Cyclization of alkoxyiminyl radicals onto Olefins : formation of 2-alkoxy-Δ1-pyrrolines, 4,5-dihydrooxazoles and 5,6-dihydro-4H-1,3-oxazines

Alkoxyiminyl radicals, generated by photolysis of N-bromo imidates, undergo exo-1,5 and exo- 1,6 cyclization onto olefins on the 0-alkyl side chains giving good yields of 4,5-dihydrooxazoles and 5,6-dihydro-4H-1,3-oxazines, respectively. exo-1,5 Cyclization onto an olefin on the iminyl side chain gives 2-alkoxy-∆ 1-pyrrolines. 4,5-Dihydrooxazole formation is more favourable than cyclization to 2-alkoxy-∆ 1-pyrrolines and both reactions are irreversible. These preferences are supported by MNDO molecular orbital calculations which predict that both processes are exothermic but cyclization onto the 0-alkenyl side chain has a lower ∆H.