded only a small amount of cyclic oligomers. The specific formation of the cyclic dimer 6 from the axial isomer was interpreted in terms of the participation of bromonium ions in the propagation.