High Temperature Bromination. Part 12: Bromination of 7-Oxabenzonorbornadiene: Synthesis of 2,3-Dibromo-7-oxabenzonorbornadiene

The electrophilic addition of bromine to 7-oxabenzonorbornadiene (8) at 0°C led in high yield to the formation of dibromoaldehyde 10. However, high-temperature bromination of 8 in carbon tetrachloride at 77°C gave non-rearranged products 17 and 18. From the elimination of non-rearranged products, 2-bromo-7-oxabenzonorbornadiene (12) was obtained. Similarly, bromination of monobromide 12 at 77°C yielded the non-rearranged tribromides 19 and 20 while bromination of 12 at 0°C gave the rearranged product 11. The dehydrobromination of tribromides (19, 20) provided the 2,3-dibromo-7-oxabenzonorbornadiene (21), which is a synthon for the trimerization, in high yield